HomeMy Public PortalAboutRES-CC-2015-31Resolution #31-2015
A RESOLUTION APPROVING THE USE OF THE
"GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES
WITHIN SPANISH VALLEY"
FOR DEVELOMENT APPLICATIONS
WITHIN THE CITY OF MOAB
WHEREAS, the City of Moab (City) is the owner of public infrastructure, and is responsible for the
design, construction, operation, and maintenance of storm drainage facilities; and
WHEREAS, the City recognizes the need to adequately design and construct both public and private
storm drainage facilities for all development activities located within the City; and
WHEREAS, the City further recognizes that sound stonn drainage design and construction is not confined
to the incorporated City limits, and should be coordinated with adjacent jurisdictions where appropriate;
and
WHEREAS, the City Engineer has reviewed the "Grand County Design Criteria for Drainage Studies
within Spanish Valley," and found these criteria to be applicable to the design of storm drainage facilities
within the City with the Amendments attached thereto;
NOW THEREFORE, we, the Governing Body of the City of Moab do hereby resolve to approve the use
of the "Grand County Design Criteria for Drainage Studies within Spanish Valley" as Amended by the
City Engineer and attached to this resolution, for the design and construction of public and private stonn
drainage facilities located within the City.
Passed and adopted by action of the Governing Body of the City of Moab in open session this
10th day of November, 2015.
ATTEST:
Kdcuad S.1&„0--4____,
Rachel E. Stenta, Recorder
/
AGENDA SUMMARY
MOAB CITY COUNCIL MEETING
November so, 2015 I
Agenda Item
#: 7-2
1
Title: Adoption of Resolution # 31-2015, approving the use of the "Grand County
Design Criteria for Drainage Studies within Spanish Valley' for Development
Applications within the City of Moab
Fiscal Impact: none
Staff Presenter(s): Phillip M. Bowman, P.E., City Engineer
Department: Engineering
Background/Summary:
The "Moab City Design Standards and Public Improvement Specifications"
(September 1999) are currently used by City staff and consulting engineers to
provide design guidance for public and private infrastructure improvements
required with proposed development within the City of Moab. In this document,
drainage design standards are provided in Section 2 — Design Standards,
Subsection 2.9 — Drainage System Plan. Since the development of these Design
Standards over fifteen years ago many storm drainage design practices and
requirements have been revised and improved, and these Design Standards for
storm drainage improvements now need to be updated.
The "Grand County Design Criteria for Drainage Studies within Spanish Valley"
(December 2011) are the design guidelines developed by Grand County for public
and private storm drainage infrastructure required with proposed development
within the unincorporated county. The Spanish Valley area lies directly adjacent to
the City of Moab, and shares the same hydrologic conditions found in the City.
These design guidelines also address the hydraulic design requirements for all
components of the storm drainage system. With relatively minor amendments as
attached, the application of the design guidelines contained in the Grand County
Design Criteria is also suitable for the design of all public and private storm
drainage improvements located within the City of Moab.
By approving the use of the Amended Grand County Design Criteria, the
expectations for storm drainage design with all proposed development will be
clarified and defined. Applicants and consulting engineers will have improved
guidance for designing the public and private storm drainage infrastructure
required, and the public storm drainage improvements accepted by the City will be
sized and designed in a manner to minimize long-term maintenance costs.
The Grand County Design Criteria document and the proposed Amendments as
recommended by staff are attached for your review.
Options:
1. Adopt Resolution # 31-2015, approving the use of the Amended Grand County
Design Criteria as attached.
2. Direct staff to make additional changes and/or amendments to the Design
Criteria, and adopt with a Resolution at a later meeting.
3. Other, as directed to staff.
Staff Recommendation: Staff recommends adopting Resolution # 31-2015,
approving the use of the Amended Grand County Design Criteria.
Attachments:
Resolution # 31-2015
Amendments to the "Grand County Design Criteria for Drainage Studies within
Spanish Valley"
Grand County Design Criteria for Drainage Studies within Spanish Valley,
dated December 2011
CITY OF MOAB
AMENDMENTS TO THE "GRAND COUNTY DESIGN CRITERIA FOR
DRAINAGE STUDIES IN THE SPANISH VALLEY" (DECEMBER 2011)
For the adoption and use of the "Grand County Design Criteria for Drainage Studies in the
Spanish Valley," the City of Moab approves the following Amendments:
1. All references to "County Engineer" are to be replaced by "City Engineer" and shall refer to the
staff position of City Engineer of the City of Moab.
2. All references to the "Spanish Valley Storm Drain Master Plan Update (2011)" are omitted,
except in the specific case(s) where any proposed development is located directly adjacent to,
upstream of, and/or downstream of areas within Unincorporated Grand County. In these cases,
the inclusion of "regional" facilities referenced in the "Spanish Valley Storm Drain Master Plan
Update (2011)" within the development area(s) inside the City of Moab will be reviewed and
coordinated with Grand County, and the necessity of those facilities will be determined by the
City Engineer.
3. All references to "post -developed discharge rate" from developed sites and/or detention ponds
shall be modified as follows:
"The allowable post -developed discharge rate for all development within the City of Moab shall
be 0.2 cfs per acre."
This reference applies to multiple pages and sections of the Design Criteria, and is intended to
replace the two regions and two allowable discharge rates discussed in Section 2.3.2 of the
Design Criteria.
4. The following requirements for a Preliminary Drainage Report and Final Drainage Report are
included with this amendment:
A Preliminary Drainage Report and Final Drainage Report are required for all new development
and redevelopment, depending on the type of development application being made. The type of
report required (Preliminary OR Final), and the number of report copies to be submitted will be
determined at the pre -application meeting. Upon review of the application materials and drainage
report submitted, one copy of the drainage report will be returned to the applicant or his
representative with comments from the City Engineer
The Preliminary Drainage Report shall contain general information regarding the proposed
drainage facilities for the development. For instance, only identify that a channel or storm drain is
proposed for conveyance, and not the size, slope, velocity or other more detailed information.
Also, it is only required to identify the location and type of detention, and not the volumes or
release rates; however, post -development flow rates shall be calculated and submitted with the
preliminary report.
The Final Drainage Report shall provide all final design and details of proposed drainage
facilities, including grading, erosion control, detention ponds, and water quality enhancement,
and is to be submitted with construction plans.
All Drainage Reports shall be prepared by a professional engineer registered in the State of Utah
in accordance with the amended Design Criteria. All Drainage Reports shall contain the
following statement of certification:
I hereby certify that this (Preliminary or Final) Drainage Report for the design of (Name
of Development) was prepared by me (or under my direct supervision) in accordance
with the provisions of the Amended Design Criteria for Drainage Studies for the owners
thereof. I understand that the City of Moab does not and will not assume liability for
drainage facilities designed by others.
(signature)
Registered Professional Engineer
State of Utah No.
