The Storm Water Management and Sediment Control section of the C-SWPPP shall comply with SCDHEC’s Regulations 72-300 and the NPDES General Permit for Storm Water Discharges from Construction Activities and contain minimum storm water control requirements to accomplish the following:
(a) Storm water management facilities to limit the 2-year, 10-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 SCS Type II distribution and pass the 100-year, 24-hour storm event. The design of these facilities shall be based on; procedures contained in the Storm Water Management Design Manual or approved by the Director.
(b) The requirements, or portions of this section may be waived by the Director if it can be shown by detailed engineering calculations and analysis which are acceptable to the Director that one of the following conditions exist:
(1) The installation of storm water management facilities would have insignificant effects on reducing downstream flood peaks; or
(2) The installation of storm water management facilities are not needed to protect downstream developments and the downstream drainage system has sufficient capacity to receive any increase in runoff for the design storm; or
(3) The installation of storm water management facilities to control developed peak discharge rates at the exit to a proposed development would increase flood peaks at some downstream locations; or
(4) The installation of storm water management facilities are not needed to control developed peak discharge rates and installing such facilities would not be in the best interest of the County.
(c) The requirements, or portions thereof, of § 8-84(a) may not be waived if the Director determines that not controlling downstream flood peaks would increase known flooding problems, or exceed the capacity of the downstream drainage system.
(d) A waiver shall only be granted after a written request is submitted by the applicant containing descriptions, drawings, and any other information that is necessary to evaluate the proposed land disturbing activity. A separate written waiver request shall be required if there are subsequent additions, extensions, or modifications which would alter the approved storm water runoff characteristics to a land disturbing activity receiving a waiver. The Director will conduct a review of the request for a waiver within 10 working days. Failure of the Director to act by the end of the tenth working day will result in the automatic approval of the waiver.
(e) For all storm water 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 storm water 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 storm water management facilities emergency spillways shall be checked using the 100-year 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.
(f) 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 Storm Water Design Manual.
(g) All storm water systems shall be designed to have 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 obtained.
(1976 Code, § 8-80) (Ord. 3551, passed 11-11-2001; Ord. 4281, passed 5-5-2009; Ord. 4698, passed 5-5-2015)