HomeMy Public PortalAbout2014_05_13_O013 DCSM Amendments for Stormwater The Town of
Leesburg,
Virginia�j' �nia PRESENTED: May 13, 2014
ORDINANCE NO: 2014-0-013 ADOPTED: May 13, 2014
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
The Town Council of the Town of Leesburg, Virginia, ordains:
SECTION I. That the majority of Article 5 of the DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG, VIRGINIA, June 27, 1990 .
amended,be and the same hereby is amended to read as follows:
SECTION 5-100 GENERAL INFORMATION
5-110 Intent
5-120 RernaflisReferences
SECTION 5-200 POLICY FOR ADEQUATE STORM DRAINAGE
5-210 General
5 220 Easements
5-230220 Hydrologic Design(For Stormwater Conveyance)
5 231221 Rational Method
5 232222 USDA—NRCS Methodology
5-240230 Closed Conduit Systems
5 211231 General
5 242232 Design Criteria
5 243233 Flow in Gutters
5 214234 Inlet Design Criteria
5 215235 Grate Inlets and Yard Inlets
5-246236 Curb-Opening Inlets
5 217237 EEnd Walls and End Sections
5 248238 General Pipe System Design
5-243239 Energy and Hydraulic Gradients
5-258240 Open Channels
5 251241 Natural Watercourses
5 252242 Man Made Stormwater Conveyance Channels
5 253243 Roadside and Median Ditches
5 254244 Lot Drainage Swales
5 255245 Stream Modification
5-256246 Flow Design
5 257247 Water Surface Profile Computations
5 258248 Riprap
5-260250 Culverts
5 261251 Design Criteria
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
SECTION 5-300 POLICY FOR STORMWATER MANAGEMENT
5-310 General
5-311 - -- . - .. _ . - ' _ -. - •- - - Applicability
5-312 - . _ - .. _ Design Storms and Hydrologic
Methods
5 313 Rational Method
5 311 USDA NRCS Methodology
5-320 Water Quality
5-321 ! _• _ _ .. . _ , _ - - - - . _' ' •- Environmental Site Design
5-322 General CriteriaHot Spots
5-323 Detention PondcStream Delineation and Buffer Criteria
5 321 Embankment Ponds
5 325 Excavated Ponds
5 326 Wet Ponds
5 327 Visual Resource Design Wet and Dry Ponds
5 328 Stormwater Management Pond Plans
5-330 Re Water Quantity
5-331 D etention Measures
5-332 Plan-PfefrarationAdequate Outfall
5-333 Pro-Rata Share Program
5-340 Stormwater Management Facilities
5-341 Design of Stormwater Management Facilities within the Tuscarora Creek
Watershed
5-342 General
5 342343 Design CriteriaExcavated Ponds
5 343344 Visual Resource Design
5-345 Stormwater Management Pond Plans
5-346 Rooftop Detention
5-347 Underground Detention
5-348 Porous Pavement
5 350 Porous Pavement
5 351 General
5 352 Design Criteria
5-360350 Regional Facilities
5470360 Waivers and Exemptions
5-380370 Inspection and Maintenance Provisions
SECTION 5-400 FLOOD PLAIN POLICY AND GUIDELINES
5-410 Applicability
5-420 Policy on Use of Flood Plain Areas
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
5-421 Warning and Disclaimer of Liability
5-422 Processing of Subdivisions and Site Plans Within or Immediately Adjacent to
Flood Plains
5-430 Preparation of Flood Plain Studies
5-431 General
5-432 Plans
5-440 Existing Construction in Flood Plain
SECTION 5-500 DAMS DESIGN AND CONSTRUCTION
5-510 Regulations
5-511 Virginia Department of Historic Resources, Division of Soil and Water
Conservation(VDHR SWC)
5-512 Town of Leesburg Regulations
5-520 Design Criteria
5-521 Embankment Ponds
. . . 1 1 Ii . . . a a
5 610 General
5 620 Water Quality Design Criteria
5 630 Exemptions to Water Quality Requirements
5 640 Hot Spots
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► -
BMP Tables
SECTION 5400600 INSPECTION AND ACCEPTANCE
5 710610 Inspection
5-729620 Acceptance
SECTION 5-700 EASEMENTS
5-701 General
5-702 Easement Applicability and Width
DETAILS
Drawing No. Title Page
DD-1 Time of Concentration, Small Basins
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
DD-3 Peak Rainfall Intensities
DD-4 Peak Rainfall Intensities
DD-5 Radius of Curvature
DD-6 Intentionally Left Blank
DD-7 Intentionally Left Blank
DD-8 Intentionally Left Blank
DD-9 Intentionally Left Blank
DD-10 Hydraulic Grade Line in Closed Conduit Junction
DD-11 Design Storm for Dams
DD-12 Danger Reach Length
DD-13 Hp and Slope at Retardance Values
DD-14 Roof Top Stormwater Detention
DD-15 Intentionally Left Blank
DD-16 Stormwater Inlet Computation Chart
DD-17 Storm Sewer Design Computation Chart
DD-18 Hydraulic Grade Line Chart
SECTION 5-100 GENERAL INFORMATION
1. Except as specified below, stormwater requirements for each land development
application shall be subject to the version of this DCSM in effect at the time of initial
acceptance.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
2. A land development application that is subject to the grandfathering provisions of
Section 14-23(e)(4)of the Town Code shall meet the technical criteria of Section 5-
311.5 of this DCSM as well as the following additional provisions:
A. The land development application shall have been approved prior to July 1, 2012.
B. The land development application shall be a proffered plan of development,
Special Exception, Preliminary Plat of Subdivision,Record Plat, Construction
Plans and Profiles, Site Plan, Minor Site Plan, Mini Site Plan, Site Plan Waiver,
Dedication Plat, Boundary Line Adjustment, Easement and Vacation Plat,
Dedication Plat as well as any Plat and Plan Revision.
C. "Layout"means a conceptual drawing sufficient to provide for the specified
stormwater management facilities required at the time of approval, as defined in
9VAC-25-870-10.
D. The land development application has not been subsequently modified or
amended in a manner resulting in an increase in the amount of phosphorus leaving
each point of discharge or the volume or rate of runoff. In order to verify this
condition, a comparison of the layouts between the original land development
application and the modified version may be utilized. If the comparison of
layouts is not conclusive, a comparison ofperformance based calculations found
in the technical criteria of Part ILC (9VAC25-870-10) as well as water quantity
engineering calculations shall be required.
E. Land development applications on parcels or lots which are part of a residential,
commercial, or industrial subdivision served by an approved stormwater
management facility designed to treat the said parcel or lot shall be deemed
grandfathered.
3. The overall drainage system for the 10 year and larger storm events for all plans
submitted to the town will consist of a major and a minor drainage system. Any plans
submitted to the town that are subject to the grandfathering provisions of Section 14-
23(e)(4)of the Town Code or those plans vested prior to July 1, 2014, shall also
consider major and minor drainage systems for the less frequent 1 and 2 year storm
events.
A. Minor The minor drainage systems will-consists of open channels and/or
closed storm sewer appurtenances and conduit drainage systems such as
inlets, manholes, street gutters,roadside ditches, swales,underground
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
pipe and small channels from the point of interception to the point of
discharge in all developments.
B. Major The major drainage systems will-consists of natural waterways;
and . .. : . - ... : • - - . . •
drainage ways such as overland relief swales and paths. The major system
_
.. - ••, . . also includes
the natural backup system which functions in case of overflow from or
failure of the minor system. Therefore,the major drainage system shall be
designed to provide overland relief to convey the 100-year rainfall event
as if the minor system has failed to function or did not exist.
C. Overland relief requirements can-beare found in Section 5-200-of-this
mariaal.
D. Either system . .. . •• . - •- . - . •• . - ., -
fasilitiesmay include stormwater quantity and/or quality management
facilities, including regional stormwater management facilities.
4. 3--When development proposes to:
a. Relocate existing storm drainage lines/stormwater management facilitiesi or.
b. Encroach upon existing storm drainage lines/stormwater management
easements with physical improvementsi or
b c.Reduce cover over existing storm drainage lines to less than that specified by
this manua1.DCSM: or
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
e- Increase cover over existing storm drainage lines to more than that specified
by this ffialittalDCSM; then.
Then the Developer-developer shall be responsible for replacement of the storm
drainage line/stormwater management facility at a new location during
development of the property. Such replacement shall be to the standards and
specifications set forth in this fnanitalDCSM, shall be approved by the Director,
and shall be at no cost to the tewnTown.
5-110 Intent
It is tThe intent of this Article is to require that all components of the drainage system the
- . - - . - _ . • • •- meet or exceed applicable
drainagesstormwater management laws and regulations.
5-120 RetnarksReferences
The following regulations and technical documents are included
by reference for storm drainage system design and performance standards within the
Town of Leesburg. Applicable provisions of these regulations and technical documents
are referenced as appropriate in this DCSM. Where there is a conflict between the
standards presented in this DCSM and the regulations and technical documents reference
in this section,the more stringent of the standards will apply unless otherwise modified
by the Director:
1. Virginia Stormwater Management Program(VSMP) Permit Regulations,
9VAC25-870 et al.
2. Virginia Erosion and Sediment Control Regulations, 9VAC25-840 et al.
3. Virginia Chesapeake Bay Preservation Area Designation and Management
Regulations, 9VAC25-830 et al.
4 4. Virginia Erosion and Sediment Control Handbook, Virginia Department of
Environmental Quality (DEO),latest edition.
2,5. The Virginia Department of Transportation Drainage Manual,Virginia
Department of Transportation(VDOT), latest edition.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
3-6. The Virginia Department of Transportation Road and Bridge Specifications,
VDOT,-Latest latest edition.
47. The Virginia Department of Transportation Road and Bridge Standards, VDOT,
latest edition. , Latest edition.
8. Urban Hydrology for Small Watersheds (TR-55), Soil Conservation Services,
Washington, D.C.,National Technical Information Service, Springfield, Virginia
22161, latest edition,-Latest-edition-or(NCRS version WIN TR-55, Latest latest
edition_)
6 9. TR-20 Project Formulation-- Hydrology Soil Conservation Service, Lanham,
Maryland,National Technical Information Service, Springfield, Virginia 22161,
ILatest edition,or ENCRS version WIN TR-20,Latest-latest edition,)
10. Virginia Stormwater Management Handbook,Published e— ginia
•-- . . - . .. . ' _ e ' DEQ,Ibatest edition_
11. Virginia Stormwater BMP Clearinghouse, DEQ and the Virginia Water Resources
Research Center(VWRRC) at Virginia Tech, latest edition.
12. Modern Sewer Design, American Iron and Steel Institute, latest edition.
13. Specification for Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe,
ASTM Standard C76, 2013a, ASTM International, West Conshohocken, PA,
2013, DOI: 10.1520/C0076-31A.
14. Design of Riprap Revetment, Federal Highway Administration(FHWA),
Hydraulic Engineering Circular No. 11 (HEC 11), FHWA-IP-89-016,
Washington, DC, 1989.
15. Hydraulic Design of Energy Dissipaters for Culverts and Channels, FHWA,
Hydraulic Engineering Circular No. 14 (HEC 14), FHWA-NHI-06-086,
Washington, DC, 2006.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
16. Design of Roadside Channels with Flexible Linings, FHWA, Hydraulic
Engineering Circular No. 15 (HEC 15), FHWA-NHI-05-114, Washington, DC.,
2005.
17. Methodology for Identification of Intermittent and Perennial Streams and Their
Origins,North Carolina Department of Environment and Natural Resources.,
Division of Water Quality, latest edition.
7,18. Other regulations and technical documents as approved by the Director.
- - I _ . -. . •
. _ • _ . _
edition
9For Additional additional References references and definition of terms used within this
Article,refer to Article 11 of the-this DCSM.
(End of Section)
SECTION 5-200 POLICY FOR ADEQUATE STORM DRAINAGE
5-210 General
1. An evaluation shall be performed for all proposed drainage systems to ensure
adequate hydraulic capacity for conveyance of the minimum ten-year event
including, but not limited to channels, stormwater management facilities, and
conduits.
2. Hydraulic capacity must be verified with engineering calculations, in accordance
with the procedures outlined in the Virginia Erosion and Sediment Control
Handbook,the Virginia Department of Transportation Drainage Manual,the
Virginia Stormwater Management Handbook,the Virginia Stormwater BMP
Clearinghouse, or other methods acceptable to the Director.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
3. The drainage system must have the hydraulic capacity to accommodate the
maximum expected flow of surface waters from a drainage area at a point of
concentration for the duration and intensity of rainfall, as specified herein.
4. Determination of the size and capacity of the drainage system shall be based on
the planned development, existing zoning, or Town Plan, whichever is greater,
within the watershed.
5. Due consideration must be given to infrequent events (100-year)resulting in
runoff quantities greater than minor system design capacity. The Therefore,the
design for the major drainage system shall provide for overland relief of the 100-
year event without flooding or damaging buildings and structures, generally
without reliance upon the minor drainage system. Specifically, all construction
plan sets shall contain provisions for the 100-year overland relief as follows:
A. If the area draining to a low point inlet is equal to or less than one(1)acre,
a minimum of one (1) foot shall be provided from the lowest opening of
any building to the ground elevation at the relief point along any point of
the path of overland relief. The designer shall also have the option to use
engineering calculations and cross sections to determine the actual 100=
year water surface for any size drainage area and provide the required
freeboard as noted below for drainage areas in excess of one (1)acre.
B. If the area draining to a low point inlet is greater than one(1) acre,
engineering calculations and cross sections shall be provided to verify that
the lowest point of entry of all buildings will-be a minimum of 6 inches
vertically above the 100:-year water surface at the relief point along the
overland relief pathways (to a minimum 6 inches of freeboard). Where
the area of overland relief is restricted and the applicant can prove that
adjacent and downstream structures will not be flooded or damaged by the
100-year overland relief flow,the Director may, on a case by case basis
reduce the amount of freeboard required for the 100 1-year overland relief
water surface to zero (0) feet such that the 100-year water surface
elevation is at the elevation of the lowest opening of the building or
structure.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
C. Unless the construction drawings depict a minimum of one foot of
freeboard has been provided from the 100-year water surface to the
lowest opening of an adjacent building(s), an as-built drawing shall be
provided,prior to issuance of an occupancy permit,to specifically depict
the controlling elevation along the path of overland relief as well as the
elevation of the lowest opening of any adjacent building(s)to ensure the
site has been constructed in conformance with the approved plans and that
the adjacent buildings will not flood during the 100:-year rain storm event.
D. On a case by case basis,the Director may approve a modification to the
requirements set forth in this section of the DCSM to permit calculations
associated with the proposed development to consider a portion of the
upstream and/or onsite 100:-year"overland"relief flow to be routed
through the underground storm pipe drainage system when:
(1) The overland relief path is restricted by site constraints beyond the
applicant's control; and
(2) The underground storm drainage system consists of a RCP pipe 48
inches (or equivalent) and larger in diameter; and
(3) The storm inlets are designed to capture the portion of the 100:
year storm event to be routed through the underground pipe; and
(4) The underground storm system pipe has been designed to have
capacity for the required 10:-year flow plus the portion of the 100:
year flow to be utilized in the underground pipe drainage system;
typically, no more than 50%of the 100-year overland relief flow
will be allowed to be routed through the underground pipe
drainage system.
*A modification request may not be required for overland relief
calculations that only analyze offsite downstream properties.
6. The drainage system shall be designed:
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
A. To generally honor all natural drainage divides and create no adverse
impact on downstream properties.
B. To account for all off-site and on-site surface water.
C. To manage, convey,and discharge surface waters as outlined in the
Virginia Stormwater Management Handbook. ,
D. To manage, convey,and discharge surface waters to a stormwater
detention-management facility of sufficient capacity and pollutant removal
efficiency to : - . ... - -- . _ ., - - , : •
. . . -: • eet the
requirements of Section 5-300 "Policy for Stormwater Management." The
Director may require additional design criteria based on the watershed's
special requirements, as identified in the Town's Stormwater Master Plan.
E. To protect residences and other occupied structures from being inundated
with stormwater.
F. To not increase stormwater sheet flow(non-concentrated) into a lower-
lying property.
G. To preserve adequate natural channel characteristics to the extent
practicable.
--
d of o 3t
LI-I. To provide (if the above conditions are not met)a drainage system
satisfactory to the Director,to provide an acceptable adequate-outfall in
accordance with the stormwater requirements in this DCSMSection 5 311
,and to preclude adverse impacts upon adjacent or
downstream properties.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
7. Except where prohibited by Chapter 14 of the Town CodeSection 14-23 of the
Town Code, the Owner owner or Developer-developer may continue to discharge
stormwater as sheet flow(non-concentrated) into a lower-lying property if at the
same location:
A. The post-development peak runoff rate based on documentation and
computations, including sheet flow, does not exceed the pre-development
peak rate; armor
B. If the above conditions are not met,the developer must provide a drainage
system satisfactory to the Director,to preclude adverse impacts upon
adjacent or downstream properties.
8. The Owner owner or eloper-developer may not discharge stormwater which
has been artificially concentrated by a pipe, culvert, channel, or other drainage
structure, onto or through lands of another without first obtaining and transferring
to the Town a permanent storm drainage easement to guarantee continuity of an
outfall from the point of discharge to the nearest natural watercourse.stormwater
conveyance system. Refer to Section 5 311 700 .
9. If off-site downstream construction and easements are required to construct-an
adequateprovide channel out€allprotection or flood protection, no plans shall be
approved until such storm drainage easement , - - - . , - - .. . •.watercourse have been obtained and recorded. It will be the responsibility of the
Developer-developer to obtain all off-site easements.
10. Storm sewer systems shall be designed in a manner such that their outfalls:
A. Are designed to reduce erosion of surrounding soils.
B. Discharge at a natural watercourse:
(1) Generally, it is better to discharge at the 100-year flood plain limits
into an adequate channel leading to the main stream bed,rather
than disturb the flood plain by extending the storm sewer system.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
(2) If there is no well_-defined adequate receiving channel at the flood
plain limits, one shall be constructed to the bed and banks of the
main channel. If wetlands are encountered,the Director may on a
case by case basis approve a modification to utilize a forebay or
other type of an energy dissipater dissipation device at the pipe
outfall to minimize the impact to existing wetlands areas.
C. Discharge at other locations:
(1) Energy dissipation devices and/or friction channel linings shall be
used when discharge velocities exceed the maximum permissible
as defined by the Virginia Erosion and Sediment Control
Handbook;or at the discretion of the Director where non-uniform
channel linings are involved. -Generally,the use of natural rock
located on the subject development site is encouraged when the
equivalency can be met by a geotechnical engineer's written
certification of the material.
a. 2 FPS to 5 FPS velocity: Sod protection(Kentucky Blue
Grass or equally resistant sod or other material as approved
by the Director)or VDOT CLI Rip Rap or current
equivalent as noted above.
b. 5 FPS to 8 FPS velocity: VDOT CLI Rip Rap or current
equivalent as noted above; Length of Rip Rap to be
determined per-in accordance with the Virginia
FASErosion and Sediment Control Handbook.
c. 8 to-18 FPS velocity: VDOT CL II dry Rip Rap(VDOT
Specification 418.04) or current equivalent as noted above;
Length of Rip Rap to be determined per-in accordance with
the Virginia E&SErosion and Sediment Control Handbook.
d. Velocities in excess of 18 FPS: Shall only be permitted
with special design energy dissipation devices
or impact basins and only with the approval of the Director.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
D. Shall not discharge in the front yard of any single family detached or
attached lot. The outfall shall discharge at the rear lot line and meet all
criteria noted in this manualDCSM.
11. Except as set forth in Section 5-420,paragraph two, Policy on Use in Flood Plain
Areas, all drainageways, including overland relief pathways, must be separated
from buildings as stated in this section of the-this DCSM.
12. Plans shall be prepared to preclude adverse impacts due to higher flow rates that
may occur during construction. Refer to Article 6 of this MantiaIDCSM.
13. Drainage structures shall be designed and constructed in such a manner that they
may be maintained at a reasonable cost and with methods and types of equipment
currently used by the town.
A. To facilitate design, construction, and maintenance, drainage facilities
shall meet and conform to the Town of Leesburg and Virginia Department
of Transportation Standards.
B. Special alternative designs for reinforced concrete special structures and
for reinforced concrete prefabricated pipe junctions (pipe 60" diameter an.
larger) shall be considered by the Director on a case-by-case basis
provided they are designed and documented.
(1) All structures and pipe junctions shall be by use of VDOT standar.
reinforced concrete structures unless a specific alternative design i
approved by the Director. Specific alternative designs will not be
approved if a standard structure will perform the function.