(Affix Seal)
5. A submittal checklist for Preliminary Drainage Reports and Final Drainage Reports will be
developed, modified, and maintained by the City Engineer. The checklist will be available to all
applicants and consulting engineers for use in the preparation of Drainage Reports. The checklist
will be used to determine the sufficiency of all reports submitted, and will be reviewed at all pre -
application meetings.
Amendments 1 through 5 noted here are approved through the Adoption of Resolution #31-2015 by the
Governing Body of the City of Moab on November 10, 2015.
GRAND COUNTY
DESIGN
CRITERIA FOR
DRAINAGE
STUDIES WITHIN
SPANISH
VALLEY
DECEMBER 2011
Prepared for
Grand County
By
Horrocks Engineers
Grand County
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
TABLE OF CONTENTS
1 Introduction 1
1.1 General Information 1
2 Design Standards 2
2.1 Hydrologic Evaluation 2
2.1.1 Runoff Peak Rate of Discharge Calculation 2
2.1.2 Runoff Volume Calculation 3
2.1.3 Design Storms 3
2.1.4 Time of Concentration 4
2.2 Hydraulic Evaluation 5
2.3 Storage Criteria 5
2.3.1 Parking Lot Ponding 5
2.3.2 Design Requirements 6
2.3.3 Storage Facilities: Maintenance and Repair 10
2.4 Streets and Curbs 10
2.4.1 Street Flooding Evaluation 11
2.4.2 Hydraulic Capacity 12
2.4.3 Maximum Velocity in Gutters 12
2.4.4 Estimating Runoff in Streets 12
2.4.5 Flow Across Intersections 13
2.5 Storm Water Inlets 13
2.5.1 Design of Inlets 13
2.6 Storm Drain Systems 14
2.6.1 Layout of the Storm Drain System 14
2.6.2 Storm Drain Location and Alignment 14
2.6.3 Hydraulics 14
2.6.4 Materials 15
2.6.5 Access Holes 15
2.7 Culverts 15
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
2.8 Open Channels 16
2.8.1 Control of Erosion 16
2.9 Other Design Considerations 16
2.9.1 Soil Erosion 16
2.9.2 Siltation and Sediment Control 17
2.9.3 Storm Water Runoff Pollution 18
2.9.4 Mosquito Control 19
2.10 Operation and Maintenance 19
REFERENCES 20
Page iii
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
FIGURE'
December 2011
Figure 2-1: Spanish Valley Allowable Release Rate per Acre per Region 7
'ABLES
Table 2-1: Design Frequency 2
Table 2-2: Spanish Valley Average Precipitation Depth/Duration/Frequency. 4
Table 2-3: Spanish Valley Average Precipitation Intensity/Duration/Frequency 4
Table 2-4: Minimum Top Widths 10
Table 2-5: Allowable Spread Criteria for 10-year Storm Event 11
Table 2-6: Manning's n for Street and Pavement Gutters 12
Table 2-7: Access Hole Spacing 15
Page iv
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
1 INTRODUCTION
ti GENERAL INFORMATION
The Grand County Design Criteria for Drainage Studies Within Spanish Valley is a document by which the
County can regulate all drainage studies and designs and ensure all drainage studies and designs are
consistent with the methodology and recommendations outlined in the Spanish Valley Storm Drain
Master Plan Update (2011). The criteria set forth in this document are not all inclusive of design criteria
and design procedures that shall be used when performing drainage studies and designs. The current
editions of the Hydraulic Engineering Circulars (HEC) and the Hydraulic Design Series (HDS) produced by
the Federal Highway Administration of the U.S. Department of Transportation shall be used as reference
and further guidance to appropriate analysis and design techniques and procedures but will not
supersede any criteria outlined in this document. All drainage studies and designs must be approved by
the County Engineer. This document does not cover drainage details and construction specifications.
APWA standard details and specifications apply.
Page 1
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
' DESIGN STANDARDS
The following Design Standards apply to the design of all storm water management improvements
whether public or private, whether within new development or drainage from off -site, whether above
or below drainage outfall locations, whether within a 100-year flood plain or not, or within a natural
channel or not. Drainage studies and designs for areas within the Spanish Valley must follow standards
outlined in this document and any additional standards and criteria set forth by the current County
Engineer. For the analysis and design standards and approach for items not discussed in this document,
refer to the current editions of the Hydraulic Engineering Circulars (HEC) and the Hydraulic Design Series
(HDS) produced by the Federal Highway Administration of the U.S. Department of Transportation or
other materials approved by the current County Engineer.
All hydrologic and hydraulic evaluation and design for a proposed development shall be performed in
accordance with sound and accepted engineering practice. All drainage studies and designs must be
reviewed and approved by the County Engineer and shall conform to the Spanish Valley Storm Drain
Master Plan Update (2011).
2.1 HYDROLOGIC EVALUATION
Watershed storm water management requires the determination of two runoff parameters: peak rate
of discharge and volume. Both parameters shall be used in the comparison of pre -development and
post -development conditions.
Peak rate of discharge calculations shall be used to determine the configurations and sizes of pipes,
channels, and other routing or flow control structures. Runoff volume calculations shall be used to
determine the necessity for, and sizing of, detention and retention facilities.
All components of the storm drainage system shall be sized based on the design frequency in Table 2-1.
A more in depth description of the design frequency and all design criteria for each component of the
storm drainage system is given later in the chapter. The size of the drainage area shall include on -site
and off -site lands contributing to the design point.
Table 2-1: Design Frequency
Design Storm Facilities
10-yea r
100-yea r
Inlets, laterals, minor trunk
lines, and roadway spread
Storage basins, culverts,
major trunk lines, and major
conveyance facilities
2.1.1 RUNOFF PEAK RATE OF DISCHARGE CALCULATION
The peak rate of runoff for areas of up to 10 acres may be calculated by the Rational Method or one of
its derivatives. The Rational Method shall be used with caution if the time of concentration exceeds 30
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
minutes and shall never be used when the time of concentration exceeds 1 hour. Computer software
adaptations of the Rational Method calculations are acceptable provided that their data and graphic
printout allow review and evaluation. The peak rate of runoff for all areas which do not use the Rational
Method, or are greater than 10 acres, shall be calculated with the Natural Resources Conservation
Service (NRCS) unit hydrograph and curve number method, also known as the SCS method. Computer
programs such as TR-20, TR-55, HEC-1, or HEC-HMS may be used.
Depending on the shape and size of the drainage basin, the quick runoff from the streets and adjacent
impervious areas may be the worst case, producing a greater peak runoff rate than from the whole
drainage basin. This scenario shall be checked and the time of concentration and drainage area shall be
adjusted as necessary if this case produces the largest peak rate of runoff.
