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4. DESIGN REQUIREMENTS FOR STORM DRAINAGE FACILITIES
4.010 General
Stormwater sewers or channels provide the facility for removing and transporting surface runoff produced from rainfall.
Design requirements differ from those for either sanitary or combined sewers.
This section gives the minimum technical design requirements of the District storm drainage facilities. In general, the
formulae presented herein for hydraulic design represent "acceptable" procedures not necessarily to the exclusion of other
sound and technically supportive formulae. Any departure from these design requirements should be discussed before
submission of plans for approval and should be justified. All construction details pertaining to storm sewer improvements
shall be prepared in accordance with the District Standard Construction Specifications unless otherwise noted.
4.020 General Requirements of Storm Sewer Construction
All storm sewers shall meet the following general requirements:
4.020.01 Size and Shape
The minimum diameters of pipe for stormwater or combined sewers shall be twelve (12) inches.
Sewers shall not decrease in size in the direction of the flow unless approved by the District.
Circular pipe sewers are preferred for stormwater sewers, although rectangular or elliptical
conduits may be used with special permission.
4.020.02 Materials
All materials shall conform to the District Standard Construction Specifications. Reinforced
concrete pipe joints shall be Type "A" or better, as required.
4.020.03 Bedding
The Project Plans and Specifications shall indicate the specific type or types of bedding,
cradling, or encasement required in the various parts of the storm sewer construction if different
than current the District Standard Construction Specifications.
Special provisions shall be made for pipes laid within fills or embankments and/or in shallow or
partial trenches, either by specifying extra strength pipe for the additional loads due to
differential settlement, or by special construction methods, including ninety percent (90%)
modified proctor compaction of fill to prevent or to minimize such additional loads.
Compacted granular backfill shall be required in all trench excavation within public (or private)
streets rights-of-way or areas where street rights-of-way are anticipated to be dedicated for public
use. Under areas to be paved, the compacted granular backfill shall be placed to the subgrade of
the pavement. Under unpaved areas, the compacted granular backfill shall be placed to within
two (2) feet of the finished surface, and generally not more than two (2) feet beyond street
pavement or curb lines. Local street jurisdiction shall govern where more stringent.
Pipes having a cover of less than three (3) feet shall be encased in concrete, unless otherwise
directed by the District.
The Storm and sanitary sewers are parallel and in the same trench, the upper pipe shall be placed
on a shelf and the lower pipe shall be bedded in compacted granular fill to the flow line of the
upper pipe.
4.020.04 Concrete Pipe or Conduit Strengths
Reinforced Concrete pipe shall be Class II, minimum. Any concrete pipe, conduit or culvert beneath a
street right-of-way, or with reasonable probability of being so located, shall be a minimum of Class III, but
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also shall account for all vertical loads, including the live load required by the highway authority having
jurisdiction. In no case shall the design provide for less than HS-20 loading of the AASHTO. For other
locations, the minimum design live load shall be the HS-10 loading.
4.020.05 Monolithic Structures
Monolithic reinforced concrete structures shall be designed structurally as continuous rigid units.
Generally these are poured in place units. Wall thickness shall be 8” minimum with one row of
reinforcement, horizontal and vertical. Wall thickness 10” and greater shall require 2 rows of
reinforcement, horizontal and vertical. (Where approved District precast structures are allowed, less steel
& thickness may be accepted).
4.020.06 Alignment
Sewer alignments are normally limited by the available easements, which in turn should reflect proper
alignment requirements. Since changes in alignment affect certain hydraulic losses, care in selecting
possible alignments can minimize such losses and use available head to the best advantage.
Sewers shall be aligned:
1. To be in a straight line between structures, such as manholes, inlets, inlet manholes and
junction chambers, for all pipe sewers thirty (30) inches in diameter and smaller.
2. To be parallel with or perpendicular to the centerlines of straight streets unless
otherwise unavoidable. Deviations may be made only with approval of the District.
3. To avoid meandering, off-setting and unnecessary angular changes.
4. To make angular changes in alignment for sewers thirty (30) inches in diameter or
smaller in a manhole located at the angle point, and for sewers thirty- six (36) inches in
diameter or larger, by a uniform curve between two tangents. Curves shall have a
minimum radius of ten times the pipe diameter.
5. To avoid angular changes in direction greater than necessary and any exceeding ninety
(90) degrees.
4.020.07 Location
Storm sewer locations are determined primarily by the requirements of service and purpose. It is
also necessary to consider accessibility for construction and maintenance, site availability and
competing uses, and effects of easements on private property.
Storm sewers shall be located:
1. To serve all property conveniently and to best advantage.
2. In public streets, roads, alleys, rights-of-way, or in sewer easements dedicated to the
District.
3. On private property along property lines or immediately adjacent to public streets,
avoiding diagonal crossings through the central areas of the property.
4. At a sufficient distance from existing and proposed buildings including footings, and
underground utilities or other sewers to avoid encroachments and reduce construction
hazards.
5. To avoid interference between other stormwater sewers and house connections to
foulwater or sanitary sewers.
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6. In unpaved or unimproved areas whenever possible.
7. To avoid, whenever possible, any locations known to be or probably to be beneath
curbs, paving or other improvements particularly when laid parallel to centerlines.
8. To avoid sinkhole areas if possible. However, if sinkhole areas cannot be avoided, see
sub-section 4.020.08 for requirements.
9. Crossing perpendicular to street, unless otherwise unavoidable.
4.020.08 Sinkhole Areas
1. Sinkhole Report
Where improvements are proposed in any area identified as sinkhole areas, a sinkhole
report will be required. This report is to be prepared by a Professional Engineer,
registered in the State of Missouri, with demonstrated expertise in geotechnical
engineering, and shall bear his or her seal.
The sinkhole report shall verify the adaptability of grading and improvements with the
soil and geologic conditions available in the sinkhole areas. Sinkhole(s) shall be
inspected to determine its functional capabilities with regard to handling drainage.
The report shall contain provisions for the sinkholes to be utilized as follows:
a. All sinkhole crevices shall be located on the plan. Functioning sinkholes
may be utilized as a point of drainage discharge by a standard drainage
structure with a properly sized outfall pipe provided to an adequate natural
discharge point, such as a ditch, creek, river, etc.
b. Non-functioning sinkholes and sinkholes under a proposed building may be
capped.
c. If development affects sinkholes, they may be left in their natural state;
however they will still require a properly sized outfall pipe to an adequate
natural discharge point.
d. An overland flow path shall be required for all sinkholes assuming the outfall
pipe and sinkhole become blocked.
Where the topography will not allow for an overland flow path:
1. The storm sewer shall be designed for the 100-year,
20-minute storm, and
2. If this storm pipe is smaller than thirty six (36) inches in diameter, a
designated ponding area shall be identified, assuming the pipe is
blocked, and
3. The ponding area shall be based on the 100-year, 24-hour storm, and
4. The low sill of all structures adjacent to the ponding area shall be a
minimum of two (2) feet above the 100-year highwater elevation.
5. Special siltation measures shall be installed during the excavation of
sinkholes and during the grading operations to prevent siltation of
the sinkhole crevice.
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2. Procedure for Utilization of Sinkholes
a. Excavation. Prior to filling operations in the vicinity of a sinkhole,
the earth in the bottom of the depression will be excavated to expose
the fissure(s) in the bedrock. The length of fissure exposed will
vary, but must include all unfilled voids or fissure widths greater
than one-half (1/2) inch maximum dimensions which are not filled
with plastic clay.
b. Closing Fissures. The fissure or void will be exposed until bedrock
in its natural attitude is encountered. The rock will be cleaned of
loose material and the fissures will be hand-packed with quarry-run
rock of sufficient size to prevent entry of this rock into the fissures,
and all the voids between this hand-packed quarry-run rock filled
with smaller rock so as to prevent the overlying material's entry into
the fissures. For a large opening, a structural (concrete) dome will be
constructed with vents to permit the flow of groundwater.
c. Placing Filter Material. Material of various gradations, as approved,
will be placed on top of the hand-packed rock with careful attention
paid to the minimum thicknesses. The filter material must permit
either upward or downward flow without loss of the overlying
material.
The fill placed over the granular filter may include granular material
consisting of clean (no screenings) crushed limestone with ten (10)
inch maximum size and one (1) inch minimum size or an earth fill
compacted to a minimum density of ninety percent (90%) modified
Proctor as determined by ASTM D-1557.
d. Supervision. Periodic supervision of the cleaning of the rock
fissures must be furnished by the Engineer who prepared the Soil
Report. Closing of the rock fissures will not begin until the cleaning
has been inspected and approved by that Engineer.
During the placement and compaction of earth fill over the filter,
supervision by the Engineer shall be continuous. Earth fill densities
will be determined during the placement and compaction of the fill in
sufficient number to insure compliance with the specification. The
Engineer is responsible for the quality of the work and to verify that
the specifications are met.
4.020.09 Flowline
The flowline of storm sewers shall meet the following requirements:
1. The flowline shall be straight or without gradient change between the inner walls of
connected structures; that is, from manhole to manhole, manhole to junction chamber,
inlet to manhole, or inlet to inlet.
2. Gradient changes in successive reaches normally shall be consistent and regular.
Gradient designations less than the nearest 0.001 foot per foot, except under special
circumstances and for larger sewers, shall be avoided.
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3. Sewer depths shall be determined primarily by the requirements of pipe or conduit size,
utility obstructions, required connections, future extensions and adequate cover.
4. Stormwater pipes discharging into lakes shall have the discharge flowline a minimum of
three (3) feet above the lake bottom at the discharge point or no higher than the normal
water line.
5. A concrete cradle is required when the grade of a sewer is twenty (20) percent or
greater. A special design and specification is required for grades exceeding fifty
percent (50%).
6. For sewers with a design grade less than one percent (1%), field verification of the
sewer grade will be required for each installed reach of sewer, prior to any surface
restoration or installation of any surface improvements.
7. The District may require the submittal of revised hydraulic calculations for any sewer
reach having an as-built grade flatter than the design grade by more than 0.1%. Based
on a review of this hydraulic information, the District may require the removal and
replacement of any portion of the sewer required to ensure sufficient hydraulic capacity
of the system.
4.020.10 Manholes
Manholes provide access to sewers for purposes of inspection, maintenance and repair. They
also serve as junction structures for lines and as entry points for flow. Requirements of sewer
maintenance determine the main characteristics of manholes. Cast in place or precast manhole
structures are generally allowable, though the former requires approved shop detail drawings.
1. For sewers thirty (30) inches in diameter or smaller, manholes shall be located at
changes in direction; changes in size of pipe; changes in flowline gradient of pipes, and
at junction points with sewers and inlet lines.
For sewers thirty-three (33) inches in diameter and larger, manholes shall be located on
special structures at junction points with other sewers and at changes of size, alignment
change and gradient. A manhole shall be located at one end of a short curve and at each
end of a long curve.
