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Description: Everett Stormwater Infiltration Infeasibility AssessmentUnder contract with Robin Kirschbaum, Inc. (RKI), Kindred Hydro, Inc. (KindredHydro) has conducted a stormwater infiltration infeasibility assessment for theCity of Everett (City). Infiltration refers to the practice of directing stormwaterrunoff to the subsurface and letting it seep into groundwater. The WashingtonState Department of Ecology has defined low-impact development (LID)infeasibility criteria in the Washington State Department of Ecology StormwaterManagement Manual for Western Washington (Ecology Manual). Theseinfeasibility criteria are described in Chapter 3.1.1 of Volume 3 and Chapters 5.3and 7.4 in Volume 5 of the Ecology Manual. The City has adopted the EcologyManual as the City of Everett Stormwater Management Manual (SWMM),effective December 31, 2016. This analysis is limited to the infiltrationinfeasibility criteria provided in the SWMM that have been mapped at this time.The SWMM details numerous factors that shall be used in determining theinfeasibility of infiltration-based LID Best Management Practices (BMPs). Thesefactors are based on site conditions, such as topography, presence ofcontamination, proximity to wells; as well as geotechnical and hydrogeologicfactors. Some, but not all of these infeasibility criteria, are amenable to mappingbased on available Geographic Information System (GIS) data. Those factorsinclude:• Surface slope,• Landslide hazards,• Erosion Hazards,• Depth to groundwater,• Landfills and contaminated soils.This map is considered applicable for LID BMPs that rely on infiltration, includingthe following:• BMP T7.10: Infiltration basins,• BMP T7.20: Infiltration trenches,• BMP T5.10A: Downspout full infiltration,• BMP T5.14A: Rain gardens,• BMP T5.14B: Bioretention,• BMP T5.15: Permeable pavement, and• BMP T7.30: Bioretention cells, swales, and planter boxes.The infiltration infeasibility map identifies the following two categories ofinfiltration feasibility within the City:• Infeasible for Infiltration: Further infiltration feasibility assessment is notrequired to demonstrate infeasibility for Minimum Requirements (MRs) #5(On-site Stormwater Management), #6 (Runoff Treatment), or #7 (FlowControl).• Potentially Feasible for Infiltration: Site-specific assessments arerequired to determine infiltration feasibility within Potentially Feasibleareas to meet MR #5-7, if applicable, in accordance with the SWMM.
Service Item Id: 9f8a6e904fc24654befeb40f2bb9deda
Copyright Text: City of Everett Public Works
Robin Kirschbaum, Inc. (RKI)
Kindred Hydro, Inc.