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West Seneca Sewer Report | Page 7 transported from land, particularly the impervious surfaces, during the initial stages of storm events. The Design Manual requires that water quality treatment be provided for the initial flush of runoff from every storm. The NYSDEC refers to the amount of runoff to be treated as the “Water Quality Volume” (WQv). Section 4.2 of the Design Manual defines the Water Quality Volume as follows: WQv = [(P )(R V )(A )] 12 Where: P = 90% Rainfall Event Number Rv = 0.05 + 0.009 (I) I = Impervious Cover (Percent) A = Contributing Area in Acres This definition ensures that, all other things being equal, the Water Quality Volume will increase along with the impervious cover percentage. This redevelopment project is proposing a combination of WQv treatment by standard practice(s) and alternative practice(s) as shown in the following tables: Table 4: Required WQv Summary P (inches) A (Ac) I (Ac) Rv WQv Required (cf) 1.00 5.68 2.98 0.29 8,672 4.3 RRv Requirements Land use change and development in the watershed increases the volume of runoff. As such, reductions in the amount of runoff from new development, accomplished through the implementation of a stormwater management plan for the site, will play an important role in the success or failure of the watershed-wide stormwater management plan. Runoff reduction techniques can be applied to manage, reduce, and treat stormwater, while maintaining and restoring natural hydrology through infiltration, evapo-transpiration, and the capture and reuse of stormwater. Volume reduction techniques by themselves typically are not sufficient to provide adequate attenuation of stormwater runoff, but they can decrease the size of the peak runoff rate reduction facilities. Section 9.2.1.B of the 2024 NYSDEC Design Manual indicates, “Although encouraged, meeting the RRv criteria is not required for the redevelopment activity portion of a project. The need to provide RRv shall be considered separately for each design point. For design points with a net increase in impervious area, RRv is required for the increase in impervious area only.” As such, bioretentions and a water quality unit have been applied to the project to meet the WQv objective for the redevelopment activity of the project. The increase in impervious cover will be treated as new development and both the WQv criteria and RRv criteria are met. The table below indicates the required RRv for the site. Page 54 of 1174