[Ord. No. 722 §1(26.010), 10-11-1999]
This Chapter provides minimum standards, controls and criteria
for stormwater management. The principal design consideration in this
Chapter is to minimize the harmful physical and economic effects of
erosion, sedimentation and flooding from stormwater runoff. This is
to be accomplished thorough the requirement of special measures to
mitigate erosion both during and after construction, the detention
and control discharge of the differential runoff from the development,
and well-designed stormwater conveyance system.
[Ord. No. 722 §1(26.020), 10-11-1999]
For the purpose of this Chapter, the following terms shall be
deemed to have the meaning indicated below.
DESIGN STORM EVENT
A storm of a specific duration expected to occur with a frequency
of once every ten (10) years.
DETENTION FACILITY
A surface water runoff storage facility that is normally
dry but is designed to hold (detain) surface water temporarily during
and immediately after a runoff event.
DIFFERENTIAL RUNOFF
The difference in peak flow rate of water anticipated to
shed from a parcel of land between the existing and improved surface
condition for each rainfall event.
FREEBOARD
The difference in elevation between the top of the detention
basin dam and the design surface water elevation.
MAJOR STORM EVENT
A storm of a specific duration expected to occur with a frequency
of once every one hundred (100) years.
RATIONAL METHOD
An empirical formula for calculating peak rates of runoff
resulting from rainfall.
RETENTION FACILITY
A surface water runoff storage facility always contains/retains
a substantial volume of water to serve recreational, aesthetic, water
supply or other functions. Surface water is temporarily stored above
the normal stage during and immediately after runoff events.
STORMWATER MANAGEMENT PLAN
The drawings, computation, data, proposed contours, reports,
etc., that identify how stormwater runoff is to be handled.
STORMWATER MANAGEMENT SYSTEM
All means, natural or manmade, used for conducting stormwater
runoff to, through or from a drainage area to the point of outlet.
STORMWATER RUNOFF
Water that results from precipitation which is not absorbed
by soil, evaporation into the atmosphere or entrapped by ground surface
depressions and vegetation.
TIME OF CONCENTRATION
An estimate of the time of surface water flow from the hydraulically
most remote part of the drainage area to the point in question.
TRIBUTARY AREA
All of the area that contributes stormwater runoff to a given
point.
[Ord. No. 722 §1(26.030), 10-11-1999]
A. General. Sediment and erosion control will be accomplished
by applying conservation practices that will reduce the potential
for damage from these hazards. Control practices use trapping, filtering
or diversion techniques to protect adjacent properties from land disturbance
activities.
B. MPDES Stormwater Permits. Effective January 1, 1992, construction
sites where the area to be disturbed is five (5) acres or more must
apply for a stormwater discharge permit from the Missouri Department
of Natural Resources.
Permit requirements are set forth in 10 CSR 20-6.200 of the
Missouri Clean Water Act.
C. When Controls Are Required. Standard vegetative and structural
practices, as specified below, that filter, divert or promote the
settlement of sediment particles from storm runoff shall be provided
in the following situations:
1. To prevent sediment-laden runoff from leaving disturbed areas.
2. To isolate disturbed areas from erosive surface runoff associated
with significant undisturbed areas.
3. To protect stormwater drainage conveyance systems at operable inlets.
D. Types Of Controls. Acceptable sediment and erosion controls
shall be either vegetative or structural as described below:
1. Vegetative practices.
a. Top soiling. Stockpiling of topsoil to enhance final
site stabilization with vegetation shall be done in such a manner
that natural drainage is not obstructed and no off-site sediment damage
results.
b. Seeding. Of the rate and type to produce dense vegetation.
d. Mulching. Is the application of grass, hay, wood
chips, wood fiber, straw, gravel or other suitable material to the
soil surface. Seeded and planted areas where slopes are steeper than
2:1 shall be stabilized with mulch.
