3310 MCDOUGALL AVE 2019-09-06 - Laserfiche WebLink

• • 7. ON-SITE STORMWATER MANAGEMENT As previously mentioned the site is currently developed and consists of a building and. parking area. The south side of the site previously contained buildings that have been removed and that area appears to have been used parking/storage of vehicles. To address the requirements for mitigation of the impervious surfaces, the applicability and feasibility of infiltration was considered first. Since the existing site is developed and has been graded the on-site soils have been compacted and are not suitable for infiltration. Dispersion was considered next. Due to the proposed parking area,vegetated flow paths cannot be met,therefore dispersion is not feasible. This project proposes to construct approximately 10,154 sf of new roof surfaces and approximately 7,348 sf of PGIS on site. Runoff from the proposed parking area will be directed toward on-site catch basins. These catch basins will pipe runoff south and connect to the existing stormwater conveyance system. This storm conveyance system directs runoff east to the right-of-way of McDougall Ave. The roof downspouts of the proposed building will also be tightlined to the proposed piped system on the project site. 8. RUN-OFF TREATMENT • Since this project is in the combined storm/sewer basin treatment of the runoff will be provided at the COE treatment plant and this project is exempt from runoff treatment per interim Public Works Policy#2-11. ,522 Qv ieA'os�i4t 2mai6 Poi' 9. FLOW CONTROL / .'ovat df iT Cons7LaThe site design for the improvementfor approximately 17,502 sf of new and replaced hard surfaces, however the(existinn site conditions consist of 17,290 sf of impervious surfaces. The existing and proposed conditions were modeled and compared using WWHM12 software. Per the Washington State Department of Ecology, the 100-year flow frequency may be analyzed using the 1-hour time-step or 15-minute time step. The 15- minute time step was used in this analysis. Therefore,the difference in the 100-year flow frequency from the threshold discharge area must not exceed 0.15 cfs. As per the interim Public Works Policy #2-11 the existing site will not be modeled as forested land cover but rather the existing site conditions. As detailed in Section 2-of this report, the existing site is currently developed and contains a building and parking area. Little vegetation exists on-site and consists of compacted material. This project proposes to remove or replace all of the existing impervious surfaces with a new building and paved parking. McDougall Warehouse 17-0212 April 2017 Page 9