[Ord. No. 662 §1, 4-3-2006]
A. 
The requirement of stormwater detention shall be evaluated for all projects submitted to the City and/or City Engineer for review and approval and may be required, if deemed necessary. Detention facilities shall be provided and designated in accordance with the requirements of this Section.
B. 
Developments that have a differential runoff of less than two (2) cfs for the 15-year, twenty (20) minute event and developments with three (3) acre or larger lot sizes shall be exempt from detention requirements unless there are known stormwater problems or special conditions downstream from a project.
C. 
If there are known stormwater problems or special conditions downstream from a project, detention shall be required.
D. 
Detention requirements shall be at the discretion of the City Engineer for projects that have a differential runoff of two (2) cfs to five (5) cfs for the 15-year, twenty (20) minute event. Projects that have a differential runoff of greater than five (5) cfs for the 15-year, twenty (20) minute event shall be required to provide detention per the following requirements:
1. 
For developments located entirely within one (1) watershed the stormwater detention requirements shall be based on the entire platted acreage of the development, including future phases. Reduction in the detention requirements for developments with several subwatersheds shall not be permitted. Detention requirements for developments located in two (2) or more major watersheds shall be at the discretion of the City Engineer.
2. 
The post-developed peak flow from the development may not exceed the existing peak flow for the 2-year, twenty-four (24) hour event and the 10-year, twenty-four (24) hour event for developments larger than ten (10) acres. For developments less than ten (10) acres, the post-developed peak flow from the development may not exceed the existing peak flow for the 2-year, twenty (20) minute event and the 15-year, twenty (20) minute event. The City and/or the City Engineer, at his/her discretion, can create stricter detention standards for watershed that are known to have stormwater management problems. The design engineer shall be made known of any stricter standards during the preliminary plat process or during the preliminary site improvement plan process for single lot developments.
3. 
The existing and post-developed peak flows shall be determined using Technical Release 55 (TR-55) for developments ten (10) acres or greater in size. All assumptions that are required for the TR-55 method shall be approved, in writing, by the City and/or the City Engineer, prior to submitting construction plans to the City for review and approval. For developments less than ten (10) acres, flow rates shall be determined using the method set forth in Section 50.30.1 "Flow Quantities" of the St. Charles County Highway Department Design Criteria for the Preparation of Improvement Plans dated February 2002.
E. 
When existing detention 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 detention requirements for the drainage area that is a tributary to the facility. Projects that cannot meet this requirement due to physical constraints shall be evaluated on a case-by-case basis.
F. 
All variances to the stormwater detention requirement must be approved in writing during the preliminary plat process or the preliminary site improvement plan process for single lot development.
[Ord. No. 662 §1, 4-3-2006]
A. 
For rates of runoff (pre-developed and post-developed) for developments larger than ten (10) acres, the 2-year, 10-year and 100-year, twenty-four (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 distributed shall be Type II.
B. 
The rates of runoff (pre-developed and post-developed) for developments less than ten (10) acres shall be determined by the Rational Method for the twenty (20) minute rainfall intensity. The 2-year, 15-year and 100-year, twenty (20) minute inflow hydrographs shall be determined as directed in Figure 50-5 of the St. Charles County Highway Department Design Criteria for the Preparation of Improvement Plans dated February 2002.
C. 
For developments larger than ten (10) acres, stormwater shall be detained on-site or off-site, as approved, and released at a rate not to exceed the release rate from the development under pre-developed conditions for the 2-year and 10-year, twenty-four (24) hour events. Note that stormwater pipes, downstream from the control structure, shall be sized to carry the runoff from the 15-year, twenty (20) minute design storm for the total tributary upstream watershed. No reduction in outfall pipe size shall be permitted because of detention.
D. 
For developments less than ten (10) acres, stormwater shall be detained on-site or off-site, as approved, and released at a rate not to exceed the release rate from the development under pre-developed conditions for the 2-year and 15-year, twenty (20) minute events. Note that stormwater pipes, downstream from the control structure, shall be sized to carry the runoff from the 15-year, twenty (20) minute design storm for the total tributary upstream watershed. No reduction in outfall pipe size shall be permitted because of detention.
E. 
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 design 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 shall be directed into the detention facility.
Flows from off-site upstream areas shall 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 off-site flows are approved by the City and/or the City Engineer to be directed into a detention facility, the design engineer must provide an analysis showing that the detention basin will adequately release stormwater under both existing and future developed off-site conditions. Modifying the release rate to accommodate off-site 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.
F. 
Detention basin volume shall be based on routing the post-developed 2-year, 10-year and 100-year, twenty-four (24) hour inflow hydrographs for developments larger than ten (10) acres or the 2-year, 15-year and 100-year, twenty (20) minute inflow hydrographs for developments less than ten (10) acres 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).
G. 
Design Of Underground Basins.
1. 
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 also shall be provided to allow for cleaning of the low-flow device from the ground surface.
2. 
The basin should be of sufficient volume and spillway capacity to pass/contain the 100-year, twenty-four (24) hour event.
