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6. Design Analysis <br />6.1. Existing Stormwater Drainage System <br />An existing hydrology and hydraulics XPS\MI|M 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, ln 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 S\ AdMEW (Ecology, 2019). The resultant flow rates for each basin <br />can be found in Appendix B. <br />6.2. Proposed BMP Design and lmprovements <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 lo be compatible with the climate and region. These facilities are <br />depicted in the following figures extracted from the S\A/lVlMEW (Ecology 2019): <br />6ttltar ro?13:. lr.rIr(qndlxn rrat$Fu:cr.t * iliE.r'.ir'3.1.r( rt'r d rt <br />' Cici..irirr*.c].-qicrlaran!(rrrta': nrraaftar. ! 'r'.d r n' rr:! ll.: - r t6..1l'odl?r(.citlparartn,.t) <br />tl-1',:i')-:.. <br />.:t-'!.rtdlr tddt a.rrr(61|f lrlrnl <br />(l2rll |rFrdl <br />0 r ln(d, <br />rtt,inl *tlrrb <br />tuft(l:J'ttr*s) <br />lll niltrcr) <br />n <br />! <br />l1 <br />(10'*tlhqr(., <br />raYtr/tr,rrr$!trrs $,rtarF.rla aa rFrl , t0 l,ata.!t{rrJ. 016 r.Ch lr{.1, <br />aggrtil.1r0 e!1.4 n! bFr <br />talrralda'dn !o',tr!rr <br />FrcuRE 1: BtoprLtRslor'r SwAl-r: <br />b