2.1.2 RUNOFF VOLUME CALCULATION
Runoff volume shall be calculated based on the NRCS method within TR-20, TR-55, HEC-1, or the HEC-
HMS computer program. The NRCS method shall be used for watersheds with drainage areas of less
than 5 square miles. The design of storage facilities shall be based on the NRCS method within a
computer program such as HEC-1, HEC-HMS, or PondPack. Other computer software adaptations of
these runoff value calculations may be acceptable provided that their data and graphic printout allow
review and evaluation.
For detention design, a 100-year 24-hour storm shall be used to calculate basin inflow. Detention basin
outflow shall be based on the 10-year historic release rate for the land within the project area and off -
site contributing areas, unless special circumstances are involved. In the case of special circumstances,
the designer must obtain approval from the current County Engineer. The 10-year historic release rate
is defined as the runoff rate that would have occurred due to a 10-year storm assuming the area is in a
natural condition before man-made changes were introduce. The 10-year historic release rate is
discussed in section 2.3.2 and shown in Figure 2-1.
2.1.3 DESIGN STORM
Table 2-2 and Table 2-3 give the precipitation depth and precipitation intensity to be used for all
projects within the portion of Spanish Valley under Grand County jurisdiction. This data was obtained
from NOAA Atlas 14 (January 2009) via the National Oceanic and Atmospheric Administration's (NOAA)
Precipitation Frequency Data Server (http://hdsc.nws.noaa.gov/hdsc/pfds/sa/ut_pfds.html).
The values given in the tables are the weighted average of several different locations within the Spanish
Valley, covering areas from the valley to the upper ridges. The data is based on the NOAA Atlas 14
partial duration series, with ARI representing the Average Recurrence Interval. In all instances, a
minimum time of concentration of 5 minutes shall be used. When using the NRCS method of analysis,
the NRCS Type II-24 hour precipitation temporal distribution and the NRCS unit hydrograph shall be
used, along with 24 hour duration precipitation values.
Page 3
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
Table 2-2: Spanish Valley Average Precipitation Depth/Duration/Frequency.
Precipitation Depth (inc
ARI
(years)
5 min
10 min
15 min
30 min
60 min
2 hr
3 hr
6 hr
12 hr
24 hr
1
0.11
0.17
0.21
0.28
0.35
0.43
0.48
0.60
0.74
0.93
2
0.14
0.22
0.27
0.36
0.44
0.55
0.60
0.75
0.93
1.17
5
0.19
0.29
0.36
0.49
0.60
0.73
0.79
0.95
1.16
1.47
10
0.24
0.36
0.45
0.60
0.75
0.91
0.96
1.13
1.36
1.74
25
0.31
0.47
0.59
0.79
0.98
1.21
1.24
1.41
1.65
2.13
50
0.37
0.57
0.71
0.95
1.18
1.48
1.52
1.65
1.90
2.45
100
0.45
0.69
0.85
1.14
1.42
1.81
1.85
1.95
2.17
2.82
Table 2-3: Spanish Valley Average Precipitation Intensity/Duration/Frequency.
ARI
(years)
5 min
10 min
15 min
30 min
60 min
2 hr
3 hr
6 hr
12 hr
24 hr
1
1.34
1.01
0.83
0.57
0.35
0.22
0.16
0.10
0.06
0.04
2
1.71
1.29
1.07
0.72
0.44
0.27
0.20
0.12
0.08
0.05
5
2.30
1.74
1.44
0.97
0.60
0.37
0.26
0.16
0.10
0.06
10
2.82
2.16
1.79
1.21
0.75
0.45
0.32
0.19
0.11
0.07
25
3.72
2.83
2.34
1.58
0.98
0.60
0.41
0.24
0.14
0.09
50
4.47
3.41
2.82
1.90
1.18
0.74
0.51
0.28
0.16
0.10
100
5.40
4.12
3.40
2.29
1.42
0.90
0.62
0.32
0.18
0.12
ENE OF CONCENTRATION
Several different methods and equations may be used to determine the time of concentration of a
watershed. Some equations, such as the NRCS lag equation, are calibrated for the entire watershed;
whereas, other methods include the summation of the travel time for sheet flow, shallow concentrated
flow, and open channel flow along the principal flow path. The designer shall select the appropriate
methods and equations for the flow path and design approach used.
When computing the travel time along natural channels, the channel shall be approximated as
trapezoidal. Values of Manning's n to be used for natural channels and floodplains shall be obtained
from Hydraulic Design Series No. 3 - Design Charts for Open Channel Flow. Note that the average slope
for a natural stream shall exclude the influence of short drops or rapid flow sections. In natural alluvial
streams, flow velocity does not exceed critical velocity except at control sections. These control sections
are usually limited in extent and represented by riffles, cascades, and waterfalls. Within the Spanish
Valley, these control sections can be identified by rock outcrops within the channels. After an initial
analysis, it may be necessary to revise the Manning's n and channel slope along the natural channel flow
path to reduce the velocity to a maximum of critical velocity. The NRCS lag equation is not influenced by
Page 4
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
the abrupt channel drops, since it is based on the average watershed slope, not the slope along the
principal flow path.
2.2 HYDRAULIC EVALUATION
The hydraulic evaluation and design for a proposed development shall be performed in accordance with
sound and accepted engineering practice. A system emphasizing a natural as opposed to an engineered
drainage strategy is encouraged. The applicability of a natural approach depends of such factors as site
storage capacity, open channel hydraulic capacity, and maintenance needs and resources. Hydraulic
capacity for open channel or closed conduit flow shall be determined by the Manning Equation.
Velocities in open channels at design flow shall not be greater than that velocity which will begin to
cause erosion or scouring of the channel. Velocities in closed conduits at design flow shall be at least 2
ft/sec but not more than the velocity which will cause erosion damage to the conduit. Refer to section
2.8.1 regarding the design for erosion control.
2.3 STORAGE CRITERIA
Provision of storage can: reduce peak runoff rates; aid in the recharge of groundwater; provide an
attenuation mechanism if storm water is to be treated; lessen the possibility of downstream flooding,
stream erosion, and sedimentation; and can be used in the development of upstream areas to avoid
increasing the runoff peaks which impact existing downstream facilities. Types of storage basins include
those used for detention and retention.
Retention facilities are used as infiltration basins, with the only outlet being the emergency spillway.
Due to concerns with sedimentation and inadequate infiltration capacities, retention basins are not
allowed in Grand County, unless approved by the County Engineer. When retention basins are allowed,
extensive soil studies of the saturated and unsaturated zones are required in order to ensure that the
retention basin will function properly over the life of the basin. Furthermore, a strict maintenance plan
must be approved by the County Engineer to ensure the basin is kept clean of sediment, and debris.
Detention facilities are used to temporarily detain storm water runoff and release it at a controlled rate.
The controlled rate is outlined in section 2.3.2 Design Requirements. These facilities shall be designed to
completely drain within 24 hours after the end of a storm event. If this is not possible due to
requirements for water quality or maximum release rate, a design exception must be approved by the
County Engineer to allow for longer detention times. Detention storage facilities may include large
basins, small landscaped basins, parking lot storage, roof tops, underground storage vaults, parks, and
recreational fields, or an appropriate combination.
