Stormwater management plans

A. Calculations. In addition to the information required for stormwater management plans, the following information must also be included with the application, where applicable:
- (1) The area of each subbasin as identified on final site plans.
- (2) The area of impervious surfaces (including all roads, driveways, rooftops, sidewalks, etc.) for each subbasin as identified in the Rhode Island Stormwater Design and Installation Standards Manual, as amended.
- (3) Weighted curve numbers (CN), as determined by the SCS TR-55 method, for each subbasin as identified in the Rhode Island Stormwater Design and Installation Standards Manual, as amended.
- (4) Invert elevations for all applicable BMPs. In addition, the elevations for permanent and/or flood pool stages, including peak discharge rates for each stage, within all basins are required.
- (5) The total volume capacity for all flood control and water quality BMPs (e.g., infiltration basin, detention basins, wet ponds, etc.). Volumes must be segregated into permanent and flood pool stage volumes where applicable. Furthermore, the volumes of all sediment storage (basins, forebays, etc.) areas must also be shown.
- (6) Predevelopment and post-development peak discharge rates and runoff volumes for the two-year, ten-year, twenty-five-year, and one-hundred-year frequency storm events for each subwatershed. The water quality volume must also be calculated for each subwatershed. All relevant variables such as curve numbers and time of concentration, along with the supporting computations and worksheets, must be included.
B. Narrative description. As part of the stormwater management plan, the applicant shall include a discussion of the protection of environmental resource functions and values. The following outline is provided as guidance for preparing a narrative description for the stormwater management plan. Depending on the size and scope of the proposed project, the amount of information required by the permitting agency may vary; therefore, it is advised to consult the appropriate permitting agency for specific requirements.
- (1) Site description: general topography, soil types, current vegetative composition and relative abundance, existing infrastructure, and/or adjacent properties, identification of major resources (e.g., wetlands, groundwater, surface waters, etc.), name of receiving water(s), potential water quality and/or hydrologic impacts on resources.
- (2) Site input data: watershed characteristics, area of all impervious surfaces, total area of site, annual mean rainfall, runoff coefficients, curve numbers for various land uses, peak discharge rates.
- (3) Land use planning and source control plan.
- (4) Best management practices: identify the type of BMP(s) employed both during and post-construction and justification for selection, including any deviation from the Rhode Island Stormwater Design and Installation Standards Manual, as amended, and the potential effect on pollutant removal efficiency.
- (5) Technical feasibility of BMPs, including sizing, location, hydraulic and environmental impacts. Alternatives, which were considered but determined not to be feasible, should also be discussed.
- (6) Maintenance schedule of BMPs to be used, both during and post-construction, including frequency of inspection and maintenance.