Basins shall be constructed to detain temporarily the stormwater runoff that exceeds the maximum peak flow rate authorized by this subchapter. The volume of storage provided in these basins shall be sufficient to control excess runoff from the 100-year storm.
(A) For on-site watershed, the 100-year, 24-hour hydrograph for the tributary drainage area, representing fully developed conditions of the watershed, and corresponding runoff coefficients and travel times, shall be routed through the stormwater basin.
(B) Runoff for upstream off-site development will flow through the site, but will not be stored. The 100-year run-off under existing upstream conditions shall be determined, and this shall be used in sizing the emergency spillway for the pond.
(C) The routing shall determine the maximum 100-year pool elevation; the maximum routed discharge, and the maximum pond storage requirement for the design storm. The maximum routed discharge shall be equal to or less than the ten-year frequency peak discharge from the watershed under pre-developed conditions.
(D) The ten-year 24-hour hydrograph for the fully developed watershed must be routed through the stormwater pond and have a routed peak discharge rate not to exceed the pre-developed two-year peak discharge rate.
(E) The lip of the emergency spillway shall be established at or above the routed 100-year pool elevation. The pond embankment shall have a minimum freeboard of two feet between the maximum routed 100-year pool elevation and the top of the facility embankment.
(F) The emergency spillway shall be designed by routing the 100-year, 24-hour hydrograph through the pond assuming that the primary spillway is not operating. This storm should be routed through the basin with the maximum pool level below the top of the embankment.
(G) All stormwater storage facilities shall be separated by a minimum of 50 feet from any primary building or structure to be occupied. A 20-feet wide easement around the perimeter of the pond, measured from the top of bank shall be required. Off-site easements may be required by the city, based on the location of primary and emergency pond spillways, and their proximity to adjacent property.
(H) Outlet control structures shall be designed to operate as simply as possible and shall require little or no maintenance and/or attention for proper operation. They shall limit discharges into existing or planned downstream channels or conduits so as not to exceed the predetermined maximum authorized peak flow rate. This facility should be designed to allow easy access from the embankment to remove trash and debris from the outlet structure.
(I) Emergency overflow facilities such as a weir or spillway shall be provided for the release of exceptional storm runoffs or in emergency conditions should the primary spillway become completely or partially inoperative. The overflow facility shall be of such design that its operation is automatic and does not require manual attention. All stormwater storage facilities shall be provided with an emergency spillway.
(J) Sod or other vegetative cover acceptable to the City Engineer shall be provided throughout the entire basin area. Grass should be cut regularly at approximately monthly intervals during the growing season or as required unless designated as a wildflower or prairie area.
(K) Debris and trash removal and other necessary maintenance shall be performed regularly by the designated body (e.g., developer or homeowner’s association) to assure continued operation in conformance to design.
(L) A report shall be submitted to the Office of the City Engineer describing the proposed development, the current land-use conditions, the method of hydraulic and hydrologic analysis used (including any assumptions or special conditions), the results of the analysis, and the recommended drainage control facilities. Hydraulic and hydrologic calculations, including input and output files, shall be included as appendices to the report.
(M) Drainage easement. All stormwater storage facility should be located in a drainage/utility easement so that the city can access the facility under emergency conditions.
(Ord. 2-2010, passed 3-8-10)