Other types of storage basins include permanent ponds, lakes and reservoirs. Design criteria for these
types of storage basins are not included in this document. The installation and design of these types of
structures must be approved by the County Engineer.
9 1 A DADKING Lcrr DnniniNr
Parking lot ponding shall be arranged so that pedestrians can reach their destinations without walking
through ponded water. The ponding shall be relegated to those portions farthest from the use served or
Page 5
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
to overflow parking areas, and shall be a reasonable portion of the total area so that sufficient parking
remains available for use. Ponding areas shall not interfere with routes and parking spaces needed for
compliance to the Americans with Disabilities Act (ADA). The maximum design depth of ponding can
vary depending upon the location. An overflow outlet shall be provided so that runoff from major
storms is limited to a seven-inch depth. Debris may accumulate at outlet drains, which may reduce the
capacity of the drain and become unsightly; therefore, provisions shall be made for periodic cleaning.
The use of semi-paved/semi-grassed areas for overflow parking which will permit infiltration of rainfall
and reduce the total runoff associated with parking lot pavements is encouraged.
2.3.2 DESIGN REQUIREMENTS
Regional detention basins are those identified in the Spanish Valley Storm Drain Master Plan Update
(2011). These detention basins are sized based on runoff from a 100-yr 24 hour storm under existing
(2009 conditions) land use. All new development must provided sufficient storage to account for the
additional runoff created by the development.
The allowable post -developed discharge rate within Spanish Valley is based on the 10-yr historic peak
runoff rate. Spanish Valley is divided into two regions, each region having a unique 10-year historic
runoff rate, designated as Region 1 and Region 2. Region 1 includes Types A and B hydrologic soil
groups and has an allowable discharge rate of 0.03 cfs per acre for the 100-yr 24 hour storm. Region 2
includes Types C and D hydrologic soil groups and has an allowable discharge rate of 0.3 cfs per acre for
the 100-yr 24 hour storm. The two different regions and their allowable discharge rate are shown in
Figure 2-1. In Figure 2-1, the shaded areas are Region 2 and the areas of the clear aerial image are
Region 1.
Adequate storage shall be provided such that each of the following applies:
• Post -developed discharge rates shall not exceed pre -developed discharge rates for the same
storm.
• The peak discharge rate from the post -developed 100-yr 24 hour storm shall not exceed 0.03 cfs
per acre for Region 1 or 0.3 cfs per acre for Region 2, when routed through the principal outlets.
• The entire 100-yr 24 hour storm shall be routed through the principal outlet without entering
the emergency spillway.
• The peak 100-yr discharge rate from the detention basin shall not exceed the capacity of the
downstream conveyance system, with considerations given to inflows occurring below the
detention basin.
• Retention basins, if allowed by the County Engineer, must be sized with a storage volume equal
to the entire 100-yr post -developed runoff volume.
Runoff greater than that occurring from the 100-year, 24-hour storm will be passed over an emergency
spillway. The emergency spillway must be designed to safely pass the entire post -developed 100-year
Page 6
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F I G U R E 2 - 1
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
flood, routed through the detention basin assuming the principal outlets are not functioning.
Furthermore, the peak emergency spillway flow rate shall not exceed the existing 100-yr flow rate
entering the detention basin (based on conditions existing before the basin is constructed). A minimum
of 1 foot of freeboard above the emergency spillway design water surface elevation is required.
The Principal Outlet Outflow Hydrograph is generated by routing the post -developed 100-yr 24 hour
storm through the principal outlet without any flow entering the emergency spillway. The Emergency
Spillway Outflow Hydrograph is generated by routing the post -developed 100-yr 24 hour storm through
the emergency spillway assuming the principal outlets are not functioning. Conveyance systems below
detention basins must have sufficient capacity for the greater of the following:
• Sufficient capacity to safely convey the 100-yr 24 hour peak flow obtained from combining the
Principal Outlet Outflow Hydrograph with downstream inflow hydrographs.
• Sufficient capacity to safely convey the 100-yr 24 hour peak flow obtained from combining the
Emergency Spillway Outflow Hydrograph with downstream inflow hydrographs.
In order to account for storage loss due to sedimentation of storage facilities, the required storage
volume below the emergency spillway crest shall be increased by 20%. Storage facilities shall be
constructed first in a project's construction sequence in order to reduce the amount of sediment carried
further downstream. Temporary structures may be necessary during the construction phase to trap
sediment before it enters the storage facility. Furthermore, storage facilities shall be cleaned at the
completion of the project construction such that the entire required storage volume is available for
runoff storage.
Storage facilities shall be located as far horizontally from surface water bodies and as far vertically from
the seasonal high-water table as possible. The bottom of storage facilities shall not intercept the post -
developed seasonal high-water table of underlying aquifers.
If the detention basin is classified as a dam, the facility shall also comply with prevailing dam safety
standards as outlined by the Utah State Dam Safety Act and the Utah Division of Water Rights.
The following list of general structural criteria shall be used to design storm water detention basins. Due
to the uniqueness of each storm water detention basin and the variability of soil and other site
conditions, these criteria may be modified or appended at the discretion of the County Engineer.
Structural Components: Principal Outlets
To minimize the chance of clogging and to facilitate cleaning, principal outlet pipes shall be at least 18
inches in diameter. Orifice plates may be used on the upstream end of the principal outlet pipe to
reduce the maximum release rate as needed. The minimum orifice plate diameter of the principal
outlet shall be no less than 6 inches. Similarly, riser pipes, if utilized, shall be at least 8 inches in
diameter.
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
Structural Components: Water Quality Outlets
A water quality storm event shall be used to maintain water quality downstream of detention facilities.
The water quality storm event is defined as a one-year frequency 24-hour storm. Water quality control
shall be maintained by providing an amount of storage at the bottom of the detention facility such that
the runoff produced by the water quality storm is routed through a 3-inch diameter water quality outlet
(orifice plate) without entering the principal outlet. The 3-inch orifice plate shall discharge into an 18-
inch minimum outlet pipe (the same outlet pipe may be used as the principle outlet). The invert of the
principal outlet shall be located at the water surface elevation resulting from routing the water quality
storm event through the detention facility and discharging through the water quality outlet only. In all
cases, the basin shall be considered initially empty (i.e., the storage provided for the water quality event
and the discharge capacity of the water quality outlet shall be utilized during the routing of the larger
100-yr design storm).
Structural Components: Outlets
This section applies to all principal and water quality outlets.
All pipe joints shall be watertight. In addition, trash racks and/or anti -vortex devices are required where
necessary. Seepage along outlet conduits shall be controlled using the latest engineering standards and
practice, and may include methods such as filters, drainage diaphragms, and anti -seep collars, etc.
Regardless of the seepage control method used, designs must be approved by the County Engineer.
