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(a)
Calculations. In addition to the information required for the site plan the following information must also be included with the application, where applicable.
(1)
The area of each subwatershed shall be 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 Storm Water Design and Installation Standards Manual, as amended.
(3)
Weighted curve numbers as determined using Urban Hydrology for Small Watersheds (USDA Soil Conservation Service, 1986 or as amended).
(4)
Invert elevations for inlets and outlets. In addition, invert elevations shall be provided for all basins including permanent and/or flood pool stages, including peak discharge rates for each stage.
(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 provided.
(6)
Predevelopment and postdevelopment peak discharge rates and runoff volumes for the two-year, ten-year, 25-year, and 100-year frequency storm events for each subwatershed to each separate water or discharge point. 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. The entire site shall be included in an evaluated subwatershed.
(7)
Supporting calculations to demonstrate that the proposed development project will meet section 22-6 of this chapter.
(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 town may vary; therefore, it is advised to consult the town 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. include 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.
(Ord. of 1-12-2010)