10315 19TH AVE SE SITE DEVELOPMENT 2016-01-01 MF Import

II � I � EVERGREEN ENGINEERING SERVICES INC. ( LAND USE PLANNING • CIVIL ENGINEERING R. Alan Murray, P.E. �IN"v�(G1 -DI`� IAugust 5, 2004 2779A °� � °� .1 ; MJR Development ��-� `1� L-t " c/o Northwest Development Advisors LLC C' � t� ' I P.O. Box 12391 - `�. �; � � MiIlCreek, Washington 98082 � l� �\ ' ' s�,' 4 1 �� . � ( Re: Silvcr Lakc Medical— Revised Storm Drainage Report ;; ��' � City of Everett SEPA 00-005 N ((�' � � ,��;� � �ar s�rs: `� � 'I his letter summarizes the revised, final stormwater analysis for the Silver Lake Medical project. The proposal would include two medical buildings and parking within a 2.05-acre site located on the east side of 19'�' Avenue Southeast at 104`h Street Southeast. I "Chere are no existing structures on the site and the vegetation is primarily blackberry bushes, � dense undergrowth and small, deciduous trees (regrowth after 1996 lo�ing). There is a small Class 3 wetland and a Class 4 stream (approximately 0.24 acres) flowing southwest across the northwest corner of the site. The stream (low is collected in an existing catch basin, located on I site, and is thcn conveyed to the enclosed system in 19'�' Avenue Southeast. See "Existing Conditions Basin Map" attached. This area is not contributory to the proposed drainage facility. IThc proposed dcvelopment will create approximately 1.49 acres of impervious surface, which is approximately 73 percent of the site. This project is required, under City of EveretYs Drainage I Ordinance, to provide streambank erosion control (detention) and runoff treatment. Because the detention analysis �vas done usiug an SCS method(Waterworks sofrware)a factor of safety must be applied to the streambank erosion control BMP (detention). See the graph attached (SF = I.48). The devebped flow rates will be restricted to match half of the existing 2-year storm flow I rate the existing 10-year and 100-year storm flow rates. ( An underground detention vault is proposed under the center of the site. See "Developed � Conditions Basin Map" attached. The vault will both contain dead storage and live storage volumes (sec attached calculations). The underground vault will consist of four cells. Tw�the cells will provide Qie primary settling of solids for the stormwater. These cells will have a dead storage depth of,3.10 feet and provide 3,986 cubic feet of storage (88.63.percent of the required Idcad storage). ! j I 8625 Evergreen Way, Suito 215 • Everett,Washington 98208-2620 • Phone:425-513-2500 • Fax:425-355-1307 O 36