Where necessary, a concrete cradle shall be provided for outlet pipes. All outlet structures shall be
reinforced concrete. All construction joints shall be watertight.
Suitable lining and energy dissipators shall be placed upstream and downstream of outlets as necessary
to prevent scour and erosion. Such lining and energy dissipators shall conform to the criteria contained
in current editions of Hydraulic Engineering Circular No. 15 — Design of Roadside Channels with Flexible
Linings and Hydraulic Engineering Circular No. 14 — Hydraulic Design of Energy Dissipators for Culverts
and Channels, both published by the Federal Highway Administration of the U.S. Department of
Transportation.
Structural Components: Emergency Spillways
Design emergency spillways to safely pass the entire post -developed 100-year flood, routed through the
detention basin assuming the principal outlets and water quality outlet are not functioning. A minimum
of 1 foot of freeboard above the emergency spillway design water surface elevation is required.
Emergency spillway side slopes shall not exceed 2 horizontal to 1 vertical or the angle of repose of the
soil or lining material, whichever is less. Vegetated emergency spillways shall have side slopes not
exceeding 3 horizontal to 1 vertical. Emergency spillways shall be suitably lined and shall comply with
criteria contained in the current edition of Hydraulic Engineering Circular No. 15 — Design of Roadside
Channels with Flexible Linings. Maximum velocities and shear stresses in emergency spillways shall be
checked based on the velocity and peak flow in the spillway resulting from the routed Emergency
Spillway Hydrograph. Suitable lining shall be provided for the maximum velocities and shear stresses
expected. Energy dissipators at the bottom of the emergency spillway shall be designed as needed
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
based on the current edition of Hydraulic Engineering Circular No. 14 — Hydraulic Design of Energy
Dissipators for Culverts and Channel.
Structural Components: Dams and Embankments
The required minimum top widths of all dams and embankments are listed in Table 2-4 below.
Table 2-4: Minimum Top Widths
Height (feet) Min. Top Width
1 (feet)
0-20
20-25
12
14
The design top elevation of all dams and embankments after all settlement has taken place shall be
equal to, or greater than, the maximum water surface elevation in the basin resulting from the routed
Emergency Spillway Outflow Hydrograph plus one foot of freeboard. Therefore, the design height of the
dam or embankment, defined as the vertical distance from the top down to the bottom of the deepest
cut, shall be increased by the amount needed to ensure that the design top elevation will be maintained
following all settlement. Consolidation tests of the undisturbed foundation soil are required to
determine the necessary increase. This increase shall not be less than 5 percent. Side slopes for all
dams and embankments are no steeper than 3 horizontal to 1 vertical. Cutoff trenches shall be
excavated along the dam or embankment centerline to impervious subsoil or bedrock, unless otherwise
approved by the County Engineer (and State Engineer if the Utah State Dam Safety Act applies).
Storage Facilities in Flood Hazard Areas
No off-line storage facilities are allowed within any flood plain. On-line storage facilities must have
adequate storage to detain the flood caused by the 100-yr 24 hour storm as previously outlined. For
storage facilities located within FEMA mapped floodways, any changes to FEMA mapping caused by the
storage facility must be considered and any appropriate CLOMAR and LOMAR must be obtained.
2.3.3 STORAGE FACILITIES: MAINTENANCE AND REPAIR
Operation, maintenance, and repair of storage facilities, including periodic removal and disposal of
accumulated particulate material and debris, is required and remains the responsibility of the property
owner. Storage facilities shall be inspected and cleaned and repaired as necessary after every storm
event. All storage facilities must provide adequate access for maintenance and cleaning.
2.4 STREETS AND CURBS
Planning a drainage system shall be done simultaneously with street layout and gradient planning, and
careful consideration shall be given to the following:
• The functions of streets as parts of the storm water management system.
• Street slopes in relation to storm water capacity and flow velocity in gutters and/or street
swales.
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
• The location and sizing of street culverts. Culverts may be sized to create temporary upstream
storage if there is proper consideration given to earth bank stability, proper freeboard,
upstream backwater effects and potential overflow effects during major flood conditions as
outlined further in section 2.7 Culverts.
• Location of streets in relation to natural streams, storage ponds and open channel components
of the system.
• Location and capacity of inlet points to pipes in relation to gutter slopes, low points, the spread
of water across streets and the flow of water across intersections.
2.4.1 STREET FLOODING EVALUATION
The design criteria for allowable spread for the 10-year storm event are based on roadway classification
as shown in Table 2-5.
Table 2-5: Allowable Spread Criteria for 10-year Storm Event
Roadway Classification Allowable Spread
Local (residential)
Flow may spread to crown of road.
Minor Collector (residential)
Major Collector
Flow spread must leave one lane free of water.
Flow spread must leave at least two travel lanes
free of water (one lane in each direction)
Arterial:
< 45 mph
> 45 mph
Allowable spread is shoulder + 3 ft.
Allowable spread is shoulder only.
In addition to the above table, the following criteria also apply for allowable spread and amount of
street flooding for the 10-year design storm:
• The allowable spread in the above table is based on a minimum shoulder width of 6 feet. A
minimum allowable spread of 6 feet shall be considered for all roadway classifications,
regardless of shoulder width.
• At no time shall street flooding extend above the height of the curb.
• Localized street flooding is acceptable in residential areas if the duration is less than 2 hours.
For the 100-year design storm, the following design criteria apply for allowable spread and amount of
street flooding:
• Streets must be able to route the 100-year flood to adequate downstream conveyance facilities.
100-year flows must be contained within the street right-of-way and adjacent drainage
easements with at least 1.0 foot of freeboard to all building structures.
• Street flooding shall not cause dangerous situations to occur with vehicular traffic.
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
• Street flooding shall at no time exceed 2 feet in depth.
• Street flooding shall not exceed 2 hours in duration.
• Pedestrian access is not required to be maintained during a 100-year flood.
• Flooding can extend into all vehicular traffic lanes.
2.4.2 HYDRAULIC CAPACITY
The hydraulic capacity of a street and gutter section to convey water can readily be calculated with a
modified version of Manning's equation as outlined in Hydraulic Engineering Circular No. 22, Second
Edition — Urban Drainage Design Manual, published by the Federal Highway Administration of the U.S.
Department of Transportation. Table 2-6 lists Manning's n values to be used for street and gutter
pavements.
Table 2-6: Manning's n for Street and Pavement Gutters.
Type of Gutter or Pavement Manning's n
Concrete gutter, troweled finish
0.012
Asphalt Pavement:
Smooth texture
Rough texture
0.013
0.016
Concrete Gutter with Asphalt Pavement:
Smooth
Rough
0.013
0.015
Concrete Pavement:
Float finish
Broom finish
0.014
0.016
For gutters with small slope, where sediment may
accumulate, increase above values of n by: 0.002
Source: USDOT, FHWA, HDS-3.