(2) Alternative designs which have received VDOT pre-approval are
required if the structure is located within a Town road right-of-way
and are preferred in other locations; however, an alternative design
prepared by a structural engineer licensed to practice in the
Commonwealth of Virginia is acceptable under the following
conditions:
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
a. The structure is not located within a Town road right-of-
way.
b. The structure or junction is prefabricated by a manufacturer
normally engaged in reinforced concrete pipe and structure
manufacture.
c. All structural design, signed, sealed and dated(original seal
affixed), is provided to the town. The structural design
shall not be included on the construction drawings as the
town will not review or approve the structural design which
shall remain the responsibility of the engineer of record.
d. Construction details delineating how the special structure
or the prefabricated pipe junction is to be incorporated into
the drainage system, and detailing the assembly of the
entire drainage structure shall be included in the
construction drawings.
(3) Manhole access shall be provided at each special structure and at
each prefabricated pipe junction:
a. Minimum 4' diameter manhole.
b. Manhole step side shall align with pipe springline to
provide standard manhole steps and straight line access
from pipe springline to ground surface. Manhole steps
shall not span any pipe opening greater than 15".
(4) Small diameter pipe (less than 60" diameter)which enter drainage
structures which contain a prefabricated pipe junction shall:
a. Enter the access manhole at the lowest elevation possible
above the prefabricated pipe junction casting.
OR
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
b. Shall be included as an integral part of the prefabricated
pipe junction casting.
c. In no case shall small diameter pipes be field connected
into the prefabricated pipe junction.
14. In order to protect downstream properties from flooding, erosion, sedimentation,
and/or other damages,the eloper developer must show, with appropriate
engineering calculations approved by the Director,that the existing off-site
downstream drainage system can accommodate the drainage from the developing
property without damage to existing facilities,properties, or buildings in
accordance with Section 5-300 of this DCSM.
-- - -- - •
• - - . •
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
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- ■ - ■'
. . • - . . . . . . - .- • • -• . . . . .. .. • ,
- - - ■ -
D. Delay development until the necessary off site facilities or improvements
are constructed by the Town or others. Other arrangements, specific to the
- -- ■•
by the Developer.
5 220 Easements
inerements:.
A. Pipes
(1) For single pipes 24 inches and less in diameter the easement width
(2) For single pipes greater than 24 inches in diameter the easement
of the pipe.
- s .
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
('1) The minimum easement width for any storm sewer shall be 15 feet.
(5) Refer to Standard WS 16 in Appendix A.
B. Channels
..
chan..e d.
C. Yards Inlets and End Sections
elevation:.
ccc=.=cccivir.
--• I! ., . - - ••- . .
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.•
- -- I!
5-230220 Hydrologic Design (For Stormwater Conveyance)
5-231221 Rational Method
1. This method is to be used for determining the design runoff for sizing all storm
sewer systems, including but not limited to, culverts, conduits and man-made
stormwater conveyance channels with drainage areas less than 200 acres. Refer
to the Virginia Department of Transportation Drainage Manual for methodology.
Refer to Detail DD-1 of this Article for "C" factors.
A. To estimate Time of Concentration(Tc),refer to Detail DD-2 of this
Article.
B. Unless otherwise specified, the prescribed design storms are the one-year,
two-year, and 10-year 24 hour storms using the site speci ier rainfall
- - . - . _ . .. - - -- rainfall peak
intensity charts for the Town of Leesburg. Refer to Detail DD-3 and DD-4
of this Article.
I 5-232222 USDA SCS �,.,...ew NRCS) �,r,.thodolog.,NRCS Methodology
1. The NRCS method shall be employed for the determination of runoff for
evaluation of all improvements to natural watercourses and storm sewer systems
with drainage areas greater than 200 acres unless an alternative method is
approved by the Director through a DCSM Medificatienmodification. This
methodology for estimating runoff is based on a more rigorous analysis of the
factors affecting runoff rates, and was developed by the United States Department
of Agriculture (USDA)—Soil Conservation Service (SCS). This method,now
known as the Natural Resources Conservation Service(NRCS)method, is
considered more accurate than the rational method for runoff determination
because more drainage shed characteristics are considered in the analysis. There
are several major methods of analysis with the NRCS method:
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A. The old version TR-55 method(manual as well as computer-based)
entitled "Urban Hydrology for Small Watersheds(Technical Release 55)"
or new NRCS WIN TR-55 (Windows Based Program).
B. The old version TR-20 (computer-based) method entitled "TR-20
Computer Program for Project Formulation-Hydrology(Technical Release
20)" or new NRCS WIN TR-20 (Windows Based Program).
C. HEC-HMS (latest version accepted by the Corps of Engineers)may be
approved by the Director on a case by case basis.
D. Other programs which utilize the methodology of TR-55 or TR-20 may be
approved by the Director on a case by case basis.
E. Shed parameters to be considered and analyzed when using the NRCS
method are as follows:
(1) Drainage area.
(2) Land use within the shed and associated imperviousness.
(3) For soil types, refer to Loudoun County Soils Maps.
(4) Shed response time(s).
I 5-240230 Closed Conduit Systems
I 5-244231 General
1. The closed drainage system will be referred to as a storm sewer system, and
normally consists of curbs and/or gutters, drop inlet structures, laterals and trunk
lines,junction chambers and manholes.
2. The purpose of a storm sewer system is to collect stormwater runoff within the
on—site and off-site drainage divides and convey the runoff to an outfall.
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
I 5-242232 Design Criteria
1. Storm sewers with drainage areas up to 200 acres may be designed using the
Rational Method only as outlined in the Virginia Department of Transportation
Drainage Manual.
2. Pipe placed in public easements or that are under public maintenance, shall be
manufactured of reinforced concrete. Other materials such as those listed below
may be used in private systems. Materials other than those listed may be
considered for use in private systems with the approval of the Director.
A. Pipe and culvert materials acceptable for storm drain construction with the
accompanying Manning's roughness coefficients "n" are shown below:
Material Manning"N"
Reinforced Concrete Pipe (RCP) 0.013
Vitrified Clay Pipe, Extra Strength(VCPX) 0.013
Cast Iron Pipe (CIP) 0.013
Polyvinyl Chloride Pipe (PVC) 0.011
I Annular Corrugated Metal Pipe (CMP) 0.0242
(Fully Paved to Unpaved)
Helical Corrugated Metal Pipe (HCMP),
Corrugations are as follows:
2-2/3"x 1/2" 3"x 1"
Diameter 18" 24" 36" 48" 60" 72" 84" 96" All Diameters
Plain or Coated .014 .016 .019 .020 .021 .021 .021 .021 .024
Paved Invert NA .015 .017 .018 .018 .018 .018 .018 .021
Smooth Interior Fully NA .012 .012 .012 .012 .012 .012 0.12 .012
Paved
*The use of PVC is restricted to drains only.
** NA=Not available.
B. Reinforced concrete pipe shall conform to ASTM Designation C-76, III
and IV; a minimum of Class III or equal is required under areas subject to
vehicular traffic. Class IV is required with cover less than two feet, areas
subject to impact loads, or where strength computation based on depth
requires the higher pipe class.
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
3. Distance Requirements. The distance between points of access in storm sewer
trunk lines shall be limited to 50 feet for 12-inch pipe; 300 feet for 15-inch to 42-
inch pipe; and 500 feet for 48-inch and larger pipe. The distance between access
points may be increased to 400 feet for 15-inch to 42-inch pipe if the flow
velocity exceeds five feet per second and the depth of flow is a minimum 25
percent of the pipe diameter.
4. Minimum pipe size. The minimum acceptable size pipe for publicly maintained
systems shall be 15-inch or its equivalent elliptical shape. For private system
designs, it is permissible to use 12-inch(or equivalent)pipe as the initial pipe in a
system, or as a lateral line when necessary, provided that the distance between
access points is 50 feet or less.
5. Pipes 15 inches in diameter and larger may be constructed on horizontal curves
with the prior approval of the Director. Refer to Detail DD-5 of this Article for
geometric information to assist in the design of concrete pipes on horizontal
curves. Prefabricated bend sections may also be used for this purpose. When this
option is approved,the designer shall provide direction during construction to
ensure the system's integrity. Refer to Standard DS-1 in Appendix A.
6. In general,there may not be a reduction in pipe size greater than one increment
moving downstream along the direction of flow unless approved by the Director.
7. Minimum cover for all round, arch, elliptical etc. storm sewer pipe as well as all
Box Culverts, Conspans and other closed conveyance systems shall be two feet
vertically from finish grade to the outside top of pipe and conveyance systems
listed herein, except where structural correction is provided, as approved by the
Director. Requests for less than two feet of cover shall be recorded on the cover
sheet.
8. Maximum cover is determined from the field supporting strength. Refer to Article
4, Section 4-140, of this Manua1DCSM.
9. Storm sewers should be designed based upon actual pipe flow to provide a
minimum velocity of 2.5 feet per second. If this minimum velocity cannot be
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achieved utilizing the actual design flows, full flow may be assumed in the pipes
to achieve a velocity when running full of not less than 2.5 feet per second.
. 10. Storm sewers shall be designed with a minimum slope of 0.5%. Slopes less than
0.5%may be considered in specific situations on a case by case basis with the
approval of the Director.
11. Storm sewers shall generally be designed so as not to exceed a design or full
velocity of 15 feet per second. Special designs shall be supported by hydraulic
grade line calculations and special design pipe to account for the additional
velocity when approved by the Director.
12. The need for concrete anchors shall be investigated on storm sewer lines with
slopes of 20 percent or greater. If anchors are required,the design engineer shall
show a detail on the plans with spacing requirements.
13. Generally storm sewers shall not have an outfall within a lot used for residential
purposes. If with specific approval of the Director, an existing storm sewer
outfalls on a lot, or adjacent to a lot, on which a building exists and which will
remain,the building must be shown with topography of the area between the
building and the outfall. The 100-year water surface elevation,the lowest point of
entry, and the floor elevations of the existing building shall be provided to
demonstrate compliance with Section 5-210 of this MantialDCSM.
14. When a trunk line passes through a structure, it shall generally match crowns and
the pipes may be adjusted to match to energy gradient as a maximum. Where
matching the energy gradient creates drops in excess of 2.5 feet,these will be
reviewed and approved by the Director on a case-by-case basis. In no case shall
the crown of the inlet pipe be lower than the crown of the outlet pipe.
15. The deflection angle from the forward projection of the centerlines of an inflow
trunk line pipe to the outflow trunk line pipe at any junction shall not exceed 90
degrees.
5-243233 Flow in Gutters
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
1. Pavement gutter is defined, for purposes of this MarittalDCSM, as the portion of a
roadway adjacent to the curb which conveys water during a storm runoff event;
gutter in this sense would include a portion of a travel lane. Gutter cross sections
generally have a triangular shape with the gutter of uniform cross slope and the
curb forming the near-vertical leg of the triangle. Refer to Detail DD-6 and DD-7
of this Article.
2. The gutter pan is defined, for purposes of this MafittalDCSM, as the portion of
integral concrete curb and gutter which slopes downward to the face of the curb.
3. Modification of Manning's Equation is necessary for use in computing flow in
triangular channels because the hydraulic radius in the equation does not
adequately describe the gutter cross section,particularly where the top width of
the water surface may be more than 40 times the depth at the curb. To compute
gutter flow, horizontal spread, or gutter depth, Manning's Equation is integrated
for an increment of width across the section. Refer to the Virginia Department of
Transportation Drainage Manual for methodology.
I 5-244234 Inlet Design Criteria
1. The spread of water on roadway pavements shall be limited to eight feet from the
face of curb or one half of the travel lane(excluding curb and gutter)whichever is
less. All design shall utilize a minimum rainfall intensity of 4.0 inches per hour.
For runoff events resulting from greater rainfall intensities,the entire roadway
section may be used for the conveyance of stormwater.
2. There are sites where it may reasonably be anticipated that the runoff from storms
with rainfall intensities greater than 4.0 inches per hour will overtax the
interception facility to the point that excess flow may result in damage to adjacent
property and roadway right-of-way. In these instances, a check storm with a
rainfall intensity of 6.5 inches per hour should be run. If all of the runoff is found
to be contained within the roadway section,both at the site and "downstream", or
if runoff escaping the road section is found to be non-damaging to adjacent
property,the interception facility may be used as originally designed under the
general criteria. If the interception facility fails to meet the check storm criteria,
it must be redesigned to accommodate the check storm.
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
3. For spacing of inlets refer to Virginia Department of Transportation Drainage
Manual.
4. Inlets located on continuous grade should be designed to intercept all of the gutter
flow. There will be cases, however, where it is desirable to reduce the length(due
to inlet inefficiency)through the use of runoff bypass. This does not infer that
the bypass volume be neglected. Inlets which have bypass flows shall be clearly
marked on the plans and bypass flow must be included in the total gutter flow
contributing to the next downstream inlet. Generally, bypass flow should not
exceed the capacity of the street gutter pan.
5. Where an inlet is located at the bottom of a sag vertical curve (referred to as a
sump or low point) for roads classified as through collector or higher, all of the
flow must be intercepted by the inlet.
6. To properly drain sag vertical curves, it is required on roads classified as through
collector or higher to place three inlets in each curve; one inlet at the low point
and one flanking inlet on each side of the low point. The flanking inlets should be
placed so that they will limit the spread in the low(flatter) gradient approaches to
the sag point and will act in relief of the sag inlet if it should become clogged.
Refer to the Virginia Department of Transportation Drainage Manual for flanking
inlet spacing.
7. Where the pavement on a continuous grade is warped in transition between super-
elevated and normal sections, water conveyed along the curb shall be intercepted
at the point in transition where the cross slope of the road section is equal to one
percent to maintain spread requirements; further,road pavement with less than
one percent cross slope towards a curb and gutter section shall not be utilized to
convey flow. All flow in areas of less than one percent cross slope, except at
median header curb, shall be confined to the gutter pan. Water concentrated in a
pavement gutter shall not escape the gutter and cross the traveiway before
interception by an inlet.
8. No reverse curb and gutter(Virginia Department of Transportation CG-6R) shall
be allowed in public rights-of-way without prior approval of the Director.
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9. Inlets shall be placed on the high side of super-elevated sections such that flow
does not leave the gutter pan.
10. Where curbs are used in cut slope areas, runoff shall be collected through a
system of ditches and inlets at the top of the cut slope prior to the flow traversing
the slope and entering the right-of-way.
11. No concentrated flow greater than two cubic feet per second based upon the two-
year event shall cross a sidewalk or curb.
12. When stormwater is being conveyed along the pavement gutter of a street with a
longitudinal slope of less than two percent, a maximum of two cubic feet per
second may cross the intersection of a street with less than 500 vehicles per day.
Where the longitudinal grade is two percent or greater, four cubic feet per second
may cross the intersection of a street with less than 500 vehicles per day.
13. No flows will be allowed to cross the intersections of streets with 500 or more
vehicles per day.
14. When bridges are located adjacent to gutter sections, it will be necessary to
coordinate the drainage design with the bridge designer. For bridges without dec i
drains, the flow from the entire bridge plus any flow crossing the bridge shall be
collected in the storm sewer system. For bridges with deck drains, it will be
necessary for the drainage designer to review each drain's size,type and location.
The flow capacity of deck drains as well as their potential for clogging shall be
fully considered and the storm sewer system designed accordingly.
15. The interception facilities for interstate highways, limited access highways and
other major arterials should be designed so that the spread (based on a rainfall
intensity of 4.0 in./hr.) does not exceed one-half of the running lane width, and
there is no significant damage to adjacent property during a storm whose rainfall
intensity is determined by a 50-year recurrence interval and the actual time of
concentration.
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
16. Under certain circumstances, such as underpasses or depressed roadway sections,
where ponded water can only be removed through the storm sewer system, the 50-
year storm(using the actual time of concentration) should be used as the check
storm and excessive depth of ponding should be avoided.
5-245235 Grate Inlets and Yard Inlets
1. Grate and yard inlets in a sump location shall be designed assuming 50 percent
clogging. These inlets operate as weirs at shallow depths and as an orifice at
greater depths. Grates of larger dimension and grates with more open area, i.e.,
with less space occupied by lateral and longitudinal bars,will operate as weirs to
greater depths than smaller grates or grates with less open area. Refer to Virginia
Department of Transportation Drainage Manual, for specific grate inlet design.
Refer to Standards DS-2, and DS-3 in Appendix A.
2. Grate inlets outside the travelway of public roads on continuous grade shall be
designed assuming 50 percent clogging. Flow passing over the grate, if
applicable, shall be collected at the next downstream inlet. Refer to the Virginia
Department of Transportation Drainage Manual.
3. Grate inlets within roadway pavement, face of curb to face of curb of public
roads, are prohibited. Standard VDOT DI-2 inlets are permitted within the
parallel parking lanes within the Crescent Design District. All other applications
for using DI-2 inlets may be used on a case by case basis with the prior approval
of the Director.
I 5-246236 Curb-Opening Inlets
1. Interception capacity of a curb-opening inlet is largely dependent on flow depth at
the curb and curb-opening length.
A. Effective flow depth at the curb and consequently, curb-opening inlet
interception capacity and efficiency, is increased by the use of a local
depression at the curb-opening or a depressed gutter to increase the
proportion of the total flow adjacent to the curb. Local depression shall be
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two inches for curb inlets with CG-6 and one inch for curb inlets with
Virginia Department of Transportation CG-2.
2. Curb-opening inlets in continuous grade situations are effective in the drainage of
pavements where flow depth at the curb is sufficient for the inlet to perform
efficiently. Curb-openings are relatively free of clogging tendencies and offer
little interference to traffic operation. Curb-opening inlets are required in the
public right- of-way. Refer to Detail DD-8 of this Article.
3. The required method for determining the length of a curb-opening inlet required
for total interception of gutter flow is located in the Virginia Department of
Transportation Drainage Manual.
4. For curb-opening inlets in a sump condition,refer to Detail DD-9 of this Article.
I 5-247237 End Walls and End Sections
1. End walls and end sections which have pipes 24 inches in diameter and larger
which are installed in residential developments within the Town of Leesburg shall
be provided with a minimum 42-inch high fence or protective railing.
2. The fence or protective railing shall be installed at the top of the end section or
integrally on top of the end wall for the portion of the wall where the drop is
greater than 18 inches.
3. The protective railing must have no opening greater than six inches.
4. The protective railing must be of corrosion resistant material and must not
obstruct the overland relief.
1 5448238 General Pipe System Design
1. Before starting the detailed design of the pipe line,the design engineer must
consider various controls which will govern the subsequent location, alignment,
depth, size, and cost of the systems.
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
A. Consideration should be given to the location of existing outfalls or
natural watercourses which are to be utilized, natural drainage divides,
proposed roadway design features such as low and high points in the
grade, super- elevated curves, street intersections, existing and proposed
utility lines; other existing and proposed storm drainage.
B. When an existing storm sewer is to be utilized, either partially or totally, it
shall be necessary to ascertain the invert elevations for all pipes, drop
inlets, catch basins, manholes, etc. This information should extend well
beyond the limits of the proposed project,both laterally and
longitudinally, at least to the next access structure, and continuing to the
point of hydraulic adequacy. The invert elevation of each pipe in a drop
inlet or manhole shall be ascertained, as well as the type of pipe.
Information obtained shall be based upon an as-built survey of record or
actual field run elevations.
C. It is essential that all utilities in the area of existing or proposed drainage
facilities be located horizontally and all gravity utilities located vertically
in order to avoid future conflicts. This is particularly important in the case
of gravity sanitary sewers because adjustment of such facilities would be
difficult and costly.
D. Test pits will be required for crossings which involve gas lines,water
mains 6 inches in diameter and larger, sanitary sewer crossings which
have minimum clearance, and all fiber optic telephone service lines.
E. New storm sewers should generally be designed to convey the ten-year
runoff without surcharge. However,the system should be checked for the
50-year runoff in situations where it would be necessary to prevent
flooding of interstate highways, limited access highways, major arterials,
and underpasses, or other depressed roadways where ponded water can
only be removed through the storm sewer system.
F. The detailed design of the storm sewer pipe line can only proceed after
taking into account the above parameters.
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VIRGINIA, June 27, 1990, AS AMENDED.
2. Size of storm sewer pipe shall be determined by the Manning's Equation. For
explanation and use, refer to the Virginia Department of Transportation Drainage
Manual and refer to Detail DD-17 of this Article.
