1800 41ST ST 2023-06-23 - Laserfiche WebLink

PREPARATION OF STORMWATER SITE PLANS The project is subject to the provisions of the City of Everett Stormwater Management Manual. This report, along with the accompanying plans, is intended to satisfy the site plan preparation requirements outlined in the regulatory documents listed above. The Stormwater Manual requires completion of the following four tasks as part of the site plan preparation process: Task 1: Define and map the study area The project study area includes the project site itself, as well as a tributary upstream basin, and a downstream flowpath for a distance of%-mile. A map of the study area is included in Appendix 1A of this report. Task 2: Review all available information on the study area The NRCS Web Soil Survey classifies the underlying soil at the project site as a combination of Everett and Indianola soils. Both of these soil units are classified as outwash soils,which are typically characterized as porous soils,with relatively small runoff rates, and a moderate to high capacity for infiltration. A geotechnical report was also performed for the project site, and has been included in Appendix 1 B, which confirmed the presence of a coarse, sandy material underlying the project site. This generally I corroborates the findings in the NRCS Web Soil Survey. However,the test pits also revealed shallow groundwater in the test pits that were excavated at the propose project site(Test Pit#'s 1 &2). In areas of shallow groundwater, surface runoff is not able to fully percolate, and will have high runoff potential. To this end,the Everett Stormwater Manual instructs this condition to be classified under Soil Group 'D',with a high runoff potential (See Vol I, Sec. 5.4.2). FEMA identifies the project site on Flood Insurance Rate Map(FIRM)Panel#53061C1030. The project is not identified as being within a flood hazard area. Appendix 1A contains a portion of the FEMA flood insurance map. ' Task 3: Field inspect the study area A field visit of the study area was conducted on January 20th, 2016. The conditions at the time of the visit were cloudy and cool, but no precipitation had occurred over the past 24-hours. No surface runoff in the parking lot or downstream flowpath during the site investigation. The project basin includes the proposed parking lot expansion area,as well as an existing pavement area which will be collected and conveyed to the public storm drainage system as part of this project. The proposed stormwater facilities will discharge to a catch basin on the project site which drains out to the City of Everett's public storm drainage system in 41st Avenue,which discharges to the northeast of the project site. The entire downstream flowpath is comprised of manmade stormwater elements(catch ' basins and underground pipes),for the full length of the 1/4-mile downstream analysis. 1800 41st Street—Parking Expansion Drainage Report