931 BROADWAY TEUTSCH STUDENT HOUSING 2018-03-07

study area are summarized in Table 8-1 below for the existing and proposed conditions. Refer to Appendix A for maps of the existing and proposed surface coverages. Toble 8-1: Existirg and Prcposed Surface Caverages Rooftops Walkways Roadway/Parking Landscaping Totallmpervious (SF) (SF) (SF) (SF) (SF) Existing Site 7,523 742 4,911 1,814 - Existing ROW 0 331 5,744 719 - Total 7,523 1,073 10,655 2,533 19,251 - Proposed Site 7,814 3,645 254 3,277 - Proposed ROW 0 1,550 938 4,560 - . Total 7,814 5,195 684 7,837 13,693 The existing and proposed conditions were modeled using the Western Washington Hydrologic Model (WWHM)to ensure that there will not be an increase in runoff from the existing conditions to the proposed conditions. If the proposed conditions are not increasing the amount of runoff from the site,the project is exempt from flow control per Public Works Interim Policy#2-11. The parameters shown in Table 8-1 above were used as inputs to the model, and the resulting flow frequency results are summarized in Table 8-2 below. Table 8-2: Existing and Proposed Peak Fiunoff Rates Existing Conditions (CFS) Proposed Conditions (CFS) Site ROW Site ROW 2-year 0.143 0.079 0.127 0.041 10-year 0.231 0.127 0.210 0.073 25-year 0.283 0.155 0.259 0.094 50-year 0.325 0.177 0.298 0.111 100-year 0.369 0.201 0.341 0.130 . The proposed conditions of the project result in a significant reduction in the peak runoff rates for all storm events. These results are consistent with the fact that the proposed project is reducing the amount of impervious surface coverage in the study area by 28.9%. The full WWHM report is � included in Appendix D. Note that the proposed site was modeled with two sub-basins under the proposed conditions. The sub-basins were delineated after determining which areas could flow via gravity to the point of connection (Basin 1a), and which remaining areas would require a pumped system (Basin 1b). Basin 1a consists of the western and southern portions of the roof, and walkways along the southern side of the building. Basin 1b consists of the northern and eastern portion of the roof, the courtyard, and any drainage collected by the footing drain system. Note that no groundwater was encountered during the subsurface explorations, and it is the opinion of the project geotechnical engineer that the amount of drainage collected by the footing drain system is negligible (barring unforeseen events such as a utility break, etc.). EVCC STUDENT HOUSING-STORMWATER SITE PLAN-REVISED.DOCX 9 