(A) If detention basins are necessary, they must be designed in accordance with the requirements of this chapter and currently accepted engineering design practices.
(B) Included in the design submission must be the following:
(1) Map showing site, its location within the watershed and the watershed limits. All detention facilities shall be analyzed with hydrograph and storage routing techniques;
(2) Examination must be made of the routing of storm water through the site and its flow path as it is discharged from the site. Sub-drainage basin limits within the watershed must be delineated;
(3) Designation of the location(s) of the proposed detention basin(s). Provide analysis for the storm water runoff from those areas tributary to the proposed detention facility. Existing and proposed conditions must be evaluated. Peak storms with return frequencies for the ten-, 25- and 100-year events shall be analyzed as follows.
(a) 1. For detention basins with drainage areas less than 200 acres, the engineer shall use the Rational Method for Detention Storage (a.k.a., the Modified Rational Method, or the Chicago Method) as described in the Illinois Department for Transportation Drainage Manual under Chapter 4, Hydrology, and Chapter 12, Detention Storage, and as further defined below.
a. Existing and proposed “C” factors for agricultural, pastures or wooded areas shall be 0.20 or less unless an extensive survey of soil types and land slopes is presented to verify that a higher value is justified.
b. Existing and proposed “C” factors for paved and building areas shall be 0.90 or more. This excludes aggregate surfaces which should be evaluated based on the condition and slope of the aggregate surface.
c. Existing and proposed “C” factors for lawns shall be 0.20 or more unless an extensive survey of soil types and land slopes is presented to verify that a lower value is justified.
d. For commercial developments, “C” factors for the proposed development shall be calculated by a full tabulation of the proposed surface areas in the development when known, or by calculating the road “C” factor within the right-of way and by calculating a composite “C” factor based on the maximum allowed coverage (pavement and building area) per lot for the development.
e. For residential developments, “C” factors for the proposed development shall be calculated by a composite section of the typical road and lot configuration showing the average amount of paved and building areas per lot, and a full tabulation of the park, other green spaces and lakes within the development.
2. Please note that the engineer may also use one of the unit hydrograph methods as described below for drainage areas under 200 acres if he or she can demonstrate that sub-watersheds have significantly non-homogeneous land use or significantly different sub-watershed timing that effect the proper design of said detention facilities.
(b) For detention basins with drainage areas greater than 200 acres, the engineer shall use a unit hydrograph routing method that is generally accepted to the Illinois Department of Natural Resources Office of Water Resources such as the Clark or TR-20 unit hydrograph methodologies inside computer programs such as HEC-1 or HEC-HMS or other computer programs accepted by the Illinois Department of Natural Resources Office of Water Resources, and as further defined below:
1. Rainfall data and distributions should be utilized as described in Bulletin 70 and Circular 173 as published by the State Water Survey. Enough durations of each required frequency must be run to determine the peak storm duration for each frequency.
2. Time of concentrations may be determined as described in either TR-55 or the IDOT Drainage Manual. Lag time if utilized may be assumed to be equal to 0.6 times the time of concentration.
3. The selected curve numbers or other runoff coefficients must be calibrated to surveys of existing flood frequencies in other local watersheds which have already been modeled. This information shall be provided to the city. If the engineer does not have calibrated curve numbers for a similar watershed, the city will provide curve numbers with such a basis.
(4) The detention basin may be designed to drain dry or to retain water for extended periods after rain events. All detention/retention basins must be properly maintained and kept free of algae growth, trash and other debris.
(5) The depth-area-volume relationship of the basin is dependent on the storage volume needed to reduce the peak inflow rate to some desired peak outflow rate. As the natural contours in the site dictate, ponding depths should be kept as shallow as possible. However, where storm water storage depths exceed four feet, a bench width of six feet should be provided around the entire perimeter of the basin (exclusive of bermed areas). Outlets from normally dry detention basins can take on a number of forms, limited only by safety and maintenance considerations. Outlet pipe must be at least 12 inches in diameter to facilitate maintenance. If less capacity is required than a 12-inch pipe would provide, flow should be throttled at the pipe entrance. Trash racks or perforated risers are required at inlets to help guard against plugging of the pipe.
(6) The side slope of the basin should be no steeper than four horizontal to one vertical (4:1) for the safety of mechanical mowing equipment and the safety of people during those times when water is being stored. All grades on the bottom of this basin should be at least 2% so that the bottom will drain quickly and leave no wet spots. A low flow concrete channel shall be provided in the basin.
(7) An emergency spillway outlet shall be provided for discharge of flows in the event the storage capacity is exceeded or the primary outlet is non-functional. The emergency spillway shall be designed for the 100-year storm frequency event assuming the basin is dry, or at normal pool for a wet basin, and the primary outlet is non-functional (plugged) at the start of the rainfall event.
(8) Easements should be included if necessary and access roads provided to allow vehicles and other equipment the access needed for maintenance of the basin.
(9) If detention is to be of the wet basin, (retention type), the volume needed for temporary storage must be provided above the normal pond elevation.
(10) The outlet structure of wet basins (retention type) must be constructed such that the pond level is maintained. The side slopes beneath the water surface of the pond should be two horizontal to one vertical (2:1), to a depth of three or four feet to discourage the growth of aquatic plants unless a wetland system is developed along the shoreline of the pond. If fish are to be stocked in the pond, at least 25% of the ponds area should be at least ten feet deep.
(11) An as-built survey of detention basin grades and outlets with an accompanying storage calculation will be required upon completion of the proposed detention basin. Any completed basin below 95% of the design storage shall be regraded and reseeded to obtain the original design storage.
(12) Detention dams which meet the criteria as a regulated dam as described in the most recent version of Rules for Construction and Maintenance of Dams by the Illinois Department of Natural Resources, Office of Water Resources, require an IDNR, Office of Water Resources, permit prior to city approval for construction
(Ord. 3319, passed 2-22-2005)