2. Spacing of manholes shall not exceed four hundred (400) feet for pipe sewers thirty-six
(36) inches in diameter and smaller; five hundred (500) feet for pipe sewers forty-two
(42) inches in diameter and larger, except under special approved conditions. Spacing
shall be approximately equal, whenever possible.
3. When large volumes of stormwater are permitted to drop into a manhole from lines
twenty-one (21) inches or larger, the manhole bottom and walls below the top of such
lines shall be of reinforced concrete. Special structural design may be required for large
pipes and/or large drops.
4. Manholes shall be avoided in driveways or sidewalks.
5. Connections to existing structures may require rehabilitation or reconstruction of the
structure being utilized. This work will be considered part of the project being
proposed.
6. When a project requires a manhole to be adjusted to grade a maximum of twelve (12)
inches of rise is allowed if not previously adjusted. When adjustments to raise or lower
a manhole is required, the method of adjustment must be stated on the project plans and
approved by the District.
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4.020.11 Overflow/Design System
The "design" components of the drainage system include the inlets, pipe, storm sewers, and
improved and unimproved channels that function during typical rainfall events. The "overflow"
system comprises the major overflow routes such as swales, streets, floodplains, detention basins,
and natural overflow and ponding areas.
The purpose of the overflow system is to provide a drainage path to safely pass flows, which
cannot be accommodated by the design system without causing flooding of adjacent structures.
The criteria for the design of the overflow and design systems shall be as follows:
1. The "design" system shall be designed in accordance with Section 4.030.
2. The "overflow" system shall be designed for the 100-year, 20-minute event, assuming
the "design" system is completely blocked. The capacity of the "overflow" system shall
be verified with hydraulic calculations at critical cross-sections. The "overflow" system
shall be directed to the detention facility, or as approved by the District.
3. The low sill of all structures adjacent to the "overflow" system swales shall be above
the 100-year highwater elevation.
4. Where the topography will not allow for an overland flow path:
a. The storm sewer shall be designed for the 100-year, 20-minute storm, and
b. If this storm pipe is smaller than thirty-six (36) inches in diameter, a
designated ponding area shall be identified, assuming the pipe is blocked, and
c. The ponding area shall be based on the 100-year, 24-hour storm, and
d. The low sill of all structures adjacent to the ponding area shall be above the
100-year highwater elevation.
5. The "overflow" system shall be designated on the drainage area map and on the grading
plan.
6. All overflow systems will be considered on a site specific basis.
7. The stormwater design for projects within designated levee districts such as Monarch-
Chesterfield, Earth City and Riverport will be based on the Stormwater Master Plan for
these districts.
4.030 Stormwater Design Criteria
4.030.01 Flow Quantities
Flow quantities are to be calculated by the "Rational Method" in which:
Q = API
where:
Q = runoff in cubic feet per second
A = tributary area in acres
I = Average intensity of rainfall (inches per hour) for a given period and a
given frequency
P = runoff factor based on runoff from pervious and impervious surfaces
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P (Runoff Factors) for various impervious conditions are shown in Table 4-1.
P.I. values for various impervious conditions are shown in Table 4-2.
1. Rainfall Frequency
A twenty (20) year rainfall frequency is to be used in the City of St. Louis and areas of
St. Louis County where combined sewers are used. A fifteen (15) year rainfall
frequency is to be used in areas of St. Louis County where storm sewers are separated
from sanitary sewers. In the design of local storm sewer systems, a twenty (20) minute
time of concentration shall be used. Figure 4-1 gives rainfall curves for 2, 5, 10, 15, 20
and 100 year frequencies.
2. Impervious Percentages and Land Use
Minimum impervious percentages to be used are as follows:
a. For manufacturing and industrial areas, 100%*
b. For business and commercial areas, 100%*
c. For residential areas, including all areas for roofs of dwellings and garages;
for driveways, streets, and paved areas; for public and private sidewalks; with
adequate allowance in area for expected or contingent increases in
imperviousness:
In apartment, condominium and multiple dwelling areas: 75%*
In single family areas:
1/4 Acre or less 50%
1/4 Acre to 1/2 Acre 40%
1/2 Acre to 1 Acre 35%
One acre or larger Calculate impervious percentage*
Playgrounds (Non-Paved) 20-35%*
d. For small, non-perpetual charter cemeteries,
allow 30%
For parks and large perpetual charter cemeteries 5%
*NOTE: Drainage areas may be broken into component areas, with the
appropriate run-off factor applied to each component, i.e. a proposed
development may show one hundred percent (100%) impervious for paved
areas and five percent (5%) impervious for grassed areas. Use of actual
component areas may be required, however, where minimum impervious
percentages are deemed misleading, or too approximate.
The design engineer shall provide adequate detailed computations for any proposed,
expected or contingent increases in imperviousness and shall make adequate allowances
for changes in zoning use. If consideration is to be given to any other value than the
above for such development, the request must be made at the beginning of the project,
must be reasonable, fully supported, and adequately presented, and must be approved in
writing before its use is permitted.
Although areas generally will be developed in accordance with current zoning
requirements, recognition must be given to the fact that zoning ordinances can be
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amended to change the currently proposed types of development, and any existing use.
Under these circumstances the possibility and the probability of residential areas having
lot sizes changed or re-zoned to business, commercial, or light manufacturing uses
should be given careful consideration.
e. Average 20-minute values of P.I. (cfs per acre) to be used are as follows:
Percent 20 Minute Duration
Imperviousnes 15 Year 20 Year
5 1.70 1.78
10 1.79 1.87
20 2.00 2.09
30 2.19 2.28
40 2.39 2.50
50 2.58 2.69
90 3.36 .. 3.50
100 3.54 3.70
*Roofs (City of St. Louis) 6.0
*Roofs 4.2 (Other than City of St. Louis)
*For Direct Connection to Sewer
3. Reduction in P.I. with Time and Area
Reduction in P.I. values for the total time of concentration exceeding twenty (20)
minutes, and for tributary areas exceeding three hundred (300) acres will be allowed only
in trunk sewers and main channels. The reduced average P.I. value for the tributary area
shall not be less than the value determined as follows on the basis of:
a. Time. As the time of concentration increases beyond twenty (20) minutes,
select the appropriate P.I. value from Table 4-1. The travel time through a
drainage channel should be based on an improved concrete section. These
reduced values shall be used unless a further reduction is allowed for area.
b. Area. As the total tributary area at any given location in a channel increases in
excess of three hundred (300) acres, the P.I. value may be further reduced by
multiplying it by an area coefficient "Ka". (The area coefficient is obtained
from data in a special study of a major storm in the St. Louis area by the U.S.
Corps of Engineers.) The average rainfall rate, for a given storm, for a given
period for the tributary area, is less than the corresponding point value as
determined from recording rainfall gauges. The curve data is as follows:
P.I. Coefficients Ka
Area (Abscissas) "Ka" (Ordinates)
300 to 449 Acres 1.00
450 to 549 Acres .99
550 to 749 Acres .98
750 to 999 Acres .97
1000 to 1280 Acres .96
1281 to 1600 Acres .95
1601 to 1920 Acres .92
1921 to 2240 Acres .91
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4.030.02 Hydraulic Grade Line for Closed Conduits
1. Computation Methods
The hydraulic grade line is a line coinciding with (a) the level of flowing water at any
given point along an open channel, or (b) the level to which water would rise in a
vertical tube connected to any point along a pipe or closed conduit flowing under
pressure.
The beginning point for hydraulic grade line computations for storm sewers shall be at
least the higher of the elevations listed in 3.030.07.1. Field-verification shall also apply.
The hydraulic grade line shall be computed to show its elevation at all structures and
junction points of flow in pipes, conduits and open channels, and shall provide for the
losses and the differences in elevations as required below. Since it is based on design
flow in a given size of pipe or conduit or channel, it is of importance in determining
minimum sizes of pipes within narrow limits. Sizes larger than the required minimum
generally provide extra capacity, however consideration must still be given to the
respective pipe system losses.
There are several methods of calculating "losses" in storm sewer design. The following
procedures are presented for the engineer's information and consideration.
It is expected that the design will recognize the reality of such "losses" occurring and
make such allowances, as good engineering judgment requires.
a. Friction Loss
The hydraulic grade line is affected by friction loss and by velocity head
transformations and losses. Friction loss is the head required to maintain
the required flow in a straight alignment against frictional resistance
because of pipe or channel roughness. It is determined by the equation:
hf =L x Sh
Where:
hf = difference in water surface elevation, or head in feet in length L
L = length in feet of pipe or channel
Sh = hydraulic slope required for a pipe of given diameter or channel of given cross-
section and for a given roughness "n", expressed as feet of slope per foot of length
From Manning's formula: Sh = [V n /(1.486 R0.667)]2
Where:
R = hydraulic radius of pipe, conduit or channel (feet)
(Ratio of flow area/wetted perimeter)
V = velocity of flow in feet per second (fps)
n = Manning's value for coefficient of roughness
Use:
n = .013 for pipes of concrete, vitrified clay, and PVC pipe
n = .012 for formed monolithic concrete, i.e., vertical wall channels, box culverts and for
R.C.P. over 48" in diameter
n = .015 for concrete lining in ditch or channel
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inverts and trapezoidal channels
n = .020 for grouted riprap lining on ditch or channel side slopes
n = .033 for gabion walled channels
Note:
"n" will have a weighted value for composite lined channels.
"n" values for unlined channels to be determined on an individual basis.
b. Curve Loss
Curve loss in pipe flow is the additional head required to maintain the required
flow because of curved alignment, and is in addition to the friction loss of an
equal length of straight alignment. It should be determined from Figure 4-2,
which includes an example.
c. Entrance Loss at Terminal Inlets
Entrance loss is the additional head required to maintain the required flow
because of resistance at the entrance. The entrance loss at a terminal inlet is
calculated by the formula:
Hti = (V2/2g)
Where: V = Velocity in flow of outgoing pipe
g = Acceleration of gravity (32.2 Ft/Sec/Sec)
d. Turn Loss
Head losses in structures due to change in direction of flow (turns) in a
structure, will be determined in accordance with the following:
Multiplier of
Change in Direction Velocity Head of
of Flow (A) Water Being Turned (K)
90 Deg. 0.7
60 Deg. 0.55
45 Deg. 0.47
30 Deg. 0.35
15 Deg. 0.18
0 Deg. 0.0
Other Angles By Interpolation
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DIAGRAM:
Formula: HL = K(VL)2/2g
Where:
HL = Feet of head lost in manhole due to change in direction of lateral flow
VL = Velocity of flow in lateral in Ft/Sec
g = Acceleration of gravity, (32.2 Ft/Sec/Sec)
K = Multiplier of Velocity Head of water being turned
e. Junction Chamber Loss
A sewer junction occurs for large pipes or conduits too large to be brought
together in the usual forty two (42) inch diameter manhole or inlet where one
or more branch sewers enter a main sewer. Allowances should be made for
head loss due to curvature of the paths and due to impact at the converging
streams.
Losses in a junction chamber for combining large flows shall be minimized by
setting flowline elevations so that pipe centerlines (springlines), will be
approximately in the same planes.
