6200 12TH DR SE 2020-10-22 - Laserfiche WebLink

Stormwater Site Plan January 9,2018 Catholic Housing Services—Everett Safe Streets Supportive Housing VIII. Permanent Stormwater Control Plan A. Off-Site Analysis The Site is located within the Pigeon Creek Number 1 watershed,and is located east across Evergreen Way from the headwaters. All runoff from the site ends up here where it flows to Possession Sound (Puget Sound). Approximately 29,000 square feet of hard surface is proposed for the site, and will increase the amount of runoff generated on-site. Due to soil conditions and the Landslide Hazard Area, immediately to the west,infiltration discharge of collected runoff shall not be used. However,soil amendment, bio-retention and underground detention are proposed on site to mitigate any increased runoff to historic levels, before being discharged into the existing 12-inch storm drain along Berkshire Drive. The proposed improvements are not expected to cause any conveyance system capacity problems. Detention is planned to mitigate the post-developed runoff to be less than pre-developed. No localized flooding is expected,as the site is to be graded to lead runoff away from proposed structures and into the water quality and flow control structures. Infiltration is not proposed as a means of discharging any collected runoff,which does not increase the risk of erosion or landslide hazards in the area. Runoff is to be discharged into the existing storm drain system along Berkshire Drive,thus creating no risk of erosion of stream banks or outfalls. The guidelines set forth in Volume I,Chapter 4 of the DOE Stormwater Management Manual were followed to avoid any violations of surface water quality standards through use of bio-retention and detention of runoff. B. Pre-Developed Hydrology It its existingd condition,the site is mostly lawn,with a gravel storage yard. The general topographic relief is mild and to the west and slightly south. Surface runoff sheet flows down the steep slope to the west into the gutter along Evergreen Way. Some of the sheet flow is captured by two city-owned catch basins, behind Jiffy Lube(6209 Evergreen Way) and Janbo Café(6125 Evergreen Way),a detention vault behind Peking Duck(6211 Evergreen Way) and five private catch basins located in the parking lots for Jiffy Lube and Janbo Cafe. Any runoff captured by these catch basins flows in a pair of converging storm drain pipes to the north where they are discharged into the underground segment of Pigeon Creek Number 1,and sent west beneath Evergreen Way. Continuous Modeling as outlined in the DOE Stormwater Management Manual was used to analyze both pre-and post-developed conditions. The land cover for the pre-developed condition was modeled as"till forest." The precipitation data used based on the project site location was the"Puget East 40 in 5 min" in MGS Flood version 4.38. C. Developed Hydrology The post-development hydrologic model was parameterized following the same sections of the DOE's Stormwater Management Manual. Along the eastern property line where the parking lot entrance and exit are proposed,the Site shall be graded in a manner to prevent offsite run-on from entering the property in addition to the use of a gutter pan to intercept off-site flows from the east. This flow shall be diverted north to the existing catch basins for the fire practice facility.A drainage basin map is provided as Figure 1 on the following page. Wage