2. Structural practices.
a. Construction entrance. Is a rock stabilization pad
located at a point of vehicular ingress and egress on a construction
site.
b. Straw bale barrier. Shall be placed on downslope
areas to intercept sediment or to reduce flow velocity. Straw bale
barriers shall not be constructed in streams or swells where there
is the possibility of washout.
c. Silt fence. Sediment shall be removed when it reaches
one-third (1/3) to one-half (½) the height of the fence.
d. Storm drain inlet protection. May consist of filter
fabric, sand bags, excavated gravel, straw bale, block and gravel
and any of the combination of the above.
e. Diversion swale or dike. These may be used to intercept
runoff and direct to a sediment control device around a disturbed
area or to an area where it can be safely released.
f. Sediment trap. Is a small storage or detention area
used to detain construction runoff long enough to allow the larger-sized
sediment particles to settle out before the runoff is released to
downstream areas.
g. Temporary sediment basin. Performs the same function
as a sediment trap, although it has a greater volume and is located
below disturbed areas generally greater than five (5) acres. They
shall be sized to provide a minimum volume of three thousand six hundred
(3,600) cubic feet per disturbed acre draining to the facility.
h. Temporary slope drain. May be plastic sheets, metal
or flexible pipe, stone, gutter, fiber mats, concrete or asphalt ditches
or half round pipe to carry runoff from one (1) elevation without
erosion of the slope.
i. Check dam. May be constructed of logs or stone across
a swale or drainage ditch to reduce the water's velocity and to trap
small amounts of sediment.
j. Level spreader. Is an excavated depression to convert
a concentrated flow to a sheet flow, allowing water to be released
at less levels.
E. Construction Plans. A contoured development map must clearly
indicate the pattern of surface water runoff, both upstream and downstream
of the development. The type of sediment and erosion control to be
utilized shall be clearly indicated. The City Engineer and/or Director
of Public Works shall have authority to require proper controls as
specified herein.
[Ord. No. 722 §1(26.040), 10-11-1999]
A. Purpose. A development's stormwater drainage system shall
be designed to:
1. Protect natural waterways.
2. Convey upstream and on-site stormwater runoff to a natural watercourse
or to a storm drainage facility.
3. Provide protection from the design storm event and address the major
storm so as to prevent major property damage and loss of life.
B. Rainfall Frequency. All facilities shall be designed to
carry a 10-year storm of a duration which produces the maximum peak
flow rate of stormwater runoff without inundation or surcharging.
All hydraulic structures shall be designed to allow for a 100-year
storm to pass through the subdivision without destroying or damaging
property or inundating dwellings. Design information which substantiates
both conditions shall be provided.
Refer to Figure "A", which is on file in City Hall, for rainfall
curves for 2-, 5-, 10-, 20- and 100-year frequencies.
C. Calculation Of Runoff.
1. The method of calculating and routing stormwater runoff shall be
a generally accepted stormwater practice. The drainage area shall
consider all on- and off-site lands contributing to the proposed development's
drainage system. Capacity for such facilities shall be based on the
maximum potential water shed development permitted by the zoning ordinance.
2. The Rational Method for calculating stormwater runoff may be used
for development up to two hundred (200) acres. The Rational Method
may be modified per MSD standards for developments greater than two
hundred (200) acres. Other common rainfall runoff calculation methods
that may be utilized include Graphical Peak Discharge, Tabular and
the Synthetic Unit Hydrograph.
3. The minimum percentage of imperviousness to be used in design shall
be based on the zoning district as shown in the following table:
|
DISTRICT
|
MINIMUM % IMPERVIOUS
|
---|
|
General Agriculture
|
45
|
|
"R-1" Single-Family
|
45
|
|
"R-lb" Single-Family
|
50
|
|
"R-2" Two-Family
|
60
|
|
"R-3" Multi-Family
|
70
|
|
"C-1" General Commercial
|
85
|
|
"C-2" Planned Shopping
|
90
|
|
"C-3" Central Business District
|
90
|
|
"I-1" Light Industrial
|
90
|
|
"I-2" Heavy Industrial
|
90
|
|
Parking, Streets, Roofs
|
100
|
4. Special uses, such as schools, churches, etc., shall have the differential
runoff computed and approved by the City Engineer (or Director of
Public Works).
