(A) The Stormwater Management and Sediment Control section of the C-SWPPP shall comply with SCDHEC’s Regulations 72-300, the NPDES-CGP and the County’s Stormwater Design Manual.
(B) Stormwater management facilities shall limit the two-year, ten-year and 25-year developed peak discharge rates to pre-developed peak discharge rates using a storm with a duration of 24 hours with a NOAA Atlas 14 distribution and pass the 100-year, 24-hour storm event. The design of these facilities shall be based on procedures contained in the county’s Stormwater Design Manual or as approved by the Stormwater Administrator.
(C) For all stormwater management facilities, a hydrologic-hydraulic study shall be done showing how the drainage system will function with and without the proposed facilities. For such studies, the following land use conditions shall be used. Existing land use data shall be determined by comparing the most recent aerial photograph to historical aerial photographs at the period of time prior to the county adopting stormwater management requirements and field checked to confirm the current site conditions.
(1) For the design of the facility outlet structure, use developed land use conditions for the area within the proposed development and existing land use conditions for upstream areas draining to the facility.
(2) For any analysis of flood flows downstream from the proposed facility, use existing land use conditions for all downstream areas.
(3) All stormwater management facilities emergency spillways shall be checked using the 100-year, 24-hour storm and routing flows through the facility and emergency spillways. For this analysis, developed land use conditions shall be used for all areas within the analysis.
(4) The effects of existing upstream detention facilities can be considered in the hydrologic-hydraulic study.
(D) A downstream peak flow analysis which includes the assumptions, results and supporting calculations to show safe passage of post-development design flows downstream. The analysis of downstream conditions in the report shall address each and every point or area along the project site’s boundaries at which runoff will exit the property. The analysis shall focus on the portion of the drainage channel or watercourse immediately downstream from the project. This area shall extend downstream from the project to a point in the drainage basin where the total area of the development comprises 10% of the total basin area. In calculating runoff volumes and discharge rates, consideration may need to be given to any planned future upstream land use changes. The analysis shall be performed in accordance with the county’s Stormwater Design Manual.
(E) All stormwater systems shall be checked to demonstrate no increase in velocity, peak flow, water surface level elevation in relationship to upstream, adjacent, and downstream property in the 100-year storm, unless an adequate permanent drainage easement is dedicated and obtained. Open channels and closed storm sewers and culverts shall be sized to carry runoff from the appropriate design storm event and to preclude the creation of backwater inundation of any area outside of a dedicated drainage easement.
(Ord. 2819, passed 6-17-19)