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6. Design Analysis <br />6.1. Existing Stormwater Drainage System <br />An existing hydrology and hydraulics XPSVl/tulM 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 lo 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\ /tvlMEW (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 to be compatible with the climate and region. These facilities are <br />depicted in the following figures extracted from the SVVI\4MEW (Ecology 2019): <br />Olllt^t loEgjl4rtt(t ta'Intr'.IIs,:ora! rr* o:jr !.r $r rt'r t.,t <br />Ci.i.F! r.t *r.r,"elqEr(qrtrq.y: sr.GDil ,I FG i n' rr:L l l. r.. hrdl'.tt:.r.r0trf.rgrdrs ' ,: I <br />,i <br />"t' <br />P1l' ltttr) <br />hrtodd fr I(l?-4itt.t) <br />l.6rilr.rtdrt t!tr(&lI'!trril1 <br />(121t1 lEal <br />f.r dr! dr..bff <br />lJ ! lr}ad, <br />:t <br />!t ,'..,,?' <br />;'i,l': "1;;') <br />ra]trr ti.n.t r!!rr.s ,,.rarx'r!. 4 r t6rl '{0 r3ilrtrl!r.rc ltlG' l.Pli lfr.ll <br />{alr{rtsdd'D Fa'tilrt, <br />FrcuRe 1: Brorrlrnattoru Swale <br />6