11517 AIRPORT RD 2025-07-24 - Laserfiche WebLink

CITY OF EVERETT STORMWATER MANAGEMENT MANUAL 8. Over-excavation of cut sections shall be necessary if the cut is in a location that will be utilized for on-site stormwater management. A depth shall be cut that will allow replacement of stockpiled native topsoil to the entire depth that was on the site up to a required maximum of 3 feet. 9. Cut sections where native topsoil replacement is required shall require ripping of any cemented till layers to a depth of 6 inches. Subsequently, the replacement of stockpiled topsoil shall be thoroughly mixed into the ripped till to provide a gradual transition between the cemented till layer and the topsoil. 10. Stockpiled topsoil shall be replaced in lifts no greater than 1 foot deep and compacted by rolling to a density that matches existing conditions. 11. For any soils removed and replaced on site as described above,the soil amendment requirements in BMP 12.20 must also be met. 12.3.2 BMP 12.20 Post-Construction Soil Quality and Depth Naturally occurring(undisturbed)soil and vegetation provide important stormwater functions including: water infiltration;nutrient,sediment, and pollutant adsorption;sediment and pollutant biofiltration;water interflow storage and transmission;and pollutant decomposition. These functions are largely lost when development strips away native soil and vegetation and replaces it with minimal topsoil and sod. Not only are these important stormwater functions lost,but such landscapes themselves become pollution- generating pervious surfaces due to increased use of pesticides,fertilizers and other landscaping and household/industrial chemicals,the concentration of pet wastes,and pollutants that accompany roadside litter. Establishing soil quality and depth regains greater stormwater functions in the post development landscape accomplishes the following goals: 1. provides increased treatment of pollutants and sediments that result from development and habitation, and 2. minimizes the need for some landscaping chemicals,thus reducing pollution through prevention. Applicability • Soil amendments are required for the disturbed areas of sites subject to Minimum Requirement #5,as outlined in Volume I. • For sites where Minimum Requirement#5 does not apply, if the site is acceptable for traditional lawn installation,then a compost-amended soil lawn is strongly recommended. A compost- amended lawn will drain equally well,while providing the incidental stormwater retention and detention benefits. • If the site being considered does not drain well, an alternative to planting a lawn should be considered. If the site is not freely draining,and turf replacement is still being attempted, compost amendment will still provide stormwater benefits but should be incorporated into the soil at a reduced ratio of no more than 30 percent by volume. This upper limit is suggested in the Pacific Northwest because extended saturated winter conditions may create water logging of the lawn. • Soil amendments shall be used in areas that are to be incorporated into the stormwater drainage system such as runoff dispersion areas,vegetated channels,rain gardens,and bioretention areas and also into the lawn and landscape areas of the development. (Note: See the design criteria Volume ill—Flow Control and Treatment 12-3