5-249239 Energy and Hydraulic Gradients
1. The hydraulic gradient for a storm sewer system shall herein be defined as a line
(water surface profile) connecting points to which water will rise in pipes,
manholes and inlets throughout the system during the design flow. The energy
gradient is a line drawn a distance V2/2g above the hydraulic gradient of the
pipes. Refer to Detail DD-10 of this Article.
A. At storm sewer junctions,the total energy loss at the junction, HL, is the
difference in elevation between the energy grade lines of the upstream and
downstream pipes. To establish these gradients for a system, it is
necessary to start at a point where the hydraulic and energy gradients are
known or can readily be determined.
B. When the energy and hydraulic gradients must be determined,the pipes
are assumed to have uniform flow. For uniform gravity flow the friction
loss in storm sewer pipes shall be determined by referring to the Virginia
Department of Transportation Drainage Manual.
(1) If the junction incorporates surface inflow and the surface inflow
comprises more than 20% of the total flow in the storm sewer,then
the Ht should be increased by 30% and the adjusted value entered
in column 17.
(2) The 50%reduction for IS-1 shaping is not applicable at upper
terminal structures.
C. Where a proposed drainage system is connected to an existing drainage
system the hydraulic gradient shall be computed through the existing
system until the stormwater contained within the system outfalls to
daylight into an adequate conveyance channel or a natural watercourse to
demonstrate hydraulic capacity.
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VIRGINIA, June 27, 1990, AS AMENDED.
(1) Information, including as-built information, as well as
development plan system computations for the existing system,
will be made available by the Town to the engineer for those
systems for which the Town has this information.
(2) The Director may waive the requirements when it has been
previously determined that the receiving system is known to have
sufficient capacity.
2. Storm sewer systems should generally be designed to convey the design year
storm as non-pressure systems.
A. However, in specific situations,primary trunk lines of storm sewer
systems may be designed for pressure flow with the approval of the
Director. All proposed pressure flow systems should be coordinated with
the Department of Plan Review in the preliminary design stage when tying
into existing storm drainage systems. The hydraulic gradient for the
design flows shall not be above an elevation of one half foot below the
established ground elevation nor more than five feet above the crown of
the pipe. For curb opening inlets the gutter flow line is considered the
established ground elevation.
B. The hydraulic gradient shall be determined by computations and then
graphically depicted on the storm profiles within the
construction plans for all storm sewer systems located within the Town or
VDOT right-of-way and for all closed pipe systems located within Town
storm drainage easements using VDOT methodology. Refer to Detail DD-
18 of this Article.
C. In general, hydraulic grade line calculations will not be required for
privately maintained lateral lines serving terminal inlets, and will not be
required for privately maintained secondary trunk lines which are flowing
less than 80%of full capacity as determined by Manning's equation.
Privately maintained storm sewer conveyance systems shall not be located
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within a publicly maintained storm sewer easement nor are they to be
located within Town or VDOT right—of-way.
D. Hydraulic grade line calculations shall be required for all storm sewer
lines which are subject to pressure flow conditions.
3. In instances where the pressure flow is due to a restricted outfall condition
entering a pond below water surface elevation,the hydraulic grade line shall not
be higher than 0.5 foot below any manhole top or inlet throat opening. In cases
where this design is unavoidable,the Director may approve an alternate design
and allow less clearance.
I 5-250240 Open Channels
I 5-254241 Natural Watercourses
1. Natural watercourses are the primary facilities for conveying stormwater runoff.
For purposes of this Section,the term natural watercourse shall refer to the total
conveyance facility,the stream(or low flow channel) and the adjacent floodplain.
2. The policy of the Town of Leesburg relative to natural watercourses shall be as
follows:
A. Generally, any encroachment into the floodplain is not permitted unless
authorized by the Director.
B. Whenever a natural watercourse must be relocated or otherwise modified,
the extent of channel reach and degree of modification shall be the
minimum necessary to provide compatibility of the channel and
development. Refer to Section 5-255 of this Article for specifics of stream
modification.
C. A narrative describing the stream's morphology (form and structure)and
environment shall be conducted and documented in addition to the
economic and engineering alternatives available for the particular location.
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D. Refer to Section 5-400, Floodplain Policy, for processing and analysis
requirements.
E. Modified and relocated channels shall duplicate the existing stream and
flood plain characteristics as nearly as possible. These characteristics
shall include the stream width, depth, slope, flow regime,pool-riffle ratio,
bank cover, side slopes and flow and velocity distribution.
F. A hydraulic analysis of the 25 and 50-year, in addition to the 100-year
frequency floods may be required by the Director to comply with the
Town's Stormwater Management Master Plan or design criteria contained
herein.
5-252242 Man Made Stormwater Conveyance Channels
1. Man-made channels are typically trapezoidal or other geometric sections and may
be either natural or artificially lined. Hydraulic capacity shall be determined by
the procedure outlined in the Virginia Erosion and Sediment Control Handbook.
The computed velocity shall approximate the assumed velocity used to determine
the Manning's "n" value.
2. All open channels shall be designed to contain the ten-year event.
- II
3ton o nts
3. The velocity of flow in open channels including bends is determined through the
use of Manning's Equation and "n" values, and Bernoulli's equation. Refer to the
Virginia Department of Transportation Drainage Manual.
4. The need, type and dimensions of lining for erosion control shall be based on the
velocity associated with the ten-year event. The lining selected shall be consisten
throughout the channel until it outfalls to a natural watercourse. For various
channel configurations, refer to the Virginia Department of Transportation
Drainage Manual.
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5. Depending upon the location, safety, damage risk and environmental
considerations, a less frequent event may be required for the design of these
channels at the discretion of the Director.
6. Where an access strip is provided, it shall have a maximum two percent cross
slope within the required easement.
7. Maximum side slope for grass lined conveyance channels shall be three to one
with a minimum longitudinal slope of one percent(two percent minimum
recommended).
8. Conveyance channels with side slopes steeper than 3:1 shall be stabilized by
paving,riprap, gabions or other approved measures.
9. Conveyance channels with longitudinal slopes less than one percent shall be
paved.
10. Paved stormwater conveyance channels or channels with side slopes steeper than
3:1 are prohibited within or through residential subdivision building lots and in no
case shall the top width of the channel be within 25 feet of a residential property
line; except that twelve feet of the required 25-foot open area may be contained
within a lot providing the additional square footage within the 12-foot area shall
not be used in the computations for determining the minimum required lot area.
11. Stormwater conveyance channels conveying more than 15 cubic feet per second
are prohibited within or through residential subdivision. Temporary stormwater
conveyance channels conveying more than 15 cfs may be located on vacant lands
which will become future residential development only if the channels are
replaced by an underground storm drainage system when the lands are developed
for residential uses.
In no case shall the top width of a permanent or temporary stormwater
conveyance channel conveying more than 15 cfs channel on an adjacent property
be within 25 feet of a residential property line.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
12. The sides of all conveyance channels shall be extended until one foot of freeboard
is provided above the ten-year event water surface elevation within the
conveyance channel.
15 253 243 Roadside and Median Ditches
1. Roadside and median ditches shall meet the standards for stormwater conveyance
channels.
2. Generally, side and median ditches shall be designed in accordance with
prevailing geometric standards applicable to the particular class of roadway,with
consideration of hydraulic capacity, erosion control and safety. Refer to the
Virginia Department of Transportation Drainage Manual.
3. The ditch should provide sufficient hydraulic capacity to contain the estimated
runoff from a ten-year frequency storm. The estimated runoff and attendant
velocity for the two-year frequency storm is to be used for determining the needs,
type and dimensions of special ditch lining for erosion control. Geometric
configurations shall conform to appropriate safety standards.
I 5-254244 Lot Drainage Swales
1. Swales for lot drainage shall conform to the standards for grass-lined conveyance
channels based on the ten-year event.
2. Within residential areas or subdivisions, an inlet shall be provided to intercept lot
drainage flow when any of the following conditions apply:
A. The lot drainage swale extends across three lots.
B. The lot drainage swale extends more than 300 feet.
C. The lot drainage swale conveys more than two cubic feet per second.
Additional flow up to 4 cubic feet per second may be permitted for large
lot subdivisions (average lot size over half an acre)with the approval of
the Director.
-37-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
3. Lot drainage swales shall not discharge in excess of two cubic feet per second
across any sidewalk or curb based on the two -year event.
4. Drainage swales located outside of residential areas or subdivisions shall be
designed to carry the flow from a 10_year storm event and maintain a non_
erosive velocity from a 2:-year storm event. The drainage design shall also
consider and provide overland relief for the 100_year storm event as per Section
5-200 of this maftualDCSM.
15 255245 Stream Modification
If stream and flood plain encroachment is unavoidable(i.e.,highway
embankment), a detailed evaluation by a registered Professional Engineer license
in the Commonwealth of Virginia shall be made and sealed.
I 5-256246 Flow Design
1. The Manning equation shall be used for open channel analysis where uniform
flow exists or can be reasonably assumed. The Bernoulli equation shall be used
to analyze flow where changes in flow resistance, size, shape or slope of the
channel occur. Refer to the Virginia Department of Transportation Drainage
Manual.
2. The computation of water surface profiles for channels involving changes in
roughness, slope, shape and discharge should not be based on a nomograph
solution.
15-257247 Water Surface Profile Computations
1. The U.S. Army Corps of Engineers, HEC-RAS River Analysis System This
computer program may be used to model open channels, culverts, and bridges.
2. The Virginia Department of Transportation Drainage Manual method may also be
used for the calculation of water surface profiles.
3. Other methods may be utilized with prior approval of the Director.
-38-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
5-258248 Riprap
Riprap for channels shall be designed in accordance with the Virginia Department of
Transportation Drainage Manual. Outlet protection shall be designed using Virginia
Erosion and Sediment Control Handbook.
I 5-260250 Culverts
1. Culverts shall be designed to account for ultimate right-of-way widths.
2. Within embankments, culvert headwalls and endwalls shall be located a minimum
of 30 feet outside the edge of pavement of traffic lanes unless traffic is separated
from the walls by a guardrail that is required due to warrants other than the walls.
5461251 Design Criteria
1. The design of culverts is dependent upon the type of control (inlet, outlet).
A. Inlet control. Deemed to be the discharge capacity of a culvert as
controlled at the culvert entrance by the depth of headwater(HW)and the
entrance geometry, including the barrel shape and cross section area, and
the type of inlet edge.
B. Outlet Control. Culverts flowing with outlet control can flow with the
culvert barrel full or partially full for part or the entire length of the barrel.
If the entire cross section of the barrel is filled with water for the total
length of the barrel,the culvert is said to be in full flow or flowing full.
C. Both inlet control and outlet control computations must be performed.
2. Culverts located beneath interstate highways, limited access highways, or major
arterials shall be designed for the 50-year event without the headwater
overtopping the roadways. The 25-year headwater shall be 18 inches below the
elevation of the adjacent travel lane edge of pavement.
3. For actual culvert design procedures, refer to the current edition of the U.S.
Department of Transportation Hydraulic Design of Highways Culverts(HDS-5).
-39-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
Culvert design procedures are also provided in the current Virginia Department of
Transportation Drainage Manual.
(End of Section)
SECTION 5-300 POLICY FOR STORMWATER MANAGEMENT
5-310 General
1. The design and construction - . -- •. . . -: for each proposed
developmentregulated land-disturbing activity to-assureshall be in compliance
with this DCSM,the Town Code, and applicable state and federal laws and
regulations State r aw and Tow«. rode concerning stormwater management.
2. The design and cGonstruction of all _ . .• . - - . .. .
stormwater management facilities for water quantity
and water quality or modifications to existing channels shall comply with all
- . . , _ . -, ., . _: . _ . the more stringent of the requirements
contained in this DCSM, the Town Code, or applicable state and federal laws and
regulations unless otherwise modified by the Director.
2,3. Evidence shall be provided to the Town Inspector to verify all required state and
federal permits have been obtained prior to the commencement of any site
sonstructieriregulated land-disturbing activity.
4. Technical guidance may be found in the Virginia Department of Transportation
Drainage Manual, the Virginia Stormwater Management Handbook,and the
Virginia Stormwater BMP Clearinghouse. Where conflicts may arise between
among design criteria manuals,the more stringent of the requirements
shall apply unless modified by the Director.
-40-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
. . . •
- - - .. - . -
5-311 Applicability
1. The provisions of this section apply to all regulated land-disturbing activities
governed by Chapter 14 of the Town Code.
2. The Director may grant exceptions only in accordance with the provisions of
Section 14-23(g)(5) of the Town Code.
-1-3. Stormwater management for water quantity and water quality shall meet the
requirements of the Virginia Stormwater Management Program(VSMP)Permit
Regulations(9VAC25-870)Part II B "Technical Criteria for Regulated Land
Disturbing Activities" except as modified by this DCSM for all land disturbing
activities one acre or greater. - - . • • _ • - - -• -. - .
2.1. Stormwater management shall meet the following requirements for all land
disturbing activities under one acre unless specifically exempt by the State or
Town Code whether or not a VSMP permit is required:
A. Water Quantity shall meet all "Adequate Outfall"requirements stated
within Sections 5-330 through 5-332 of this DCSM as well as all other
applicable sections of this DCSM.
B. Water Quality shall meet all required pollutant loading reductions as stated
within Section 5-320, Article 6 and all other applicable sections of this
DCSM.
-41-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
5. Notwithstanding the subsection 3 above, regulated land-disturbing activities that
are grandfathered pursuant to Section 14-23(e)(4) of the Town Code shall meet
the applicable criteria found in the VSMP Permit Regulations Part II C "Technical
Criteria for Regulated Land-Disturbing Activities Grandfathered Projects and
Projects Subject to the Provisions of 9VAC25-870-47 B"as modified by more
stringent requirements contained in the DCSM and the Town Code in effect on or
before June 30, 2014.
•
. - - •• I • • -
•
. A- --
-- s
- y . .
-42-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
- . - -
E. The selection of A, B, C or D above shall be as listed in the following
• -
•
•...
!!
.. -
erodhng-its-bank
-43-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
.. - -
-- D' - -
---
- S .. .. -
-
s - = - -
„f e isc a«go
-
C. On Site field shot cross sections.
• - - - - - - - - •: - - . - ., . . : : .:•--:
-44-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
• - ..
- , -
321 of this Article for the various watersheds.
• - -
pufposes.
I 5-344312 Design Storms and Hydrologic Methods
-45-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
1. The Rational method Method or modified-Modified rational Rational method
Method (as applicable) shall be used for determining the peak runoff for small
drainage areas of twenty acres or less. ' - - • _ -•: !- ::. - -
.. . . • .-
. • • •
-2. The Natural Resources Conservation Service (NRCS)method er--el&Seil
- . _ •_ _ _ • , :: shall be used for the determination of runoff
for drainage areas larger than twenty acres. The use of the NRCS er-SCS
methodology shall be applied but not limited to large dams,major culverts, and
all ponds and dams with a permanent pool. - .•. - .- •, - .•, ., -
A. The NRCS WIN TR 55 (Windows Based Program) or old SCS TR 55
.. -
- -
B. The NRCS WIN TR 20(Windows Based Program)or old SCS TR 20
- -
- -
a. ! - •
•
methefl-afe-as-fellewsi
(1) Drainage area.
-46-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
(5) Design storm (21 hour, Type II rainfall distribution)
9. . •
- - . • L • •
. It
_ - " • • AL • j • •
or old SCS "Technical Release 20".
1. The required storage volume shall be determined by the "Storage Indication
." -
•
• . . ,•
• -• • • - - • • I. •
•
. • • •• • I
i - .
basis.
(1) Develop an elevation discharge curve for the structure. For
(2) Develop an elevation storage curve for the structure. The storage
• - : - -
-47-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
('I) Select the routing interval. The shorter the interval selected,the
B. Use the NRCS or old SCS computer method or the storage indication with
•
Time Rates Volumes
Inflow Outflow inflow Outflow Storage
Hours efs efs cfs hrs cfs hrs cfs hrs
Days efs efs cfs days cfs days cfs days
I Days AF/day AF/day A AF AF
Hours 4141f inihr incites inches inches
Days ity iay inches iftehes inches
(1) cfs hours— 12.1 (acre feet);
(2) cfs days— 0.501 (acre feet); and
(3) inches — acre feet/53.3 (drainage area in square miles)
I 5-620320 Water Quality-Design-Criteria
1. Indigenous vegetation should be preserved to the maximum extent practicable
consistent with the proposed use, development, or redevelopment.
-48-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
2. Impervious surface cover shall be minimized consistent with the proposed use,
development, or redevelopment.
23. All types of construction drawings, such as but not limited to residential
construction plans,public improvement plans, and all types of site plans
(excluding applications for new residential homes on lots of record not associated
with a bonded subdivision, site plan waivers and zoning permits), shall provide
water quality in accordance with the minimum requirements of the Virginia
Stormwater Management Regulations as set forth in this DCSM regardless of lot
size or amount of disturbed acres, even when a formal VSMP permit is not
required for the project(such as those exempt under Town Code Section 14-
23(e)(6)).
5-321 - • . - . •. . . • ! • • . . .. • . I- ! . • e . . - Environmental Site
Design
•
1. Each application for a regulated land-disturbing activity shall include a written
assessment for the potential for the use of -: .. _ !- - - . - _ ! :- •_-
teeimiquesEnvironmental Site Design(ESD)., should elude atten
•• - - D
applisatien
- -- - - .
• �. -
2. The Virginia Stormwater BMP Clearinghouse shall be the sole source of
efficiencies and design of ESD practices being considered to meet the water
quality requirements of this DCSM. However, iIn addition to other 14.14-ESD
-49-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
resources that may be available,the following mayshall be considered in the
development of the written assessment:
A. Low-Impact Design Strategies: An Integrated Design Approach, United
States Environmental Protection Agency, Office of Water, EPA 841-B-00-
003 dated June 1999 and subsequent modifications and updates thereof;
and
B. Low-Impact Development Hydrologic Analysis, United States
Environmental Protection Agency, Office of Water, EPA 841-B-00-002
dated June 1999 and subsequent modifications and updates thereof.
5-640-322 Hot Spots
1. The Director may determine that a proposed : - - . . -- - , -. . . -- - ,
useland use or activity associated with a regulated land-disturbing activity
constitutes a pollution hot spot, and that a greater level of stormwater treatment
quality management is necessary to prevent pollutant wash-off after construction.
2. A stormwater hot spot is defined as a land use or activity that generates higher
concentrations of hydrocarbons,trace metals or toxicants than are found in typical
stormwater runoff or that generates a pollutant that is subject to a TMDL Waste
Load Allocation(WLA) assigned to the Town. The maximum level A greater
level of stormwater treatment-quality management, which assumes pre-
development greenfield conditions regardless of actual existing site conditions.,
may-be neededshall be required at hot spot sites to prevent pollutant wash off afte
construction.
3. Developments-Land uses or activities that are deemed by the Director as a Hot-ho
Spot-spot shall not be exempt from the - .. •- -. . - - .. - :
regulations—maximum requirements for water quality treatment in this DCSM
even if the limits of disturbance is less than an acre and/or if the site is considered
re-development.
4. - -: .. - . - . . _ . .. Hot spots may include, but are not
limited to,the following:
-50-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
A. Vehicle salvage yards and vehicle recycling facilities;
B. Vehicle service and vehicle maintenance facilities;
C. Vehicle equipment cleaning facilities;
D. Fleet storage areas;
E. Industrial sites;
F. Outdoor liquid container storage;
G. Outdoor loading and unloading facilities;
H. Commercial container nurseries;
I. Golf courses;
J. Storing or dispensing of petroleum products and F(aar-deus-hazardous
Substancessubstances.
. .. ., . . - .. . . _ - • . . . .. - - _, .. - •-_
e. •
•
. - . . - , •--- -- : . .
1.
..
- - -- + . • .
-51-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
- s -
• a •-
' - ' -
- '
K. Dry cleaning operations-
L. Public works storage areasi
M. Facilities that generate or store hazardous materials;.
N. Chemical storage areas., and
O. Areas known for the sale or transfer of contaminants
5. On making a written determination that a proposed developmentrreelewlepmeft
or seland use or activity constitutes a pollution hotspot,the Director shall require
the creation and implementation of a stormwater pollution prevention plan
(SWPPP) •. . __. . .. - . _ . :! . -• • - - to reduce the generation of
pollutants at the source. The - . -- _ - ': . •. ' - _ •. ' ., SWPPP)
shall be in addition to all other requirements in this Articleother eguirea BMPs.
At the discretion of the Director, a SWPPP developed in conformance with
-52-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
9VAC25-151-80 may be deemed sufficient to satisfy the requirements of this
section.
