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 / Public Works / Storm/Surface Water Program / Watershed-based Stormwater Retrofit Plan & Pre-design / Infiltration Feasibility Assessment

Infiltration Feasibility Assessment

Pursuant to Mukilteo Municipal Code 13.12.040, the City has adopted the most recent Washington State Department of Ecology Stormwater Management Manual for Western Washingon (SWMMWW) which includes stormwater development standards.  The SWMMWW requires Low Impact Development (LID), including stormwater infiltration facilities, as the preferred alternative.  It also requires site specific exploration for development and redevelopment projects. In order to establish a baseline for screening future LID approaches, a City-wide assessment of infiltration feasibility was performed and included the feasibility of both shallow and deep infiltration. The assessment refines the understanding of Mukilteo's geology which affects stormwater infiltration facilities but does not replace the requirement for site-specific  analysis.  The Infiltration Feasibility Report (link to PDF) is Appendix B of the Pre-Design Report. 
Shallow infiltration generally relies on vertical infiltration directly from the LID facility (typically a bioretention swale, tree-box, or pervious pavement) and is generally suitable in relatively flat areas with permeable surface soils. For the shallow infiltration assessment, feasibility was considered a function of surficial permeability, surface slope gradient, and steep slope hazards factors. Broadly speaking, glacial till soils in Mukilteo were found to be less permeable and thicker than current Washington State Department of Natural Resources maps show.  Additionally, the advance outwash is thinner than originally believed. The assessment also incorporated guidelines found in the SWMMWW regarding steep slope setbacks and location of infiltration facilities on surface slope gradients.  In Mukilteo, the low-permeability glacial till soils at the surface combined with the proximity to steep slope hazards led to the finding that few areas are categorized as “good” for shallow infiltration. However, the presence of glacial till soils and/or proximity to steep slopes do not alone mean a site is infeasible for LID best management practices.  It is possible the results of the broad scale of assessment may in fact end up being not applicable at the site scale. For each new development and redevelopment project, a site-specific investigation is still required, in accordance with the SWMMWW.

Deep infiltration is considered suitable when a permeable, unsaturated soil zone (referred to as a receptor horizon) exists beneath low-permeability surface soils. Deep infiltration systems use a deep well or trench to convey treated stormwater from the LID facility to the deeper permeable soils. For this assessment, deep infiltration feasibility was considered a function of steep slope hazards and potential for deep infiltration receptor horizon factors.
For each factor of shallow and deep infiltration feasibility, geographic information system (GIS) maps were created and the infiltration feasibility of combinations of the factors described above (referred to as hydrogeomorphic units) was evaluated. The results are summarized below:
  • Shallow Infiltration Feasibility: Most of the City does not appear to be suitable for shallow infiltration due to the presence of low-permeability glacial till soils at the surface and/or proximity to steep slope hazards including landslides. There are small areas considered moderate or good for shallow infiltration scattered throughout the City. ​However, the presence of glacial till does not in itself make a site infeasible for LID best management practices. For each new development and redevelopment project a site-specific investigation would still be required.  Also, the NPDES permit and the stormwater manual already address steep slope issues by creating default distances from steep slopes that automatically determine LID is not feasible for a site if it is located closer to a steep slope than the default distance. This may or may not be reflected in the City-wide shallow infiltration feasibility assessment.

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  • Deep Infiltration Feasibility: Assessment of deep infiltration feasibility is uncertain in any specific City area because of the limited availability of reliable subsurface information. However, recently acquired regional data on the geology of the City’s ravine slopes and deep explorations conducted for the City’s Stormwater Retrofits program suggest a low potential for deep infiltration below most uplands portions of the City. Because of the potential for steep slope hazards including landslides, deep infiltration is generally not feasible along the City’s shoreline and within or near the steep ravines and gulches found in the City.  However, a site-specific analysis could result in more a certain finding that deep infiltration is feasible. 

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These feasibility assessments provided are suitable for identification and evaluation of potential stormwater infiltration solutions. Site-specific geologic and geomorphic mapping and subsurface explorations, infiltration testing, and additional analysis are recommended for site-specific activities.

Given the results of the infiltration study and analysis done to date it appears that for much of the study area and perhaps for most of the city typical low impact development BMPs generally will not be appropriate.  However, a tailored approach that focuses on retrofitting existing detention facilities, but also takes advantage of the existing data and analysis, could reveal the feasibility of using LID-related approaches customized for Mukilteo that can be effective in limiting peak flows and improving water quality.  These customized approaches will in all likelihood be similar to what is proposed for the Staybridge Pond.  In fact, the city has already partially embarked on this path by retrofitting two existing stormwater detention ponds using a different Ecology low impact development grant. 

Despite the appearance that Mukilteo’s landscape generally does not support widespread implementation of low impact development BMPs, the city will not abandon efforts to construct typical low impact development BMPs within its jurisdiction. It’s entirely possible site specific analysis can reveal locations where those BMPs would be both feasible and effective.​

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