2.4.3 MAXIMUM VELOCITY IN GUTTERS
The velocity in the deepest part of the gutter shall not exceed 10 feet per second. This velocity is readily
computed by the Manning equation using the depth at a point six inches from the face of the curb as the
hydraulic radius. The mean velocity for the entire cross section is not a good measure. If the calculated
velocity exceeds ten feet per second, the allowable discharge in the gutter shall be reduced until velocity
is within the limit.
2.4..4 ESTIMATING RUNOFF IN STREETS
The peak flow contributed to a gutter or Swale from impervious areas and adjacent contributing areas of
less than 10 acres shall be estimated by the Rational Method. Inlets are usually sized so that a portion
of the flow is bypassed; the actual flow in the next reach of gutter includes this bypass flow. The flow
reaching the second inlet is a portion of the flow contributed from its drainage subarea plus the flow
bypassing the previous upstream inlet. The low point or sump inlet catches the remaining flow from
Page 12
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
both directions and shall be sized accordingly. Locations and required capacities of inlets are
established by computing estimated flow rates, depth and velocity of flow, and spread across street.
The procedures and equations which shall be used for estimating runoff in streets as well as calculating
inlet intercept and bypass flows are found in the latest edition of Hydraulic Engineering Circular No. 22 —
Urban Drainage Design Manual.
2.4.5 FLOW ACROSS INTERSECTIONS
Flow across collector or arterial street intersections or cross -walks is not allowed for the 10-year event
and more frequent storms. Controlled flow across local streets is acceptable as long as the Street
Flooding Evaluation criteria in section 2.4.1 is met and the maximum velocity does not exceed 10 feet
per second.
2.5 STORM WATER INLETS
Storm water inlets provide the transition between open surface flow and a closed conduit system. They
are either constructed as part of the street's curb and gutter system, located in street swales or used to
drain open areas. The inlets shall be located to remove runoff from surfaces when the flows exceed the
criteria for velocity, spread of water across streets, or flow across intersections or cross -walks. Inlets
shall also be located upstream of superelevations in order to prevent flow from crossing the roadway.
Inlets in street swales also remove flow when it exceeds swale capacity. Drainage of open areas is often
picked up by an inlet in a depressed area.
9 5 4 1Frinr.i nr Ir.0 rTq
Inlets shall be analyzed and designed according to the type of inlet being used. Inlet capacity shall be
calculated as outlined in the current edition of Hydraulic Engineering Circular No. 22 — Urban Drainage
Design Manual. Inlets on grade in gutter sections shall consist of grates depressed 2 inches below the
gutter flow line. Inlets at sump locations shall include an open curb in order to reduce the effect of
clogging. The design and analysis of all inlets shall include a 50 percent clogging. The clogging factor is
applied to inlets on grade by reducing the effective length of the grate by 50% when calculating the
splash -over velocity. The clogging factor is applied to inlets at sump locations by reducing the effective
perimeter by 50% when in weir flow and by reducing the clear open area by 50% when in orifice flow.
Flanking inlets at sump locations may be necessary, depending on the potential damage and flooding
that may occur if the primary low -point inlet becomes clogged or its capacity exceeded. When flanking
inlets are determined to be necessary, they shall be designed according to the current edition of
Hydraulic Engineering Circular No. 22 — Urban Drainage Design Manual.
Catch basins shall not include a sediment trap formed by lowering the floor of the box below the
elevation of the outlet pipe. Inlets, catch basins and manholes shall be designed in accordance with the
latest APWA details and standard specifications. Frames and grates shall be bicycle safe in areas of
potential bicycle traffic.
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
2.6 STORM DRAIN SYSTEMS
2.6.1 LAYOUT OF THE STORM DRAIN SYSTEM
Storm drain systems shall be used when runoff can no longer be carried in open channels, swales or
gutters due to hydraulic capacity and the limitations and criteria outlined in section 2.4. Storm drain
systems shall also be used when the installation of channels is not feasible due to limited right-of-way
and other constraints. The underground storm drain system consists of a series of inlets, pipes, and
manholes.
2.6.2 STORM DRAIN LOCATION AND ALIGNMENT
The storm drain system shall be located within the street right-of-way. Storm drain alignments outside
of the street right-of-way shall be approved by the County Engineer and a perpetual drainage easement
for the county shall be obtained. The size of the drainage easement shall be dictated by working needs.
In general, the minimum width of drainage easements shall be 20 feet for one utility and five additional
feet, if practicable, for each additional utility located in the same easement.
9 F11YDRAULIr'
Storm drain systems shall be designed to convey the peak 10-yr 24 hour runoff when flowing full. The
capacity of major trunk lines and pipe conveyance systems shall be sufficient for the 100-yr 24 hour
post -developed flow without surcharging at structures. Major trunk lines and pipe conveyance systems
include those which convey flow from upstream culverts, channels, and storage basins. Trunk lines and
pipe conveyance systems conveying flow from upstream storage basins shall have the capacity outlined
in section 2.3.2. Hydraulic capacity shall be determined by the Manning Equation with the use of
appropriate Manning's n for the pipe material used.
The hydraulic grade line shall be computed for all storm drain systems, accounting for all head losses
throughout the system, including head losses due to pipe friction, momentum changes, and losses at
junctions, bends, and structures. These calculations shall conform to the latest edition of Hydraulic
Engineering Circular No. 22 — Urban Drainage Design Manual.
For minor storm drain systems controlled by the 10-yr 24 hour storm event, it shall be shown that the
hydraulic grade line does not rise above the pipe soffit in the 10-yr event. Larger events may surcharge
the pipe. However, an analysis combining the capacity of street and storm drain flow must show that
the 100-yr 24 hour peak flow is contained within the street section as outlined in section 2.4.1 Street
Flooding Evaluation. For major trunk lines and pipe conveyance systems, it must be shown that the 100-
yr hydraulic grade line does not surcharge at any structure along the storm drain system.
The minimum allowable pipe size in the storm drain system is 18-inch diameter. The minimum diameter
of major trunk lines and pipe conveyance systems is 24-inches. Storm drain pipes shall not decrease in
size in the downstream directions.
Storm drain pipes shall be designed to ensure self -scouring velocities. The minimum allowable velocity
shall be 2.5 feet per second when flowing full or 2.0 feet per second when flowing at the design flow,
whichever results in a steeper pipe slope. The minimum pipe slope shall be 0.3 percent.
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
2.6.0 MATERIALS
Storm drains are usually constructed of reinforced concrete, or high density polyethylene, or corrugated
metal pipe; however, other materials may be used. The selected pipe material shall be consistent with
the loads generated from the bury depth and highway loading. Pipe materials shall be analyzed to be
appropriate for the soil characteristics. The selected pipe material shall be approved by the County
Engineer. All storm drain pipes shall include water tight joints.