At junction points for combining large storm flows, a manhole with a slotted
cover shall be provided.
A computation method for determining junction chamber loses is presented
below:
Hj = y + Vh1 - Vh2
Where:
Hj = junction chamber loss (ft.)
Vh1 = upstream velocity head
Vh2 = downstream velocity head
y = change in hydraulic grade line through the
junction in feet
Where:
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y = [(Q2V2)-((Q1V1) + {(Q3V3Cos θ3) + (QnVnCosӨn)})]
0.5(A1+A2)g
Where:
Q2 = Discharge in cubic feet per second (cfs) at the exiting conduit
V2 = Velocity in feet per second (fps) at the exiting conduit
A2 = Cross sectional area of flow in sq. ft. for the exiting conduit
Q1 = Discharge in cfs for the incoming pipe (main flow)
V1 = Velocity in fps for the incoming pipe (main flow)
A1 = Cross sectional area of flow in Sq. Ft
for the incoming pipe (main flow)
Q3,Qn = Discharge(s) in cfs for the branch lateral(s)
V3,Vn = Velocity(ies) in fps for the branch lateral(s)
Ө3, Өn = The angle between the axes of the exiting pipe and the branch
laterals(s)
g = Acceleration of gravity (32.2 ft/sec/sec)
Where:
Ө = is the angle between the axes of the outfall and the incoming laterals
f. Losses at Junctions of Several Flows in Manholes and/or Inlets
The computation of losses in a manhole, inlet or inlet manhole with several flows
entering the structure should utilize the principle of the conservation of energy.
This involves both the elevation of water surface and momentum (mass times the
velocity head). Thus, at a structure (manhole, inlet or inlet manhole) with
laterals, the sum of the energy content for inflows is equal to the sum of the
energy content of the outflows plus the additional energy required by the
turbulence of the flows passing through the structure.
DIAGRAM:
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The upstream hydraulic grade line may be calculated as follows:
Hu = [VD2/2g]-[((QU/QD)(l-K)(VU2/2g))+((QL1/QD)(l-K)(VL12/2g))
+((QLN/QD)(l-K)(VLN2/2g))] + HD
Where:
HU = Upstream hydraulic grade line in feet
QU = Upstream main line discharge in cubic feet
per second
QD = Downstream main line discharge in cubic feet
per second
QL1-QLN = Lateral discharges in cubic feet per second
VU = Upstream main line velocity in feet per second
VD = Downstream main line velocity in feet per
second
VL1-VLN= Lateral velocities in feet per second
HD = Downstream hydraulic grade line in feet
K = Multiplier of Velocity of Water being turned
g = Acceleration of gravity, 32.2 ft/sec/sec
The above equation does not apply when two (2) almost equal and opposing flows, each
perpendicular to the downstream pipe, meet and no other flows exist in the structure. In this case
the head loss is considered as the total velocity head of the downstream discharge.
g. Transition Loss
The relative importance of the transition loss is dependent on the velocity
head of the flow. If the velocity and velocity head of the flow are quite low,
the transition losses cannot be very great. However, even small losses may be
significant in flat terrain. The sewer design shall provide for the consideration
of the necessary transitions and resulting energy losses. The possibility of
objectionable deposits is to be considered in the design of transitions.
For design purposes it shall be assumed that the energy loss and changes in
depth, velocity and invert elevation, if any, occur at the center of the
transition. These changes shall be distributed throughout the length of the
transition in actual detailing. The designer shall carry the energy head,
piezometric head (depth in an open channel), and invert as elevations, and
work from the energy grade line. Because of inherent differences in the flow,
transitions for closed conduits will be considered separately from those for
open channels.
(1) Closed Conduits
Transitions in small sewers may be confined within a manhole.
Special structures may be required for larger sewers. If a sewer is
flowing surcharged, the form and friction losses are independent of
the invert slope; therefore, the transition may vary at the slopes of the
adjacent conduits. The energy loss in a transition shall be expressed
as a coefficient multiplied by the change in velocity head (V2/2g) in
which V is the change in velocity before and after the transition.
The coefficient may vary from zero to one, depending on the design
of the transition.
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If the areas before and after a transition are known, it is often
convenient to express the transition loss in terms of the area ratios
and either the velocity upstream or downstream.
For an expansion:
HL = K(V1-V2)2/2g≈[K(V1)2/2g][1-(A1/A2)]2
in which HL is the energy loss; K is a coefficient equal to 1.0 for a sudden
expansion and approximately 0.2 for a well-designed transition and the subscripts
1 and 2 denote the upstream and downstream sections, respectively, i.e., A1 =
Area Before Transition and A2 = Area After Transition.
For a contraction:
HL = [K(V2)2/2g][(1/Cc)-1]2≈[K(V2)2/2g][1-(A2/A1)]2
in which K is a coefficient equal to 0.5 for a well-designed transition, Cc is a
coefficient of contraction, and the other terms and subscripts are similar to the
previous equation. Losses in closed conduits of constant area are expressed in
terms of (V2/2g).
The above equations may be applied to approximate the energy loss through a
manhole for a circular pipe flowing full. If the invert is fully developed, that is,
semi-circular on the bottom and vertical on the sides from one-half depth up to
the top of the pipe, for the expansion (A1/A2) = 0.88, and for the contraction
(A2/A1) = 0.88. The expansion is sudden; therefore, K = 1. The contraction may
be rounded if the downstream pipe has a bell or socket. In this case, K may be
assumed to be 0.2.
The expansion energy loss is 0.0l4 [(V1)2/2g] and the contraction energy loss is
0.010 [(V2)2/2g]. If the invert is fully developed, the manhole loss is small, but if
the invert is only developed for one-half of the depth, or not at all, the losses will
be of considerable magnitude.
(2) Open Channel Transitions
The hydraulics of open channel transitions are further complicated
by possible changes in depth. As a first approximation to the energy
loss, unless a jump occurs, the equations given above may be used
with a trial-and-error solution for the unknown area and velocity.
The K value for a well-designed expansion should probably be
increased to 0.3 or 0.4. Whether the properties of the upstream or
downstream section will be known will depend on the characteristics
of the flow and the channel, but can be determined by a profile
analysis. In transitions for supercritical flow, additional factors shall
be considered. Standing waves of considerable magnitude will be
produced in transitions. The height of these waves must be estimated
to provide a proper channel depth. In addition, in long transitions,
air entrainment will cause bulking of the flow with resultant greater
depths of the air-water mixture.
4.030.03 Hydraulic Grade Line Limits
The hydraulic grade line shall not rise above the following limits as determined by flow quantities
calculated per Section 4.030.01. Stormwater conveyance systems are usually designed for 15 year or 20
year, 20 minute rainfall frequencies per 4.030.01.1.
Revised 12/15/2014 56
1. The hydraulic grade line at any inlet or storm manhole shall not be higher than two (2)
feet below the inlet sill or top of manhole.
2. Storm sewers shall not flow with greater than three (3) feet of head.
3. The hydraulic grade line for combined sewers shall not rise above the pipe intrados.
4. The beginning point for the hydraulic grade line computations shall be the higher (i.e.
more conservative) elevation as determined below:
a. For connection to existing pipe system:
(1) Top of pipe intrados of at least two reaches downstream of the connection
point of the existing system; or
(2) The hydraulic grade line computed for the existing system.
b. For connection to channels or ditches:
(1) Top of pipe intrados of the proposed pipe, or
(2) The hydraulic grade line computed for the channel or ditch as approved
by the District.
c. For upstream system pipe connection to dry and wet detention basins:
(1) The starting hydraulic grade line for all incoming pipes shall be the 100
year-24 hour blocked low flow water surface elevation, where County
maintained streets are located adjacent to or upstream of the basins.
(2) The starting HGL for all other situations may be the 100 year – 24 hour
unblocked low flow water surface elevation, unless the local road
authority requires something higher.
5. When storm sewers are designed to convey 100 year flows, effusion at low lying inlets is
not allowed, unless 100 year ponding easements are so delineated, granted, and recorded.
Those associated temporary “ponding” easements however, should not be confused with
100 year overland flow paths, for which no conveyance area easements are presently
required. Also, such intentional effusive designs may be prohibited for St. Louis County
maintained streets or highways.
4.030.04 Inlets
Inlets function entirely as entry points for stormwater flow. They also may be constructed to serve as a
manhole on separate stormwater sewers, and are then termed inlet-manholes. Steep gradients may give
such low inlet capacities that additional inlets should be located at more favorable grade locations or
special inlets designed for steep gradients must be used. Provision must be made to control by-pass flow
and to provide additional capacity in the inlet and line affected by such increased flow. Six (6) inch open
throat inlets should be used at all times. The open throat should not be obstructed or otherwise restricted
by bars, wires or screens.
Grated inlets, without an open throat or other provision for overflow shall be avoided except under
exceptional conditions, and are prohibited in grade pockets. Any exceptions shall be used only with
District approval.
Curb inlets shall be placed at street intersections or driveways such that no part of the inlet structure or
sump is within the curb rounding.
Revised 12/15/2014 57
1. Inlets are shown in the Standard Details of Sewer Construction. The minimum depth
of a terminal inlet is four (4) feet from the top of the inlet to the flowline of the
outlet pipe. Greater depth shall be used for intermediate inlets if necessary for the
required depth of the hydraulic grade line. Trapped inlets shall have the depth
shown in the Standard Details of Sewer Construction.
2. Inlet capacity should not be less than the quantity flow tributary to the inlet and by-
pass flow shall be avoided whenever possible. “Multiple type” inlets, used in the
past, had one integral chamber shallower than the other chamber, and tops of
different size stones, as well. Use of “multiple type” inlets is prohibited for new
construction, as are bars, screens, or wires across inlet openings. Use “double inlets”
instead, if necessary for capacity. Bypass, if unavoidable, must be identified,
including amount and spread; local road jurisdiction approval must also be provided.
Inlets at low points or grade pockets should have extra capacity to compensate for
possible flow by-pass of upstream inlets.
Figure 4-3 shows inlet capacity/maximum gutter capacity with a given gutter line
grade and flow. Inlets angled in opposition to direction of vehicular travel may be
dangerous and are to be avoided.
3. Connections to existing structures may require rehabilitation or reconstruction of the
structure being utilized. This work will be considered part of the project being
proposed.
4. Grated trough drains will not be accepted for dedication to the District. Where
appended to the District curb inlet structures, design shall provide clear and
workable separation for purposes of maintenance responsibility by others for their
part of the drain.
4.030.05 Open Channels
*NOTE: This section contains some excerpts relating to design and are attributed to Open
Channel Hydraulics by Ven Te Chow, a McGraw-Hill work published in 1959.
All open channels shall meet the following requirements:
1. Size and Shape
Open channels shall not decrease in size in the direction of flow. Open channels shall be
vertical walled except in special cases where other approved materials are being
considered.
2. Materials
Channels may be constructed with reinforced concrete or other approved material.