5. Special circumstances may dictate that the developed impervious area
may differ from that shown in the above table. An example may be single-family
lots of larger than ten thousand (10,000) square feet. Calculations
prepared by a registered professional engineer may be submitted to
the City Engineer (or Director of Public Works) for his/her evaluation
to determine if a minimum impervious area which differs from that
shown in the above table may be utilized. Likewise, the City Engineer
(or Director of Public Works) may determine that the percent of impervious
area for a particular development differs from that shown in the above
table.
See Figure "B", which is on file in City Hall, for the runoff
factor to be utilized for various impervious conditions and rainfall
durations.
D. Open Channels. Open channels consist of swales, ditches
or depressions, both natural and manmade, that convey water. Channels
shall be protected from scour and erosion by providing a channel lining
adequate to sustain the velocity of the 10-year design storm. If velocities
in channels exceed five (5) feet per second during ten 10-year design
storms, then erosion control other than vegetation shall be provided
in channel construction. Channels shall have hydraulic capacity to
carry the 10-year design storm runoff within the channel bed and banks.
Out-of-bank flow may be permitted on land slopes parallel to the channel
where it can be shown that no erosion damage or serious property damage
will result. Channels function as part of the major drainage system
and shall be evaluated for the 100-year design storm to determine
the impacts of runoff on adjacent property. The channel's hydraulic
capacity shall be increased where adjustments to channel geometry
provide significant protection to adjacent properties during 100-year
event.
E. Closed Conduit Storm Sewers.
1. Pipe sizes for closed conduit flow shall be based on the 10-year
design storm runoff and minimum allowable velocities. The system shall
provide for the cleaning of sediment and other deposits by maintaining
a minimum velocity of two (2) fps during the 10-year storm.
2. Manning's Equation is the most common method of estimating the capacity
and flow resistance in conduits, although the Kutter, Hazen-Williams
and Darcy Weisbach formulas are also acceptable.
3. Closed conduit storm sewer systems shall convey the 10-year design
to a point of discharge by gravity or pressure flow. In pressure flow
conditions, the hydraulic grade line shall be calculated to reflect
losses in pipes and structures and shall not rise to an elevation
greater than the sewer structure tops during design storm.
4. Generally, gravity flow occurs where the capacity of pipe run exceeds
the design flow and the outfall point does not control discharge.
Storm sewer systems may be designed for pressure flow when the hydraulic
grade line is above the crown of the pipe. The decision to design
a pressure flow system may be based on aesthetics, the need to submerge
outfalls, economics, limitations associated with the reduced pipe
sizes or grade constraints in outfalling the system.
F. Inlets. Calculations shall be submitted to demonstrate the
capacity of all inlets, such calculations must consider the cross-slope
of the pavement, depth of water at the curb face, size of opening
and the longitudinal grade of street. Street inlets and inlets in
parking areas shall reduce the spread and depth of flow to acceptable
levels during the 10-year design storm. The acceptable level of flow
for a minor access or local access street would maintain an eight
(8) foot travel lane with a maximum one (1) inch depth. One (1) clear
ten (10) foot travel lane must be maintained for a collector street.
Two (2) clear ten (10) foot travel lanes mist be maintained for a
major street. Any area inundated by water ponding at an inlet during
the 10-year storm event shall be located within an easement or right-of-way.
The effects of the 100-year storm event shall also be analyzed to
insure no property damage or dangerous conditions grades may be designed
to permit a portion of flow to bypass the structure, however, calculations
for the downstream structure must consider the bypass.
1. Plans and calculations. A drainage map shall be
developed from a base reproduction of the site plan or grading plan.
The existing and proposed contours shall be shown, normally at two
(2) foot intervals, for the subject property, extending off-site one
hundred (100) feet or less, as determined by the department for proper
design of the proposed improvements. Contour intervals other than
the above shall be used as determined by the site topography. Only
USGS datum shall be used.
