8615 7TH AVE SE 2021-02-26 - Laserfiche WebLink

5. SOURCE CONTROL (MR #3) This project proposes to construct a remodel/addition to the existing church. Per Volume 1 of the 2014 DOE manual, this development does not require source control. 6. PRESERVATION OF NATURAL DRAINAGE.SYSTEM (MR #4) The runoff from the site improvements will be connected to the existing on-site conveyance/collection system. As a result, this will ensure that the runoff drains to the natural downstream location. 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 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 the new and replaced impervious surfaces for this project are required to meet Minimum Requirements 1-5. As a result, Section 2.5.5 of Volume I in the 2012 Stormwater Management Manual for Western WA, this project is required to use On-site Stormwater BMPs from List#1 or demonstrate compliance with the Low Impact Development Performance Standard and BMP T5.13, (applicant option). The applicant chooses to use List#1. To address the requirements for mitigation of the new roof surfaces (no proposed replaced roof surfaces), the applicability and feasibility of Stormwater Management BMP's found in List#1 were considered first. Per List#1, 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, the site has been previously cleared and does not consist of native vegetation. Therefore, full dispersion was found to be infeasible. 2. Full Infiltration and Bioretention were considered next. The soil type for this area is Alderwood gravelly sandy loam with a hard pan layer at 20-40" and water table at 18-36". In addition, there are places throughout the site with undocumented fill and areas of cut/grading. Since infiltration is determined to not be feasible in undocumented fill and the hardpan/water table is within 36" of the ground surface in the areas where there is no fill, this cannot be used for this project. Zillah 2-Lot SFR's 18-0605 June 2018 Page 10