Gabions, articulated mattresses or other systems may be approved. However, the District
shall have the right to approve or disapprove any channel material and shall select the
appropriate channel material if a proposed material is rejected. Swales shall be sodded
unless velocities are excessive (greater than 5 fps or where velocities are less than 2 fps
causing deposition of soil particles, then concrete swales may be used. Swales used as
BMP’s shall be appropriately vegetated and maintained, or otherwise stabilized in an
approved manner.
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3. Bedding
Special provisions shall be made for channels or paved swales laid over fill on non-
supportive soils to support the channel on paved swales. Pipes extended to the channel in
a fill area shall have compacted crushed limestone bedding for support.
4. Structural Considerations
Provision must be made for all loads on the channel.
5. Alignment
Open channel alignments may be limited by available easements, physical topography,
existing utilities, buildings, residential development, maintenance access and roadways.
6. Locations
Storm channel locations are determined primarily by natural drainage conditions. It is
also necessary to consider accessibility for construction and maintenance, site availability
and competing uses, and evaluating effects of easements on private property.
Storm channels shall be located:
a. To serve all adjacent property conveniently and to best advantage.
b. In easements or rights-of-way dedicated to the District.
c. In easements on common ground when feasible.
d. On private property along property lines or immediately adjacent to public
streets, avoiding crossings through the property.
e. At a sufficient distance from existing and proposed buildings and underground
utilities or sewers to avoid future problems of flooding or erosion.
f. To avoid interference between stormwater sewers and house connections to
foulwater or sanitary sewers.
g. In unpaved or unimproved areas whenever possible.
h. Crossing perpendicular to streets, unless unavoidable.
7. Flowline
The flowline of open channels shall meet the following requirements:
a. Gradient changes shall be kept to a minimum and be consistent and regular.
b. Gradient designations less than the nearest 0.001 foot per foot shall be avoided.
c. Channel and swale depths shall be determined primarily by the requirements of
the channel size, utility obstructions and any required connections.
8. Other Open Channel Considerations and Requirements
a. All natural channels and ditches shall be improved unless otherwise authorized
by the District.
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b. Drainage within private property should be controlled to prevent damage to the
property crossed. Swales, or broad shallow grass lined ditches with non-erosive
slopes, are generally located at or near rear lots and along common property
lines. If a paved gutter is utilized, then appropriate erosion protection shall be
used at both ends.
c. Drainage channels and water courses draining through a subdivision shall be
enclosed if the required pipe size does not exceed sixty (60) inches unless a BMP
plan is approved which incorporates the channel specifically. Drainage channels
originating within a project site are encouraged to be incorporated into the
development’s stormwater management plan. When it is undesirable or
impractical to enclose a channel with a pipe across a road or street, a suitable
bridge or culvert shall be required.
d. For flows greater than 4 cfs, area inlets or inlet manholes are required to intercept
the gutter or swale flow unless part of a workable, recognized and approved
BMP.
e. All improved concrete channels shall have a forty eight (48) inch chain link fence
on each side of the channel, or other protective measures as directed by the
District.
f. Channels and water courses draining large areas shall be located in rights-of-way
or easements previously approved by the District as a part of an adequate overall
plan for drainage.
9. Design Limitations
a. The flow quantity shall be calculated by the method presented in Section
4.030.01 of this manual.
b. If the channel is within an area designated in a community's flood insurance
study, then the channel shall also meet all District and the community's floodplain
requirements.
c. Other agencies of jurisdiction, for example FEMA or MoDNR, may have
requirements which must be met. A U.S. Army Corps of Engineers permit may
be required for any construction affecting a watercourse.
10. Hydraulic Grade Line
a. Computation Methods
In open channels the water surface is identical with the hydraulic grade line. The
hydraulic grade line shall be computed throughout the channel reach to show its elevation
at junctions with incoming pipes or channels and at the ends of the channel reach under
consideration. It shall also provide for the losses and differences in elevations as
required below. Since it is based on design flow in a given channel, it is of importance in
determining minimum sizes within narrow limits. The depth at which the actual flows
will occur is controlled by the two end conditions of the reach considered, and by the
relationship between the energy available and by the energy required to overcome the
losses that are encountered along the channel.
There are several methods of calculating "losses" in channel design. The following
procedures are presented for the engineers information and consideration.
Revised 12/15/2014 60
It is required that the design recognize the reality of such "losses" occurring and make
such allowances as good engineering judgment indicates.
(1) Control Sections
The engineer should locate all possible control sections for the reach in question.
A control section refers to any section at which the depth of flow is known or can
be controlled to a required stage. At the control section, flow must pass through
a control depth which may be the critical depth, the normal depth or any other
known depth. Three types of control sections include (a) Upstream Control
Section; (b) Downstream Control Section; (c) Artificial Control Section, which
occurs at a control structure, such as a weir, dam, sluice gate, roadway
embankment, culvert, bridges or at the confluence with a major river or stream.
(2) Friction Loss
The friction loss may be calculated by the same procedure as is presented in
(3) Flow in Curved Channels
The centrifugal force caused by flow around a curve produces a rise in the water
surface on the outside wall and a lowering of the inner wall. This phenomenon is
called superelevation. The flows tend to behave differently according to the state
of flow.
In subcritical flow, friction effects are of importance, whereby in supercritical
flow, the formation of cross-waves is of major concern.
(a) Curve Losses
Curve losses may be estimated from Figure 4-2 by replacing D, diameter,
with b, width of channel.
(b) Superelevations
In addition to curve losses, an evaluation of superelevations should be
considered and, if required, an allowance made in the top elevation of
outside wall. Equations are presented below which may be used to
determine the superelevation at channel bends.
1) Trapezoidal Channels
Subcritical Flow:
ΔHw = 1.15(V2/2grc)[b+D(ZL+ZR)]
Supercritical Flow:
ΔHw = 2.6(V2/2grc)[b+D(ZL+ZR)]
2) Rectangular Channels
Subcritical Flow:
ΔHw = (V2b/2grc)
Supercritical Flow:
ΔHw = (V2b/grc)
Where:
ΔHw = Change in water height above the
Revised 12/15/2014 61
centerline water surface elevation.
V = Average velocity of design
flow in Fps
g = Acceleration of gravity (32.2
Ft/Sec/Sec)
rc = Radius of curve on horizontal
alignment in feet
b = Base width of channel in feet
D = Depth of flow in straight channel
ZL = Left side slope (ft/ft)
ZR = Right side slope (ft/ft)
(4) Transitions
Transitions should be designed to accomplish the required change in
cross section with as little flow disturbance as possible.
The following features are to be considered in design of transition
structures.
(a) Proportioning
For a well designed transition, the following rules should be used:
1) The optimum maximum angle between the channel axis
and a line connecting the channel sides between the
entrance and exit sections is 12.5o.
2) Sharp angles in the structure should be avoided.
(b) Losses
The energy loss in a transition consists of the friction loss and the
conversion loss. The friction loss may be estimated by the
Manning Formula. The conversion loss is generally expressed in
terms of the change in velocity head between the entrance and
exit sections of the structure.
Ht = Kt ΔVH
Where:
Ht = Conversion loss
Kt = Coefficient of head loss
in transition
ΔVH = Absolute change in
velocity head
Average design values for Kt are presented in the table below:
Contracting Expanding
Type of Transition Section Section
Warped 0.10 0.20
Wedge 0.20 0.50
Cylinder-quardrant 0.15 0.25
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Straight Line 0.30 0.50
Square End 0.40 0.75
See Figure 4-4 for sketches of each type of transition.
(c) Freeboard
A transition shall have a minimum of one (1) foot of freeboard
above the hydraulic grade line.
(d) Hydraulic Jump
The existence of a hydraulic jump in a transition may become
objectionable, and the design of the transition should be checked
for such.
(e) Sudden Enlargement and Contraction
A sudden enlargement results when an intense shearing action
occurs between incoming high-velocity jet and the surrounding
water. As a result, much of the Kinetic energy of the jet is
dissipated by eddy action. The head loss at a sudden enlargement,
HLe, is:
HLe = Ke( ΔV2/2g)
Where:
Ke = Coefficient of head loss
for enlargements = 1
ΔV = Change in velocities
between incoming and
outgoing sections
g = Acceleration of gravity
(32.2 Ft/Sec/Sec)
The flow in a sudden contraction is
first contracted and then expanded
resulting in high losses as compared
to a sudden enlargement. Thus the
head loss at a sudden contraction,
HLc, is:
HLc = Kc( ΔV2/2g)
Where:
Kc = Coefficient of head loss
for contractions = 0.5
ΔV = Change in velocities
between incoming and
outgoing sections
g = Acceleration of gravity,
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(32.2 Ft/Sec/Sec)
(5) Constrictions
A constriction results in a sudden reduction in channel cross section. The effect
of the constriction on the flow depends mainly on the boundary geometry, the
discharge and the state of flow. When the flow is subcritical, the constriction will
induce a backwater effect that extends a long distance upstream. If the flow is
supercritical, the disturbance is usually local and will only affect the water
adjacent to the upstream side of the constriction. A control section may or may
not exist at a constriction. The control section, when it exists, may be at either
side of the constriction (upstream or downstream), depending on whether the
slope of the constricted channel is steep or mild. The entrance and outlet of the
constriction then acts as a contraction and an expansion, respectfully.
(6) Obstructions
An obstruction in open-channel flow creates at least two paths of flow in the
channel. Typical obstructions include bridge piers, pile trestles, and trash racks.
The flow through an obstruction may be subcritical or supercritical.
b. Hydraulic Grade Line Limits
(1) The hydraulic grade line at any point along a channel shall not be higher
than one (1) foot below the top of the channel wall.
(2) The hydraulic grade line at any point along a channel shall not cause the
hydraulic grade limits of the storm sewer system to be exceeded as stated
in Section 4.030.03 of this manual.
11. Hydraulic Jump
When flow changes from the supercritical to subcritical state, a hydraulic jump may
occur. A study should be made on the height and location of the jump, and for discharges
less than the design discharge, to ensure adequate wall heights extend over the full ranges
of discharge.
12. Open Channel Junctions
a. General
(1) Consideration shall be given in the design of open channel
junctions to the geometry of the confluence of flows in order to
minimize undesirable hydraulic effects due to supercritical
velocities.
b. Confluence Design Criteria
(1) The momentum equation can be applied to the confluence design
if the below stated criteria is used.
(2) The design water-surface elevations in the two joining channels
should be approximately equal at the upstream end of the
confluence.
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(3) The angle of the junction intersection can vary from 0-12 degrees.
(4) The width of the main channel shall be expanded below the
junction to maintain approximate flow depths throughout the
junction.
(5) Flow depths should not exceed ninety percent (90%) of the
critical depth.
13. Erosion Protection
Properly designed rock blankets, minimum one (1) foot thick, shall be required at each
end of the improved channel. The minimum length of the rock blanket shall be twenty
five (25) feet. A rock toe wall, minimum two (2) foot deep, shall be constructed at the
free end of each blanket. Alternative erosion protection products may be considered,
such as articulated block mattresses, etcetera.