6. In order to adequately protect surface water and groundwater quality, land uses
and activities that propose storing, handling and/or dispensing petroleum products
and hazardous substances shall at a minimum meet the following standards:
A. Oil/water separators shall be required for all facilities that engage in
activities (other than agricultural)that potentially generate oily
wastewater, including but, not limited to, vehicle
maintenance/washing/detailing, fuel storage/dispensing, and machine and
paint shops. When available, the discharge generated from activities not
exposed directly to stormwater runoff(such as but not limited to car
washing facilities and interior building operations) shall be to the Town's
sanitary sewer system. If this a sanitary sewer outfall is not available or if
the facility is directly exposed to stormwater runoff(such as a refueling
station or outdoor storage area),and the discharge must be to the storm
sewer and a Virginia Pollutant Discharge Elimination System(VPDES)
permit will be required.
B. Secondary containment shall be required for activities that propose
storing,handling and/or dispensing of petroleum products(except for
liquefied petroleum gas) and hazardous substances. The secondary
containment shall be designed to provide a means of detecting material
loss from the primary container; sufficient/compatible containment of the
loss; retrieving the loss; and correcting the deficiency. For groups of
tanks/containers,the secondary containment must be able to hold the
contents of the largest container plus precipitation (if there is no roof).
This precipitation shall not be re-directed to the storm sewer. Temporary
secondary containment shall be provided for construction sites that use
petroleum products or hazardous substances.
C. The applicant shall provide evidence that an approved Emergency
Response Plan has been filed with and approved by the Town as well as
the Loudoun County Department of Fire and Rescue Services.
-53-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
7. Specifically, all types of site plans for new(or proposed renovations to) gas
stations, other type of fuel dispensing facilities or fuel storage sites shall at a
minimum meet all hot spot requirements stated herein and include the following
items:
A. Plans shall Include the following BMP Hot Spot Narrative
"This site will operate as a fuel storage and/or dispensing station and is
considered a Hot Spot per DCSM(5-640.4J). An oil water separator such
as a Baysaver or equivalent approved product is required to contain minor
spills on site. The designed system will provide a primary separation
manhole, secondary storage manhole (or facility) and oil water separator
such as a Baysaver separator(or approved equivalent). A shutoff valve,
visible to on duty staff, easily accessible and properly signed, is to be
installed on the downstream side of Baysaver or equivalent. In the event
of a fuel spill, the valve is to be closed immediately and the spill contained
within the onsite (water tight)pipes and structures which will be over-
sized to hold the contaminated rainwater and material until HAZMAT
teams reach the site. These"secondary" storage systems shall be designed
to contain a minimum of 2,500 gallons."
B. Plans shall Include the following Notes & Details:
1) BMP Hot Spot structure details to be provided on Final Site Plan.
2) BMP Hot Spot Maintenance Plan to be provided on Final Site Plan.
3) BMP Hot Spot site specific Narrative to be provided on Final Site
Plan.
4) BMP Hot Spot structure sizing computations to be provided on Final
Site Plan.
5) BMP Hot Spot structure capacities to be provided on Final Site Plan.
C. Emergency Response Plan:
The Applicant shall provide evidence that an approved Emergency
Response Plan has been filed with and approved by the Town as well as
the Loudoun County Department of Fire and Rescue Services.
5-632358 Stream Delineation and Buffer Criteria
-54-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
1. All : - . -- - , - - . - - •. - - - regulated land-disturbing activities
subject to this article-Article shall clearly delineate perennial and intermittent
streams on or directly adjacent to the site. Such determination shall be made
using a reliable, site-specific, and scientifically valid system of in-field indicators
acceptable to the Director such as but not limited to determinations from the
Army Corps of Engineers or the Virginia Department of Environmental Quality
or determinations based upon and in accordance with Identification Methods for
the Origins of Intermittent and Perennial Streams (latest version)published by the
North Carolina Division of Water Quality. The condition of the water features,
including whether they are natural or engineered, shall also be noted. -ate
- - - - - -_ - .' _- . . -- .. - : . s - .
- - - - • - -• - .. . .•
noted.
- -- . •- - -- - : __L: - - - !' - - - - -
as-amen
buff a shad be enhancea
q 2. • . • - .., -1 - . .. - - .' - --- • , :•ny site with a perennial or
intermittent stream within a natural channel shall meet the following additional
performance criteria:
-55-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
A. Measures shall be taken to protect the perennial stream from non-
concentrated stormwater runoff from adjacent impervious surfaces.
(1) A minimum 50 foot wide vegetated area preserved where
present, or established where not present, shall be provided on
both sides of the stream(measured from the scar linecenterline of
the stream). Larger buffers may be required for a Creek Valley
Buffer as defined by the Zoning Ordinance in specific situations.
(2) If the required vegetated buffer area is in poor condition, as
determined by the Director,the vegetated buffer area shall be
enhanced to prevent erosion and ensure proper functioning of the
area as a buffer to pollution.
(3) If the required vegetated buffer area does not exist or cannot be
provided by a proposed development, an adequate buffer may
(with prior approval of the Director) alternatively be met through
the use of a smaller vegetated buffer area in combination with
equivalent on-site stormwater treatment as long as such a reduction
is not prohibited by other Town Ordinances and/or Regulations
such as but not limited to the Creek Valley Buffer criteria as
defined by the Zoning Ordinance.
04(4) See Section 5-700 for vegetated buffer easement requirements.
. .• . . . - . - .. . - :•- . . . - .. . . . s -
5,3. Establishment, enhancement, or replacement of the buffer area shall be in
accordance with Chapter 5.1 `Buffer Area Establishment, Replacement, and
Restoration" of the most recent version of Riparian Buffer Maintenance and
Mitigation Guidance Manual published by the ` - .•. •-- - . - .. •
-56-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
- . -, !• . - - . . .. - : . -.t: • • :.- --Department of
Environmental Quality, as amended or as modified by the Director.
674. Physical relocation, alteration, or undergrounding of a perennial or intermittent
stream will be considered on a case-by-case basis.
7 Construction plans shall not be approved until proof is provided to the Director
that all required federal, state, and local environmental permits have been
obtained.
8
5. _ .. . . . - . . . . . .
. . .. _ . . , ., . . . . . . . s - .
!' - - - , . . . - . . - . . .. s
Owe:
5-330 Water Quantity
In addition to the water quantity requirements of Section 5-311, the following design criteria
shall be applied to all types of subdivision and site plans regardless of whether a VSMP permit is
required or not, unless specifically exempted within this DCSM:
1. The determination of flooding and channel erosion impacts to receiving streams
shall be measured at each point of discharge and based upon:
A. Runoff volumes that consider proposed land use and includes runoff from
the balance of the upstream watershed contributing to each specific point
of discharge.
B. Complete calculations and analysis.
C. On-Site field shot cross sections.
D. Offsite field shot cross sections (where permission cannot be obtained to
obtain these sections,the Town will consider alternate means of obtaining
-57-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
the required topography such as but not limited to aerial topography and
field shot topography transposed from approved plans of record).
1-2. Subdivision plans and site plans shall be designed such that properties and
receiving waterways downstream of any land-disturbing activity shall be
protected from erosion and associated damage due to changes in runoff rate of
flow and hydrologic characteristics, including but not limited to changes in
volume, velocity, frequency, duration, and peak flow rate of stormwater runoff.
5-331 Detention Measures
1. On-site detention of stormwater is desirable in many cases to alleviate existing
downstream drainage problems and to preclude the development of new ones. In
some areas of the watershed, detention may cause increased peak flows to occur
on the major streams and tributaries. Therefore, the downstream impact must be
carefully investigated. The Director may prohibit detention of stormwater for
larger stormwater events including but not limited to the 10 year and or 100 year
storm events where and when it is identified in the Town's Stormwater
Management Master Plan(in the Town Branch and Lower Tuscarora
subwatersheds only) as not being in the best interest of the Town.
2. Special stormwater management design is required within the Tuscarora drainage
shed to address downstream flooding concerns in accordance with the Town's
Stormwater Management Master Plan(see DCSM Section 5-341). Stormwater
5-332 Adequate Outfall
-58-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
1. Where an adequate channel or an adequate closed stormwater conveyance system
is not available adjacent to the site,the developer shall provide a drainage system
satisfactory to the Director to preclude an adverse impact(e.g. soil erosion;
sedimentation; yard flooding; duration of ponding water; inadequate overland
relief) on down-stream properties and receiving channels. In addition,the
c, erstormwater management Faeility--facility shall be
required at a minimum to restrict the flow to any outfall channel to
predevelopment conditions. . - - . .- - .. - . •• .. . . .. .,
2. Concentrated stormwater runoff leaving a development site shall not aggravate or
create a condition where an existing dwelling or a building constructed under an
approved building permit floods from storms less than or equal to the 100-year
storm event. If such a dwelling or building exists, detention for the 100-year
storm event shall also be provided; and
3. Concentrated surface waters shall not be discharged on adjacent or downstream
property, unless an easement expressly authorizing such discharge has been
granted by the owner of the affected land or unless the discharge is into a natural
watercourse, or other appropriate discharge point as set forth above.
'1. The selection of A, B, C or D above shall be as listed in the following paragraphs
for the. atershcd
5-4. An adequate outfall within the Town shall be defined as:
A. A well--defined(i.e., with bed and banks)natural or man-made channel
which is capable of conveying the post development runoff for the design-
year event, as defined herein for the particular shed in which the
development is proposed,without eroding or overtopping its banks.
B. A well-defined (i.e.,with bed and banks)natural or man-made channel
shall be considered adequate at any point where the total contributing
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
drainage area is at least 100 times greater than the drainage area of the
development site in question.
C. An analysis shall be performed downstream of the site subject to proposed
development to verify the adequacy of the receiving system. This analysis
shall be performed for a minimum distance of 300 feet downstream, and
shall continue until the flow is discharged into a natural watercourse of
sufficient capacity to convey the design-year event without overtopping or
eroding its banks.
6 5. Concentrated stormwater leaving a development site shall only be discharged into
a well-defined(i.e., with bed and banks)natural or man-made outfall channel of
sufficient hydraulic capacity, such that there is no overtopping or erosion
downstream of the subject development for the release rate of the concentrated
stormwater. This shall be required regardless of whether or not the peak
discharge rate is changed by the development.
A. Where an adequate channel or an adequate closed stormwater conveyance
system is not available adjacent to the site,the developer shall provide an
outfall drainage system satisfactory to the Director to preclude an adverse
impact(e.g. soil erosion; sedimentation; yard flooding; duration of
ponding water; inadequate overland relief) on downstream properties and
receiving channels. • : _ . . -, • - - . •- • - •. _ ---
B. Concentrated surface waters shall not be discharged on adjacent or
downstream property, unless an easement expressly authorizing such
discharge has been granted by the owner of the affected land or unless the
discharge is into a natural watercourse, or other appropriate discharge
point as set forth above.
5-333 Pro-Rata Share Program
1. The developer may choose to:
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
However, In lieu of or in addition to on-site stormwater quantity management
required in this Article,the Town may require pro-rata share contributions in
areas where downstream pro-rata share improvements have been installed and/or
stormwater detention is not in the best interest of the overall drainage system as
defined in the Stormwater Management Master Plan. The-developer-must
2. Notwithstanding(1) above, in lieu of on-site stormwater quantity management
required in this Article, a developer may:
A. Contribute his proportionate share toward the correction of off-site outfall
deficiencies in those instances where pro-rata share policies have been
adopted by the Town and construction of the downstream improvements is
completed or where construction of the downstream improvements will
occur concurrently with the subject development.
B. Construct or to provide the funds for the construction of more than his
proportionate share of the downstream off-site drainage improvements, so
that he may proceed with the improvement of his land without damaging
the properties of others.
In such cases, at the request of the Developer and based upon calculations
prepared by the Developer's engineer and approved by the Director,the
Town may establish a pro-rata share policy to collect, on a pro-rata basis,
any excess funds expended beyond the Developer's proportionate share of
the cost of such improvements from other properties within the watershed
served by such drainage improvements. whe suci. properties are
3. Any requests for nutrient offsets shall be done in strict accordance with the Town
Code.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
- -
5-320340 ' Stormwater Management Facilities
5-324341 Design of Stormwater Management Facilities within Tuscarora Creek
Watershed
Design of Steffnwater-stormwater Management management Facilities-facilities within
the Town of Leesburg's Tuscarora Creek watershed shall be based upon all of the
following three criteria:
1. Flood Control Criteria
A. All designs for stormwater management and determination of adequate
outfall for flood control shall at a minimum be in compliance with the
Town of Leesburg Stormwater Management Master Plan adopted June
1990 as well as all criteria set forth below.
--
•• • •
a .• - ._ 1
■• : . - - -
B. Design Criteria for the Tuscarora Creek Watershed
m •
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
• - -
m • - -
two and ter .o ntte
('I) Tuscarora Creek.
(1) Town Branch Subbasin
Town Branch, Channel. Stormwater management within the
channel of Town Branch shall be provided such that the channel
will be protected for the 25-year event. In areas designated by the
Director as having a high potential for flooding,the channel and
adjacent properties shall be protected from the 100-year event.
Town Branch, On-site. Stormwater management facilities
(detention) shall be provided for all areas within the watershed
exclusive of the Town Branch conveyance channel such that the
post development peak runoff will not exceed the predevelopment
peak runoff for the one twe;ten and 25-year storm events.
Further, additional stormwater management measures shall be
incorporated into the detention facility design so as not to exceed
the allowable post development peak runoff from the site for the
more frequent one and two year(24 hour) storm events as defined
within the Virginia Stormwater Management Handbook in
accordance with this DCSM as well as other applicable state and
federal regulations.
04(2) Upper Tuscarora Creek Subbasin
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
Upper Tuscarora Creek, Channel. Stormwater management for the
Upper Tuscarora Creek shall be by the use of regional facilities as
delineated in the Master Plan.
Upper Tuscarora Creek, On-site. Stormwater management
(detention) shall be provided by utilization of the regional facility
for all areas within the watershed which drain to a regional facility.
Design of regional facilities shall be such that the post
development peak runoff will not exceed 0.1 cfs/acre, 0.3 cfs/acre,
and 0.4 cfs/acre for theenertwe3 ten, and 25-year events,
respectively. Further, additional stormwater management
measures shall be incorporated into the regional facility/site
design so as not to exceed the allowable post development peak
runoff from the site for the more frequent one and two year(24
hour) storm events as defined within the Virginia Stormwater
Management Handbook in accordance with this DCSM as well as
other applicable state and federal regulations.
a. Where a proposed development is served by an existing
regional stormwater management facility, Stormwater
management measures(detention) shall be provided for all
areas of the development within the watershed exclusive of
the Tuscarora Creek conveyance channel such that the post
development peak runoff shall not exceed the allowable
post development peak runoff from the site for the more
frequent one and two year(24 hour) storm events as
required by the Virginia Stormwater Management
Handbook and this DCSM. et e eea the predevelopment
b. If a regional facility does not exist to serve a proposed
development and if the Director concurs in writing that it is
not feasible-required to construct an on-site regional
facility,then an on-site Stormwater management facilities
(detention) shall be provided for all areas of the
development within the watershed exclusive of the
Tuscarora Creek conveyance channel such that the post
development peak runoff will not exceed the
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
predevelopment peak runoff for the ener twe7 ten and 25-
year storm events. Further, additional measures must be
incorporated into the onsite stormwater management design
so as not to exceed the allowable post development peak
runoff from the site for the more frequent one and two year
(24 hour) storm events as defined within the Virginia
Stormwater Management Handbook in accordance with
this DCSM as well as other applicable state and federal
regulations.
(-)(3) Lower Tuscarora Creek Subbasin. On-site stormwater
management measures(deter)shall be provided for all areas
within the watershed exclusive of the Tuscarora Creek conveyance
channel such that the post development peak runoff will shall not
exceed the allowable post development peak runoff from the site
for the more frequent one and two year(24 hour) storm events as
defined within the Virginia Stormwater Management Handbook in
accordance with this DCSM as well as other applicable state and
federal regulations. - --. - . . . . - . -• , = -
year events.
2. Discharge Control Criteria
In addition to the flood control criteria above, all concentrated discharges of
stormwater from road or site drainage systems, designed for the ten-year event,
from all developing sites shall be discharged into an adequate channel(extending
from the point of discharge to the main channel of the watershed listed above)for
the ten-year storm event or greater at arterial road crossings, or stormwater
management(detention) shall be provided. Refer to Section 5-240 X58
concerning required channels for concentrated discharges.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
3. Erosion Control Criteria
In addition to the flood control and discharge control criteria above, all
concentrated discharges of stormwater from all developing sites shall be
discharged in a manner that complies with the requirements set forth by this
DCSM as well as other applicable state and federal regulations. Vie- -adequate
hided
5-323342 General-criteria
1. . _ •_ - .. - - -
. . The
Director may preclude the onsite use of any stormwater management facilities, or
require more stringent design, construction, and/or maintenance provisions if
based on a review of the stormwater management plan the proposed facility
cannot function as designed due to actual project site conditions.
2. Stormwater management facilities shall not be located on any single family
attached or detached residential lot unless specifically approved, in writing,by the
Director. However, lots may extend into a wet pond to the permanent pool
elevation. All lots which are constructed in accordance with the above shall
comply with the setbacks required for the 100-year flood plain.
3. If existing structural stormwater management facilities are employed to meet the
water quality criteria in whole or part, an onsite inspection report certified by a
person who is licensed as a professional engineer, architect, landscape architect,
or land surveyor pursuant to Article 1 (54.1-400 et seq.) of Chapter 4 of Title 54.1
of the Code of Virginia shall be provided as evidence to prove that the existing
stormwater management facility was '- •, • - - •, . , - •. - -- -- t:
exist...,.BMP has been-designed-and-constructed in accordance with proper
design standards and specifications, and that facilities are currently in good
working order,properly functioning and performing at their designed levels of
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
service. A review of both the original structural design and maintenance plans
may be required to verify this provision. A new maintenance agreement may also
be required to ensure compliance with Town codes and ordinances,
Type-e€ Owner-of
Zoning-Use Aesthetics Drainage Drainage Facility Town
SFB T ewers Let-Owsers Yes
er--140A
SFA 1=1OA Town Town 140A Yes
Multi Family Lot-Owner Let-Owner Town Lot-Owner Yes
140A 140A 1404
Commercial Let-Owner Lot-Owner Town Lot-Owner Yes
Industrial
Institutional
TyPe-o€ Maintenance Cuariaater Owner-of Easement Maint
Zoning-Use Aesthetics Drainage of FaeiW to Agreement
Drainage Town Required
SR) Let Town Lot-Owners bet Yes* Yes
Owners HOA Owners
1404 140A
SFA bet Town Lot Owners bet Yes* Yes
Owners HOA Owners
140A 1404
Multi- Let-Owner Let-Owner Let-Owners bet Yes* Yes
Family FIOA 140A 140A Owner
HOA
Commercial Let-Owner Let-Owner bet Yes* Yes
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
Industrial Ownef
Institutional
maintainer e f last r or.
SFA: Single Family Attached
4. Detention facilities (wet and dry), except those which are underground, shall be
designed utilizing both a primary spillway and a separate, independent emergency
spillway. Combined spillways shall only be permitted when approved by the
Director and shall conform to section 5 324.9.DSection 521.9.D, Combined
Spillways
4.5. Any embankment constructed for purposes of impounding stormwater shall be
designed in accordance with the requirements in Section 5-500, Dams Design and
Construction.
323 Detention Ponds
•
•
Article.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.•
-- Y• • • • . .... ..
•
. D Y• . .. -
suitable-for-its-intended-use;•. . . , _- •. _ - • - • . -, .•-- - • , . . . . -, , -•!, • . . . - - - - -- - , .. . - _ S•- - - -- . - . -- - . -• - .. - . . .
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
• - A •
•
•
5 324 _--
Embanken♦bends
•
v •_ . . , . .. . , . . - - . . - ; _ .•..� - - _ -
E. Approval is required by the Director for use of any impoundments
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
Safety) (VR 625 01 00), dated February 1, 1989, as revised under 1VAC
50 20 10, effective July 1, 2002 et.seq.
. • ■
F. Side Slopes: Side slopes shall be stable and shall not be steeper than three
_ ■
- ■ • o
-
•
far-design.
(1) Class"Low Risk" Structures located in rural, agricultural, or urban
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
» • .
b3`
(1) Increasing the retarding storage, and/or
of Leeslufg:
, .. .. . ., s __ • - . ., -- ., . . - .
(2) Class "b" Structures.
(3) Class "c" Structures.
3. Impoundment Laws
•
- _. - ' •- •
.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
• N.•
-,ix feet and having less than 50 acre feet of storage. Refer to Detail DD
11 of this Article.