2. The location of existing and proposed property lines, street, sinkholes,
railroads, areas within the tract subject to inundation by stormwater
and other significant natural features, such as wooded areas and rock
formations etc., shall be included on the map. All existing and proposed
stormwater facilities, such as inlets, manholes, pipes, culverts,
bridges, channels, etc., and all existing and proposed improvements
required for proper design review, such as pavement, buildings, etc.,
shall be included on the map.
3. The runoff details shall be required showing individual flows for
each existing and proposed structure and cumulative flows in pipes
and gutters, including "Q" and area. The map shall show all bodies
of water, such as ponds or lakes (including surface area and elevation),
and all waterways (including their names or the names of creeks or
rivers they flow into).
4. Lots shall be laid out so as to provide positive drainage away from
all buildings. Individual lot drainage shall be shown and coordinated
with the drainage pattern for the area and designed so that runoff
from one (1) lot will not adversely affect an adjoining lot. All necessary
grading to direct stormwater runoff shall be located within a drainage
easement.
5. All computations, plans and specifications related to the implementation
of this Chapter must be prepared and sealed by a professional engineer
registered in the State of Missouri.
[Ord. No. 722 §1(26.050), 10-11-1999]
A. Maintenance. Each owner of the property being developed
has the responsibility and duty to properly operate and maintain any
stormwater management system which has not been accepted for maintenance
by the City. The responsibility of maintenance of the system in subdivision
projects shall remain with the developer until such time as the stormwater
management system escrow for such development has been released. Upon
release of escrow, the maintenance responsibility shall be vested
in the trustees of the subdivision by virtue of a trust indenture.
The indenture of trust shall clearly indicate resident responsibility
for maintenance. The responsibility for maintenance in single lot
development shall remain with the general contractor and owner until
final inspection of the development is approved and an occupancy permit
is issued. After occupancy, the maintenance of the management systems
shall be vested in the owner of the project. All such privately owned
and maintained systems shall be subject to periodic inspection by
the City Engineer (Director of Public Works or his/her representative).
B. Storage Capacity.
1. The rates (pre-developed and post-developed) of runoff shall be determined
for the 10-, 20- and 100-year rainfall frequencies. The minimum storm
duration shall be twenty (20) minutes.
2. Stormwater shall be detained on site or adjacent property under agreement and metered out at the rate of an undeveloped site for the above frequencies and minimum duration to prevent possible flooding and erosion downstream. Design criteria to establish this differential runoff rate shall be as provided in Section
515.040 "Design Criteria". Note that stormwater pipes shall be sized to carry the total developed tributary upstream water shed. No reduction in pipe size shall be permitted because of detention.
3. Detention basin volume will be based on routing each post-developed
runoff hydrograph through the detention facility while satisfying
the appropriate allowable release rate. The routing computation shall
be based on an application of the continuity principle. The discharge
rate shall be based on the maximum head conditions in the detention
facility.
C. When Required. The City retains the right to require detention
storage in all cases in which the proposed development will generate
excess runoff that adversely affects the carrying capacity of the
receiving watercourse and/or adversely affects adjoining property
owners. Therefore, every development shall have stormwater detention,
except as specified herein.
1. Off-site facility two (2) or more developments. If
two (2) or more developments, including that of the applicant, have
provided for a common system.
2. Off-site facility by City. If an off-site stormwater
management system has been either constructed or programmed or identified
for construction by the City and the applicant has agreed to contribute
to or participate in the construction thereof.
3. Other management techniques. Management techniques
other than detention facilities may be utilized by the development
provided the techniques proposed meet the intent of this Chapter and
provide a benefit to the watershed that equals or exceeds the benefit
that a detention facility would provide.
[Ord. No. 1264, 1-14-2019]
D. General Design Features.
1. Dry bottom basins. May be constructed to temporarily
detain the stormwater runoff so that the rate at which it is released
is the same as before development. The following features shall be
incorporated into the design of any detention basin:
Freeboard. Detention storage areas shall have
adequate capacity to contain the storage volume of tributary stormwater
runoff with at least two (2) feet of freeboard above the water surface.
