5406 S 1ST AVE 2021-02-25 - Laserfiche WebLink

5. SOURCE CONTROL (MR #3) This project proposes to construct two new single-family residences and two new driveways. Per Volume 1 of the 2014 DOE manual, residential development does not require source control. 6. PRESERVATION OF NATURAL DRAINAGE SYSTEM (MR #4) The runoff from the site improvements will either be infiltrated on-site or collected and released to the downstream system. However, by not grading or compacting the majority of the pervious surfaces to remain after construction in the area of disturbance a maximum amount of site runoff from the developed project will be absorbed and infiltrated into the existing on-site soils. This will mimic the existing conditions by maintaining the existing direction and condition of flow from this site. 7. ON-SITE STORMWATER MANAGEMENT (MR #5) As documented in Section 1 of this report this project is required to meet Minimum Requirements 1-9. As a result, Section 2.5.5 of Volume I in the 2012 Stormwater Management Manual for Western WA, this project is required to meet the requirements in Table 2.5.1. Since this parcel is less than 5 acres this project is required to meet Low Impact Development Performance Standard and BMP T5.13; or List #2 (applicant option), per Table 2.5.1. The applicant chooses to use List#2. To address the requirements for mitigation of the roof surfaces, the applicability and feasibility of Stormwater Management BMP's found in List #2 were considered first. Per List #2, this project must to the extent feasible implement Full Dispersion (BMP T5.30), Full Infiltration (BMP T5.10A), Bioretention, Basic Dispersion (BMP T5.10B), or Perforated Stub-outs (BMP T5.10) in the order listed. Roofs: 1. Therefore, Full Dispersion was considered first. Due to the size of the project site, a flow path of 100 feet would not be viable. Furthermore, it appears the site have previously been cleared and does not consist of native vegetation. Therefore, full dispersion was found to be infeasible. 2. Full Infiltration and Bioretention were considered next. Based on the on-site soils, infiltration-was found feasible on Lot 1. Therefore, the roof surfaces proposed on lot 2 will be infiltrated via downspout infiltration trenches located on Lot 1 within an easement which contains the feasible soils. Based on the geotechnical recommendations a design infiltration rate of 0.63 inches/hour was used to determine the size of the infiltration trench. The roof surfaces on Lot 2 total approximately 1,710 sf and the minimum bottom area of the infiltration trench for each lot was found to be 270 sf. The trench has been designed to be 45 linear feet long and 6 feet wide with 3 feet of gravel. Please see Appendix A for WWHM calculations. Zillah Lot 2 SFR 18-0605 Aug.2019 Page 10