[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; Ord. No. 20-0368, 8-28-2020; Ord. No. 22-0185, 3-16-2022; Ord. No. 24-0371, 10-15-2024]
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 MOR109 issued in compliance with the Missouri Clean Water Law (Chapter 644, RSMo., as amended) and Missouri and Federal regulations pursuant thereto.
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 24-hour 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.
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.
Exhibit 1 Design Storm
Stormwater and Erosion and Sediment Control
Design Storm Detention
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
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:
Q
=
peak runoff rate in cubic feet per second (cfs)
A
=
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
a. 
P.I. values for various impervious conditions are shown in Tables 1-1 through 1-3.[1]
[1]
Editor's Note: Tables 1-1 through 1-3 are included as attachments to this Chapter.
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.
*
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.