2320 106TH ST SW 2022-08-29 - Laserfiche WebLink

8.0 PERMANENT STORMWATER CONTROL PLAN Part A- Off-Site Analysis An Off-Site Analysis is required for projects that contain 5,000 square feet or more of new impervious surface, which this project does. An analysis was made of the downstream conveyance system, the majority of which occurred through wetland areas and open channel creek areas. There are a couple of minor crossings of roads within one-quarter mile of the project site and there were no problems found in this downstream drainage course. There is no upstream basin contributing runoff to the project site as the site tends to slope towards the east, thereby precluding any runoff from the east entering the site. The site is bound on the north by 106th Street S.W., on the west by existing development, and the site south of the project site tends to drain to the east as well. Please refer to the Off-Site Analysis Drainage System Table and Downstream Drainage Course Map on the following pages of this report. Since this project is providing flow control to City of Everett standards utilizing the Western Washington Hydrology Model (WWHM) 2012 methodology the pre-developed condition was utilized as forested land. Road improvement areas were modeled as bypass since that area will not drain to the detention vault. Runoff rates from the project site will actually be less than they are under existing conditions. Part B - Pre-Developed Site Hydrology As mentioned previously, the pre-developed condition on this project site consists of forested land which is thickly vegetated almost over the entire development area. Of this development area, 3.19 acres will be on site and 0.11 acre off site impervious surface under the proposed condition. 0.18 acres of pervious area will drain to the detention vault. There will be existing forested land remaining in perpetuity on site along the eastern property line of the site. These are located in the wetland buffers and will be mitigated for areas encroaching on the buffer with new development. Please refer to the Pre-Developed Conditions Map on the following pages of this report. Part C - Developed Site Hydrology For purposes of sizing the flow control and water quality facilities on this site, there are 3.19 acres of impervious and 0.18 acre of till grass contributing to detention and 0.11 acre of road improvement impervious surface bypassing the detention on site. Please refer to the Developed Conditions Basin Map on the following pages of this report. The detention vault will discharge along the southeast corner of the project site into a dispersal trench which will then sheet flow into the wetland buffer immediately adjacent to it. The finished floor of the proposed building is elevation 554. The dispersal trench grade board will be set at elevation 547.5 and the detention vault discharges at 541.5 as an invert of the outfall pipe from the vault, which drains directly into a modular wetland system water quality vault. This modular wetland system vault has General Use Level Designation from the Department of Ecology for Enhanced Water Quality as well as phosphorus control and Basic Water Quality. Runoff from the modular wetland system will discharge into a pump station which will direct these minor release rates directly into a dispersal trench as mentioned previously. Part D -Stormwater Flow Control Plan The flow control facility proposed for this project site is a detention facility with more than 6 inches of dead storage located below the live storage in a vault that has live storage available of 156 feet by 60 feet by 8 feet of live storage as required by the flow control calculations sized with the WWHM 2012 model without using LID requirements. 17576.002.doc