The following shall govern the design of any improvement with respect to the temporary storage of stormwater runoff:
(A) Acceptable storage methods. Increased stormwater runoff resulting from a proposed development should be detained on-site by the provision of appropriate stormwater storage ponds. Storage of stormwater shall not be allowed to occur within any swales unless such storage is part of a plan approved by the Planning Director and Superintendent of Stormwater.
(B) Design storm. Design of stormwater storage facilities shall be based on a return period of once in 100 years. The storage volume and outflow rate shall be sufficient to handle stormwater runoff from a critical duration storm, as defined in this subchapter.
(C) Allowable release rate.
(1) The allowable release rate of stormwater originating from a proposed development shall not exceed a two-year pre-developed rainfall event rate for a ten-year post-developed storm, and as described by the determination of storage volumes through hydrograph methods. In any case, the system must assure adequate positive outfall to a recognized waterway or drainage facility.
(2) In the event that the natural downstream channel of the storm sewer system is inadequate to accommodate the release rate provided above, then one of the following alternative solutions must be selected:
(a) The allowable release rate shall be reduced to that rate permitted by the capacity of the receiving downstream channel or storm sewer system, and additional storage volume determined by the Board shall be required to store that portion of the runoff exceeding the capacity of the receiving sewers or waterways.
(b) The developer must correct the downstream restriction.
(3) If more than one stormwater basin is involved in the development of the area upstream of the limiting restriction, the allowable release rate from any one basin shall be in direct proportion to the ratio of its drainage area to the drainage area of the entire watershed upstream of the restriction.
(a) Drainage system overflow design. Drainage systems shall have adequate capacity to convey the stormwater runoff from all upstream tributary areas through the development under consideration for a storm of 100-year design return period calculated on the basis of the upstream land in its present state of development.
(b) The Stormwater Utility Board, the City Engineer or Superintendent of Stormwater may approve, upon review of evidence submitted by the developer, an allowance, equivalent to the reduction of low rate provided, for upstream storage.
(c) When such upstream storage and release rates have previously been approved by the Board and evidence of its construction can be shown.
(d) Determination of storage volume - hydrograph method. For developments requiring that stormwater storage be provided, the hydrograph methods shall be used to determine the required volume of stormwater storage.
(Ord. 2-2010, passed 3-8-10)