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6. Design Analysis <br />6.1 Existing Stormwater Drainage System <br />An existing hydrology and hydraulics XPSWMM model from the Plan was used for the analysis of <br />this study. Before using this model, various hydrological parameters ware reassessed and <br />updated to model the existing conditions of the drainage basins associated with this study. <br />Subbasins within the project site were modified to follow the topography of the City rather than <br />grouped tax parcels. In reassessing the subbasins, new flow paths were created for each <br />subbasin. Land uses within each subbasin were updated and used to calculate new Curve <br />Number (CN) values and new times of concentration (Tc). The resulting subbasin parameters can <br />be found in Appendix A <br />For the stormwater retrofits, the water quality design storm is based on the 6 -month, 3 -hour storm <br />event in accordance with the SWMMEW (Ecology, 2019). The resultant flow rates for each basin <br />can be found in Appendix B. <br />6.2. Proposed BMP Design and improvements <br />The preferred alternative includes the implementation of biofiltration swales and vegetated filter <br />strips for stormwater treatment in the Treatment Assessment Area. This option allows the <br />stormwater treatment facilities to be dispersed around the City, treating runoff as close to the <br />source as possible. Additionally, the preferred alternative provides the least amount of <br />disturbance to the historical integrity of the City by adhering to the historical preservation <br />guidelines of preserving sidewalks, fences, and trees when at all possible. These vegetated <br />facilities will be designed to be compatible with the climate and region. These facilities are <br />depicted in the following figures extracted from the SWMMEW (Ecology 2019): <br />MINIMAL IOMM <br />• rY. MlCIrJiChrdY iY!!eN <br />9:L�r,iv Wgea'tirY1i1G 4!'Itl•+r <br />C,[:r r9*I Iter YGni.•r: • <br />• E��R�am ll,aY,ae��eN ., k <br />iJ is <br />'v <br />clpcw! r41 yclr <br />Ivrewril Yrt I <br />JIII' n n In rNJ 7 <br />Rt•6a1'rpr Glr iwla• � � } <br />1e!nWb'i faNr]:ri'r <br />Bpwn f0 <br />Cnl eira ! <br />Gin goR+ll 1 <br />s.IRh di..kOr 11 <br />t] 11(ptil, ! <br />r <br />I <br />4 <br />-a <br />IhNi.Wacre <br />rh two, <br />xrnFiO ana tuOrnR ri J \ �— M9rr3N0 hrn aneEemay t,e, <br />wOgrrle trs l.pin ryr•al <br />e/loeat uneniw�+ <br />tiferNrs 0•.n p04rrc7, <br />FIGURE 1: Sior-ELTRATION SIAVALE <br />6 <br />