14. Sanitary Sewer Crossings
The characteristics of any sanitary sewer crossing shall be given consideration in the
design of the channel floor.
4.030.06 Culverts
The design of culverts shall include consideration of many factors relating to requirements of hydrology,
hydraulics, physical environment, imposed exterior loads, construction and maintenance.
With the design discharge and general layout requirements determined, the design requires
detailed consideration of such hydraulic factors as shape and slope of approach and exit
channels, allowable head at entrance (and ponding capacity, if appreciable), tailwater levels,
hydraulic and energy gradelines, and erosion potential.
1. Hydraulic Design
The hydraulic design of a culvert for a specified design discharge involves (1) selection of a type
and size, (2) determination of the position of hydraulic control, and (3) hydraulic computations to
determine whether acceptable headwater depths and outfall conditions will result. Hydraulic
computations will be carried out by standard methods based on pressure, energy, momentum and
loss considerations.
2. Entrances and Headwalls - Outlets and Endwalls
Where an existing culvert is to be extended, the possibility for maintaining or improving existing
capacity should be investigated. Marked improvement may be obtainable by proper entrance
design. All culverts shall be designed for possible extension unless there are extenuating
circumstances.
4.040 Bridges
Bridges shall be designed to meet the current criteria of the governing agencies.
4.040.01 Waterway Capacity and Backwater Effects
Sufficient capacity will be provided to pass the runoff from the design storm determined in accordance
with principles given elsewhere in this manual.
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4.040.02 Clearance
The lowest point of the bridge superstructure shall have a (freeboard) clearance of two (2) feet above
design water surface elevation for the 15-year frequency in St. Louis County (20-year frequency in
the City of St. Louis) and one (1) foot for the 100-year frequency.
4.040.03 Waterway Alignment
The bridged waterway will be aligned to result in the least obstruction to stream flow, except that for
natural streams consideration will be given to future realignment and improvement of the channel.
4.040.04 Erosion Protection
To preclude failure by scouring, abutment and pier footings will usually be placed either to a depth of not
less than five (5) feet below the anticipated depth of scour, or on firm rock if such is encountered at a
higher elevation. Large multispan structures crossing alluvial streams may require extensive pile
foundations. To protect the channel, revetment on channel sides and/or bottom, consisting of concrete or
grouted rock blanket should be placed as required. The governing authority should be contacted
regarding their design requirements.
4.050 Outlet Erosion Protection
If outlet velocities exceed 5 fps, an appropriate erosion protection must be provided. Erosion protection may be required at
outlets where velocities are less than 5 fps if soil conditions warrant.
For paved channels a cutoff wall will be required at the termini with appropriate protection. The cutoff wall shall extend a
minimum depth of two (2) feet into the existing ground line.
4.060 Limitations on Areas Draining Across Sidewalks or Driveways [Deleted by Amendment 4]
In the City of St. Louis, per Ordinance No. 60664, up to three thousand (3,000) square feet of parking area may discharge via
a driveway to the public or private street. An additional three thousand (3,000) square feet may discharge into a public alley.
Areas larger than this must have any excess area discharge into interceptor basins as set forth in the (City of St. Louis)
Plumbing Code.
In unincorporated areas of St. Louis County, area inlets shall be required to intercept overland flows greater than 1 cfs to
prevent that flow from crossing sidewalks or curbs.
4.070 Impervious Areas - In City of St. Louis [Deleted by Amendment 4]
Any area which is to be paved, repaved, expanded or otherwise improved, that is over three thousand (3,000) square feet in
area, whether presently paved or not, shall at such time as it is to be paved, repaved, expanded or be otherwise improved, be
provided with storm water drainage facilities constructed in accordance with plans and specifications submitted to and
approved by the District.
Such approval is a pre-requisite of the City of St. Louis for acceptance of an application for the paving or any other
construction necessary to bring an impervious area into compliance with City of St. Louis Ordinance 52552 as amended. Any
impervious area, less than three thousand (3,000) square feet in area, shall not require on-site storm water drainage facilities.
The area draining toward the rear alley is counted separately from the area draining toward the street, in actual practice, if
both are receiving flow.
4.080 General Performance Criteria for Stormwater Management
4.080.01 When Required
1. The requirements of stormwater quantity and quality management shall be evaluated for all
projects submitted to the District for review and approval. Stormwater management facilities
shall be provided and designed in accordance with the requirements of this section. If another
Revised 12/15/2014 66
local jurisdiction requires more stringent design standards, then they shall govern in that locale.
A Stormwater Management Facilities (BMP) Operation and Maintenance Design Report and
Plan, including specific continuing resources, procedures and schedules to be used, shall be
submitted for approval. If required and approved, the Plan shall be included in a recorded
Maintenance Agreement by reference.
2. Stormwater quantity and quality management requirements shall be evaluated for all projects,
and specifically, will be required for projects including:
a. All new development and redevelopment projects that disturb greater than or equal to
one acre, including projects less than 1 acre that are part of a larger common parcel or
project that is greater than one acre. However, existence of downstream stormwater
problems may require quantity detention on the proposed site, where less than 2 cfs
differential is proposed.
b. Projects which have a differential runoff of 2 cfs or greater for the 15-year, 20-minute
event (in St. Louis County).The differential runoff is calculated by the Rational Method
using PI factors.
Subsequent development or redevelopment of sites without prior stormwater detention
shall provide detention or retention, when cumulative differential increase, since January
15, 2000, equals 2 cfs or greater. Projects with prior detention shall provide additional
detention or retention for increasing runoff irrespective of the 2 cfs threshold. The degree
of commonality between subsequent or concurrent projects, sites or parcels within same
watershed shall be as determined by the District for purposes of this section.
3. When existing stormwater management facilities are going to be used to accommodate additional
runoff from building or parking lot expansions or subdivision additions, the facilities shall be
retrofitted to meet the current stormwater management requirements for the drainage area, served
by the facility. Projects, which cannot meet this requirement due to physical constraints, will be
evaluated for alternatives on a case by case basis.
4. The stormwater design projects within designated levee districts such as Monarch-Chesterfield,
Earth City, Howard Bend, and Riverport will be based on the Stormwater Master Plan for these
districts. If the Stormwater Master Plan does not address water quality, the requirements of this
manual shall apply.
5. [Added by Amendment 3. See Section 16] Controls shall be designed and implemented to
prevent or minimize water quality impacts by reasonably mimicking pre-construction runoff
conditions on all “new development” projects to the maximum extent practicable. (This is a
Small MS4 NPDES Stormwater Discharge Permit requirement. It necessitates controls and
practices that reduce runoff volume through infiltration, evapotranspiration, and/or rainwater
harvesting.)
On “redevelopment” sites, controls shall be designed and implemented to prevent or minimize
water quality impacts by effectively utilizing water quality strategies and technologies, including
those that reduce runoff volume, to the maximum extent practicable. When micro-detention is
required in the combined sewer area to address sewer capacity problems, these controls should
also apply runoff reducing strategies and technologies.
Sites with 20 percent or less existing imperviousness are considered new development for
determining if the “mimicking pre-construction runoff conditions” requirement is applicable.
Any subsequent or additional development or expansion projects on those sites will also be
considered new development. Subdividing does not affect that requirement.
Revised 12/15/2014 67
4.080.02 Unified Stormwater Sizing Criteria
1. General
This section presents a unified approach for sizing stormwater Best Managements Practices
(BMP’s) to meet pollutant removal goals, reduce channel erosion and prevent flooding and pass
extreme floods. A very brief summary is listed below.
SUMMARY OF THE KEY COMPONENTS AND STORMWATER CRITERIA
Water Quality Volume
(WQv ) (acre-feet)
WQv = [(P)(Rv)(A)]/12
P = rainfall depth in inches and is equal to 1.14
Rv = volumetric runoff coefficient, and
A = area in acres
Channel Protection Storage Volume
(Cpv)
Cpv = 24 hour extended detention of post-developed
one-year, 24 hour storm event
Flood Protection Volume
(Qp2 & Qp100)
The post-developed routed peak flow from the site
may not exceed the existing routed peak flow for the
2-year and 100-year, 24-hour events, or the
allowable release rate for differential runoff greater
than 5 cfs.
The following sub-sections provide more expanded information, directories, explanations and
resource references.
2. Water Quality Volume (WQv) [See Amendment 4]
a. The Water Quality Volume (denoted as the WQv) is the storage needed to capture and
treat the runoff from 90% of the recorded daily rainfall events. In numerical terms, it is
equivalent to 1.14 inches of rainfall multiplied by the volumetric runoff coefficient (Rv)
and site area. The WQv is directly related to the amount of impervious cover created at a
site. A minimum WQv of 0.2 inches per acre shall be met at all sites where WQv is
required.
b. As a basis for determining water quality treatment volume the following assumptions may
be made:
(1) The water quality volume WQv for offsite areas is not required.
The following equations are used to determine the storage volume, WQv (in acre-
feet of storage):
WQv = [(P)(Rv)(A)]/12 P = 1.14 inches of rainfall
Where: WQv =water quality volume (in acre-feet)
Rv =0.05 + 0.009 (I) where I is percent impervious cover
A =drainage area to BMP in acres
(2) Measuring Impervious Cover: The measured area of a site plan that does not have
vegetative or permeable cover shall be considered total impervious cover.
(3) Multiple Drainage Areas: When a project contains or is divided by multiple
drainage areas, the WQv volume shall be addressed for each drainage area.
(4) Offsite Drainage Areas: The WQv shall be based on the impervious cover of the
proposed site. Offsite existing impervious areas may be excluded from the
calculation of the water quality volume requirements.
Revised 12/15/2014 68
(5)(4) BMP Treatment: The final WQv shall be treated by an acceptable BMP(s) from
the list presented in Section 4.080.05, or as approved by the District.
(6)(5) Subtraction for Non-structural Practices: When non-structural practices are
employed in the site design, the WQv volume can be reduced in accordance with
the conditions outlined in Section 4.080.06.
(7)(6) Extended Detention for Water Quality Volume: The water quality requirements
can be met by providing a 24-hour draw down of a portion of the water quality
volume (WQv) in conjunction with a stormwater pond or wetland system.
Referred to as ED, this is different than providing the extended detention of the
one-year storm for the channel protection volume (Cpv). The ED portion of the
WQv may be included when routing the Cpv.
3. Channel Protection Storage Volume Requirements (Cpv) [See Amendment 1, Chapter 16]
a. General
To protect channels from erosion, a 24-hour extended detention of the one-year, 24-hour
storm event shall be provided. The rationale for this criterion is that runoff will be stored
and released in such a gradual manner that critical erosive velocities during bankfull and
near-bankfull events will seldom be exceeded or prolonged in downstream channels.
A detention pond or underground vault is normally needed to meet the CPv requirement
(and subsequent flood protection criteria Qp2 and Qp100).
b. As a Basis for determining Channel Protection Storage Volume the following
assumptions may be made:
(1) The model TR-55 (or approved equivalent) shall be used for determining peak
discharge rates. (See 4.080.02 Paragraph 4.b.(1) for additional TR-55
information.
