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Ravnik & Associates, Inc ,.,. /1, <br /> CIVII, ENGINEERING 8�T LAND-USE PLANNI �, �w �t4o <br /> STORM DRAINAGE MEMO <br /> 2,011 for �, \ • apt , <br /> NOV 0 220215 Street <br /> (BLD-2017 0349) jbtiAL <br /> LANDED GENTRY HOMES RESIDENTIAL DEVELOPM} <br /> Revised: October 26, 2017 <br /> September 29, 2017 <br /> The residential development proposed herein located at 2202 15th Street in Anacortes, <br /> Washington is generally proposed to include construction of a new house and driveway <br /> which will create an increase in the stormwater runoff from the site. The new roof area <br /> (3,376 sf), driveway (1,190 sf), and new sidewalk (470 sf) for a total of 5,036 sf of new <br /> impervious surfaces on a 10,475 square foot lot where an existing building, gravel and <br /> lawn areas currently exist. Stormwater runoff from the proposed 3,376 sf house roof area <br /> will be collected by downspouts and conveyed via a tight-lined system to a proposed <br /> infiltration trench to be installed within the northerly area of the lot. The driveway will <br /> be cross-sloped downhill to the west to promote dispersion and infiltration of runoff <br /> waters onto adjoining lawn areas which area large enough to infiltrate up to the 100-year <br /> storm as noted further on within this report. The sidewalk with collect runoff via a new <br /> gutter and convey it east, then north along the west side of"D" Avenue where it will <br /> enter an existing catch basin, as generally occurs today. Additional small areas of <br /> impervious surfaces, such as walkways, etc will be graded to disperse runoff via sheet <br /> flow onto adjoining areas of lawn/landscaping. <br /> Initially the DOE Flow Chart 1-2.4.1 Flow Chart for Determining Requirements for New <br /> Development has been used to determine that all minimum requirements apply to the new <br /> and replaced hard surfaces and converted vegetation areas. Most notably are Minimum <br /> Requirements #5-Onsite Stormwater Control, #6-Runoff Treatment, &#7-Flow Control. <br /> This project proposes to infiltrate runoff generated from a majority of the new and <br /> replaced hard surfaces as noted within the following sections of this report. Since the <br /> design within this report notes impervious surfaces (with the exception of the sidewalk) <br /> will be infiltrated as confirmed by use of a continuous model software (WWHM)they are <br /> considered "ineffective impervious surfaces", and thereby is Flow Control exempt per <br /> DOE Volume I Section I-2.5.7. <br /> As required by the 2014 Department of Ecology Stormwater Manual, the Western <br /> Washington Hydrology Model(WWII),a continuous model software, has been used to <br /> calculate pre-developed runoff conditions, post-developed runoff conditions, and the <br /> combined performance of storage volume and infiltration discharge. As further described <br /> herein, a underground detention/infiltration facility will be used to infiltrate the runoff <br /> from the new house. When sizing infiltration trenches using WWHM software, a <br /> theoretical riser outlet structure is purposely included within the analysis. If water is <br /> P:4Projects1170204DOCSDrainage Memo 10.26.17.doc 1 <br /> 1633 LINDAMOOD LANE / P.O. BOX 361 • BURLINGTON, WA 98233 <br /> PH: (360) 707-2048 • FAx: (360) 707-2,216 <br />