10617 ROSEWOOD AVE 2022-01-25 - Laserfiche WebLink

• KUKHOTSKIY DUPLEX BMP T5.10A Full Infiltration System has been selected for approximately 457 square feet of the proposed roof areas fronting the northern duplex unit. Based on BMP T5.10A in the • 2019 WADOE stormwater manual, a 2'x125LF infiltration trench is required for every 1,000 sf. of contributing roof runoff or an equivalent 250 sf. infiltration trench footprint. With 457 sf. of proposed roof area draining to an infiltration system, a minimum of 114.25 sf of infiltration trench footprint would be required for mitigation. A 8'x15' (120 sf.) infiltration trench has been proposed north of the proposed driveway area with an overflow • connection to an existing open ditch near the southwest corner of the site. BMP T5.10B Downspout Dispersion Systems has been selected for approximately 3,238 square feet of the proposed roof areas and will consist of 5 splash blocks located on the northeast side of the proposed duplex and one near the southwest corner. Each splash block • will have a 50' vegetated flow path. See below for sizing: Proposed Duplex Splash Block Sizing Proposed Roof to Splash Block Area=3,238 SF 1 Splash Block Required Per 700 SF Roof Area • Required Splash Pads= 3,238 SF/700 SF=4.62 Designed Splash Pads= 5 Proposed Duplex Infiltration Trench Sizing Proposed Roof Area=457 SF 2 x 125 LF of Trench Required Per 1,000 SF Roof Area • Total Footprint of Trench Needed= (457 SF/ 1,000 SF) * (125 LF * 2') = 114.25 SF Designed Trench Lengths= 8'x15' or 120 SF For proposed driveway area, BMP T5.12 Sheet Flow Dispersion has been selected and a gravel flow spreader has been specified along the downslope side. The site contains • adequate area for the 10' to 19' vegetated flow path required for driveway runoff. Refer to the drainage plan for flow path placement. If the proposed driveway is to be composed of gravel and concentrated flows are not expected, then a gravel flow spreader is not required, but required dispersion flow path length must be maintained. Additionally, all disturbed pervious areas are to be amended per BMP T5.13. • Minimum Requirement # 6: Runoff Treatment Due to the site proposing less than 5,000 square feet of new/replaced pollution generating impervious surfaces, less than 3/ acres of forest to lawn conversion, and less than 2.5 • acres of forest to pasture conversion,runoff treatment is not required. Minimum Requirement# 7: Flow Control The project has proposed to infiltrate 457 square feet of the total roof runoff area. This brings the total net effective hardscape is below 5,000 square feet therefore; Due to the • site proposing less than 5,000 sf of new effective impervious surfaces and less than a 0.10 cfs. increase in the undeveloped 100 year storm(based on 1 hour time step analysis), • WESTERN ENGINEERS,INC. 17 PROJECT:19-1825-A