.. . . •
Service TSC Technical Note Engineering UD16,which was issued on
. . •
•
•
th
- - •
F. Snyder.
C. The graph, as shown in Detail DD 12 of this Article,has the width of the
stem-flood-plain,
.• . . . - - • . - - ... .
Example 1
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL(DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
.. --
- -' - - -
. .. - - —
. __ Y.
. Y. • ••
• .• !
.. ..
9 11 -
... ! • • !•
Example-2 •
•'' ' _ ., . : .. ' ,: • e "- - , _ . ... -- - •
_•- :- • " •_. -. , •• _ . •• • .. - . , •.. •, ,-n.n . . - - - - - - - _
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.•
.. . . : . ,
•
•
. - • - -•, , • . ..• -• . ', . . . . .,•• - , . . .. :, .. . '.
•
approved,
F. A chcck should be madc to cnsur" . . . •• • . • - , -• • - ,
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
. e
• . .•
!! -
- !! : -
11
6. Engineering Surveys. Once the location of the pond or reservoir has been
7. Geologic Investigations
• -
• --
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
• . . .
s .. - •- •
• -
• • - - s - -
•
s •- . _.. •
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
8. Earth Embankment
to,,. dtt, as determined below
Total Minimum
Top-Width
(Feet) (Feet)
11 or less 4-2
15---1-9 }2
20 24 4-2
25 34
33
• -• -- - -
Slope
Upstream Downstream
Clayey Sand, Clayey Gravel, 34 34
Gravel...
Silty Clay, Clayey Silt>>> 34 34
•
•
• • . -
•
_ :.., '49 ., - - -• - - - - - • ; . . . . , . - - - ..
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
s . •. . _ • . . .,• . . : .. .
OF
�. •
II- -
a. . - • • •
G. Seepage Control.
(1) Seepage control is to be included:
a. If pervious layers are not intercepted by the cutoff;
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
c. If needed to ensure a stable embankment; or
d. If special problems, such as fractured rock, etc.,
for a stable dam
c. If recommcndcd by the geotechnical engineer.
b. Reservoir blanketing; or
c. A combination of these measures.
9. Spillway)
•
used
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
•
be 1.2 or greater.
. ,
• .-
•-
•_ • , . _ , . . •.• ., , .. , . .
. Y._ • .
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.•• •
- -_ - -- - . • . •. . . . • _ • •
shall-be-water--tight,
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
al3o be p rmittea
• •---
-
• • -
submitted♦e he Tow«
C. Emergency Spillways
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.•
.. . . -- . .. . - .. - .. - . -- I , , - - , !, !
cnd of this Article.
net-be•endangered.
.. _ .
design-flew,
(6) Permissible Velocities
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
Vegetation
Easily-Er-edeta
Slope-of-Exit-Channel Slope-of-Exit-Channel
pct 0 5 pet-54-0 pet-04 5 pet 5-1-0
ftis ftis s ftis
Kentueky
Bluegrass
Smooth-Broome 6 5 4
T-all c l Fescue
Reed
/ ass
Sod-Ferming
Grass-Legume 5 4 4 3
Mixtures
Lespedeza
Sericea
Weeping
Levegrass 34 34 24 24
Yellow B1uestem
Native-Grass
MixtlifeS
} SCS TP 61 •
•
clay, and clay.
. .. . -.. , .. . -
b. The capacity of the spillway shall be determined using
Stand Average-Height Degree-ef Stand Average-Height e Height
of-Vegetation-in Rem of-Vegetation-in Retardants
lashes lashes
Higher-than A Hither 13
Geed 30 Fair 30
11 to21 H 11 to21 G
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
64040 F 6404-0 l�
2443-6 D 240-6
D
Less-than-2 B Less than 2
- - - - -
channel. Refer to Detail DD 13 of this Article.
- - - ► . - . -
drainage.
(9) The inlet channel may be curved to fit existing topography, but exit
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
•
enibartlErnent
(1) With the required discharge capacity,the degree of retardance,
- . .
- - �.
possible:
D. Combined Spillways
• - - 1!.
- .- . _
Maximum Flood (PMF) or the 100 year storm event-or-the
- D ' D • -
_ , _ • , , _
-
facility desig -)
(3) The combined spillway (when approved by the Director) shall
provide for a minimum of 24 inches of freeboard from the
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
flood (whichever is applicable to the specific stormwater
management facility design)to the Top of the Dam.
• • -
E. Structural Emergency Spillways,
-
- A.A
Developer.
5 325343 Excavated Ponds
1. General. Excavated ponds, with contributing watershed areas of more than ten
acres which create a failure hazard, shall be designed as embankment ponds.
Excavated ponds;that are normally dry(SWM ponds) and include a pipe outlet
control system; shall be designed using the principal and emergency spillways
design criteria as that for embankment ponds.
2. Side Slopes. Side slopes of excavated ponds shall be such that they will be stable
and shall not be steeper than three horizontal to one vertical.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
3. Pond bottom(Dry Ponds) shall be graded to provide a minimum 1% slope to the
lowest opening of the pond structure unless a lesser slope is approved by the
Director.
4. Inlet Protection. Where surface water enters the pond in a natural or excavated
channel, the side slope of the pond shall be protected against erosion.
5. Placement of Excavated Material. The material excavated from the pond shall be
placed in one of the following ways so that its weight will not endanger the
stability of the pond side slopes and where it will not be washed back into the
pond by rainfall: Refer to Standard DS-5 in Appendix A.
A. Uniformly spread to a height not exceeding three feet with the top graded
to a continuous slope away from the pond.
B. Uniformly placed or shaped reasonably well with side slopes no steeper
than three horizontal feet to one vertical foot for the excavated material
behind a berm width equal to the depth of the pond but not less than 12
feet.
C. Shaped to a designed form that blends visually with the landscape. D.
Used for low embankment and leveling.
D. Hauled away.
5 326 Wet Ponds
•
requirements:
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
(2) The minimum surface area should be one half acre. A minimum
� . . - - -
• - -
€ems
• . • . . . . . , . • . . . ! _
•
•
A. Landscape Planning. A pond's apparent size is not always the same as its
• - -- . ., . _. . . '_ - . . , •- - , . • ., . _.,
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
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•
•- -
- , .. -- . - , . .. -
• - - - --
- - -- - . - - - _., - ., .
transition. Refer to Standard DS 8 in Appendix A.
E. Density and height of vegetation can be increased progressively from the
_ .. -1, - . - - _ - .. 9 - --
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
A. If waste material is not removed from the site, it must be placed so that its
•• •.
.
• - -- - I . •
5 in Appendix A.
-- - - s - -
I 5-328344 Stormwater Management Pond Plans
The following information shall be shown on the design drawings for all stormwater
management ponds:
1. Watershed Map. Watershed maps shall be shown on plans submitted for
approval. Bar scales shall be used. The map shall show the watershed boundary;
the drainage pattern; location of bridges and culverts and other structures that
affect the flow of water; location of roads, buildings,property lines and fences
and north arrow.
2. Plan View. The plan view should indicate the, center line of dam and cross
section of the dam as well as a profile of the outlet structure and outfall pipe with
corresponding station numbers. The plan will contain an accurate contour map of
the structure site and adjacent area. When this map is shown on a plan-profile
sheet,the plan scale should be the same as the profile. The contour interval
should be one or two feet. The plan view should show in detail the following:
A. The spillways and fill locations.
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B. All benchmarks, soil borings,borrow pits, fences, buildings,roads,
bridges, springs, wells or other improvements that influence the design or
construction of the proposed work.
C. Property lines.
D. Name of property owners.
E. Contour at normal pool and design stormwater surface elevations.
F. Contour map of flooded area. The scale of this map may be one inch=20
feet where it can incorporate the details of the local plan,to one inch=
100 feet where a separate map would be required. The size, as determined
by the scale and required details, vary from a one sheet combination of all
maps and the profile, to an individual sheet for this map alone. The object
of this map is to indicate with reasonable accuracy:
(1) Contours of the normal pool elevation at a maximum contour
interval of two feet when needed for design.
(2) Contour of the spillway design flood water surface elevation.
These contours shall be on a one foot vertical interval.
G. The applicant shall on the construction plan set,provide a reference to the
geotechnical report and include all applicable geotechnical notes regarding
liner, embankment, seepage controls, etc.
3. Profile of Principal and Emergency Spillway. The profile shall be plotted at a
scale of one inch equals ten feet horizontally and one inch equals five feet
vertically on profile paper. The following shall be shown:
A. Principal Spillway. The spillway must be shown on the profile at the
correct station and elevation. The following elevations shall be clearly
labeled:
(1) The crest of the spillway.
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(2) The top of outlet apron or invert of pipe spillway at the outlet.
(3) The water surface at maximum stage of design storm.
(4) The top of the earthfill (settled height).
B. Emergency spillway. The spillway must be shown on the profile at the
correct station and elevation. The following shall be clearly shown:
(1) Crest of the spillway.
(2) Length of the control section.
(3) Slope of the approach channel.
(4) Slope of the exit channel.
(5) The existing ground elevation along the centerline.
C. For both spillway profiles,the location and elevation of rock, gravel or
soil strata that affects the design or location of the structure shall be
shown.
4. Cross Sections. Cross sections should be plotted as viewed looking downstream.
The profile station for each cross section plotted should be centered directly
below the cross section on the data sheet. When possible,the cross section
should be plotted to the same horizontal and vertical scale,thereby giving an
undistorted cross section.
A. The cross section along the centerline of the proposed earthfill shall show
the following information:
(1) Elevations and important dimensions of the principal spillway in
proper relation to the cross section showing the elevations of crest,
apron, and top of weir or headwall.
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(2) The top of the earthfill as constructed and its final settled height.
(3) The elevation of earthfill berms, if applicable.
(4) The elevation and dimensions of the emergency spillway.
(5) The location and descriptions of soil borings taken along or near
the centerline of the proposed structure.
(6) The dimensions of and the depth of the cutoff trench.
B. Cross sections through the fill or embankment area shall show the
following information:
(1) The side slopes of the fill.
(2) The top width of the embankment.
(3) The elevation of top of fill as constructed and its settled height.
(4) The existing and proposed ground lines.
(5) Property line, if applicable.
(6) Core trench dimensions and location.
(7) Foundation drain.
(8) The elevation of earthfill berms, if applicable.
(9) The location of any fences,
(10) Stations and centerline or baseline.
It may be possible to show this information on the profile of the principal
spillway.
5. Other Details as Needed.
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A. Seeding and mulching specifications for the fill, spillway, and borrow
areas.
B. Pipe and riser construction details supplemented with details of
appurtenant structures including but not limited to trash racks, anti-seep
collars,propped outlet, concrete reinforcing, fencing.
C. Construction notes required to assist in layout, construction, and checking
of the completed practice.
D. Notes and details from qualified soils engineer.
5-330345 Rooftop Detention
5 331 Design Criteria
1. Design Criteria
A. Roof top storage shall be an appropriate design to detain up to the ten-
year,two-hour event, and emergency overflow provisions shall be
adequate to discharge the 100- year, 30-minute event.
(1) A roof design in the Town of Leesburg is currently based on a
snow load of 30 pounds per square foot or 5.8 inches of water.
Assuming a reasonable factor of safety,properly designed roofs
are structurally capable of holding three inches of detained
stormwater.
(2) Roofs calculated to store depths greater than three inches shall be
required to show structural adequacy of the roof design and to be
approved by the Loudoun County Building Official prior to
approval by the Director.
B. No less than two roof drains shall be installed in roof areas of 10,000
square feet or less, and at least four drains in roof areas over 10,000 square
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feet in area. Roof areas exceeding 40,000 square feet shall have one drain
for each 10,000 square foot area. Refer to Detail DD-14 of this Article.
C. Emergency overflow measures adequate to discharge the 100-year, 30-
minute event without unnecessary risk to life or property must be
provided.
(1) If parapet walls exceed three inches in height,the designer shall
provide openings(scuppers) in the parapet wall sufficient to
discharge the 100-year, 30-minute storm flow at a water level not
exceeding five inches.
(2) A scupper shall be provided for every 20,000 square feet of roof
area, and the invert of the scupper shall not be more than 3.5
inches above the roof level. (If such openings are not practical,
then detention rings shall be sized accordingly).
D. Detention rings shall be placed around all roof drains that do not have
controlled flow. Refer to Standard DS-9 in Appendix A.
(1) The number of holes or size of openings in the rings shall be
computed based on the area of roof drained and runoff criteria.
(2) The minimum spacing of sets of holes is two inches center-to-
center.
(3) The height of the ring is determined by the roof slope and in no
case shall the height of the ring exceed three inches.
(4) The diameter of the rings shall be sized to allow the 100-year
design storm to overtop the ring(overflow design is based on weir
computations with the weir length equal to the circumference of
the detention ring).
(5) Conductors and leaders shall also be sized to pass the expected
flow from the 100-year design storm.
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E. The maximum draw down time of the roof shall not exceed 17 hours.
F. The Town of Leesburg will accept Josam Manufacturing Company and
Zurn Industries, Inc. market "controlled-flow" roof drains, or other
approved equivalents.
G. Access for inspection shall be provided for all roof drains.
5 332 Plan Preparation
2. Plan Preparation
RA. Computations required on all development plans:
(1) The roof area in square feet.
(2) The storage volume provided at three-inch depth.
(3) The maximum allowable and calculated discharge rate.
(4) Inflow-outflow hydrograph analysis or acceptable charts(For
Josam Manufacturing Company and Zurn Industries, Inc. standard
drains,the peak discharge rates as given in their charts are
acceptable for drainage calculation purposes without requiring full
inflow-outflow hydrograph analysis).
(5) The number of drains required.
(6) The size of openings required in detention rings.
(7) The size of rings to accept openings and to pass the 100-year
design storm.
I 5-340346 Underground Detention
5 341 C eneral
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1. General
A. All underground detention facilities that are to be publicly maintained or
associated with a residential subdivision shall have prior approval of the
Director.
B. Private underground stormwater management facilities utilizing
corrugated metal pipe (CMP)will be approved, if the conditions listed
below are satisfied:
(1) Pipe must be aluminum CMP or aluminized steel CMP. Other
materials may be considered on a case by case basis with prior
approval from the Director.
(2) All access structures,manholes, inlets, and control structures must
be reinforced concrete meeting Virginia Department of
Transportation standards and conform with OSHA Confined Space
Regulations.
(3) Calculations must be submitted to demonstrate that the structure
will withstand the expected traffic loading in a paved area.
(4) All construction details must be provided.
(5) Structures must not be placed under the main site accessways or
adjacent to the public right-of-way.
C. Underground systems conveyed to the Town must be constructed entirely
of reinforced concrete.
5 342 Design Criteria
2. Design Criteria
DA. Underground detention shall be a closed tank or pipe system.
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&B. Sediment traps and trash racks shall be provided. These should be placed
near maintenance access points.
l:C. All underground facilities shall have at least two points of access to
facilitate maintenance. The Director, on the recommendation of the
Director of Public Works, may require additional access points if it is
necessary for the required maintenance.
G D. All underground detention facilities shall have spillways designed to
accommodate the design-year event,while providing detention for the
one-year, two-year and ten-year event(s) as required.
14E. All facilities shall provide for adequate overland relief for runoff in excess
of the ten-year event. Routing of the 100-year event through the
underground detention facility is not required.
F. Peak runoff rates from the facility shall meet detention requirements as set
forth in this DCSM. - •. • . •. . .. - . • . .. ' . -
development r. „ff rotes
LG. See Section 5-700 for easement requirements.
•
(1) Pipes
a. For single pipes 24 inches and loss in diameter the
the-013e_
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b. For single pipes greater than 21 inchcs in diameter the
c. For multiple pipes at the same or different elevations the
I -
c. Rcfcr to Standard WS 16 in Appendix A.
5343 Plan-PTeparation
3. Plan Preparation -
IAA. Plans shall accurately show the alignment of the structure and all
appropriate easements.
LB. All corners and junctions of conduits shall be shown on all plans. These
shall include the invert elevations of the tank if applicable.
M C. A profile of the tank or conduit, including sufficient sections, shall be
shown.
ND. Details of gravel bed, tank, or conduit construction and entrance and
outfall structures shall be shown.
0,E. Plan views of structures consisting of multiple sections shall include flow
arrows.
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I 5-358347 Porous Pavement
5 351 General
1. General
A. The use of the following porous pavement surfaces shall be allowed on
private property with all required design elements as appropriate for
individual site conditions and a privately maintained maintenance
agreement acceptable to the Town:
(1) Pervious Pavers
(2) Pervious Concrete
(3) Other materials may be considered on a case by case basis and
shall require prior approval of the Director.
B. The use of porous pavement shall not be allowed for any public facility
without prior approval of the Director of Public Works.
5 352 Design Criteria
2. Design Criteria
G A. Soil tests shall be conducted in accordance with Section 9-300.
(1) In addition to these tests, a percolation test shall be performed.
(2) An outflow "hydrograph" shall be developed based on the
absorption and percolation rate of both subgrade and pavement and
supplemental subdrainage.
D B. Projected traffic counts and live loading calculations are required.
E,C. The mix (gradation) including density of both the subgrade and the porous
pavement, shall be designed and sealed by a professional registered
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engineer certified by the State of Virginia with background in pavement
design.
-D. Preparation of the site and the placement of the pavement shall be done
under the direct supervision of a professional engineer registered in the
State of Virginia.
6,E. Pavement density tests shall be made within three hours of placement and
shall be compared with engineer's calculations.
14,F. Prior to final inspection, a second density test shall be made and results
compared with the previous test.
17G. The construction plans shall contain:
(1) Engineering calculations, including design mix criteria and
specifications.
(2) Calculation of the ten-year peak inflow rate.
(3) Storm routing calculations shall be provided as required by the
Director.
(4) Calculations for nominal percolation rates based on the closure of
voids due to sediment deposition.
(5) Provisions for freeze/thaw action and approved deicing chemicals
shall be noted.
J:H. All future repairs (sealing, overlays) shall be done only with the prior
approval of the Director of Plan Review and shall be in accordance with
guidelines outlined above.
I 5-360350 Regional Facilities
1. Regional facilities shall be determined based on the area of the drainage shed
associated with the specific structure as follows:
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A. The drainage area to the structure shall be no less than 100 acres to be
considered for a regional facility.
B. The facility shall serve more than two sites.
2. The Director of Plan Review shall ultimately decide, based on the above criteria
and the future benefit to the public, whether the facility is to be accepted as a
regional facility.
3. These facilities shall be located such that they conform to those areas outlined
within the Stormwater Management Master Plan for possible regional facilities.
Other sites may be approved at the discretion of the Director of Plan Review.
4. Sites which are conveying their stormwater to a regional facility shall convey
their stormwater through closed conduit, or adequate open channels. These
systems shall have duly recorded easements.
5. Regional facilities shall be designed such that they retain a permanent pool of
water with continuous release, and be of adequate volume and depth to become an
amenity to the community.
6. For guidelines for dam design, refer to Section 5-520 of this Article.
7. If buildings are to be constructed downstream from the facility where failure may
result in loss of life, the appropriate portion of the Probable Maximum Flood
(PMF) or the 100 year storm event or the spillway design flood (whichever is
applicable to the specific stormwater management facility design) elevation shall
be determined and a building restriction zone set, similar to the restrictions for
flood plains. Refer to Section 5-420 of this Article.
5-370360 Waivers and Exemptions
1. Stormwater management waivers and exemptions shall be considered in
accordance with the process established in Section 14-23 of the Town Code. All
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-
-
-1-acre 5 acres
r acre 2 acres
1/3 acre -1-acre
ih_aere were
1/8-acre 4-acre
acre or less.
is 2/3 acre or less and included therein, 1/3 acre or less of impervious area.
. .. , . . , - - , - . ... - - , . . .. , ..
end-per-king-lets.
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11 .. .It .. • - --
, .. . .
4.2. _ . . , - . , . :, . . - - ... - •. . , _ _ . . .. .
- •: -. . :The following are examples of when a waiver
may be considered by the Director:
A. The hydraulic characteristics of the receiving stream or the environmental
characteristics of the existing stream and the site are such that on-site
management or detention of flows are contrary to sound engineering
practices or detrimental to the environment,;
B. Existing off-site stormwater management facilities provide the required
control. In such cases, on-site stormwater management may be waived
provided that the delivery system from the developing site to the off-site
stormwater management facility is designed based on Articles 5 and 6 of
this MafittaIDCSM.
C. An off-site stormwater management facility has been identified for
construction in the Capital Improvements Program, and the applicant will
agree to a financial contribution or dedicated an easement or land for the
construction thereof.
