(A) Dry bottom stormwater storage facilities should be designed where possible to serve a secondary purpose for recreation, open space, or similar types of uses which will not be adversely affected by occasional intermittent flooding and will not interfere with stormwater management.
(B) (1) Minimum grades for turf areas within the basin shall be 2%, except that the minimum grade shall be 0.5% if tile underdrains are adequately installed underneath turf areas.
(2) Storage facility side slopes shall not be steeper than three to one and shall provide for safe maintenance practices.
(3) Storage basin excavations shall follow the natural land contours as closely as practicable. The geometry of dry bottom stormwater storage basins shall be authorized by the Village Engineer.
(C) (1) The outlet control structure shall be provided with an interceptor for trash and debris, and it shall be designed and constructed to minimize soil erosion and not require manual adjustments for proper operation. The control structure shall be designed to operate properly with minimal maintenance or attention. The control structure shall be provided with safety screens for any pipe or opening, and shall be constructed to allow access to it at all times, including times of flood flow.
(2) Stormwater storage facilities that receive runoff from tributary areas outside the development site that are utilized primarily for agricultural purposes shall be provided with permanent sediment trapping basins upstream from the stormwater storage area and a screened outlet structure to prevent clogging from crop residuals.
(D) Subsurface low flow conduits shall be provided in dry bottom stormwater storage basins. These conduits shall be constructed to reduce the frequency of time that the storage area will be covered with water and shall facilitate dewatering of the soils in the stormwater storage area to avoid saturated soil conditions. Low flow conduits shall be designed to intercept trickle flows from storm sewers and other drainage facilities which outlet into the basin. Low flow conduits shall not be less than eight inches in diameter. Tile underdrain systems may be combined with the low flow conduits.
(E) Pipe outlets of less than eight inches in diameter shall not be allowed. Multiple outlet pipes from a stormwater storage area shall be avoided if they are designed to be less than 12 inches in diameter.
(F) Seeding, silt fencing, straw bale dikes, or other soil stabilization measures shall be established in the stormwater storage area immediately following construction. During construction of the overall development, sediment may buildup in the storage area due to erosion. All sediment shall be removed before construction is considered completed.
(G) Permanent erosion control measures such as mulching, hydroseeding, conventional seeding, nurse crops, fertilizing, or sod installation shall be utilized to control soil movement and erosion within the storage area. The subdivider is referred to § 153.099. The installation of these permanent measures shall take place only after the majority of construction and other silt and sediment producing activities have been completed.
(H) Adequate impact stilling basins shall be provided at the downstream side of any outlet structure to ensure that downstream soil erosion is mitigated and the regime of the downstream drainage facility is not disturbed.
(I) The maximum planned depth of stormwater stored shall not exceed six feet.
(J) Tile underdrains shall be provided for all detention basins serving a watershed area of more than five acres, and for all detention basins located in soils with groundwater elevations of less than three feet below ground surface. Tile underdrains shall be constructed so that they shall facilitate dewatering of the soils in the stormwater storage area to avoid marshy or saturated soil conditions, and may be combined with the low flow conduits.
(Ord. 97-7, passed 7-1-1997, § 5.19)