Erosion, Flood, Stormwater & Watershed Standards 12.5 Stormwater Management
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rate for the 1-year, 24-hour and the 10-year, 24-hour storms. Runoff volume drawdown time shall be a minimum of 48 hours, but not more than 120 hours. Stormwater control measures (SCM) shall be designed, constructed, and maintained so that the project achieves the “runoff volume” or “runoff volume match” defined as the volume of stormwater runoff generated from all the built-upon area of a project during a storm per the minimum required storm depth. SCM shall meet the minimum design criteria (MDC) of 15A NCAC 02H.1050 through .1062. C. Suspended Solids: All structural stormwater treatment systems used to meet the requirements of the program shall be designed to have a minimum of 85% average annual removal for Total Suspended Solids. D. Design Criteria: General engineering design criteria for all projects shall be in accordance with 15A NCAC 2H .1008(c), as explained in the Stormwater Design Manual published by the NCDEQ, or as amended. E. Impervious Surface Maximum: New development shall not exceed 70% impervious surface on a project-by project basis. For the purpose of calculating the impervious surface area, total project area shall include total acreage in the tract on which the project is to be developed. The Administrator or approving authority may reduce the impervious cover requirement at the time of plan approval based on plans adopted for specific areas of the town or where it is impractical to meet current requirements. F. Watercourse Buffers (Minimum Each Side): See Section 12.7 12.5.4. Additional Standards For The Falls Lake Watershed A. Applicability: The requirements of this section shall apply within the Falls Lake Watershed. B. Nitrogen and Phosphorous Loading 1. Nitrogen and phosphorus loads contributed by the proposed new development shall not exceed the following unit-area mass loading rates: a. 2.2 pounds per acre per year for nitrogen, and b. 0.33 pounds per acre per year for phosphorus. 2. Notwithstanding 15A NCAC 2B.104(q), redevelopment subject to this ordinance that would replace or expand existing structures or improvements and would result in a net increase in built-upon area shall have the option of either meeting the loading standards identified in 12.5.4.B.1, above, or meeting a loading rate that achieves the following nutrient loads from the net increase in built-upon area: a. 40% reduction for nitrogen, and b. 77% reduction for phosphorus. 3. The developer shall determine the need for engineered stormwater controls to meet these loading rate targets by using the accounting tool for nutrient loading approved by the Environmental Management Commission for the relevant geography and development type under review. The accounting tool is available through the NC DWQ website at: https://deq.nc.gov/about/divisions/water-resources/planning/nonpoint- source-management/nutrient-offset-information#stormwater-nutrient- accounting-tools. C. Control and Treatment of Runoff Volume: Stormwater systems shall be designed to control and treat the runoff generated from all surfaces by the first inch of rainfall in the one-year, 24-hour storm event. The treatment volume shall be drawn down pursuant to standards specific to each practice as provided in the Design Manual. To ensure that the integrity and nutrient processing functions of receiving waters and
Town of Wake Forest Unified Development Ordinance – July 19, 2022
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