D. Two or more developments, including that of the applicant,have provided
jointly, through reciprocal easements, or other means, for the management
of the stormwater facility.
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5:3. Any new projects which are additions, extensions and modifications to those
developments listed in the above categories which have been granted a prior
waiver under this policy shall be required to provide stormwater management for
the entire site where the acreage limitations listed for each are exceeded by the
subsequent addition, extension and modification thereto.
6.4. Owners and elopers developers who have projects falling within these
categories or the following conditions and who desire not to provide stormwater
management for the site, must request in writing to the Director,that the
requirements be waived.
5. Each request will be considered individually by the Director. All applications for
a waiver will receive a written response outlining the reasons for approval or
denial of the application within 45 days of receipt of a complete application.
&:6. It should be noted that in reviewing the waiver application, all storm drainage out
falls,receiving channels and channel capacities, velocities and other related storm
drainage discharge considerations will be closely examined to determine the need
for additional outfall treatment and/or channel protection needs. Further,the
developer's engineer shall furnish the Director a signed and sealed document prior
to granting a stormwater management waiver, stating that the receiving storm
drain system in question is adequate.
5-380370 Inspection and Maintenance Provisions
•
271. Engineers in the preparation of plans for construction shall include
Inspectiensinspections, maintenance and operation of these facilities as one of the
primary design considerations. All construction and site plans shall Abe
accompanied by a separate arIinspection Schedule and Maintenanc-
plan-Plan that identifies the owner,the responsible parties for inspections and
maintenance as well as the inspection requirements and maintenance plan
schedule that must be approved and recorded prior to plan approval. A separate
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Maintenance Agreement shall also be required to have been approved in a format
approved by the Town Attorney and recorded prior to plan approval.
3:2. The following shall be included in the design of detention facilities:
A. Access-ways shall be designated on plans and cleared, graded, and
constructed along with the facility. These access-ways shall be a minim
of 12 feet in width with a maximum cross slope of 2%and a maximum
longitude slope of 12%.
B. Proximity of facilities to public right-of-way shall be determined in order
to minimize the length of required access-way.
C. Access shall be provided such that all portions of a facility are accessible.
D. Standard drainage easement agreements are not acceptable for access;
therefore, special access easement agreements are to be executed which
shall preclude planting of shrubs, construction of fences and other
structures within the easement.
E. Grading of access ways to facilities and grading around facilities shall
leave slopes which do not exceed eight percent to allow for access by
maintenance vehicles.
F. Major facilities including wet ponds, underground chambers, etc., shall be
accessible with at least one all-weather access roadway to include a mini-
mum of a 12-foot wide surface to the satisfaction of the Director.
G. As these facilities are generally in close proximity to dwellings and may
be subject to vandalism, principal spillways and other devices shall be
designed to minimize unauthorized entry or tampering.
H. Underground chambers shall provide for two or more access points for
ventilation and cleaning and be large enough to accommodate cleaning
equipment. Generally, the access,where possible, shall be a minimum of
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24 inches in diameter to facilitate maintenance and conform to Town
standards for access.
3. Maintenance responsibility for stormwater management facilities that control only
water quantity shall be as listed below:
Type of Maintenance Guarantor of Owner of Easement to
Zoning Use Aesthetics Drainage Drainage Facility Town
SFD Lot Owners Town Town Lot Owners Yes
or HOA
SFA HOA Town Town HOA Yes
Multi-Family Lot Owner Lot Owner Town Lot Owner Yes
HOA HOA HOA
Commercial Lot Owner Lot Owner Town Lot Owner Yes
Industrial
Institutional
SFD: Single Family Detached
SFA: Single Family Attached
4. Maintenance responsibility for stormwater management facilities that control both
water quantity and water quality or just water quality shall be as listed below:
Type of Maintenance Guarantor Owner of Easement Maint
Zoning Use Aesthetics Drainage of Facility to Agreement
Drainage Town Required
SFD Lot Town Lot Owners Lot Yes* Yes
Owners HOA Owners
HOA HOA
SFA Lot Town Lot Owners Lot Yes* Yes
Owners HOA Owners
HOA HOA
Multi- Lot Owner Lot Owner Lot Owners Lot Yes* Yes
Family HOA HOA HOA Owner
HOA
Commercial Lot Owner Lot Owner Town Lot Yes* Yes
Industrial Owner
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Institutional
*Private Stormwater Management easement with clause for Town to be
maintainer of last resort.
SFD: Single Family Detached
SFA: Single Family Attached
(End of Section)
SECTION 5-400 FLOOD PLAIN POLICY AND GUIDELINES
5-410 Applicability
Without prior approval of the Director and all other required approvals under Town
ordinance,there shall be no construction permitted within flood plains. This shall include
all stormwater management facilities. Floodplain, for the purpose of this EnamialDCSM,
shall mean a drainage area of 50 acres or more that is inundated by the 100-year water
surface elevation along any natural watercourse permanent or intermittent. Wet ponds
with a permanent pool whose embankments cross the flowline of a watercourse will be
permitted with Director approval. However, adequate calculations must be provided
indicating that the embankment construction will create no adverse impact to downstream
property from flows based on a 100-year event. These calculations shall include
adequate hydraulic and flood plain limit computations(HEC RAS or other method
approved by the Director run for both existing and proposed conditions). These
computations will assume that the entire drainage area upstream of the structure is in its
ultimate developed condition(based upon the current Town and County, where
applicable, (Comprehensive) Plans,the current Town and County,where applicable
Zoning Map and the existing development of properties within the watershed,whichever
depicts the highest use that generates the highest potential for stormwater runoff).
5-420 Policy on Use of Floodplain Areas
1. Flood plain areas are primarily intended to remain as open or common areas.
These areas may be utilized to provide space for recreational activities.
2. Under no circumstances shall any residence be located adjacent to a 100-year
water surface such that its lowest point's nearest edge is within two vertical feet
and 15 horizontal feet of the defined flood plain. Commercial and industrial
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buildings may be located closer than 15 feet horizontal and 2 feet vertical with
appropriate floodproofing and prior approval of the Director.
3. The Developer must provide factual information that any proposed development
will not adversely affect the existing 100-year water surface elevation. The
Developer also must provide emergency access to the development during the
100-year flood.
4. In any case,where a road,public or private, which provides access to a
development, subdivision, or residence is inundated by the 100-year floodplain as
identified by,the Federal Emergency Management Agency(FEMA); or is
inundated by more than twelve inches for the 100-year overland relief for the
storm drainage system,the Developer shall provide an emergency vehicle access
study. This study shall demonstrate that an alternate emergency vehicular access
route is available to bring emergency services to the area beyond the flooding of
the road during the 100-year storm event and include the following:
A. The alternate emergency vehicle access route must be along public streets
or private streets only.
B. The alternate emergency vehicle route must be less than one mile long.
C. The alternate emergency vehicle route must be above the 100-year flood
elevation at all points.
5-421 Warning and Disclaimer of Liability
1. The degree of flood protection to be required by the Town of Leesburg Design
and Construction Standards Manual is considered reasonable for regulatory
purposes. Lower frequency floods may occur or flood heights may be increased
by man-made or natural causes, such as bridge openings restricted by debris.
2. Therefore,this Article does not imply that areas outside the flood plain areas, or
land uses permitted within such areas, will be free from flooding or flood
damages under all conditions.
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3. Additionally, the grant of a permit or approval of a subdivision or land
development plan in an identified flood plain area or flood hazard area shall not
constitute a representation, guarantee, or warranty of any kind by any official or
employee of the Town of Leesburg of the practicability or safety of the proposed
use, and shall create no liability upon the Town of Leesburg, its officials or
employees.
5-422 Processing of Subdivisions and Site Plans Within or Immediately Adjacent to
Floodplains
1. If a Subdivision Plan, or any type of Site Plan or Capital Improvement Plan or
land disturbing activity proposes to modify the ground surface,the channel
alignment, or proposes construction within or contiguous to the Town defined 100
year floodplain(a point at which the drainage area is equal to or greater than 50
acres) of any natural water course,permanent or intermittent. The following
processing procedure shall apply:
A. A floodplain study shall be submitted for Town review in accordance with
Section 5-430 of this article.
(1) A floodplain study-shall be submitted for Town review and
comments for all land development activities associated with
parcels that contain 100 year floodplain as well as for parcels that
are directly adjacent to parcels containing 100 year floodplain;
Studies submitted in conjunction with Site Plans or Construction
Plans may be included on Plan Review Schedule. Except for
Town Capital Improvements Plans,the Department of Plan
Review(DPR)reviews and approves all drainage related Town
floodplains not regulated by FEMA. For all Capital Improvement
Plans, the Department of Capital Projects Management reviews
and approves all drainage related Town floodplains not regulated
by FEMA.
(2) A floodplain alteration study shall be submitted for Town review
and comments for all land development activities associated with
parcels that propose construction activities that are performed
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
within the limits of the 100 year floodplain as noted above. Studies
submitted in conjunction with Site Plans or Construction Plans
may be included on Plan Review Schedule. Except for Town
Capital Improvements Plans, DPR reviews and approves all
drainage alterations related Town floodplains not regulated by
FEMA. For all Capital Improvement Plans,the Department of
Capital Projects Management reviews and approves all drainage
related Town floodplains not regulated by FEMA.
(3) After Town review and resubmission by the Applicant, if
necessary, the floodplain study and/or floodplain alternation study
may be approved by the Town if all outstanding comments have
been properly addressed and resolved with DPR or CPM
depending upon the type of plan submission as noted above.
(4) Prior to Final Town Approval of the Floodplain and or Floodplain
Alteration Study,the Applicant shall provide the Town with an
electronic version of the study in a format to be determined by the
Town.
2. If the floodplain study or alteration is determined to lie within a designated
FEMA floodplain,the applicant shall first obtain an approval of the studies
through DPR(for all projects not associated with a Town Capital Improvement
Project)and then be responsible to prepare and package the study in a format
acceptable to FEMA for the Town to submit to FEMA for their final review and
approval.
For all Town managed Capital Improvement Projects,the Department of Capital
Projects Management shall be responsible to prepare and package the study in a
format acceptable to FEMA and to submit to FEMA for their final review and
approval.
Specifically:
A. A floodplain study shall be submitted for Town review in accordance
section 5-430 of this article.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
(1) After the Town has approved the study(ies)the Applicant shall
prepare the FEMA floodplain study package for submission to the
Town. For all Town Capital Improvement Projects,the
Department of Capital Projects Management shall be responsible
to prepare the appropriate FEMA Floodplain study packages;
(2) The Appropriate Town Staff then prepares and sends a transmittal
to FEMA requesting their review of the study; including the
applicant's package and a Town endorsement.
(3) Where the applicant's or Town's study proposes to change the
flood elevation that FEMA has on record, FEMA will notify the
Town of their approval by letter authorizing a FEMA "Conditional
Letter of Map Revision(CLOMR)".
(4) Upon the Town's receipt of the CLOMR, a copy of FEMA
notification will be transmitted to the applicant and/or the
appropriate Town Staff.
(5) The Applicant's or Town's Engineer shall furnish the appropriate
Town Staff with any revisions and correspondence related to the
Study that they send directly to FEMA during the FEMA approval
process. In addition the applicant's or Town's engineer shall
furnish the appropriate Town Staff with a copy of the final version
of the study that was approved by FEMA and a letter certifying
that no other changes were made to the study other than those
required by FEMA.
(6) The associated construction plans shall not be approved nor shall
the Applicant be permitted to begin any construction on their site
until such time as the CLOMR has been issued for the project and
copies have been made available to the appropriate Town and
County Staff.
(7) Upon completion of all construction activities involving the 100
year floodplain,the Applicant's or Town's engineer shall submit
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as-built construction drawings of the final floodplain study with
field shot topography of the new land contours as well as any
proposed infrastructure within or directly adjacent to the
floodplain.
(8) Once the appropriate Town Staff confirms the floodplain study as-
built is consistent with the approved plans and the originally
approved floodplain and floodplain alteration study,the as-built
floodplain study is routed to FEMA for review and approval along
with the Town's approval endorsement.
(9) FEMA will notify the Town of their approval by letter authorizing
a FEMA "Letter of Map Revision(LOMR)".
(10) Occupancy of proposed buildings associated with a related
subdivision plan or site plan will not be issued until such time as
the Town has been provided a copy of the approved LOMR from
FEMA.
3. Construction drawings or site plans submitted for Town review shall conform to
any floodplain and or floodplain alteration studies that have been approved or
prepared for floodplains that exist on or are directly adjacent to the proposed
development or land disturbing activities.
A. All types of Site Plans, Construction Plans and/or Capital Improvement
Plans containing floodplain, shall not be deemed acceptable for review
until such time as the floodplain study(and if applicable the floodplain
alteration study)have been submitted to the Town for review. Concurrent
processing of floodplain studies and Site Plans or Construction Plans is
permitted.
B. The associated Site Plans (all types), Construction Plans and/or Capital
Improvement Plans shall not be approved nor shall the Applicant be
permitted to begin any construction on their site until such time as all
Town comments have been resolved and the CLOMR has been issued for
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VIRGINIA, June 27, 1990, AS AMENDED.
the project and copies have been made available to the appropriate Town
and County Staff.
5-430 Preparation of Floodplain Studies
5-431 General
1. Floodplain studies shall be required for subdivisions or any type of Site Plan or
developments or areas subject to a Capital Improvement Plan which contain or ar-
contiguous to natural watercourses,whether permanent or intermittent,with
drainage areas greater than 50 acres upstream of the subject site. Floodplain
studies may be deemed necessary for smaller drainage areas as required by the
Director.
2. Flows shall be determined by the SCS or new NRCS methodology or rational
method up through twenty acres (see Section 5-231 of this Manxa1DCSM)unless
otherwise approved by the Director.
3. Water surface elevations shall be determined using the standard step or HEC-RAS
or other approved method. Computations shall be based on physical properties of
the drainage shed and sound engineering judgment. The Manning "n" values for
each cross section and method used shall be approved by the Director prior to
submission of computed water surface elevations. Refer to the HEC-RAS
Hydraulic Reference Manual, *Latest Edition) for an acceptable methodology of
determining the Manning's "n" values for natural channels. -
4. Spacing of cross sections shall not exceed 300 feet and shall be cut at all changes
in:
A. Horizontal alignment,
B. Channel gradient,
C. Channel width, and
D. At any obstruction in the channel which significantly affects the flow.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
5. Cross sections shall be field run for final flood plain studies or the engineer may
provide certified topography with a two-foot interval contour.
6. Cross sections shall extend both upstream and downstream of the subject site to
the point where the post development water surface elevations are identical to the
existing predevelopment water surface elevations and shall continue 300 feet
beyond said point.
5-432 Plans
1. Floodplain study plan views shall be drawn at a horizontal scale of one inch
equals no more than 50 feet and no less than 25 feet. The accompanying profiles
shall utilize the same horizontal scale with a vertical scale of one inch equals no
more than ten feet and no less than five feet.
2. The floodplain limits (100-year)as calculated by the study shall be shown
accurately on the plans.
3. Limits of potential construction shall also be designated on the plans. These
limits shall preclude residences within two vertical feet and 15 horizontal feet of
the computed 100-year water surface elevations except as previously noted.
4. The baseline and section lines shall be shown on the plans. Mathematical ties
between the baseline, flood plain easement lines and property lines shall also be
shown.
5. Profiles for all cross sections shall be shown to a scale of one inch equals no more
than five feet vertically and one inch equals no more than 50 feet horizontally.
These profiles shall include the following information:
A. Cross section identification consistent with plan review.
B. Manning's "n" values used and where applied.
C. Computed 100-year water surface elevation with station callout.
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
D. Stationing shall be consistent with HEC-2 or HEC-RAS input data(if
applicable).
6. Profiles shall show the 100-year water surface elevation and invert elevation of
the stream at every cross section for the entire length of the study.
5-440 Existing Construction in Flood Plain
Federal Flood Insurance criteria dictates that the effects of the 100-year storm on
buildings insured under the Flood Insurance Program must be investigated. Such
cases would only be encountered when a structure encroaches upon an existing
flood plain, thus creating a backwater condition.
(End of Section)
SECTION 5-500 DAMS DESIGN AND CONSTRUCTION
5-510 Regulations
5-511 Virginia Department of Historic Resources,Division of Soil and Water
Conservation (VDHR SWC)
1. Construction of impoundments requires compliance with this gal-DCSM as
well as the State of Virginia standards under-the Virginia Dam Safety Act,Article
2, Chapter 6, Title 10.1 (10.1-604 et seq) of the Code of Virginia and Dam Safety
Regulations established by the Virginia Soil and Water Conservation Board
(VS&WCB). Permits for construction and operation of dams regulated by DCR
shall be issued by the Virginia Department of Conservation and Recreation.
2. All dam designs regulated by DCR can be submitted directly to DCR by the
applicant as long as the applicant simultaneously copies all correspondence
submitted to DCR to the Town of Leesburg. The Applicant shall provide proof to
the Town of Leesburg that all required dam permits have been issued before
scheduling a preconstruction meeting. All dam designs not regulated by DCR
shall be made part of the site plan or construction plans submitted to the Town of
Leesburg.
5-512 Town of Leesburg Regulations
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VIRGINIA, June 27, 1990,AS AMENDED.
1. It is the policy of the Town of Leesburg,that compliance with the criteria set forth
within this Article shall be required for the design and construction of dams
within Town limits that are not under the jurisdiction of the Virginia Department
of Conservation and Recreation-.
2. The design procedures and criteria in this Article have been compiled for the use
of persons involved in the design and construction of impoundment structures of
sufficient size to represent a potential hazard to downstream properties.
3. The Town of Leesburg will review all dam designs and regulate those intended to
impound water except as exempted below:
A. Any existing or proposed dam regulated by the Federal Government or the
Virginia Department of Conservation and Recreation.
B. All dams formed by highway embankments
(1) The Virginia Department of Transportation has special design
criteria for permanently impounding water upstream of highway
embankments.
(2) The Director shall approve such impoundments only upon
favorable recommendation from the Virginia Department of
Transportation.
4. Refer to Detail DD-11 of this Article for a graphical representation of
impoundments which are regulated by the Town of Leesburg or regulated by the
Virginia Department of Conservation and Recreation.
5. Except as exempted above, highway embankments shall not be used as dam
embankments within the Town. This does not restrict the use of culverts with a
headwater condition during rainfall events without a permanent surface elevation.
6. An inspection and maintenance agreement shall be executed with the Town by the
owner and recorded among the land records of the Town of Leesburg prior to plan
approval.
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
7. A permit from the Town shall also be required for dam construction.
8. Dams regulated by the Town of Leesburg shall be designed by a Professional
Engineer licensed in the State of Virginia with expertise in the fields of
geotechnical engineering, hydraulics, and dam design.
9. During construction, the owner shall employ an engineer licensed in the State of
Virginia to inspect the construction of the dam,to file weekly reports with the
Director covering construction progress including soil and compaction test data.
10. Record drawings with as-built information shall be submitted to the Director at
the completion of construction and shall include soil classification, compaction
and density test results, and concrete test results,to document the physical;and
structural soil characteristics of the facility.
11. After completion of construction,the owner's construction engineer shall certify,
in writing,that the dam was constructed in accordance with the approved plans
and specifications.
5-520 Design Criteria
1. All dam and embankment designs shall conform to the practices accepted by the
Virginia Department of Conservation and Recreation(DCR),the Army Corps of
Engineers, or others as approved by the Director. All dam and embankment
designs shall conform to the requirements of Sections: 5-323, 5-324, 5-325, 5-
326, 5-327, and 5-328 of this ManualDCSM.
2. Storage volume. An evaluation of the-topography of the drainage area to the
proposed wet pond is necessary to ensure that an adequate base flow exists to
maintain a permanent pool of water in accordance with DCR guidelines.
3. Dams shall be designed based on hydrology methods developed in the old SCS
TR-55,NRCS WIN TR-55 (Windows Based Program),the old SCS TR-20,
NRCS WIN TR-20 (Windows Based Program), or HEC-HMS as well as other
programs which utilize the general methodology of TR-55 or TR-20 which may
be approved by the Director on a case by case basis.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
A. All designs shall incorporate emergency spillways,the design of which
shall provide the required stormwater management detention and shall
pass the full volume of that portion of the Probable Maximum Flood
(PMF) shown in DD-11 based upon dam height and impoundment. (The
PMF is defined in Article 11). In no case shall the emergency spillway
elevation be lower than the computed ten-year water surface elevation.