Outlet control works. Outlet works shall be
designed to limit peak outflow rates from detention storage areas
to or below peak rates that would have occurred prior to the proposed
development.
Outlet works shall not include any mechanical components or
devices and shall function without requiring attendance or control
during operation unless specifically approved by the City Engineer
(or Director of Public Works).
Emergency overflow/spillway. Emergency structure
shall be provided to permit the safe passage of runoff generated in
excess of the design storm event. Anti-vortex measures shall be provided.
Maximum depth. The maximum planned depth of
stormwaters stored shall not normally exceed five (5) feet.
Side slopes. The maximum side slopes for grassed
basins shall not normally exceed one (1) foot vertical for three (3)
feet horizontal.
Limits of ponding. In no case shall the limits
of maximum ponding be closer than thirty (30) feet horizontally from
any basin and less than two (2) feet vertically below the lowest sill
elevation.
Interior drainage. The basin bottom should
be designed to drain expeditiously. Flows through the detention basin
should be handled by paved ditch from inflow structure to outflow
structure to minimize erosion.
Multi-purpose basins. If the detention basin
is to have other uses, the design of the basin bottom should include
underdrains to expedite drying of the bottom between runoff events.
Aesthetics. Designs should result in aesthetically
pleasing configurations which will enhance public acceptability.
2. Wet bottom basins. Wet bottom basins may also be
used to temporarily detain the differential runoff from the development.
In addition to the general design features enumerated above for dry
bottom basins, the following features should also be incorporated
into the design of any wet bottom basin:
Normal pool depth. In order to minimize weed
growth, the normal pool depth should be four (4) feet minimum.
Depth of fish. If fish are to be kept in the
pond, at least one-quarter (¼) of the permanent pool should
have a minimum depth of ten (10) feet.
Facilities for emptying. In order to ease
cleaning of the pond or shore line maintenance, the pond design should
include provisions for emptying the pond.
Low flow by-pass. The design of any pond may
include a low flow by-pass channel or pipeline to divert runoff that
can be accommodated by downstream drainageways.
Side slopes below normal pool. The side slopes
below the normal pool elevation may exceed the maximum side slope
permitted above normal pool (3:1 slope). The design shall, however,
include provisions for a safety ledge having a depth of water not
greater than three (3) feet immediately adjacent to the shoreline.
Forbay. In order to minimize siltation of
the ponding, a forbay should be included in the design.
3. Rooftop storage. Detention storage may be met in
total or in part by detention on roofs. Details of such design, which
shall be included in the building permit applications, shall include
the depth and volume of storage details of outlet devices and downdrains,
elevations of overflow provisions. Direct connection of roof drains
to sanitary sewers is prohibited.
4. Parking lot storage. Paved parking lots may be designed
to provide temporary detention storage of stormwater on all or a portion
of their surfaces. Outlets will be designed so as to slowly empty
the stored waters and depth of storage must be limited so as to prevent
damage to parked vehicles.
5. Other detention methods. All or a portion of the
detention storage may also be provided in underground or surface detention
facilities to include basins, tanks or swales, etc. Emergency overflow
conditions shall be considered in all methods.
E. Stormwater Management Improvement Fund. A Stormwater Management
Improvements Fund shall be and is hereby created. Said fund shall
be reserved for funding improvements to stormwater systems owned and
maintained by the City and for no other purposes unless authorized
in the ordinance.
[Ord. No. 1264, 1-14-2019]
F. Plans And Calculations. Topographic map outlining the limits
of the contributing watershed and site plan of suitable scale and
two (2) foot contour interval showing the land to be developed and
such adjoining land whose topography may affect the layout or drainage
patterns for the site.
1. The location of streams and other floodwater runoff channels, calculations
supporting the method and capacity needed for the safe and temporary
storage of increased runoff resulting from the proposed development,
if temporary storage is needed.
2. Basic information regarding the receiving watercourse into which
the proposed stormwater will discharge. This information should include
a general cross section and existing downstream culverts, bridges
and other waterway openings.