H. 
The design engineer must submit the following for review of a detention facility:
1. 
Pre-development and post-development drainage area maps. The drainage area maps along with calculations for pre-development and post development shall be used to determine the differential runoff for each basin within the development.
2. 
Watershed specific time of concentration for each detention basin.
3. 
Elevation versus discharge tables of curves for all design frequencies.
4. 
Elevation versus storage tables of curves for all design frequencies.
5. 
Inflow calculations and data for all frequencies.
6. 
Maximum ponding elevation with the low-flow blocked for the 100-year, twenty-four (24) hour event if the development is larger than ten (10) acres, or the 100-year, twenty (20) minute event if the development is less than ten (10) acres.
7. 
Hydraulic gradeline computations for pipes entering and leaving the basin for all frequencies.
8. 
If the embankment contains fill material, a geotechnical report may be required.
9. 
Site plan showing appropriate design information.
10. 
Structural calculations for the outlet control structures. Structural calculations shall be provided for cast in place structures. The calculations shall be certified by a registered Missouri professional engineer. Shop drawings shall be provided for precast structures prior to installation.
11. 
Cross sections defining size, shape and depth of the detention basin shall be required. At a minimum, three (3) sections, one (1) at each end and one (1) in the middle of the basin shall be required. These sections shall be used to compute the as-built volume of the basin and thus must be tied to a known physical structure or baseline.
I. 
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 paved swale. The paved swale shall be non-reinforced concrete, six (6) inches thick, with a minimum two percent (2%) slope to the center and a minimum two-tenths percent (0.2%) longitudinal slope. Paved swales shall be a minimum of six (6) inches deep and four (4) feet wide or one and three-tenths (1.3) times the diameter of the pipe entering the basin, whichever is greater, and be keyed to a structure or channel. The bottom of the basin shall be sloped a minimum of two percent (2%) towards the concrete swale.
J. 
Railroad tie walls cannot be used where water will be in contact with the railroad tie wall.
K. 
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 three (3) feet.
L. 
The maximum side slopes for dry basins or ponds and the fluctuating area of permanent ponds or lakes shall be three to one (3:1) (three (3) feet horizontal, one (1) foot vertical) without fencing. A fencing detail shall be provided on plans if side slopes require fencing.
M. 
Dry basins or ponds and the fluctuating areas of permanent ponds or lakes are to be lined with placed revetment, sod or commercial erosion control blanket and kept mowed. Type of material proposed shall be noted on plans.
N. 
Control structures and overflow structures are to be reinforced concrete.
O. 
The outflow pipe shall be sized for the developed flow rate.
P. 
An emergency overflow is required for all detention basins. The emergency overflow shall pass the 100-year, 24-hour storm event for developments larger than ten (10) acres or pass the 100-year, 20-minute storm event for developments less than ten (10) acres.
Q. 
In basins with concrete walls or rock blanket covered slopes, the bottoms should be paved or provisions should be made for mowing equipment to reach the bottom (ramps, etc.).
R. 
Retaining walls and any required safety features must be designed and constructed as regulated by the Foristell Building Department.
[Ord. No. 662 §1, 4-3-2006]
A. 
The maximum depth of water in a dry detention basin or pond shall not exceed six (6) feet. Projects that need a deeper basin to attain the required detention volume due to physical constraints may be evaluated on a case-by-base basis. The design and construction of dams greater than eight (8) feet or as directed by the City and/or City Engineer 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.
[Ord. No. 662 §1, 4-3-2006]
A. 
The maximum ponding elevation shall be calculated based on a routing of the 100-year, twenty-four (24) hour design storm for developments greater than ten (10) acres and the 100-year, twenty (20) minute design storm for developments less than ten (10) acres.
B. 
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. Lowest sill building elevations for lots adjacent to the detention basin shall be provided on the site improvement plans.
C. 
The limits of maximum ponding in parking lots shall not be closer than ten (10) feet horizontally from any building and not less than one (1) foot vertically below the lowest sill elevation of any building. Lowest sill building elevations shall be provided.
D. 
A minimum of two (2) feet of freeboard shall be provided from the top of the basin to the maximum ponding elevation.
[Ord. No. 662 §1, 4-3-2006]
The detention basin, access roads or paths, control structure and outfall pipes are to be located in common ground dedicated to the subdivision trustees.
[Ord. No. 662 §1, 4-3-2006]
The subdivision trust indentures shall provide for maintenance responsibility, operation, ownership and funding for the basin(s). Maintenance, operation, ownership and funding for detention basins on privately owned sites shall be the responsibility of the owner. The City of Foristell shall not be responsible for maintenance, operation and ownership of the detention basins, access roads or paths, control structures and outfall pipes.
[Ord. No. 662 §1, 4-3-2006]
The low elevation of the detention basin shall be above the 15-year, twenty (20) minute hydraulic elevation of the receiving channel or pipe system.
[Ord. No. 662 §1, 4-3-2006]
Dams with a height of thirty-five (35) feet or greater require approval from the Missouri Department of Natural Resources.