(2) The rainfall depth for the one-year, 24-hour storm event is 2.50 inches. Use Type
II rainfall distribution.
(3) The length of overland flow used in time of concentration (tc) calculations is
limited to no more than 100 feet for post project conditions.
(4) The 24-hour extended detention is defined as providing a 24-hour
detention lag time (T) for the one-year storm. The lag time is defined as the
interval between the center of mass of the inflow hydrograph and the center of
mass of the outflow hydrograph.
(5) A Cpv orifice diameter of less than 3.0” will require a special internal control for
orifice protection. For Cpv orifice between 3” and 1 1/2” diameter, an internally
controlled orifice shall be used with slot width less than or equal to 1/3 of orifice
diameter. Less than 1 ½” orifice will not be allowed.
(6) Cpv shall be addressed for the entire site. If a site consists of
multiple drainage areas, Cpv may be distributed proportionately to each drainage
area.
(7) Extended detention storage provided for the Cpv does not fully
meet the WQv requirement (that is Cpv and WQv should be treated separately).
Revised 12/15/2014 69
(8) The stormwater storage needed for Cpv may be provided above the WQv storage
in stormwater ponds and wetlands; thereby meeting all storage criteria in a single
facility with appropriate hydraulic control structures for each storage requirement.
(9) Infiltration is not recommended for Cpv control because of large storage
requirements. If proven effective, appropriate and desirable however, in some rare
situations it may be permissible.
c. Exemptions
To protect channels from erosion, a 24-hour extended detention of the one-year, 24-hour
storm event shall be required at all sites that do require Flood Volume Detention (Qp).
Also, the existence of downstream stormwater problems may require treatment for
channel protection (CPv), regardless of any other possible exemptions.
At some sites however, the provision of traditional extended detention (Cpv) may not be
effective or may be best achieved by other means. Specifically, the following sites may
be exempted from the channel protection storage requirement:
A.) For sites 5 acres or larger, exempt if:
1.) No detention for Flood Protection Volume (Qp) is required, and
2.) The project is a redevelopment site (at least 20% of the existing site was
impervious coverage as of January 15, 2000) and
3.) The watershed is a less sensitive, i.e. “zero increase” watershed (per
Table 4-5)
And either 4.) or 5.) below
4.) Surface BMP’s are utilized to treat the required WQv and more pervious
area is created on the site and within the tributary area.
OR
5.) Underground devices are utilized to treat the required WQv but the
proposed site will be at least 20% more pervious than the existing site.
B.) All sites less than 5 acres will normally be exempt.
C.) For all sites, regardless of size, exempt if any of the following
apply:
1.) The project site will be 100% pervious, and does not increase annual or
more frequent discharge from site (for a 1-year event per TR-55).
2.) The project is a redevelopment site (at least 20% of the existing site was
impervious coverage as of January 15, 2000) and the project reduces the
site impervious area to less than 10% of the total site area.
3.) The project is located within levee district boundaries or within the very
flat portions of the FEMA defined flood plain of the Mississippi,
Missouri, or Meramec Rivers.
4.) The project is located upstream of permanent lakes, concrete lined
channels, or enclosed pipe systems. However, the presence of any
intervening reach or downstream, natural channel which does need
extended detention channel protection may then nullify this exemption.
Revised 12/15/2014 70
a. To protect channels from erosion, a 24-hour extended detention of the one-year, 24-hour
storm event shall be provided. The rationale for this criterion is that runoff will be stored
and released in such a gradual manner that critical erosive velocities during bankfull and
near-bankfull events will seldom be exceeded in downstream channels.
A detention pond or underground vault is normally needed to meet the CPv requirement
(and subsequent flood protection criteria Qp2 and Qp100).
As a Basis for determining Channel Protection Storage Volume the following
assumptions may be made:
(2) The model TR-55 (or approved equivalent) shall be used for determining peak
discharge rates. (See 4.080.02 Paragraph 4.b.(1) for additional TR-55
information.
(2) The rainfall depth for the one-year, 24-hour storm event is 2.50 inches. Use Type
II rainfall distribution.
(3) The length of overland flow used in time of concentration (tc) calculations is
limited to no more than 100 feet for post project conditions.
(10) The 24-hour extended detention is defined as providing a 24-hour detention lag
time (T) for the one-year storm. The lag time is defined as the interval between
the center of mass of the inflow hydrograph and the center of mass of the outflow
hydrograph.
(11) Cpv is not required at sites where the one-year post development peak discharge is
less than or equal to 2.0 cfs. A Cpv orifice diameter of less than 3.0” is not
allowed.
(12) Cpv shall be addressed for the entire site. If a site consists of multiple drainage
areas, Cpv may be distributed proportionately to each drainage area.
(13) Extended detention storage provided for the Cpv does not fully meet the WQv
requirement (that is Cpv and WQv should be treated separately).
(14) The stormwater storage needed for Cpv may be provided above the WQv storage
in stormwater ponds and wetlands; thereby meeting all storage criteria in a single
facility with appropriate hydraulic control structures for each storage requirement.
(15) Infiltration is not recommended for Cpv control because of large storage
requirements. If proven effective, appropriate and desirable however, in some rare
situations it may be permissible.
4. Flood Protection Volume Requirement (Qp2 & Qp100)
a. To protect downstream areas from flooding stormwater shall be detained on site or
offsite as approved and released at a rate not to exceed the allowable release rates for
the 2-year and 100-year 24-hour events, as determined by the District for the watershed
in question. The allowable release rates have been determined by watershed modeling
(see Table 4- 5). The engineer has the option to calculate a site specific release rate
based on procedures provided by the District's Engineering Department (Volumetric
Method). Note that stormwater pipes, downstream from the control structure, shall be
sized to carry the runoff from the 15-year 20-minute design storm for the total tributary
upstream watershed. No reduction in outfall pipe size shall be permitted because of
detention.
Revised 12/15/2014 71
b. As a Basis for Determining the Flood Protection Volume the following assumptions
may be made.
(1) The 2-year and 100-year, 24-hour inflow hydrographs shall be determined by
using Technical Release 55 (TR-55), "Urban Hydrology for Small Watersheds"
from the Natural Resources Conservation Service, formerly Soil Conservation
Service (SCS). The inflow hydrograph shall be developed based on the actual
flow and timing characteristics upstream of the detention facility. The rainfall
distribution shall be Type II. The rainfall quantities to be used are from the
Illinois State Water Survey Bulletin 71, and shall be as follows: 3.1” for the 2-
year, 24-hour storm and 7.2” for the 100-year, 24-hour storm.
(2) The volume of detention may be provided through permanent detention facilities
such as dry basins or ponds, permanent ponds or lakes, underground storage
facilities or in parking lots. The engineer shall make every effort to locate the
detention facility at or near the lowest point of the project such that all of the on-
site runoff will be directed into the detention facility. Multiple use of detention
basins is encouraged. Multiple use may include parking lots, ball fields, tennis
courts, play grounds and picnic areas. This is subject to the approval of the
District.
Flows from offsite, upstream areas should be bypassed around the detention
facility to ensure that the proposed detention facility will function as designed and
will provide effective control of downstream flows with development in place. If
offsite flows are directed into a detention facility, the allowable release rates shall
not be modified without District approval. Modifying the release rate to
accommodate offsite flows may reduce or eliminate the effectiveness of the
detention facility, because it will no longer control the increased volume of runoff
during the critical time period of the watershed.
As stated in Item 4.a above, the engineer has the option to calculate a site specific
release rate based on procedures provided by the District's Engineering
Department. The engineer shall provide detailed modeling to prove that the
increase in runoff volume has been limited to existing conditions during the
critical time period of the watershed (For Volumetric Method see Appendix –
forthcoming).
(3) Detention basin volume will be based on routing the post-developed 2-year and
100 year, 24-hour inflow hydrographs through the detention facility while
satisfying the appropriate allowable release rate. The routing computations shall
be based on an application of the continuity principle, (i.e., level pool routing).
Municipalities or St. Louis County may by ordinances, have other volume
requirements. The more stringent volume requirements will govern.
4.080.03 Limits of Maximum Ponding in Stormwater Ponds
1. The maximum ponding elevation shall be calculated based on a routing of the design storm
(100-year, 24-hour event) assuming the low flow outlet is blocked with water ponded to the
overflow structure’s sill.
2. The limits of maximum ponding in dry basins or ponds and permanent lakes or ponds shall
not be closer than thirty (30) feet horizontally to any building, and not less than two (2) feet
vertically below the lowest sill elevation of any building.
Revised 12/15/2014 72
3. The limits of maximum ponding in parking lots shall not be closer than ten (10) feet
horizontally from any building and not less than (1) foot vertically below the lowest sill
elevation of any building.
4. A minimum of one (1) foot of freeboard shall be provided from the top of the basin to the
maximum ponding elevation.
5. Micro-Detention will be looked at case by case but in general the following apply:
a. 12” Freeboard Requirement
Exceptions allowed: (Freeboard 6” or less)
(1) Upper terraces of multiple in-line detention basin(s).
(2) If the basin’s outfall is directed onto a natural water way with no downstream
property to be impacted. This could be obtained by providing swales and/or
using existing swales that are around the downstream perimeter of the site and
which direct flows to natural waterways or property with no future build out
possibilities (6” freeboard still preferred).
b. Other Exceptions:
30’ Requirement for Horizontal Distance From HW Elevation
(1) If the basin is very shallow
(2) If the basin’s H.W. elevation is 2’ below the low sill of building.
It is recommended that additional construction and grading notes be put on the plans, as
well as a landscape schedule of plantings. Grading tolerances should be kept at + 0.1 and
as-builts provided. Infiltration rates should be accurate for basins without outfall structure.
Copy of maintenance and operations notes should be included on the plans.
4.080.04 General Stormwater Basin Design Requirements
1. Underground Basins’ Special Requirements:
a. Adequate access for basin maintenance and inspection shall be provided. A means of
visual inspection from the ground surface of the low flow device, overflow weir, and
outlet structure is necessary. Access shall also be provided to allow for cleaning of
the low flow device from the ground surface.
b. The basin should have sufficient volume and spillway capacity to pass/contain the
100-year 24-hour event with the low flow outlet blocked. In some situations it may be
desirable to have control structures with at least 2 outlet openings, one above the
other.
c. Underground basins shall be acceptable for non-residential projects only, except by
special municipal request, cooperation and a lack of other technical options. (See also
sub-section 4.080.05.1.c also).
d. Acceptable materials for underground basins are poured in place
reinforced concrete, RCP and aluminized CMP. The CMP gauge shall be approved
by the District prior to installation. Restricted use of other systems may be allowed in
the future.
Revised 12/15/2014 73
e. Provide immediate manhole access from ground surface for both sides of the low
flow device. Also provide a manhole at upstream end of underground basins, for
access, inspection, to facilitate maintenance and air release.
f. Adequate flowline spot elevations, sections and profiles including pipe length and
slope shall be labeled to define basin and pipe geometry.
