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

I Existing Site Hydrology I The project site occupies a single drainage basin which historically appears to have flowed to the north. The existing site is occupied by a commercial office building and surrounding parking lot. Most of proposed parking stalls are situated on existing landscape/lawn areas. However, approximately 15 of the I proposed stalls will be situated in an area that that has not previously been cleared. Surface runoff from the existing parking lot is currently collected and routed to a combined storm/sanitary sewer system that extends through the subject properties. Basin mapping for the project has been provided in Appendix 5A of this report. I Developed Site Hydrology The proposed development activity involves some grading activity, and construction of several banks of new parking stalls. The stalls are not all situated together, so collection and management of stormwater I from the new pavement areas cannot be reasonably achieved in a single stormwater management system. With that in mind, the project is proposing the installation of stormwater conveyance facilities that collect runoff from the new pavement areas, and convey it to the combined sewer system. I The City will not allow any net new flow into the combined sewer system, as it contributes additional load to the wastewater treatment plant. With this in mind, the project is proposing the installation of additional storm improvements to collect, detain, and treat runoff from an area of existing pavement equal or I greater than the new proposed pavement area. This will remove that existing areas contribution to the combined sewer system, so that the combined system will see no net increase in flow. IPerformance Standards The project is subject to the provisions of the current City of Everett Stormwater Management Manual. The project proposes less than one-acre of improvements, and will utilize the Santa Barbara Urban I Hydrograph runoff model for stormwater detention sizing. The City's stormwater manual requires that the peak 2-yr discharge rate from the detention vault does not exceed 50%of the pre-developed 2-yr flowrate from the tributary basin area. The manual further requires that the peak 10-yr and 100-yr discharge rate I from the detention vault do not exceed the pre-developed 10-yr and 100-yr runoff rate from the tributary basin area. The stormwater model for the project will be constructed and analyzed using Stormshed 3G hydrologic modeling software, and will utilize a Type 1 A rainfall distribution, as prescribed by the City's stormwater manual. IThe project is also subject to Basic Stormwater Treatment requirements, as outlined in the City's stormwater manual. The goal of basic treatment is to achieve 80% removal of total suspended solids for flows up to and including the peak flowrate from the 6-month design storm event. This water quality design flowrate will also be calculated using the Stormshed model described above. IOn-site Stormwater Management BMPs Minimum Requirement#5 in the Everett Stormwater Manual requires the evaluation of low-impact stormwater solutions for this project. This project site is almost fully built-out, and there is not enough I open area remaining to utilize dispersion BMPs. The underlying soils present at the site are typically considered feasible for infiltration, but on this particular site, very shallow groundwater was observed in a recent geotechnical study. This does not allow enough vertical room in the soil column to accommodate an infiltration or bioretention facility. With this in mind, conventional detention and treatment facilities appear to be the most feasible stormwater management solutions. It is also required that all disturbed project areas meet the requirements of BMP 12.20 Post-Construction Soil Quality and Depth. 1800 41 st Street-Parking Expansion Drainage Report I