Sec. 16.2-21. Computation methodologies.
(a) Stormwater quality: All stormwater quality control facilities shall meet the requirements of either a) technology-based water quality criteria as documented in the Virginia stormwater management handbook, b) performance-based water quality criteria as documented in the Virginia stormwater management handbook, or c) other criteria developed and published by the city department of community development.
(b) Stormwater quantity:
(1) Stormwater design events
(i) For stormwater detention, the post-development peak runoff rate from a two (2)-year storm and a ten (10)-year storm, considered individually, shall not exceed their respective pre-development rates.
(ii) All components of stormwater conveyance systems, including pipes, inlets, gutters, and open channels or swales shall be designed to convey a ten (10)-year rainfall event.
(iii) All structures conveying a perennial flow shall be designed to convey a twenty five (25)-year rainfall event.
(iv) All structures conveying the flow of a designated floodplain shall be designed to convey a one hundred (100)-year rainfall event.
(v) All impoundments, including detention and retention ponds, shall be designed to withstand a one hundred (100)-year rainfall event and shall meet all other applicable Virginia dam safety requirements.
(vi) Design storms shall be based on either a) type II, a twenty-four (24)-hour storm using the rainfall distribution recommended by the U.S. Soil Conservation Service and U.S. Soil Conservation Service calculation methods (TR-55 or TR-20), b) the rational method, or c) another calculation method, only with prior approval by the plan approving authority.
(2) For purposes of computing runoff, all lands in the site shall be assumed prior to development to be in good condition if the lands are pastures, lawns, or parks, with good cover if the lands are woods, or with conservation treatment if the lands are cultivated; regardless of conditions existing at the time of computation. Runoff coefficient values should be assigned according to Table 5-2 of the Virginia erosion and sediment control handbook.
(3) All stormwater management facilities which incorporate storage shall be verified by submitted routing calculations. (Ord. No. O-01-079, 4-24-01, eff. 5-1-01; Ord. No. O-07-047, 4-10-07)
Last updated date: 4/13/2007 3:02:27 PM