Laserfiche WebLink
9610 19th Avenue SE <br /> July 27, 2012 (A4 <br /> Revised September 18, 2012 <br /> Page 7 <br /> IV. PERMANENT STORMWATER CONTROL PLAN <br /> The permanent stormwater control facilities on the site are designed in accordance <br /> with the City of Everett Stormwater Management Manual, February 15, 2010 and the <br /> 2005 Washington State Department of Ecology Stormwater Management Manual. <br /> "Figure 1 - Net Exposed Subgrade Area" shows that the net exposed subgrade area on <br /> the site (not counting those areas where subgrade will not be changed) is 0.904 acre. <br /> Therefore, the stormwater detention system is sized using the Santa Barbara Urban <br /> Hydrograph method. The performance of the system meets the required performance <br /> by limiting the peak developed rate of runoff from the 2-year, 24 hour design storm to <br /> 50-percent of the existing condition 2-year, 24 hour design storm while maintaining <br /> the existing condition peak rate of runoff for the 10-year, 24 hour and the 100-year, <br /> 24 hour design storms. <br /> A. Off Site Analysis <br /> The existing drainage infrastructure for the McDonalds Restaurant drains through a <br /> stormwater detention vault, an open swale, and into a 12-inch ductile iron pipe <br /> flowing westerly to the easterly edge of Interstate 5 and into a wetland. <br /> B. Pre-Developed Hydrology <br /> "Figure 2 - Predeveloped Basin Areas" depicts the pre-developed conditions at the <br /> McDonald's Restaurant site (pervious surface consists of 0.173 acre and impervious <br /> surface consists of 0.731 acre.) However, since the McDonald's Restaurant site also <br /> includes a stormwater detention system, the existing site is required to be modeled as <br /> per the conditions depicted by the detention system (pasture.) Therefore, in <br /> accordance with the original 1995 McDonald's Restaurant SBUH calculations site, the <br /> existing conditions are modeled as follows: <br /> Existing Condition: <br /> 0.95 acre, curve number of 85, and a time of concentration based on 236-feet of <br /> sheet flow with a mannings 'n' of 0.4, a slope of 0.036 and a P2 = 1.55". <br /> Note: copies of the pertinent McDonald' Restaurant calculations are provided in the <br /> appendix for reference. <br /> C. Developed Hydrology <br /> "Figure 3 - Developed Basin Areas" depicts the developed conditions entered into the <br /> SBUH hydrograph model. Although the actual developed area (excluding areas where <br /> subgrade will not be exposed) is equal to 0.904, the area modeled in the SBUH model <br /> is 0.95 acre in order to match the original McDonald's Restaurant site area. This <br /> consists of 0.08 acre pervious and 0.87 acre impervious. In accordance with <br /> hydrologic soil group "C", the pervious area is assigned a CN=86 and impervious areas <br /> 98. To be conservative, the minimum time of concentration of 6.3 minutes is used for <br /> the developed condition. <br /> Ph: (206)403-0933, emait:jayd@deckerce.com <br />