All discharge pipes and culverts shall terminate with a precast concrete, high density polyethylene, or
corrugated metal end section matching the material of the pipe. Alternatively, a cast -in -place concrete
headwall with or without wing -walls as conditions require may be installed. All outlets greater than or
equal to 48 inches in diameter shall have a cast -in -place concrete headwall, with or without wing -walls
as conditions require.
To allow the County to plan better for system management, the name of the county, the year installed,
and the words "STORM SEWER" shall be cast integrally on manhole covers.
2.6.E 4CCESS HOLEF
The principal purpose of access holes is to provide access for cleaning and inspection. Access holes shall
be provided at all junctions of two or more storm drains, where pipe sizes change, and at changes in
grade or alignment. Inlet boxes shall also provide access through the grate or manhole openings. The
maximum access hole spacing per pipe size is given in Table 2-7.
Table 2-7: Access Hole Spacing
Pipe Diameter (inches) Maximum Spacing
24 or less
27 — 36
42 — 54
60 +
300 ft
400 ft
500 ft
1000 ft
2.7 CULVERTS
All culverts shall be designed to convey the 100-yr peak runoff. The minimum culvert diameter allowed
is 24-inches. The minimum allowable slope is 1.0 percent. Culverts shall be designed to have a minimum
velocity of 2.5 feet per second at the design flow rate.
The maximum allowable headwater shall be the minimum of the following criteria:
• Shall be at least 1 ft below upstream structures and shall be non -damaging to upstream
property.
• Shall be below the outside edge of the shoulder.
• Shall be equal to the elevation where the flow begins to divert around the culvert.
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
Any culvert may cause an increase in water level in the upstream channel. This backwater can flood
property and overflow into the streets. The channel backwater surface upstream of culverts shall be
limited to at least 1 ft below upstream structures and property lines.
All culverts shall have flared end sections or cast -in -place headwalls and wing -walls at both ends of the
culvert for good appearance and hydraulic characteristics. The criteria outlined in section 2.6.4 apply.
Suitable lining and energy dissipators shall be placed upstream and downstream of culverts and storm
drain outlets as necessary to prevent scour and erosion. Such lining and energy dissipators shall
conform to the criteria contained in latest editions of Hydraulic Engineering Circular No. 15—Design of
Roadside Channels with Flexible Linings and Hydraulic Engineering Circular No. 14 — Hydraulic Design of
Energy Dissipators for Culverts and Channel.
2.8 OPEN CHANNELS
Natural drainage channels shall be preserved as much as possible in order to maintain natural drainage
patterns and hydrology. When natural drainage channels are used, considerations shall be given to the
affects of added runoff from upstream developments. Adequate access for maintenance of natural
channels shall be provided. Existing abandoned irrigation channels shall also be maintained and used
for runoff conveyance. However, irrigation ditches shall not be used as outfall channels for culverts or
storm drain systems, unless approved by the owner and the County Engineer.
Open channels shall have capacity for the 100-yr design storm with minimum 1 foot of freeboard. Small
roadside channels conveying runoff from less than 10-acres shall have capacity for the 10-yr design with
1 foot of freeboard to the outside edge of the shoulder. Open channel capacity shall be evaluated and
designed based on Manning's equation. Values of Manning's n to be used for man-made channels shall
be obtained from the latest edition of Hydraulic Engineering Circular No. 15 — Design of Roadside
Channels with Flexible Linings. Channel side slopes shall not exceed 2 horizontal to 1 vertical or the
angle of repose of the soil or lining material, whichever is less. Adequate access for maintenance shall
be provided.
2.8.1 CONTROL OF EROSION
Erosion control design applies to man-made channels as well as natural channels when flows are
increased above the natural capacity. Channels shall be designed for erosion control based on the
permissible tractive force method as outlined in the latest edition of Hydraulic Engineering Circular No.
15 — Design of Roadside Channels with Flexible Linings. Supercritical velocity shall not be allowed in
channels unless they are concrete lined. Drop structures and/or energy dissipators shall be installed as
needed to maintain velocities below subcritical in all other channels.
2.9 OTHER DESIGN CONSIDERATIONS
9 (Li COIL ERncinN
Control of erosion during residential construction requires an examination of the entire site to pinpoint
potential problem areas, such as steep slopes, highly erodible soils, soil areas that will be unprotected
for long periods or during peak rainy seasons and natural drainage ways. Steps shall be taken to assure
Page 16
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
erosion control in these critical areas. After a heavy storm the effectiveness of erosion control measures
shall be evaluated. Periodic maintenance and cleaning of the facilities is also important.
Construction activities that disturb one or more acres of land must be authorized under the UPDES
General Permit for Construction Activities as administered by the Utah Division of Water Quality. The
UPDES permit requires control and elimination of storm water pollution through the development and
implementation of a Storm Water Pollution Prevention Plan (SWPPP) which must include appropriate
Best Management Practices (BMPs)
The basic requirements for controlling erosion during and after construction include:
• Earth slopes: concentrations of water at the top of the slope flowing down an unprotected bank
shall be avoided or controlled. Runoff shall be diverted to safe outlets. Slopes shall be protected
from erosion by quick establishment of vegetative cover, benches, terraces, slope protection
structures, mulches, or a combination of these practices as appropriate.
• Waterways or Channels: Waterways shall be designed to avoid erosion by limiting maximum
velocities and providing appropriate erosion protection. These will both vary based on the
character of the channel material. Every effort shall be made to preserve natural channels.
• Erosion Control Methods: Erosion may be controlled by the use of vegetative and rock linings,
grade control structures, energy dissipators, special culverts, and various types of pipe
structures. Structures are expensive and should be used only after it has been determined that
recommended vegetation, rock revetment or other measures will not provide adequate erosion
control.
• Existing Vegetation: Existing vegetation shall be preserved wherever possible.
• Soil Treatment, Seeding and Mulching: The use of vegetation for erosion control is limited to
situations where the vegetation can be sustained through irrigation. The determination of
proper soil treatment, seed mixture and long-term maintenance requirements shall be done by
a licensed landscape architect.
• Outfall Design: Every effort shall be made to keep the elevation of an outfall pipe as close to the
downstream grade as possible. Outfall pipes shall not be elevated above the level of the
downstream ground without providing appropriate erosion protection and energy dissipation.
The velocity of the outfall pipe shall be minimized as much as possible. Appropriate erosion
protection and energy dissipation shall be provided. Special attention must be given if the
outfall is to an overland flow area with a steep slope.
9 q " ^.0 rgrir'm ^Nr? Ccr"RAFNT C(1NTRO'
Proper control of soil erosion during and after construction is the most important element of siltation
and sediment control. However, it is physically and economically impractical to entirely eliminate soil
erosion. Secondly, erosion is a natural function and is required in certain portions of the drainage system
Page 17
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
to provide future stream capacity. Therefore, provisions shall be made to trap eroded material at
specified points. Some measures that shall be implemented as appropriate are:
• Temporary ponds which store runoff and allow suspended solids to settle out can be used
during construction and may be retained as part of the permanent storage system after
construction.
