(A) Standards generally. Design standards are established for the purpose of promoting sound development practices which respect, preserve and enhance the county’s watercourses and are not intended to prohibit the use of innovative and alternative techniques which can be demonstrated to have the potential for successfully achieving the objectives stated in § 52.01.
(B) Design storm.
(1) The measures shall control and treat the difference in stormwater runoff volume leaving the project site between the pre- and post-development conditions for, at a minimum, the one-year, 24-hour storm. Runoff volume drawdown time shall be a minimum of 48 hours, but not more than 120 hours.
(2) All structural stormwater treatment systems used to meet the requirements of this chapter shall be designed to have a minimum of 85% average annual removal for total suspended solids (TSS).
(3) The hillside development standards of Ch. 70 of the county’s code of ordinances (§ 20-70-68) apply. This chapter limits the density of disturbed area and impervious surfaces on steep slopes.
(4) The design of drainage facilities in flood hazard areas shall be consistent with the requirements of the county’s flood damage prevention ordinance. No stormwater controls shall be allowed within the floodway. No built upon area shall be within 30 feet landward of any perennial and intermittent surface water.
(5) The computation of stormwater runoff shall follow established engineering practice. Acceptable methods of computation include the rational method, the peak discharge method as described in U.S.D.A. Technical Release Number 55 (TR-55), and U.S. Geological Survey regression equations, where applicable. If an alternate method is proposed, the method should be described and justification for using this method should be provided. The same method must be used for both the pre- and post-development conditions.
(6) Runoff coefficients shall be based on full development of the project and of the watershed to the extent of the current zoning or land use patterns, and shall include the complete development of the site through build-out, including roof tops and other impervious areas that may be proposed.
(7) (a) Stormwater detention shall be provided to ensure that the rate of discharge does not exceed the pre-development rate of discharge.
(b) In order to demonstrate this, pre- and post-development hydrographs will be submitted that demonstrate no increase in flow leaving the site during the one-year 24-hour storm. Inflow-outflow calculations shall also be submitted for any stormwater detention ponds.
(8) Stormwater controls that drain in whole or part to designated trout waters shall be designed
and shall implement the best stormwater practices that do not result in a sustained increase in the receiving water temperature, while still meeting the other requirements of this chapter.
(2013 Code, § 20-12)