[Ord. No. 17-0244, 4-24-2017]
The stormwater design requirements specifies and authorizes
performance and design standards to reduce the potential detrimental
effects associated with urbanization of land. The applicant's engineer
shall select and design appropriate stormwater controls adequate to
meet these requirements.
[Ord. No. 17-0244, 4-24-2017]
A.
Designs for erosion and sediment controls shall meet the following
performance standards.
1.
Compliance with all standards imposed by Missouri Department
of Natural Resources Missouri State Operating Permit MO-R100 or MO-RAXXXX
or if the land disturbance area is within a valuable water resource
area, as determined by the Missouri Department of Natural Resources,
a Missouri State Operating Permit MO-R109 issued in compliance with
the Missouri Clean Water Law (Chapter 644, RSMo., as amended) and
Missouri and Federal regulations pursuant thereto.
[Ord. No. 20-0368, 8-28-2020]
2.
Compliance with the following additional standards stated herein.
a.
Stormwater shall be detained and released at a rate not to exceed
the release rate from the site under the existing (pre-developed)
conditions for the 2- and 100-year storm events. This will require
a larger volume of stormwater to be maintained on-site and released
at a slower rate for major frequency storms. Detention/retention basins
must be located in common ground that is not located in the floodway
area. Detention/retention basins must also contain an overflow structure
capable of passing a 100-year, 20-minute design storm.
[Ord. No. 22-0185, 3-16-2022]
b.
The minimum maintenance access to a detention/retention facility
shall be a ten (10) foot strip of common ground for maintenance. 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) and the depth is four
(4) feet or more. 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.
c.
For all low impact development subdivisions, open drainage swales
along the streets may be acceptable, if they are conducive to the
area, non-erosive and approved on the preliminary plat. Plans must
be approved by the Director of Public Works.
d.
No land disturbance activity shall result in the impounding
of surface water on property other than the permittee's unless the
permittee obtains easements or licenses for that purpose.
e.
Temporary discharges into receiving streams from any area undergoing
land disturbance activities shall not result in the accelerated erosion
of those streams' channels at the point of discharge.
3.
If temporary facilities and erosion and sediment controls installed
pursuant to approved plans fail to meet the performance standards
set out herein, the Director of Public Works may require the permittee
to submit modified plans.
[Ord. No. 17-0244, 4-24-2017]
A.
The erosion and sediment controls and temporary facilities shall
be designed to accommodate at a minimum the runoff for the design
storm specified below.
B.
If installed erosion and sediment controls, designed in accordance
with these design storm and coefficients, fail to meet its performance
standards above, the permittee shall be required to correct the deficiency
in question.
1.
Design Storm. Designs for erosion and sediment controls and
temporary facilities constructed during land disturbance activities
shall be based on the design storms shown in Exhibit 1.
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Exhibit 1 Design Storm
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|---|---|
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Stormwater and Erosion and Sediment Control
|
Design Storm Detention
|
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Stormwater conveyance systems — permanent
On-site drainage ditches and diversions
Storm sewers and structures
Entrance road culvert
* Storm sewers shall flow at 80% capacity
|
15-year/20 minute
15-year/20 minute
15-year/20 minute
|
|
Storm inlet sediment protection and temporary facilities
|
2 cfs/drainage area acre
|
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Sediment basin:
Basin size
Basin overflow
|
3,600 cubic feet/acre of disturbed area
2 cfs/drainage area acre
|
2.
Peak Runoff Rate Calculation Method. The Rational Method, as
developed by Mulvaney in 1851, shall be used to determine the peak
(maximum) runoff rate. The Rational Method (also known as the Rational
Formula) is:
|
Q = API
|
|
Where:
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|
Q
|
=
|
peak runoff rate in cubic feet per second (cfs)
| |
|
A
|
=
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drainage area in acres
| |
|
P
|
=
|
runoff factor based on runoff from pervious and impervious surfaces
| |
|
I
|
=
|
average intensity of rainfall (inches per hour) for a given
period and a given frequency
| |
b.
Land disturbance areas shall be considered stripped of all vegetation
in determining runoff for erosion and sediment controls.
c.
After cut and fill operations are completed, land disturbance
areas shall be considered stripped of all vegetation and pavement
installed in determining sediment controls, runoff conveyance systems
and erosion prevention devices.
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*
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Drainage areas may be broken into component areas, with the
appropriate runoff factor applied to each component, i.e., a proposed
development may show one hundred percent (100%) for impervious for
paved areas and five percent (5%) impervious for grassed areas. The
runoff coefficients shall be determined for each drainage area to
design erosion and sediment control and temporary facilities based
on the following criteria.
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