• Protection of inlets to the underground pipe system can be accomplished during construction by
placing straw bales or drop inlet barriers around the structure. Temporary erosion control must
be removed prior to final acceptance.
• Silt fences along the down slope edges of construction sites to trap sediments which are carried
by overland flows.
• Storm drain systems shall be protected from Aeolian (or wind blown) sediments by the proper
use and placement of silt fencing.
• Egress points from construction sites shall be controlled, so that sediment is not carried off -site
by construction traffic.
STORM WATER RUNOFF POLLUTIOr
In 1990, Phase I of the US Environmental Protection Agency's (EPA) Stormwater Program was
promulgated under the Clean Water Act. In 1999, EPA issued the Final Rule for Phase II of its Stormwater
Program.
Phase I relies on National Pollutant Discharge Elimination System (NPDES1) permit coverage to address
stormwater runoff from: (1) "medium" and "large" municipal separate storm sewer systems (MS4s)
generally serving populations of 100,000 or greater, (2) construction activity disturbing 5 acres of land or
greater, and (3) ten categories of industrial activity. The Stormwater Phase II Final Rule is the next step
in EPA's effort to preserve, protect, and improve the Nation's water resources from polluted stormwater
runoff. The Phase II program expands the Phase I program by requiring operators of certain regulated
small MS4s in urbanized areas, operators of small construction sites, as well as small MS4s outside the
urbanized areas that are designated by the permitting authority, to obtain NPDES permit coverage for
their stormwater discharges and to implement programs and practices to control polluted stormwater
runoff.
Generally, Phase I MS4s are covered by individual permits and Phase II MS4s are covered by a general
permit. Each regulated MS4 is required to develop and implement a stormwater management program
(SWMP) to reduce the contamination of stormwater runoff and prohibit illicit discharges.
Grand County does not currently meet the EPA definition of an urbanized area and is not considered a
regulated MS4 under the EPA Phase II rules. However, it is certain the stormwater regulations will
1 Note that in Utah, the Division of Water Quality administers the NPDES program via DWQ's UPDES program.
Page 18
GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
become increasingly stringent on a state and federal level. Grand County can position itself for future
regulations by considering the following areas which represent Minimum Control Measures under the
Phase II program. Additional information on each of these areas can be found by following the link
provided.
1) Public education and outreach: Develop a public education program that uses a mix of local strategies
and reaches a diverse audience. (http://www.epa.gov/npdes/pubs/fact2-3.pdf)
2) Public participation and involvement: Include the public in developing, implementing, updating, and
reviewing a stormwater management programs. (http://www.epa.gov/npdes/pubs/fact2-4.pdf)
3) Illicit discharge detection and elimination: Implement and enforce program to eliminate non-
stormwater discharges via the storm water system. (http://www.epa.gov/npdes/pubs/fact2-5.pdf)
4) Construction site runoff control: Develop, implement, and enforce a program to reduce pollutants in
stormwater runoff from construction activities that result in a land disturbance of greater than or equal
to one acre. (http://www.epa.gov/npdes/pubs/fact2-6.pdf)
5) Post construction runoff control: Develop, implement, and enforce a program to reduce pollutants in
post -construction runoff from new development and redevelopment projects that result in the land
disturbance of greater than or equal to 1 acre. (http://www.epa.gov/npdes/pubs/fact2-7.pdf)
6) Pollution prevention and good housekeeping: Develop and implement a good storm water system
operation and maintenance program, employee storm water training program.
(http://www.epa.gov/npdes/pubs/fact2-8.pdf)
2.9.4 MOSQUITO CONTROL
The design of all storm drainage system design shall consider mosquito control measures. Some
mosquito species in Spanish Valley can, during hot weather, complete their development in standing
water in as little as 4 days. However, most species, including the major West Nile virus vectors (Culex
spp.), require at least 7 days for development. At least one species of Culex is known to breed in
enclosed and underground water containers that it can access.
• Storm water detention ponds shall be designed so that they empty completely within 96 hours.
• Mosquito control shall be considered in the design of manholes and other subsurface storm
drainage structures.
2.1 O OPERATION AND MAINTENANCE
Adequate provision for short- and long-term maintenance of the storm water system is an important
design consideration. Maintenance and replacement needs and costs shall be part of economic
analyses. Maintenance shall be included for all drainage facilities in order to ensure they function
properly during storm events. Proper right-of-way and easements shall be provided for maintaining all
drainage facilities.
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GRAND COUNTY DESIGN CRITERIA FOR DRAINAGE STUDIES WITHIN SPANISH VALLEY
December 2011
REFERENCES
Horrocks Engineers. Spanish Valley Storm Drain Master Plan Update. Grand County, Utah. 2011.
U.S. Department of Agriculture, Natural Resources Conservation Service. Soil Survey Geographic
(SSURGO) Database. Canyonlands Area, Utah — Parts of Grand and San Juan Counties. Obtained from
the Soil Data Mart (http://soildatamart.nres.usda.gov/).
U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather
Service. NOAA Atlas 14, Precipitation -Frequency Atlas of the United States, Volume 1, Version 4,
Semiarid Southwest. 2006. Extracted from the Precipitation Frequency Data Server January 2009
(http://hdsc.nws.noaa.gov/hdsc/pfds/sa/ut_pfds.html).
U.S. Department of Transportation, Federal Highway Administration. Design Charts for Open Channel
Flow, Hydraulic Design Series No. 3. FHWA-EPD-86-102. 1961.
U.S. Department of Transportation, Federal Highway Administration. Design of Roadside Channels with
Flexible Linings, Hydraulic Engineering Circular No. 15, Third Edition. FHWA-NHI-05-114. Sep 2005.
U.S. Department of Transportation, Federal Highway Administration. Hydraulic Design of Energy
Dissipators for Culverts and Channels, Hydraulic Engineering Circular No. 14, Third Edition. FH WA-N H I-
06-086. July 2006.
U.S. Department of Transportation, Federal Highway Administration. Urban Drainage Design Manual,
Hydraulic Engineering Circular No. 22, Second Edition. FHWA-NHI-01-021. Aug 2001.
U.S. Environmental Protection Agency, Office of Wastewater Management, Water Permits Division,
National Pollution Discharge Elimination System (NPDES). (http://cfpub.epa.gov/NPDES/).
Utah Department of Environmental Quality, Division of Water Quality, Utah Pollutant Discharge
Elimination System (UPDES). (http://www.waterquality.utah.gov/UPDES/stormwater.htm).
Utah Department of Administrative Service, Division of Administrative Rules. Title R655 — Natural
Resources, Water Rights. (http://www.rules.utah.gov/publicat/code/r655/r655.htm).
Utah State Legislature. Utah Code -Title 73-Chapter 05a-Dam Safety.
(http://le.utah.gov/code/TITLE73/73_05a.htm).
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