(A) Definitions. For the purpose of this section the following definitions shall apply unless the context clearly indicates or requires a different meaning.
DETENTION BASINS. Dry surface storage areas created by constructing an excavated or embankment basin.
DETENTION/RETENTION OF STORM WATER. Storage of excess run-off on the site of a development and gradual release of the storm run-off at an acceptable rate.
RETENTION BASINS. Permanent ponds where additional storage capacity is provided above the normal water level.
(B) Objective. The objective of a detention/ retention facility is to regulate the run-off from a rainfall and to control discharges to downstream areas in order to reduce the impact on downstream drainage systems.
(C) Requirement. Detention/retention of storm water will be required for each subdivision or land development unless specifically exempted.
(D) Exemptions to detention/retention requirements. The developer may apply to the village for exemption from construction of detention facilities. Each request will be reviewed on its own merit and as it affects the entire drainage area in which it lies and into which it flows.
(E) Design.
(1) Run-off and volume calculation methods. The methods outlined in "Ohio Supplement to Urban Hydrology for Small Watersheds: Technical Release No. 55 Engineering Division, Soil Conservation Service, (SCS), U.S. Department of Agriculture," will be used to determine run-off.
(2) Dams in excess of ten feet are subject to Ohio Department of Natural Resources (ODNR) review.
(3) In some instances, as determined by the Village Engineer, a hydrograph method may be necessary to predict the length of time of high discharge from a detention/retention basin. This determination will be made at the preliminary plan stage. It is the developer's responsibility to obtain this determination prior to beginning design procedures.
(4) Allowable run-off. The volume and peak rate of run-off from an area after full development shall not exceed the volume and peak rate of run-off from the same area before development for all 24-hour storms with frequencies of 5, 10, 25, 50, and 100 years.
(5) For those areas where a study of the downstream area indicates the extended time of high discharge or velocity due to restricted release rate and storage may cause flooding or excessive erosion, the Village Engineer in cooperation with the Hamilton County Soil and Water Conservation District may require additional controls.
(6) Basin construction.
(a) The side slopes of a detention/ retention basin shall not exceed 4:1 and shall be seeded or sodded.
(b) The bottom of the basin shall be seeded or sodded and sloped to the outlet flow control device. A method of carrying low flow through the basin shall be provided and include appropriate erosion control.
(c) The maximum water depth for detention basins shall be six feet, unless otherwise approved by the village.
(d) The top of the embankment shall have a minimum width of eight feet.
(e) Outlet flow control devices may be either single-stage or multi-stage, with provision for emergency overflow.
(f) Other requirements may be imposed for specific cases.
(g) The basin shall be provided with vehicular access for service vehicles.
(h) Public easements shall be granted for each basin and access road, on all single-family developments.
(i) Operation and maintenance of basins in commercial and industrial zones shall be the responsibility of the property owner. Private drainage easements will be provided specifying long-term maintenance responsibilities.
(F) Submission requirements. Plans and supporting data to verify storage volumes, release rates, and the like shall be submitted. The submission shall include, but is not limited to, the following:
(1) A plan, which may be the Improvement Plan, Drainage and Grading Plan, or similar plan at a scale of one inch equaling 100 feet or larger, shall be submitted and contain at least the following information:
(a) The outline and designation of the drainage area.
(b) All existing and proposed drainage facilities.
(c) Existing and proposed contours (two foot minimum interval).
(d) Existing structures.
(e) Downstream restrictions (may be on a separate plan).
(f) The detention/retention basin with outlet structures, including emergency overflow features.
(g) Pertinent elevations such as water surface, flowline of flow control devices, and the like.
(h) Any other information required by the village to clarify intent or design features.
(2) All calculations and other supporting data in sufficient detail and form to facilitate an expedient and accurate review.
(Ord. C-604, passed 9-8-86)