2. General Detention Report Requirements
For detention facilities in general, the Engineer must submit 2 copies of a report sealed by
a Missouri Registered Professional Engineer containing the following for review of the
detention facility: (see 4.100 also)
a. Elevation vs Discharge tables or curves for all frequencies.
b. Elevation vs Storage tables or curves for all frequencies.
c. Inflow calculations and data for all frequencies.
d. Hydraulic gradeline computations for pipes entering and leaving the
basin for all frequencies.
e. If the embankment contains fill material a geotechnical report may be required.
f. Site plan showing appropriate design information.
g. Structural calculations for the outlet control structures (if required).
h. Cross sections defining the size, shape and depth of the detention basin shall
be required. At a minimum, three sections, one at each end and one in the
middle of the basin will be required.
3. All ends of pipes discharging into a dry basin or pond shall be connected with the low
flow pipe or control structure, by means of a permeable swale. The swale shall a
minimum 4:1 lateral (25%) slope to the center and a minimum 2.0% longitudinal slope to
ensure positive drainage. Swales shall be a minimum of six (6) inches deep and four (4)
feet wide or 1.3 times the diameter of the pipe entering the basin, whichever is greater. .
The bottom of the basin shall be sloped a minimum of two percent (2%). towards the edge
of the swale.
No concrete swales will be allowed unless approved by the District. Permanent erosion
control protection must be provided at the ends of discharging pipes. Lengths of the swale
in which the velocity exceeds 2 feet per second analyzed by the 15 year –20 minute storm
event, shall have appropriate permanent erosion control protection. Aggregate, porous
concrete blocks or appropriate vegetation are required, unless otherwise approved.
4. Railroad tie walls cannot be used where water will be in contact with the railroad tie
wall.
5. Permanent detention ponds or lakes are to be designed to minimize fluctuating lake levels.
Maximum fluctuation from the permanent pool elevation to the maximum ponding
elevation shall be six (6) feet.
6. The maximum side slopes for dry basins or ponds, and the fluctuating area of permanent
ponds or lakes shall be 3:1 (three feet horizontal, one foot vertical) without fencing.
7. Dry basins or ponds and the fluctuating areas of permanent ponds or lakes are to be
appropriately vegetated to the maximum high water elevation. . Areas above that elevation
Revised 12/15/2014 74
shall be appropriately stabilized and vegetated. Sod and mowing above that elevation may
be approved and is required for dam embankment slopes and downstream toe areas for
wet basins where riprap is not appropriate.
8. Control structures and overflow structures are to be reinforced concrete, including precast.
Only for underground proprietary or aluminized CMP basins may other approved
materials be allowed.
9. The outflow pipe shall be sized for the developed flow rate.
10. In basins with concrete walls or rip rap covered slopes, provisions should be made for
mowing equipment to reach the bottom (ramps, etc.).
11. Maximum Depths:
a. The maximum depth of water in a dry detention basin or pond shall not
exceed eight (8) feet. Projects which need a deeper basin to attain the
required detention volume due to physical constraints may be evaluated on a
case by case basis. The design and construction of dams greater than eight (8)
feet or as directed by the District must be sealed and certified by a
Professional Engineer registered in the State of Missouri with
demonstrated expertise in geotechnical engineering.
b. Parking lots used for automobiles shall have a maximum depth of eight (8)
inches of water.
c. Parking lots used for trucks or truck trailers shall have a maximum depth of
water of twelve (12) inches.
12. Detention Basin Fencing
A four (4) foot (minimum height) approved fence shall be provided around the perimeter
of any basin where the side slopes exceed 3:1 (three (3) feet horizontal, one (1) foot
vertical). Fencing such as post and rail, or fencing which prevents easy observation of
required detention basin maintenance, such as tall privacy fencing, should not be used.
13. Detention Basin Elevation
If the detention basin discharges to a piped sewer system, the low elevation of the
detention basin shall be above the 15-year, 20-minute hydraulic elevation of the
receiving storm system, or the 20 year, 20 minute hydraulic elevation of the receiving
combined systems, as applicable. Basin backflow contamination must be prevented, if
the downstream combined system is known to surcharge.
14. If the detention basin discharges to an open channel, or to a piped sewer system affected
by flood levels in a nearby downstream open channel, then the low elevation of the basin
is desirable to be above the 100-year flood elevation in the open channel as established
by the FEMA Flood Insurance Study or the District SSMIP, whichever is greater.
15. In all cases mentioned above, if the low elevation of the basin is below the receiving
system hydraulic grade or channel flood elevation, then the basin shall be sized to store
the entire design storm volume, unless directed otherwise by the District.
4.080.05 Acceptable Urban BMP Options [See Amendment 4]
1. This section sets forth five acceptable groups of BMPs that can be used to meet the
Water Quality volume criteria (WQv). The design and selection of these BMPs shall
comply with the Maryland Stormwater Design Manual, Volumes I & II (October 2000;
Revised 12/15/2014 75
effective date July 1, 2001), as prepared by the Center For Watershed Protection and
the State of Maryland Department of the Environment (MDE). Where the District
criteria or requirements are more stringent, then they shall govern. Adaption to local
Missouri environment and natural conditions should be expected but shall be as
approved by the District or a higher authority. The Manual can be purchased through
MDE’s website. A simple search for Maryland Stormwater Design Manual will provide
a direct link.
a. The acceptable BMP designs are assigned into five general categories for stormwater
quality control (WQv):
BMP Group 1 stormwater ponds
BMP Group 2 stormwater wetlands
BMP Group 3 infiltration practices
BMP Group 4 filtering practices
BMP Group 5 open channel practices
b. To be considered an effective BMP for stand-alone treatment of WQv, a design shall be
capable of:
1. capturing and treating the required water quality volume (WQv)
2. removing 80% of the TSS, and
3. having an acceptable longevity rate in the field
c. A combination of BMPs and/or credits is normally required at most development sites to
meet all three stormwater sizing criteria.
d. NOTE: Groundwater sump pump discharge may be problematic if directed towards
storm water BMPs. In general this practice should be avoided unless otherwise allowed
by the District.
1. BMP Group 1. Stormwater Ponds
a. Practices that have a combination of permanent pool, extended detention or shallow
wetland equivalent to the entire WQvs include:
P-1 micropool extended detention pond
P-2 wet pond
P-3 wet extended detention pond
P-4 multiple pond system
P-5 pocket pond
2. BMP Group 2. Stormwater Wetlands
a. Practices that include significant shallow wetland areas to treat urban stormwater but
often may also incorporate small permanent pools and/or extended detention storage to
achieve the full WQv include (Modification of existing wetland areas will require a CWA
404/401 permit: Corps 404 permit):
W-1 shallow wetland
W-2 ED shallow wetland
W-3 pond/wetland system
W-4 pocket wetland
b. U.S. Army C.O.E. Jurisdictional Wetlands shall not be used for control of water quantity
(i.e. the flood protection volume). Wetlands shall not be used for control of water
quantity (i.e. the flood protection volume).
Revised 12/15/2014 76
c. Wetlands shall be designed by a qualified and experienced team.
3. BMP Group 3. Infiltration Practices
a. Practices that capture and temporarily store the WQv before allowing it to infiltrate into
the soil over a two day period include:
I-1 infiltration trench
I-2 infiltration basin
b. Infiltration practices will be allowed on sites where it is proven that infiltration will
work. This must be supported by a soils report.
4. BMP Group 4. Filtering Practices
a. Practices that capture and temporarily store the WQv and pass it through a filter bed of
sand, organic matter, soil or other media are considered to be filtering practices. Filtered
runoff may be collected and returned to the conveyance system. Design variants include:
F-1 surface sand filter
F-2 underground sand filter
F-3 perimeter sand filter
F-4 organic filter
F-5 pocket sand filter
F-6 bioretention*
F-7 proprietary filtering system
*may also be used for infiltration
b. Filtering practices may be allowed on commercial projects. They are not allowed on
residential projects.
c. A maintenance agreement and maintenance schedule shall be required.
5. BMP Group 5. Open Channel Practices
a. Vegetated open channels that are explicitly designed to capture and treat the full WQv
within the dry or wet cells formed by checkdams or other means include:
0-1.01 dry swale
0-2 wet swale
0-3 filter strips
b. Open channel practices shall be designed with the proper plantings. They are not allowed
on single-family residential projects. They may be allowed on condominium or
apartment projects if maintenance is provided by a management company.
c. Wet swales shall be designed to drain out over time.
4.080.06 Stormwater Credits
Non-Structural BMPs are increasingly recognized as a critical feature of stormwater BMP plans,
particularly with respect to site design. In most cases, non-structural BMPs shall be combined with
structural BMPs to meet all stormwater requirements. The key benefit on non-structural BMP is that they
can reduce the generation of stormwater from site; thereby reducing the size and cost of structural BMPs.
In addition, they can provide partial removal of many pollutants. The non-structural BMPs have been
classified into seven broad categories. To promote greater use of non-structural BMPs, a series of credits
and incentives are provided for developments that use these progressive site planning techniques. Further
Revised 12/15/2014 77
details can be found in Chapter 5 of the Maryland Stormwater Design Manual, Volumes I & II (October
2000).
• natural area conservation
• disconnection of rooftop runoff
• disconnection of non-rooftop impervious area
• reserved buffers
• open channel use
• environmentally sensitive development
• impervious cover reduction
4.080.07 Easement Required
In subdivisions, the detention basin, BMP’s, access roads or paths, control structures, and outfall pipes
are to be located in easements dedicated to the subdivision trustees. Lack of appropriate easement(s) will
not relieve trustees of responsibility for required maintenance of the Stormwater Management System
BMP’s.
4.080.08 Maintenance Agreement
Prior to plan approval the property owner(s) of the Detention Basin site(s) shall execute a District
Maintenance Agreement for the urban BMPs and the detention basin or pond to insure the urban BMPs
and the detention area will be kept in working order, to the satisfaction of the District. The District will
not be responsible for maintenance of detention basins or BMPs.
Annual trustee or non-residential Property Owner’s certification and reporting of performance of required
maintenance, operation and repairs shall commence upon MSD Construction Approval of detention
facilities; final closeout of the subdivision or project SWPPP; or as otherwise specified. The annual
report will be required for those projects where the recorded Maintenance Agreement requires the
reporting directly or by reference included in the Agreement (see Sub-section 4.080.01 Item 1. regarding
need for BMP Maintenance Plan). The annual report shall be submitted to the Engineering Department,
Design Division, Development Review at 2350 Market St., St. Louis MO, 63103 (return receipt
requested).
4.090 Dam Permit Requirements
Dams with a height of thirty five (35) feet or greater will require approval from the Missouri Department
of Natural Resources.