An examination and determination of the flood plain created by the
passing of the DD-11 PMF shall also be conducted and the limits of the
area so flooded shall be delineated on submitted plans.
B. All embankments shall be designed with a top width and side slopes
appropriate for the material used to construct them.
C. Compaction standards to be employed are to be stated on the plans.
D. The embankment design shall take into account settlement based on
compaction and type of material used.
E. Side slopes above the permanent pool elevation shall be no steeper than
three to one.
F. The top of the embankment shall be a minimum of one foot and a
maximum of two feet above the computed water surface elevation
when passing through the emergency spillway the full volume of that
portion of the Probable Maximum Flood(PMF) shown in DD-11 based
upon dam height and impoundment. (The PMF is defined in Article 11).
G. Core trench, anti-seep collars or alternate measures, erosion protection on
upstream face and outlet protection shall be considered in the design and a
detail per the recommendations within the approved geotechnical report
shall be included on the plans.
H. Any riser employed shall be designed to overcome buoyant forces. Risers
shall also incorporate trash racks with anti-vortex devices.
II
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VIRGINIA, June 27, 1990, AS AMENDED.
4. Dam Failure.
A. As determined by the Director, a dam failure analysis is required for
facilities with embankments between six and 25 feet with a capacity
greater than 15 acre feet.
B. As part of the overall dam design,the engineer shall determine the
segment of stream valley downstream from the dam that would experience
an increased flood depth resulting from a potential dam failure.
C. Two types of danger reach analyses are to be investigated.
(1) For the first analysis,the engineer shall route the next highest
design storm through the proposed spillway system. Refer to
Detail DD-11 of this Article.
a. If the dam may fail as a result of overtopping, a danger
reach analysis shall be performed.
b. If overtopping does not occur, a downstream analysis is not
required.
(2) The second analysis shall consider a dam failure as a result of
internal erosion with the pond or lake level at normal pool
elevation. Analysis of this type will not be required for dams
without a permanent pool.
D. Where required,the analysis shall be conducted to a point downstream
where the dam break flood depth, danger reach length, has attenuated to
within one foot or less of the flood depth that would be experienced
without the dam.
E. If the dam break analysis shows a potential for flooding of
habitable structures,the engineer and owner shall increase the spillway
capacity and downstream channel capacity where applicable.
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
F. References used in dam design, construction, and maintenance include the
latest versions of:
(1) Virginia Stormwater Management Handbook, (Latest edition).
(2) Virginia Erosion& Sediment Control Handbook, (Latest edition).
(3) Army Corps of Engineers Technical Manuals.
(4) Virginia Department of Conservation and Recreation Technical
Manuals
(5) Nonstructural Urban BMP Handbook,Northern Virginia Regional
Commission Technical Manuals, (Latest edition).
(6) Northern Virginia BMP Handbook;Northern Virginia Regional
Commission Technical Manuals, (Latest edition).
G. Easements
Easements shall be provided for vehicular access for maintenance of the facility
and its appurtenances.
5-324521 Embankment Ponds
4-0:1. Embankment ponds shall conform to all of the following Town Standards as well
as the"Virginia Impounding Structures Regulations(Dam Safety)",Virginia's
updated"Dam Safety and Floodplain Management Programs and Regulations"
and all related SCS or new NRCS "Technical Bulletins"or other written State
requirements as applicable:
A. Failure of the dam will not result in loss of life, in damage to homes,
commercial or industrial buildings,highways classified as through
collectors or higher, railroads; or in interruption of the use or service of
public utilities.
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B. The product of the storage times the effective height of the dam is less
than 2,000. Storage is the volume, in acre-feet, in the reservoir below the
elevation of the crest of the emergency spillway. The effective height of
the dam is the difference in elevation in feet between the lowest open
channel emergency spillway crest and the lowest point in the original
cross section on the centerline of the dam. If there is no open channel
emergency spillway,the top of the dam becomes the upper limit.
C. The Maximum Dam height allowed in the Town of Leesburg shall be less
than 25 feet without prior approval from the Director.
D. The Geotechnical Engineer shall determine if a site has the characteristics
to support a dam and shall certify to any methods required to remediate
the site to a condition that would support a dam.
E. Approval is required by the Director for use of any impoundments
regulated : -- _ .. - . • - . .. - -under the Virginia pia
_. . - Impounding Structures Regulations
4VAC 50-20-10 et seq.) ,effective July 1, 2002 et.seq.
1. Permits for construction and operation of State regulated dams are
issued by the Virginia Soil and Water Conservation Board.
2. A copy of any state-approved design also must be submitted to the
Director in order to receive Director approval for the construction
plans.
F. Side Slopes: Side slopes shall be stable and shall not be steeper than three
horizontal feet to one vertical foot without Director approval.
G. Pond bottoms (Dry Ponds) shall be graded to provide a minimum 1%
slope to the lowest opening of the pond structure unless the Director
approves a flatter slope.
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-14 2. Structure Classification. All structures (dams) will be reviewed and classed
according to factors and procedures outlined in the National Engineering Manual
and supplemented herein. The class of risk hazard as contained in this
document is related to the damage that might result from a sudden major breach
of the earth embankment. Structure classification and land use for runoff
determination must take into consideration the anticipated changes in land use
throughout the expected life of the structure. The valley downstream;and the
relationship of the site to industrial and residential areas all have a bearing on the
amount of potential damage in the event of a failure. The classification of a dam
is determined only by the potential hazard from failure, not by the criteria selected
for design.
A. Classification factors in the National Engineering Manual
(1) Class"Low Risk" Structures located in rural, agricultural, or urban
areas dedicated to remain in flood tolerant usages where failure
may damage non-resident buildings, agricultural land, flood plains,
or Town and County roads.
(2) Class"Significant Risk" Structures located in predominantly rural
or agricultural areas where failure may damage isolated homes,
main highways, or minor railroads or cause interruption of use or
service of relatively important public utilities.
(3) Class "High Risk" Structures located where failure may cause loss
of life, serious damage to homes, industrial, and commercial
buildings, important public utilities, main highways, or railroads.
B. When structures are spaced so that the failure of an upper structure could
endanger the safety of a lower structure,the possibility of a multiple
failure must be considered in assigning the structure classification of the
upstream structure. Additional safety can be provided in either structure
by:
(1) Increasing the retarding storage, and/or
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(2) Increasing the emergency spillway capacity.
C. The following types of embankment structures are prohibited in the Town
of Leesburg:
(1) Class "a" structures with a storage height product of 2000 or
greater, and/or an effective dam height of 35 feet and greater.
(2) Class "b" Structures.
(3) Class "c" Structures.
4-2:3. Impoundment Laws
A. Virginia Impounding Structures Regulations (Dam Safety)-criteria
requires that dams rRegulated by the State of Virginia;=must be certified
by the State-state agency responsible for dam safety(Currently currently
the Virginia Department of Conservation and Recreation(DCR)in
pia).
B. This requirement excludes impoundments having a dam height less than
six feet and having less than 50 acre-feet of storage. Refer to Detail DD-
11 of this Article.
13.1. Approximate method for determining Dam Breach Inundation Zone:
A. This method is based on information contained in the Soil Conservation
Service TSC - Technical Note - Engineering UD16,which was issued on
July 3, 1969, and shall be performed for all embankment structures which
have an embankment height exceeding 15 feet and or those which
impound more than 25 acre-feet of water. Refer to Section 5-520 for dam
failure analysis requirements.
B. This method is based on the following:
(1) The dam is assumed to fail when the water depth is at the top of
the dam.
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(2) The peak rate of the breached hydrograph is based on data supplied
by the Bureau of Reclamation for actual dam failures.
(3) The method is based upon a valley flood routing method taken
from the Journal of the Proceedings of the ASCE, Hydraulics
Division, May 1964, "Hydrology of Spillway Design",by Franklin
F. Snyder.
C. The graph, as shown in Detail DD-12 of this Article, has the width of the
valley below the dam in feet versus the length of reach per acre-foot of
storage behind the dam for a depth(above bank full stage) at the lower end
equal to one foot. Actual storage is to be calculated from the top of dam
and the width of the valley would normally be the 100-year frequency
storm flood plain.
D. Two examples of how to use this graph are as follows:
Example 1
A Developer wishes to build a lake for stormwater management and
recreation. It has been determined that the height of the dam will be ten
feet, and that there would be approximately eight acre-feet of storage
behind the dam. From visual observation, it is noted that there are some
homes located on the flood plain 1,500 feet below the dam site. It has also
been determined that the average width of the valley is 400 feet. An
analysis must be made to determine if there would be a danger to these
homes if the dam failed.
Using the above information, enter the left side of the graph with a valley
width of 400 feet,move horizontally to the curve labeled H=ten feet, go
down vertically and read 160 feet at the bottom of the graph. This value is
for one acre-foot of storage. It is determined that there are eight acre-feet
of storage, so multiply 160 feet by eight and obtain 1,280 feet. This is the
distance below the dam where the depth of flow in the flood plain would
be one foot if the dam would fail.
Since the homes were located 1,500 feet downstream from the dam,this
would indicate that there would be little, if any, damage to these homes as
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a result of a sudden breach of the dam. This would indicate that the dam
would be a low hazard, and that Class "a" design criteria could be used in
the design of the dam if the flood plain is to remain in flood tolerant usage.
Example 2
Same as Example 1, except that the height of the dam is 15 feet. Entering
the graph again with a valley width of 400 feet and going across to the
curve labeled H= 15 feet and then going vertically down,the length of
reach would be equal to 210 feet for one acre-foot of storage for depth
(lower) =one foot. Eight acre-feet of storage is necessary,therefore
multiply 210 times eight and obtain 1,680 feet. This is the distance below
the dam where the depth of flow in the flood plain would be one foot if the
dam would fail.
Since the homes were located 1,500 feet downstream from the dam,this
would indicate that the depth of flow at the homes would be greater than
one foot and would probably cause serious damage to these homes. This
would indicate that the dam would be a higher hazard structure than Class
"a" and would therefore be prohibited within the Town of Leesburg.
This is an approximate method and more detailed valley routings will give
more precise answers. This method should not be used if there is not a
uniform valley width, or if there is any downstream obstruction, such as a
road fill, an undersized pipe, etc.
44:5. Selecting the Stormwater Management Pond Site
A. The selection of a suitable stormwater management pond site should begin
in the preliminary stage of the development, with a view of selecting the
site that proves most practical and economical.
B. A pond with a normal pool planned as a site amenity,may incorporate the
required stormwater management function; or a stormwater management
pond may,with modification,be utilized as a site amenity.
C. A wet pond utilized as a site amenity should be located at a site where the
valley is narrow, side slopes are relatively steep, and the slope of the
valley floor will permit a large deep basin. Such sites tend to minimize
the area of shallow water if a permanent pool is being considered;
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however,they should be examined carefully for adverse geologic
conditions. In urban and suburban areas, large areas of shallow water
should be avoided due to excessive evaporation losses and the growth of
aquatic plants.
D. Consideration must also be given to any legal requirements. The
landowner is responsible for obtaining all necessary and required
easements of rights to discharge.
E. As previously noted the pond should not be located where sudden release
of the water, due to failure of the dam, would result in loss of life, injury
to persons, damage to residences or industrial buildings, railroads or
highways, or cause interruption of use or service of public utilities. A site
which presents one or more of these hazards is unsuitable and will not be
approved.
F. A check should be made to ensure that no buried pipelines, cables, or
other utilities exist in the construction area. Where such a site must be
used,the utility owners shall be contacted prior to foundation investigation
and utility relocation out of the embankment and impoundment area will
be the responsibility of the developer.
G. No part of any pond shall be located within a 100-year Town and/or
FEMA Floodplain without specific prior approval from the Director(and
FEMA within FEMA floodplains).
H. The outfall from all BMP and/or SWM Facilities shall be at an elevation
equal to or greater than the 100-year Floodplain elevation. With approval
from the Director,the outfall pipe may extend into the 100 year floodplain
as long as provisions are made for the pipe to have gaskets and the 100
year water surface elevation is lower than the bottom of the SWM facility.
16. Engineering Surveys. Once the location of the pond or reservoir has been
determined, sufficient engineering surveys shall be performed so that the
information required for stormwater management pond design can be obtained.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
4-6,7. Geologic Investigations
A. All designs for wet ponds shall have a geologic investigation performed.
Analysis shall be performed for dry ponds which have an embankment
height greater than 15 feet and/or those which impound more than 25 acre-
feet and/or those whose draw down time exceeds 24 hours.
B. The requirements of a foundation for an earthfill dam are that it provide
stable support for the embankment under all conditions of saturation and
loading, and that it provide sufficient resistance to seepage to prevent
excessive loss of water. Adverse foundation conditions can lead to failure
of a dam due to cracking,piping, sliding, settlement or uplift.
C. The foundation conditions under the proposed dam sites shall be
investigated to ensure that the site is suitable and that a safe structure can
be designed. The extent of the foundation examination will depend on the
complexity of the conditions encountered and on the height of the dam.
The "Unified Sys—tem of Soil Classification" shall be used in foundation
investigations and these logs shall be accurately located and shown on the
final design plans.
D. Borings should be taken or test pits excavated at intervals along the
centerline of the dam. The depth and spacing of the borings or pits should
be sufficient to determine the suitability of the foundation.
E. Borings should also be taken along the centerline of the principal spillway
to ensure an adequate foundation for the pipe and riser.
F. If a permanent pool is being considered, adequate soils investigations will
be needed in the proposed pool area to be assured that excessive seepage
will not be a problem.
G. In most cases, it is necessary to bypass excess storm runoff around the
embankment of a pond through an excavated spillway. For economic
reasons, suitable material excavated from the spillway should be used in
the earthfill. Therefore, soil borings should be made along the approximat:
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
centerline of the proposed spillway to determine the type of material that
will be encountered, its erodibility, and its suitability for use in the
embankment. If additional borrow is needed, soil borings should be made
in the selected borrow areas in order to estimate the kinds and amounts of
suitable fill materials available.
H. Materials selected for construction of a dam must have sufficient strength
for the dam to remain stable and provide sufficiently low permeability,
when compacted,to prevent harmful seepage through the dam.
A record or log of each boring or test pit should be made showing the
location depth and classes of materials encountered. The location of each
boring should be marked on the ground, so it can be referenced to other or
more detailed surveys.
J. All information developed during the design process should be recorded in
the form of an engineering plan for the pond.
4-778. Earth Embankment
A. Top Width. The minimum top width of the dam is shown below. When
the embankment top is to be used as a maintenance access road,the
minimum top width is to be the width of the proposed access road plus the
top width as determined below.
Total Minimum
Height of Embankment Top Width
(Feet) (Feet)
14 or less 12
15 - 19 12
20 -24 12
25—34
35
* or larger as may be required by the State approval agency
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
B. Side Slopes. The upstream and downstream side slopes of the settled
embankment shall not be less than:
Fill Material Slope
Upstream Downstream
Clayey Sand, Clayey Gravel, 3:1 3:1
Sandy Clay, Silty Sand, Silty
Gravel...
Silty Clay, Clayey Silt>>> 3:1 3:1
C. Wave Erosion Protection. Where needed to protect the face of the dam,
special wave protection measures such as berms, riprap, sand-gravel, soil
cement or special vegetation shall be provided. Refer to the Virginia
Department of Transportation Drainage Manual.
D. Freeboard. The vertical interval between the elevation of the water surface
in the reservoir with the emergency spillway flowing at design depth and
the minimum elevation at the top of the settled embankment is the
freeboard, and shall equal or exceed one foot;-in addition,the minimum
difference in elevation between the crest of the emergency spillway and
the settled top of dam shall be two feet.
E. Allowance for Settlement. The design height of the dam shall be
increased by the amount needed to ensure that the design top elevation
will be maintained after all settlement has taken place. If a minimum
required density is specified,the increase shall be five percent.
F. Foundation Cutoff. A cutoff trench of relatively impervious material shall
be provided under the dam and into the abutments, as required, and be
deep enough to extend into a relatively impervious layer except:
(1) In those cases where a layer of relatively impervious material thick
enough to provide stability exists at the surface of the foundation;
or
(2) In those cases where a layer of such material does not exist at a
reasonable depth.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
(3) Where the Geotechnical Engineer certifies(to the satisfaction of
the Director)that it is not necessary.
The cutoff shall be located at or upstream from the centerline of the dam.
Where such a layer does not exist at a reasonable depth,the engineer
responsible for the technical design shall provide a geotechnical analysis,
demonstrating that the site is feasible for the construction of a dam.
The cutoff trench shall have a bottom width adequate to accommodate the
equipment used for excavation, backfill and compaction operations,with
the minimum width being four feet, and shall have side slopes no steeper
than one foot horizontal to one foot vertical.
G. Seepage Control.
(1) Seepage control is to be included:
a. If pervious layers are not intercepted by the cutoff;
b. If seepage may create swamping downstream;
c. If needed to ensure a stable embankment; or
d. If special problems,-such-as-fractured-rock,-etc.,
require drainage for a stable dam
e. If recommended by the geotechnical engineer.
(2) Seepage control may be accomplished by
a. Foundation, abutment or embankment drains;
b. Reservoir blanketing; or
c. A combination of these measures.
Foundation drains are to be considered when the normal water
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
depth in the pond is greater than 15 feet(measured from the low
point at the centerline of the dam).
18 9. Spillways
A. Emergency spillways for all ponds will be designed to provide the
required detention and to pass the full volume of that portion of the
Probable Maximum Flood(PMF) shown in DD-11 based upon dam height
and impoundment. (The PMF is defined in Article 11).
B. Principal Spillways. A conduit, with needed appurtenances, shall be
placed under or through the dam except where a weir type structure is
used.
(1) The principal spillway shall be designed to provide the detention
required and control the release rate for those design-year events
stipulated for each major water-shed.
(2) The crest elevation of the inlet or riser shall be at least one foot
below the crest elevation of the earth emergency spillway.
(3) The inlet or riser size for pipe drops shall be such that the flow
through the structure goes from weir-flow control to pipe control
flow without going into orifice flow control in the riser. The inlets
and outlets shall be designed and analyzed to function
satisfactorily for the full range of flow and hydraulic head
anticipated. The riser shall be analyzed for flotation,using water
at the principal spillway crest elevation, and assuming all orifices
and pipes are plugged. The factor of safety against flotation shall
be 1.2 or greater.
(4) Size. The capacity of the pipe conduit shall be adequate to
discharge long duration, continuous, or frequent flows without
flow through the emergency spillways. The diameter of the pipe
shall not be less than 12 inches.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
(5) Conduits under or through the dam shall be reinforced concrete.
The conduits shall be capable of withstanding the external loading
without yielding, buckling or cracking. Conduit strength shall not
be less than Class III. The inlets and outlets shall be structurally
sound and made from materials compatible with the pipe. All
conduit joints are to be made watertight by the use of gaskets.
(6) Excavation for Placement. Where excavation into existing or
compacted ground is required in order to obtain the proper
elevation for the conduit,this excavation shall be of sufficient
width to accommodate the conduit, anti-seep collars, earth hauling
and hand operated compaction equipment. The side slopes of the
excavation shall not be steeper than one to one.
(7) Multiple Conduits. Where multiple conduits are used,there shall
be sufficient space between the conduits and the installed anti-seep
collars to allow for backfill material to be placed between the
conduits by the earth moving equipment and for easy access by
hand operated compaction equipment. This distance between
conduits shall be equal to or greater than the pipe diameter or
width opening but not less than two feet.
(8) Anti-Seep Collars. Anti-seep collars shall be installed around all
conduits through earth fills of all wet ponds and for all dry ponds
whose draw down time exceed 24 hours unless the Geotechnical
Engineer certifies (to the satisfaction of the Director)that it is not
necessary. Impoundment structures incorporating anti-seep collars
shall use the following criteria:
a. Sufficient collars shall be placed to increase the seepage
length along the conduit by a minimum of 15 percent of the
pipe length located within the saturation zone.
b. The assumed normal saturation zone shall be determined by
projecting a line with a slope of four horizontal to one
vertical from the point where the normal water elevation
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
touches the upstream slope of the fill to a point where this
line intersects the invert of the conduit. All fill located
below this line may be assumed to be saturated.
c. For ponds that are normally dry,the starting elevation shall
be the maximum water surface elevation in the pond when
the principal spillway storm is routed through the structure.
d. Maximum collar spacing shall be 14 times the minimum
projection of the collar measured perpendicular to the pipe.
e. Minimum collar spacing shall be five times the minimum
projection of the collar measured perpendicular to the pipe.
f. All anti-seep collars and their connections to the conduit
shall be water tight.
g. Alternate designs(designed to the satisfaction of the
Director and certified by the Geotechnical Engineer)will
also be permitted.