3. A layout of the proposed stormwater management system including the
location and size of all drainage structures, storm sewer, channels,
channel sections, detention basins and analysis regarding the effect
said improvements will have upon the receiving channel and its high
water elevation.
4. All computations, plans and specifications related to the implementation
of this Chapter must be prepared and sealed by a professional engineer
registered in the State of Missouri.
[Ord. No. 722 §1(26.060), 11-10-1999]
A. Storm
pipes shall be protected from excessive bearing pressures by placing
them outside the forty-five degree (45°) influence zone of building
structures unless an engineering calculation shows the pipe material
or soil condition to be adequate for the subjected load.
B. Pipes
on twenty percent (20%) slopes or greater shall be anchored securely
with concrete anchors or equal to prevent the pipe from creeping downhill.
C. Pipes
constructed on fill shall be stable and protected against settlement
by compacting fill material to ninety-five percent (95%) of the modified
proctor maximum dry density.
D. Pipes
thirty-six (36) inches or larger may be placed on a curve alignment
utilizing alignment radii established by pipe manufacturer.
E. The
receiving surface where pipes discharge shall be protected from erosion
by evaluating the discharge velocity for the 10-year design storm.
The use of energy-dissipating devices may be necessary to reduce the
velocity to acceptable levels for the receiving surface. Grouted revetment
used shall be a minimum length of ten (10) times the diameter of the
discharge pipe.
F. A manhole,
inlet or junction box shall be located at changes in pipe size, grade,
alignment or material.
G. The
angle between influent and effluent pipes shall be not less than ninety
degrees (90°) and the drop between inverts shall be not less than
one-tenth (0.1) foot.
H. Manhole
and inlet castings located in travelways shall be capable of withstanding
traffic loads and shall be constructed flush with the finished surface.
I. All
materials and appurtenances for stormwater management systems shall
conform to current standards in the American Society for Testing and
Material (ASTM).
J. Manholes
shall be pre-cast or cast-in-place concrete, brick, concrete block
with concrete or brick risers and approved manhole covers.
K. A new
drainage channel or pipe shall intersect an existing drainage channel
at a maximum angle of sixty degrees (60°).
L. All
trenches under roadway pavement shall be backfilled with MHTD Type
I aggregate in six (6) inch layers and compacted to ninety-five percent
(95%) of the modified proctor maximum dry density.
M. All
piping shall be bedded per the manufacturer's requirement.
N. Grated
inlets will not be allowed without special approval by the City Engineer
(or Director of Public Works).
O. All
materials used in the construction of storm sewers shall be subject
to inspection and approval of the City Engineer (or Director of Public
Works).
P. Acceptable
pipe material shall be reinforced concrete pipe, corrugated metal
pipe and corrugated polyethylene pipe. Reinforced concrete pipe shall
conform to the requirements of the Specifications for Reinforced Concrete
Culvert, Storm Drain and Sewer Pipe, ASTM C76. Strength class or classes
shall be as required per designed specifications of the latest editions
of the Concrete Pipe Handbook as published by the American Pipe Association.
Q. Corrugated
metal pipe shall conform to the requirements of AASHTO M36 "Standard
Specification for Zinc Coated (Galvanized) Pipe" or the "Standard
Specifications for AASHTO M196 Corrugated Aluminum Pipe". Structural
design requirements shall be per the latest edition of the Handbook
of Steel Drainage and Highway Construction Products as published by
the American Iron and Steel Institute.
R. The
City Engineer (or Director of Public Works) may consider for approval
a written request to use other types of pipe. Corrugated polyethylene
pipe shall conform to the requirements of AASHTO M294 "Standard Specifications
for Corrugated Polyethylene Pipe". All polyethylene pipe should be
installed according to ASTM D2321 "Standard Practice for Underground
Installation of Thermoplastic Pipe for Sewers and Other Gravity Flow
Applications".
S. All
construction details pertaining to stormwater drainage shall be in
accordance with the Metropolitan St. Louis Sewer District requirements
unless otherwise noted herein.