Permit File 2202 15th Street (3)

Home Browse Search

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