(9) Antivortex Devices. Drop inlet spillways are to have adequate
antivortex devices in accordance with the latest edition of the
Virginia Stormwater Management Handbook.
(10) Safety Guardrails and Trash Racks. Trash racks shall have
openings no larger than 3/4 of the conduit diameter or width
opening,but in no case less than six inches in its smallest
dimension. Racks and rails should be used when it is necessary to
prevent clogging or when a safety hazard exists. Flat grates for
trash racks are not acceptable, side openings must be provided.
(11) All ponds in urban areas shall be analyzed for safety. Low stage
inlets on ponds that are normally dry shall have adequate trash
racks. Velocity of water through the trash rack opening at design
flows shall not exceed three feet per second.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
(12) Drain Pipe. A pipe with a suitable valve should be provided to
drain the pool area where needed for maintenance. The principal
spillway conduit may be used as a pond drain when so located as
to accomplish this function.
(13) A narrative detailing how the pond is to be drained for
maintenance and who is responsible shall be part of all plan sets
submitted to the Town.
C. Emergency Spillways
(1) A separate, independent emergency spillway shall be provided for
each dam,the purpose of which is to provide for safe passage of
the appropriate portion of the Probable Maximum Flood(PMF)or
the 100 year storm event or the spillway design flood(whichever is
applicable to the specific stormwater management facility design)
without damage to the embankment.
(2) Capacity. The minimum capacity of emergency spillways shall be
that required to pass the peak flow expected from the appropriate
portion of the Probable Maximum Flood(PMF) or the 100 year
storm event or the spillway design flood(whichever is applicable
to the specific stormwater management facility design). The
routing shall start with the design water surface at the elevation of
the crest of the principal spillway. Refer to Detail DD-11 at the
end of this Article.
(3) Emergency spillways are to provide for passage of the design flow
at a non-erosive velocity to a point downstream where the dam will
not be endangered.
(4) Cross Section. Excavated earth spillways shall be trapezoidal and
shall be located in undisturbed earth. The side slopes shall be
stable for the material in which the spillway is to be constructed
but not steeper than 3:1. For dams having effective heights
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
exceeding 20 feet,the emergency spillway shall have a bottom
width of not less than ten feet.
(5) When natural spillways are used, a dike shall be constructed from
the end of the dam to prevent the flow from impinging on the toe
of the dam. The dike shall have a freeboard of one foot above
design flow.
(6) Permissible Velocities
a. Earth spillways shall be designed for non-erosive velocities
through the control section and for a reasonable distance
below the spillway. The maximum permissible velocity for
the grass or grass mixture to be used shall be selected from
the following table:
PERMISSIBLE VELOCITY FOR VEGETATED SPILLWAYS'
Vegetation Permissible Velocity
Erosion-Resistant Soils2 Easily Eroded3
Slope of Exit Channel Slope of Exit Channel
pct 0-5 pct 5-10 pct 0-5 pct 5-10
ft/s ft/s ft/s ft/s
Kentucky
Bluegrass
Smooth Broome 7 6 5 4
Tall Fescue
Reed
Canarygrass
Sod-Forming
Grass-Legume 5 4 4 3
Mixtures
Lespedeza
Sericea
Weeping
Lovegrass 3.5 3.5 2.5 2.5
Yellow Bluestem
Native Grass
Mixtures
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
1 SCS-TP-61
2 Those with higher clay content and higher plasticity. Typical soil textures are silty clay, sandy
clay, and clay.
3 Those with a high content of fine sand or silt and lower plasticity, or non-plastic. Typical soil
textures are fine sane, silt, sandy loam, and silty loam.
b. The capacity of the spillway shall be determined using
vegetal retardants representing an unmowed condition. The
maximum velocity shall be determined with-a vegetal
retardants representing a closely mowed condition.
GUIDE TO SELECTION OF VEGETAL RETARDANTS
Stand Average Height Degree of Stand Average Height Degree of
of Vegetation in Retardants of Vegetation in Retardants
Inches Inches
Higher than Higher than B
30 `� 30
11 to 24 B 11 to 24 C
Good 6 to 10 C Fair 6 to 10 D
2to6 D 2to6 D
Less than 2 E Less than 2 E
(7) Excavated earth spillways shall have an inlet channel,control
section, and an exit channel. Upstream from the control section,
the inlet channel shall be level for the minimum distance of 25 feet
and shall have side slopes equal to three to one or greater.
(8) The flow shall enter the spillway through the inlet channel. The
maximum depth of flow(Hp) located upstream from the level part
shall be controlled by the inlet channel, level part, and exit
channel. Refer to Detail DD-13 of this Article.
Excavation of the inlet channel or the exit channel, or both,may be
omitted where the natural slopes meet the minimum slope
requirements. The direction of slope of the exit channel must be
such that discharge will not flow against any part of the dam.
Wing dikes, sometimes called kicker levees or training levees, can
be used to direct the outflow to a safe point of release. The
spillway should be excavated into the earth for the full length and
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
width of the spillway. Refer to Standard DS-4 in Appendix A.
If this is not practical,the end of the dam and any earthfill
constructed to confine the flow; shall be protected by vegetation or
riprap. The entrance to the inlet channel should be widened so it is
at least 50 percent greater than the bottom width of the level part.
The inlet channel should be reasonably short and shall be planned
with smooth, easy curves for alignment. It shall have a slope
toward the reservoir of not less than two percent to ensure
drainage.
(9) The inlet channel may be curved to fit existing topography,but exi
channels shall be straight for a minimum distance well beyond the
downstream toe of the dam at the lowest point in the valley.
(10) The grade of the exit channel of an-excavated earth spillway shall
fall within the range established by discharge requirements and
permissible velocities. The exit channel shall terminate only where
the design flow may be discharged without damage to the earth
embankment.
(--3-)(1) With the required discharge capacity,the degree of retardance,
permissible velocity, and the natural slope of the exit channel
known,the bottom width of the level and exit sections and the
depth of the flow(Hp) can be computed from figure in Detail DD-
13 of this Article which show discharge per foot of width. The
natural slope of the exit channel should be altered as little as
possible.
B. C. Combined Spillways
(1) Combined spillways (a single structure that combines the primary
and emergency spillways) shall require approval of the Director
and shall only be permitted when adequate provisions for the
release of flows based upon the appropriate portion of the Probable
Maximum Flood (PMF) or the 100 year storm event or the
spillway design flood(whichever is applicable to the specific
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
stormwater management facility design) can be accommodated
downstream of the structure's outfall and when protection of the
embankment is employed. Refer to Detail DD-11 of this article for
spillway design requirements.
(2) The combined spillway(when approved by the Director) shall be
designed to provide the detention required and control the release
rate for those design-year events stipulated for each major water
shed and adequately control the outflow of the less frequent events
of the appropriate portion of the Probable Maximum Flood(PMF)
or the 100 year storm event or the spillway design flood
(whichever is applicable to the specific stormwater management
facility design).
(3) The combined spillway (when approved by the Director) shall
provide for a minimum of 24 inches of freeboard from the
elevation of the appropriate portion of the Probable Maximum
Flood(PMF) or the 100 year storm event or the spillway design
flood (whichever is applicable to the specific stormwater
management facility design)to the Top of the Dam.
(4) Any design which utilizes a combined spillway shall incorporate a
secondary all weather access route for the facility.
G:C. D. Structural Emergency Spillways
(1) Pipes, culverts, chutes or drops, when used for principal spillways
or principal-emergency or emergency spillways, shall be designed
in accordance with the principles set forth in the National
Engineering Handbook, "Drop Spillways"; and "Chute Spillways."
The minimum capacity of a structural spillway shall be that
required to pass the peak flow expected. The routing shall start
with the water surface at the elevation of the design storm.
(2) Structural emergency spillways may only be approved after an
independent structural review of the design is completed by a
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
structural engineer familiar with hydraulic structures, selected by
the Director. All costs of this review shall be borne by the
Developer.
(End of Section)
5 610 General
• - - . . - , - - - - - , . . . _ _ .
• I .
w
(2) Chapter 11 of the Town Code, and
•
•
• .
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
•
pareel,
- - -
- - ,
p
•
. _ •. • •' , - . • - . .. , • - . .• •••-
- - - - -
- - - • , , . -
. _
-• - . ,•- -• - -
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
A. In average land cover condition is defined as the measure of the
Y.. •-
percent-er
D• • • - -•
• • .' - S -
a. In no case shall the post development non point source
' m - -
- - '
c. If BMPs are structural,evidence(certified by a professional
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
-- - '
ordi ces;
• • -- -
• -
.
• -. . . I .
• - .. .
utilized.
_.. .
- • , • •
- - - - - - -
• • - -
BMP.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
. .. . . - s - - - . , . .. - - • - -• - • - .,•
. . . -• ., _ w-- D• - - - ' , - _ - -
3. Design Considerations
•
•
B. Credit for treating off site developed sites is not permitted by right.
•
.- - _ •
•
. 111'" -
. . -
(1) Drainage area served;
developmenti
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. .
_.. • •
the Directo )
(7) Whether the system is on line or off line;
.- - -
•
•
an
• -".
E. Additional BMP(Water Quality) Design Options
•
.
• - •
- • • - - ..
• - • - , . . . ...
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
,• :, a b the pl: nt
• - -
. _ • .- _
. •
performance-criteria.
. _ • - •- . -, - . . _ , ,
fedevelepment
Criteria:
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
a. A minimum 50 foot wide vegetated area preserved
•- - - - , . - . . _ . . _ _ , .. .
- . . . •-
speeifie-situations.
_.. ■•
. t •
_ ■
r •
• • '
•
•
■_ _ ■ - • •-
• ■ . .
•
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
. . .. _ . . •• - _ _ .. - . •- _ ., _ - _ . _ .
-- . . . . . . .. . .. . • • - • • - Z. •• ..,
. •
• •
• - •
• •tr
• • • . •-
- - - - •
• - • .
•• •• •••thereof,
i • . • •
- •' - • - - -
a. Developments that are part of a larger existing or
• ,
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
b. Developments that are deeded"Hot Spots"as defined in 5
640 of this article.
previAed rti
water-shedr and
rates,and
..
ease-basis.
5 640 HOT SPOTS
. . . - - ..
- -
. .• _
8. The following are examples of such hot spots:
A. Vehicle salvage yards and vehicle recycling facilities;
•
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
C. Vehicle equipment cleaning facilities;
D. Fleet storage areas;
E. Industrial sites;
F. Outdoor liquid container storage;
• • .
H. Commercial container nurseries;
I. Golf courses;
•
•
- .. .
• . .•
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.•. . - . . . • - - • . - - Y-. . - . ..
Y. .
- -- - .
K. Dry cleaning operations.
L. Public works storage areas
N. Chemical storage areas
-
- ..
requir-e
s - -
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STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
•- - -- .•
- - . . -.. .
' .. . • - .
as-amended:
.. . , ., _•
•
- «. • .- -
I ' ' s -
• • . _
. . - - --
• - - .• .
•
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
Ordinafice�
• - . . , - - • • , _ . . •.. _ _ . - • - . , -. -
. • • .•
.. • I. .... • . I • • . •
•
• - • - -
.. . •
- • •
. • . • - - • • •
t. • • • - • - - - - -
•
• • , • • •- • • '• -
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
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E. A schedule for BMP implementation.
•
• - -
.• . - - ---
Table 5 630 1 BMP Efficiencies and Considerations
BMP °%Range-of Positive Negative
Efficiency Imperviees Gonsidefations Considerations
Vegetated 4-0% 16 21% 1. Increases 1. Low removal
Filter vegetative efficiency.
Strom cover, 2. Small drainage area
2. Can serve small, 3. Water quality only.
•Gassed 15% 16 21% 1. Easy to 1. Low removal
Swale incorporate, efficiency.
2. May be 2. Water quality only.
3. Requires
Gonstrueted 20% 22 37% 1. May be used for 1. Space intensive.
Wetland*
2. May be used 3. May be visually
Extended 35% 22 37% -1,-Well-established 1. Space intensive.
Detention (2*WQV) design 2. Sometimes
prineiples: perceived-as-net
Retention 40% 22 37% 1. Well established ,,Increased
Basin (3*WQV) design maintenance
principles, costs.
50% 38_66% 2. Can incorporate 2. Increased
(1*WQV) quantity-controls, liability-due-te-stafiding
3. Can serve a large water,
65% 67 100% area 3. Space intensive.
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
Infiltration 50% 38-66% 1. High removal 1. Requires
(Basins-OF (1*WQV) effieiene permeable
Trenches)* 2. Effective soil .
65% 67 100% 2. Quantity control
(2*WQV) limier
3. Not to be used
gioretention 50% 38-66% 'w be 1. Small drainage
Basin/Filter* aesthetically area,
2. Not to be used
near-marine-cla3Ger
2. Appropriate for karst.
Sand Filter* 6544 67 100% 1. Primarily water
very-dense-urban-areas,control:
2. May be placed 2. Maintenance is
M op alPage-of Positive Negative
Efficiency Impervious Considerations
5. More expensive to
Manufacture Variable Variable, 1. Easy to install and 1. Proprietary device.
d depending- pendesign.. 2. Water quality only.
BMP-s Manufacturer 2. Manufacturer 3. Design and
15 20% recommendatie may-perform removal-efficiencies-not
(Stermeepter 11-s maintenanee, standardized,
, Baysaver, 3. Can be
vertechs;
Green Roof* Variable Variable 1. Secondary benefits 4. Best f r new
( construction:
reduces 2. De „a al
pollution), not
2. Aesthetically standardized,
pleasing
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
BMP Measures
Permeable Variable Variable 1. Does not take up 1. Are easily clogged or
Pavers/ compacted.
pervious 2. Reduces overall 2.Not for high traffic
concrete/ areas.
3. Most
appropriate-for-low-use soils-or-tinderdrain
Tree Box Variable Variable:- 1. May be used for
Filter depending upon retrofit area:
74% Manufacturer situations 2,-Water-quality-only,
(Pi ltere-Or reeemmendatie 2. Small
similar)* n-s area-needed cost,
Street Variable Variable 1. Aesthetically 1. Equipment is
Sweeping* pleasing expensive
2, Reduces and labor intensive.
2. Water quality only.
- , . . . •
- .
AKA, -L-=--Water-Quality-Value
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
I SECTION 5-700 600 INSPECTION AND ACCEPTANCE
I 5410610 Inspection
1. All storm sewers shall be inspected by the Town of Leesburg Inspectors(or their
designee) and/or the Virginia Department of Transportation at periodic intervals
during construction.
2. These inspections shall include a visual check of all storm sewer and
appurtenances for damage related to construction.
1 5- 20620 Acceptance
1. All damage as determined by the above inspection shall be corrected, (replaced or
repaired)to the satisfaction of the Director before acceptance.
2. Testing as required by the Director prior to acceptance shall be done in the same
manner as that in Article 4, Section 150 of this ManualDCSM.
3. Storm sewers shall be clean and free of debris and sediment prior to acceptance
by the Town.
4. The Director shall approve and recommend for acceptance accept all storm sewer,
appurtenances, and detenrstormwater management facilities when it has been
determined that the field engineering and construction has been completed as
stated in the approved plans,the structures are in place,the ground around them
stabilized in accordance with the final plans,a stormwater facility maintenance
agreement has been executed,the performance bond for maintenance is provided.,
and a site as-built drawing has been submitted by the applicant and approved by
the Town. Once the paper copies of the Final Site As-Built has been approved by
the Town,the applicant shall, prior to acceptance and bond release, provide the
Town with additional paper copies as well as an electronic version(in a format
determined acceptable by the Town)of the As-Built drawings.
(End of Section)
I SECTION 5-700 EASEMENTS
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
5-701 General
1. The easement requirements of this section are in addition to other easement
requirements contained in this Article.
2. Storm drainage, stormwater management facility, natural channel, and vegetated
buffer area easements shall be required in accordance with the table in Section 5-
702.
3. Flood plain easements shall be required in accordance with Section 5-400.
4. All easements shall be conveyed to the Town and shall be in a form approved by
the Town Attorney. Easement widths shall be in one foot increments.
5. All stormwater management facility easements and vegetated buffer area
easements must include a separate maintenance agreement in accordance with this
Article and the Town Code.
4-6. The dedication of a vegetated buffer area easement is not to be construed as
requiring the Town to maintain the vegetated buffer area.
2,7. Only publicly maintained structures shall be located in the Town's right-of-way,
Town-owned property, or public easements unless granted written permission
from the Director.
3.8. All privately maintained structures shall be located within private easements that
guarantee the Town maintenance rights and access as the maintainer of last resort.
4.9. Standard easements are not acceptable for access to detention facilities; therefore,
special access easement agreements are to be executed that preclude planting of
shrubs and the construction of fences and other structures within the easement.
5-220702 Easement Applicability and Width Easements
-160-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
Easement Type
Applicability
Width
Storm Drainage —
Minimum
15 feet
Pipes
Maximum
Single Pipes — 30 feet
Multiple Pipes — 15 feet each side
Single pipes less than
1:1 side slope from the pipe invert to the
or equal to 24 inches
elevation of the finished grade on both sides
in diameter
of the pipe
Single pipes greater
1:1 side slope from the pipe invert to the
than 24 inches in
elevation of the finished grade on both sides
diameter
of the pipe plus the outside diameter of the
pipe
Multiple pipes
1:1 side slope from the lowest pipe invert to
the elevation of the finished grade on the most
outside pipe, plus the combined outside pipe
diameters, plus the width of space between
each pipe.
Storm Drainage —
Top width of the
Minimum 15 feet
Channels
channel bank less than
or equal to 5 feet
Top width of the
Top width plus a ten foot access strip
channel bank between
immediately adjacent to one side of the
5 and 10 feet
channel
Top width of the
Top width plus a ten foot access strip
channel bank greater
immediately adjacent to both sides of the
than 10 feet; side
channel
slopes exceeding 3:1
Top width of the
Top width plus a ten foot access strip
channel bank greater
immediately adjacent to one side of the
than 10 feet; side
channel
slopes not exceeding
3:1
Storm Drainage —
Yard inlets and end
Minimum width set at the limits of the 10-
Yard Inlets and End
sections (or
year water surface elevation
Sections
headwalls)
Storm Drainage —
100-year overland
Minimum width set at the limits of the 100-
100-year Overland
relief flow path, not
year overland relief flow path
Relief
including ponded
areas contiguous to
the flow path
-161-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990, AS AMENDED.
Easement Tv in
Applicability
Width
Stormwater
Above ground
10 feet beyond engineered structural
Management Facility
structures
components and the 100-year water surface
elevation
Underground
10 feet beyond periphery of the structure
structures
Access roadways
1 foot on each side of the entire length of the
roadway
Natural Channel
Minimum
Limits of the 100-year water surface
Vegetated Buffer
Minimum
Width of vegetated buffer area required in
Area
Section 5-323
inerements
A. Pipes
(1 l Fer- single p s 2 ;nehes and less in diameter the easement width
shall be determined by a 1 .1 side slope extending from the
elevation of the ert to the ele at; o f the posed
(2)--Fer-single-pipes-gfeater-4han-2-44nehes-in-diameter-the-easement
•idtl, shall be determined bye 1 .1 side slope extending f;.om tl,e
elevation of the r ert to the elevation a the proposed
f:n shed ade .. botl. ide of'the plus the utside- dia eter
ewe -pig
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL(DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
15 feet each side for multiple pipes.
(5) Refer to Standard WS 16 in Appendix A.
B. Channels
•
A. -
ehel-is-required
C. Yards Inlets and End Sections
elevation,
�r
-163-
AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA, June 27, 1990,AS AMENDED.
A. Pipes
(2) For single pipes greater than 21 inches in diameter the easement
ewe pipe.
(5) Refer to Standard WS 16 in Appendix A.
--• -- I -
m
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AN ORDINANCE: TO REVISE ARTICLE 5 OF THE DESIGN AND CONSTRUCTION
STANDARDS MANUAL (DCSM) OF THE TOWN OF LEESBURG,
VIRGINIA,June 27, 1990, AS AMENDED.
CONSTRUCTION DETAILS ASSOCIATED WITH CHAPTER 5
SECTION II. This Ordinance shall become effective July 1, 2014; and
SECTION III. If any provision of this Ordinance is declared invalid,the decision shall
not affect the validity of the Ordinance as a whole or any remaining provisions of the Ordinance.
PASSED this 13th day of May,2014.
Kristen C. U _ stand, Mayor
Town of Leesburg
ATTEST:
i't ' -
Clerk of Co 11144
P:\Ordinances\2014\0513 DCSM Stormwater Amendment.doc