A. General Information:
1. The purpose of this section is to explain the city's policy regarding the ownership, design, construction, and maintenance responsibility for detention basins. Detention basins are used to collect and hold stormwater runoff for a period of time to compensate for increases in stormwater runoff caused by reduced ground surface perviousness due to activities such as paving or building construction.
B. Design:
1. General:
a. Use section 15.4.4.4, "Hydrologic Design Standards", of this chapter, to determine design storm and hydrologic method of determining peak inflows - 10-year design storm of most critical duration (minimum 60 minute storm) as determined by the city engineer.
b. Detention basin sizing shall follow the methodology in "IDOT Drainage Manual", Pond Pack™ software, Hydroflow Hydrographs™ software, or AutoCAD Civil 3D™ software.
c. Outflow shall be limited to the calculated rate of runoff for the sites existing topography as if it was entirely covered with grass; assuming normal soil conditions, reasonable grass coverage, and normal ground saturations for a 5-year design storm of the most critical duration (minimum 60 minute storm) as determined by the city engineer.
d. Parking lot and other such surfaces shall not be permitted to be used for stormwater detention.
e. Plans shall clearly indicate normal and high water elevations, design storage volume, minimum, maximum and typical slopes.
f. Detention basins shall release the 10-year storage volume within twenty four (24) to forty eight (48) hours after the end of the rainfall event.
g. Emergency overflow routes shall be clearly designated for runoff in excess of the 10-year storm.
h. Minimum outlet storm sewer size shall be twelve inches (12"); smaller diameter restrictions (e.g., orifice plate or short pipe length) are acceptable.
i. Inlet and outlet pipes shall be provided with concrete end sections and erosion protection.
j. Detention ponds and their appurtenances (i.e., storm sewer systems, weirs, dams, orifices, and specialized outflows) shall be designed by a professional engineer registered in the state of Illinois.
k. "Bubble up" outlets are prohibited.
l. Pumped outlets and other active control structures are discouraged and must be preapproved on a case by case basis by the city engineer.
m. Temporary erosion techniques shall be used as required to ensure a full stand of cover vegetation in minimum time.
n. Detention basin side slopes above normal pool shall be designed with permanent erosion protection consisting of grass, nongrass vegetation, or other permanent finish. Permanent erosion protection shall be aesthetically suitable to the development or existing surrounding land use.
2. Dry Bottom Basins:
a. Side slopes shall be a maximum of 4H:1V. If retaining walls are used, their height is limited to four feet (4'). Retaining wall design and material type shall be approved by the city engineer.
b. Dry bottom basins shall have one percent (1%) minimum bottom slopes or underdrain systems as approved by the city engineer.
c. Dry bottom basins shall include a low flow channel with some form of erosion protection, for example a concrete "V" gutter.
3. Wet Bottom Basins:
a. Above water side slopes shall be a maximum of 4H:1V. If retaining walls are used, their height is limited to four feet (4'). Retaining wall design and material type shall be approved by the city engineer.
b. In accordance with IDOT standard specifications section 204, below water slopes shall be a maximum of 4H:1V except that slopes of 2H:1V will be permitted below a point where the proposed water depth will be eight feet (8') or greater.
c. Wet bottom basins shall have a natural or artificial means of aeration.
d. If fish or other aquatic wildlife are desired, a minimum depth of eight feet (8') shall be maintained over at least twenty five percent (25%) of the pond's surface area.
e. So called "safety shelves" are not required as they promote the growth of algae and may cause entrapment.
f. To minimize erosion caused by wave action, shoreline stabilization shall be required around all wet bottom detention basins. Shoreline stabilization shall extend down the side slope to an elevation one foot (1') below the normal water surface elevation and up the side slope to an elevation one foot (1') above the normal water surface elevation. Approved shoreline stabilization methods include riprap revetment constructed in accordance with the requirements of the Illinois department of transportation "Standard Specifications For Road And Bridge Construction", latest edition; cast in place portland cement concrete retaining walls; or modular concrete block retaining walls. Alternative shoreline stabilization methods may be submitted to the city engineer for review and approval. Any proposed shoreline stabilization method shall be reviewed and approved by the city engineer prior to installation.
g. An outlet structure shall be provided to allow dewatering of the pond for maintenance. Gravity dewatering is strongly preferred.
h. Wet bottom basin design shall include an evaluation of soil permeability. A basin liner shall be included in the design if needed to ensure water retention to normal pool elevation.
4. Underground Detention Systems:
a. Underground systems may be permitted subject to the approval of the city engineer.
b. All underground systems shall be designed so that inspection and cleaning of these systems is easily accomplished.
c. Normal and accepted engineering practices shall be used for the design and construction of underground detention systems.
C. Diagrams And Checklists:
1. Basin plans and specifications for review and approval.
2. Design calculations for basin hydrology and sizing.
a. Hydrologic calculations shall include areas tributary to the basin and their land use types.
b. Basin stage-storage-discharge tables shall be included in the submittal.
c. Emergency overflow routes shall be included in the submittal.
d. Inflow and outflow hydrographs both, for both pre- and post- conditions, shall be included in the submittal.
e. A statement of engineering assumptions (i.e., soil conditions, grass coverage percentages, impervious area percentages, soil saturations, design storms, etc.) shall be included in the submittal. (Ord. 2013-20)