4.100 Detention Report
1. A Detention Report shall be submitted.
2. The Detention Report shall contain a complete table of contents and a summary.
3. The Detention Report shall be signed, dated and sealed by the Missouri Professional Engineer
who is responsible for its preparation. If the report is prepared by another person, a note on the
cover shall state the preparer’s name and that he or she is under the direct supervision of the
Missouri Professional Engineer whose seal is shown.
4. The Detention Report shall be in a binder, not loose leaf.
5. The engineer shall submit two copies of the Detention Report.
6. A copy of the current “Checklist for Review of Storm Water Detention” shall be completed by
the design engineer and submitted with the reports. The checklist is shown on Exhibit 4-A
hereinafter, or on the website if updated in the future.
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EXHIBIT 4-A
Check List for Review of Stormwater Detention
The following check list is intended to provide guidance in reviewing detention designs. It is not intended to supersede any
criteria stated in the District’s “Rules and Regulations and Engineering Design Requirements for Sanitary Sewage and
Stormwater Drainage Facilities”, February 2006.
the District P-No
Project Name:
Location:
1. Check planning maps for downstream problems.
2. Check SSMIP report for downstream problems.
3. Detention required if differential runoff, per 4.080.01.2(b), is greater than 2 cfs, or if required by the
District due to flooding problems.
4. If differential runoff is greater than 5 cfs, use watershed release rates if applicable (see release rate
table, figure 4-5)
_______ 5. For building and parking lot additions, detention is required if there is existing detention. Check existing
calculations to see if addition is covered.
6. No reduction in outfall pipe size permitted because of detention.
7. Basin located at or near lowest point of site such that on-site runoff will be directed to basin.
8. Offsite flows bypassed around basin.
_______ 9. If site specific release rate is used (volumetric procedure), no increase in runoff volume during the critical
time period of the watershed.
10. Underground basin has adequate access for maintenance.
11. Provide means of visual inspection of both sides of low flow device from ground surface for
underground basin.
12. Underground basin has volume and spillway capacity to pass/contain 100-year, 24-hour event with low
flow blocked. It is further recommended that the control structure have at least 2 openings, one
above the other.
13. Check flow capacity of downstream pipe with inlet control nomographs found in Hydraulic Design of
Highway Culverts, U.S. Department of Transportation publication.
_______ 14. Check inlet control constants as entered in the analysis to match the Hydraulic Design of Highway
Culverts per US Department of Transportation.
15. Aluminized corrugated metal pipe (CMP) allowed for commercial projects only, gauge specified.
16. No underground basins allowed for residential projects, except by special municipal request, cooperation
and a technical lack of other options.
Revised 12/15/2014 79
17. TR-55 or similar SCS method used for hydrology; Type II rainfall distribution.
18. 2-year and 100-year, 24-hour rainfall amounts of 3.1” and 7.2”, respectively.
_______ 19. Legible Detention Basin Area maps showing flow paths used in time of concentration calculations and
CN values for existing and proposed conditions, 24” X 36” exhibits are preferred, (Existing
Conditions Map is not applicable to a “Fixed” Release Rate Analysis).
20. Elevations vs. Discharge table, including modeling data.
21. Elevation vs. Storage table.
22. Hydraulic gradeline calculations for incoming and outgoing pipes.
23. Provide a copy of the SCS soils map and label site.
24. Geotechnical report may be required for embankment.
25. Structural calculations may be required for control structure.
26. Details of control structure showing reinforcing.
_______ 27. Minimum of 3 cross sections through basin for as-built calculations tied to a baseline or known point or
to Property Line.
28. Incoming pipes should discharge at the toe of the slope in dry basins.
29. Earthen Pilot swale provided from incoming pipes to control structure.
30. Earthen Pilot swale is a minimum of 6 inches deep.
31. Details for permanent erosion control for earthen swales with slopes greater than 3%.
32. Minimum longitudinal slope of earthen swale is 2.0%, slope called out on plan.
33. Bottom of basin is sloped a minimum of 2% laterally towards earthen swale and called out on plan.
34. Rock blanket provided along outside of curved swale downstream of incoming pipe to prevent
erosion.
35. Concrete headwall provided around protruding low flow pipe.
36. Trash rack provided for low flow openings less than 6” wide.
37. No railroad tie walls within ponding area of basin.
38. Maximum fluctuation above permanent pool is 6’.
39. Maximum side slopes are 3:1 without fencing.
_______ 40. Dry basins and the fluctuating areas of lakes are to be appropriately vegetated to the maximum high
water elevation, call out on plan. Sod and mowing may be approved above that level and on dam slopes
(required, if not riprapped).
_______ 41. Control structures are to be reinforced concrete; no brick allowed; wall thickness is at least 8” w/one row
of steel or 10” w/two rows of steel. Underground basins, as appropriate. [Deleted by Amendment 5]
42. In basins with walls, provide access ramp.
Revised 12/15/2014 80
43. No wetland mitigation in detention basin.
44. Maximum depth of water in a dry basin is 8’ exceptions on a case-by-case basis.
45. Maximum depth of water in a parking lot is 8”, 12” for truck parking lots.
46. Maximum ponding elevation calculated with low flow blocked and water ponded to sill of overflow
structure.
_______ 47. Limits of maximum ponding are 30’ horizontally and 2’ vertically from a building; 10’ horizontally and 1’
vertically for parking lot detention.
48. Freeboard from top of berm to maximum ponding elevation is at least 1’.
49. Basin is located in common ground or easement dedicated to subdivision trustees.
50. Owners of the basin execute a District Maintenance Agreement.
51. Four foot high fence required if side slopes are steeper than 3:1.
52. Low elevation of basin is above 15-yr, 20-min hydraulic elevation of receiving system; or detain whole
storm and provide a backflow preventer (Tideflex) normally installed inside of the outfall structure.
53. Dams with a height of 35’ or greater require MDNR approval.
_______ 54. Hydraulic calculations showing 100-year flow is conveyed to basin; calculations at ditch sections, sills of
structures set above 100-yr elevation.
55. Smallest low flow opening is 3” diameter or 4” x 2” slot. However, special trashrack or opening protection
may be required.
56. Detention cannot cross watershed boundary.
57. Discharge pipe into wet basin shall be a minimum of 3’ above bottom, or flowline of pipe shall be no
higher than the normal pool elevation.
58. The report is sealed by a Missouri Professional Engineer.
59. The report has a Table of Contents, a summary and is bound.
_______ 60. The starting hydraulic grade line for all incoming pipes shall be the 100 year – 24 hour blocked low flow
water surface elevation, or an elevation approved by the District.
_______ 61. For some channel and wetlands work, a 404 and/or 401 permit may be required from the Corps and
MoDNR, respectively.
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TABLE 4-5 [See Amendment 4]
WATERSHED RELEASE RATES FOR DESIGN OF ROUTED DETENTION FACILITIES
FOR DEVELOPMENTS WITH A POST-DEVELOPMENT DIFFERENTIAL RUNOFF
GREATER THAN 5 CFS*
Note: *Differential runoff based on the 15-year, 20 minute storm. See “Rules and Regulations and
Engineering Design Requirements for Sanitary Sewage and Stormwater Drainage Facilities”, Chapter 4, Section
4.080.02. Differential runoff of 5 cfs or less, use “Zero Increase” for all watersheds and both routing frequencies.
**Differential runoff and detention routings should be computed from a completely undeveloped
condition for projects in the Harlem & Baden watersheds, Grand & Bates (March 9, 2001) sewershed, and other
sewersheds as may be directed by the District, in the City of St. Louis. System capacity has been found overcharged for
rainfall events as small as once every two (2) years. Physical limitations of the site may be considered in special cases.
**Projects which have any increase in differential runoff in the Harlem & Baden watersheds, Grand & Bates sewershed,
and other sewersheds as may be directed by the District will have stormwater management requirements as shown in
items A.) through D.) as follows:
WATERSHED FINAL ROUTED
RELEASE RATE
100-YEAR
24 HOUR STORM
FINAL ROUTED
RELEASE RATE
2-YEAR
24 HOUR STORM
VOLUMETRIC
METHOD
CRITICAL TIME
PERIOD OF
WATERSHED
Baden ** Zero Increase Zero Increase NA
Bonfils
(Cowmire)
1.0 cfs/acre 0.4 cfs/acre Hour 12.0-14.0
Bonhomme 1.8 cfs/acre 0.25 cfs/acre Hour 12.0-15.5
Caulks 1.4 cfs/acre 0.2 cfs/acre Hour 12.0-14.0
Coldwater Zero Increase Zero Increase NA
Creve Coeur 1.2 cfs/acre 0.13 cfs/acre Hour 12.0-17.2
Deer Zero Increase Zero Increase NA
Dunn 1.0 cfs/acre 0.4 cfs/acre Hour 12.0-13.0
Fee Fee 1.3 cfs/acre 0.15 cfs/acre Hour 12.0-14.1
Fenton Zero Increase Zero Increase NA
Fishpot 1.5 cfs/acre 0.3 cfs/acre Hour 12.0-14.7
Grand Glaize Zero Increase Zero Increase NA
Gravois Zero Increase Zero Increase NA
Harlem ** Zero Increase Zero Increase NA
Kiefer 2.2 cfs/acre 0.7 cfs/acre Hour 12.0-13.0
Maline Zero Increase Zero Increase NA
Martigney Zero Increase Zero Increase NA
Mattese Zero Increase Zero Increase NA
Mill 1.5 cfs/acre .13 cfs/acre Hour 11.5-15.5
River Des Peres Zero Increase Zero Increase NA
Spanish Lake 1.0 cfs/acre 0.37 cfs/acre Hour 11.0-16.0
University City Zero Increase Zero Increase NA
Watkins Zero Increase Zero Increase NA
Williams 0.7 cfs/acre 0.2 cfs/acre Hour 12.0-15.1
Yarnell 1.3 cfs/acre 0.3 cfs/acre Hour 12.0-14.0
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STORMWATER MANAGEMENT REQUIREMENTS FOR
HARLEM AND BADEN WATERSHEDS AND OTHER WATERSHEDS
AS DIRECTED BY THE DISTRICT
A.) For project differential increase 0 to < 2 cfs: Volume reduction type BMPs sized for 75% of the 90th %
rainfall 1.14” WQv calculation as shown in 4.080.02.2.b(1). In general, the minimum amount of tributary
impervious area to be treated by the BMPs shall be the acreage of added impervious area. The BMPs shall be
designed to capture 75% of the WQv from the total drainage area tributary to the BMP. The BMP shall be
designed to drain down completely in 2 days. The BMP overflow path shall be designed for the 100-year, 20-
minute storm. For an example, see MSD website BMP toolkit.
B.) For project differential increase 2 cfs or >: TR55 analysis for 2-year and 100-year events, with level pool
routing to meet release rates as specified in Table 4-5.
C.) In the case of new directly sewer connected roof area replacing existing pavement area, MSD will evaluate
these on a case-by-case basis.
D.) For all cases, where localized downstream flooding or localized downstream sewer surcharging occurs,
differential runoff calculations and detention routings shall be computed from a completely undeveloped
condition as directed by the District.
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