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February 26, 2015
Polluted stormwater runoff is the largest threat to Puget Sound’s water quality, according to the Puget Sound Partnership.
The state of Washington has mandated that Puget Sound be restored to a healthier condition by 2020. The work will be carried out through the Puget Sound Partnership’s “action agenda.” Its strategic initiatives include a focus on stormwater management for basins and watersheds.
The urbanization of watersheds has caused degraded water quality, deteriorated habitat and increased flood flows. As rivers and streams continue to fall short of water quality standards, communities have shifted away from efforts to control point sources of pollution, such as from factories and wastewater plants, to efforts to reduce non-point sources of pollution. For urban watersheds, this means better treatment of stormwater runoff, often through retrofit projects of previously developed lands.
Watershed-scale stormwater planning provides a useful framework to integrate the many state and federal regulatory drivers that communities face surrounding clean water.
Stormwater management at the watershed scale is dictated by the Washington State Department of Ecology’s Phase I and Phase II municipal stormwater permit. Each permittee is required to participate in the development of strategies to prevent future impacts and address existing impacts.
As with any mandate, funding is always a concern. Fortunately, Ecology was selected by the U.S. Environmental Protection Agency as a lead organization in the watershed protection and restoration area of emphasis. With this selection came grant funding through the National Estuary Program (NEP) to fund watershed protection and restoration projects.
Two communities covered under the Phase II permit, the city of Kirkland and Thurston County, were each awarded NEP grants administered through Ecology.
The projects that were funded were stormwater retrofits within basins that had documented water quality issues. Kirkland and Thurston County each selected the consultant team of AHBL and Northwest Hydraulic Consultants to identify stormwater retrofits using a basin-wide approach for two distinctly different watersheds.
Thurston County’s project evaluated the sensitive Woodard Creek Basin that drains to a protected shellfish district, Henderson Inlet. Urban land uses, with the residential, commercial, and industrial development concentrated in Lacey and Olympia, cover 16 percent of the drainage basin, with the remaining areas being more rural in nature. Its primary water quality issue is bacteria.
The city of Kirkland’s project focuses on the Totem Lake sub-basin, which was developed between the 1960s and 1980s without the benefit of stormwater treatment. It is part of the larger Juanita Creek Basin and one of the most densely developed portions, with residential and commercial uses. High flows have caused flooding and water quality problems and limit the ability of the area to receive more of Kirkland’s anticipated growth.
While the two basins are quite different, the AHBL-NHC team determined that the following six-step framework was adaptable to each project.
1. Define watershed restoration objectives. First, the shortcomings of the existing basin and stormwater system must be understood.
Many sources are available, including previous characterization studies, water quality assessments, drainage complaints and documented reports of flooding. The goals for the retrofits will then center around three primary areas: water quality (i.e., reduce pollutants of concern), physical/hydrological (i.e., reduce flood damage), and community (i.e., increase downstream shellfish harvesting opportunities).
2. Apply GIS-based desktop analysis to evaluate sites with restoration potential. With basins covering hundreds to thousands of acres, using a systematic desktop analysis with existing GIS datasets is a key to efficiency.
The desktop analysis begins by defining the types of retrofit site parcels, like existing stormwater facilities, right-of-way segments or pollution hot spots. Because this GIS analysis can create a pool of hundreds of potential sites, additional criteria for each type of retrofit parcel are created, such as eliminating parcels with critical areas.
Using this process on Woodard Creek took the potential retrofit sites from 400 to 66.
3. Conduct site feasibility and detailed restoration assessments on sites identified during the GIS analysis.
Once the potential retrofit sites are identified, the field work begins. The detailed site assessment includes a field investigation by experienced stormwater engineers based upon pre-established feasibility criteria. Collecting stakeholder input at this stage is also important because residents often have valuable information about the sites.
Next, screening factors are developed with scores in order to qualitatively rank the sites. For Totem Lake, screening factors included infiltrative capacity of the soil and ownership of the parcels (city-owned vs. private).
Once the potential sites are scored, those with the best scores continue on in the prioritizing process.
4. Develop location-specific retrofit concepts. Now that the scoring process has produced the most promising sites, a conceptual stormwater retrofit design is identified.
Inputs to determine which best management practices are appropriate include the area available on the parcel, upstream tributary area, soil type and site observations from the field investigations in step 3. Best management practices may include biorentention with infiltration, bioretention without infiltration, vegetated filter strips or others.
5. Evaluate location-specific retrofit concepts. This step evaluates if the retrofit design will meet the basin retrofit objectives in step 1.
Because both Woodard Creek and Totem Lake had water quality goals to reduce pollutants, a model was used to calculate expected pollutant loads and their anticipated reductions based on the best management practice selected.
The goal for Totem Lake is also to reduce flooding, and so a hydraulic model was developed to calculate the reduction in flow from the various retrofit alternatives.
6. Rank projects and select a preferred list of capital improvement projects for pre-design. The final ranking of the top sites that remain occurs in this step.
For Woodard Creek, land ownership and pollutant load reduction were among the final criteria. Public land ownership presented a better near-term opportunity to implement the retrofit for Thurston County. However, in Totem Lake, Kirkland was open to approaching private landowners to consider a partnership to accomplish the retrofit.
Thurston County and Kirkland’s efforts to address stormwater issues are paving the way for a sustainable approach to planned redevelopment. Through a six-step site selection and pre-design process, these two projects provide the foundation for a functional stormwater retrofit program that addresses current problems while best taking advantage of local conditions and opportunities.