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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 were 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 />[M[,aLL %0iE6' <br />u,ararnerl r✓ary ' <br />] � r-aa.na �ar.a♦.vas l <br />�%✓ Ar.aa♦f avprvN ✓f J `N9e�nr a'ry aefee rry Nra, <br />M'rM IJ Gripin`iypa�y ybn,vrtra•a• <br />IfirygCMpYrr,fr <br />FIGURE is 810FILTRATION SWALE <br />ta'•rY�r[✓�rrp tryr <br />- <br />� Oae%ri <br />Ruy.0 <br />yyarrl NX 9µrY �.• <br />Y,lu ifttt <br />�%✓ Ar.aa♦f avprvN ✓f J `N9e�nr a'ry aefee rry Nra, <br />M'rM IJ Gripin`iypa�y ybn,vrtra•a• <br />IfirygCMpYrr,fr <br />FIGURE is 810FILTRATION SWALE <br />