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STORMWATER SITE PLAN May 10,2017 <br /> OAKES AVENUE APARTMENTS PAGE 6 <br /> MR 1: PREPARATION OF STORMWATER SITE PLANS <br /> DRAINAGE PLAN DESCRIPTION <br /> Direct precipitation is the source of stormwater runoff. The buildings consume approximately <br /> 33% of the lot and yield non pollution generating impervious surface (NPGIS) runoff from the <br /> roof. The paved drive aisle accounts for approximately 20% of the parcel and all of the <br /> pollution generating impervious surfacing (PGIS). The remainder of the site is lawn or <br /> landscaping and shall be treated with.BMP T5.13 soils. <br /> The frontage requires no traditional improvements. An existing driveway apron will be <br /> demolished and replaced with standard curb and planter strip, and some of the frontage will <br /> also contain improved landscaping or sidewalk. <br /> The site is surrounded by developed lots; paved parking abuts the north property line and the <br /> southern property line has a few feet of lawn or landscaping, beyond which lies a sidewalk and <br /> a home. Oakes fronts the west face of the parcel, and an existing retaining wall lines.the east <br /> property face with an alleyway lying below. Approximately 430 sf of the north property will <br /> sheet flow onto this site. This 500 sf offsite contribution currently discharges to the combined <br /> sewer system. <br /> WATER QUALITY MEASURES <br /> Please see MR 3. <br /> STORMWATER TREATMENT BMP'S <br /> The site contains less than 5,000 sf of new or replaced PGIS, MR6 is not required. <br /> DETENTION SIZING <br /> The site contains less than 5,000 sf of new or replaced PGIS, MR7 is not required. <br /> CONVEYANCE CALCULATIONS <br /> The site naturally flows eastward towards the alley. Although existing conditions contain some <br /> organic duff and soft soils, the soils are shallow and underlain with Vashon till/hardpan; <br /> perched flow is expected during heavy rains. <br /> The existing, 8" combined sewer main exists in the alley, to which a private 8" combined lateral <br /> will be connected. On-site, the proposed development provides 6" private sewer stubs that <br /> connect to the proposed 8" combined sewer lateral. A parallel, 8" private storm system <br /> conveys roof, footing and drive courtyard underdrain discharge to a catch basin. This same <br /> catch basin serving the drive aisle and is fitted with a floating material separator and discharges <br /> to the 8" private combined sewer lateral. <br /> The 6" and 8" lines specified are minimum standard sizes for the applications used and contain <br /> ample capacity. <br /> P:\Work(Share£ile)\Proieets\2016;l6-208 Rainier Building Co\CE'DOCS\Drain,2017 05 10 16-208 Oakes Drain Rpt.doc <br />