Salish Sea Ecosystem Conference

Washington State Phase I county watershed-scale stormwater planning studies: a long term plan to identify stormwater management strategies to improve receiving waters

Washington State Department of Ecology’s 2013-2018 NPDES Municipal Stormwater permittees conducted detailed hydrologic modeling studies to demonstrate how planned development could be accommodated while restoring the beneficial and designated uses to the receiving waters in urbanized watersheds. The jurisdictions used modeling tools including HSPF hydrologic modeling, in-stream ecological targets, and cost optimization tool to determine the most cost effective set of infrastructure to achieve in-stream ecological targets). This talk focusses on the lessons learned from those plans, looking across the plans for similarities and differences. Each of the four counties (Snohomish, King, Pierce, Clark) selected a medium sized (10+ square mile) watershed which included urban growth areas designated pursuant to the Growth Management Act, and therefore known to be under pressure for development in the near future. Counties and cities in Washington are required to plan for and accommodate growth, cities are required to allow high intensity to meet growth targets dictated by the State of Washington. The watersheds had unique characteristics, but all are already partially urbanized. The counties created models to test a suite of strategies in various scenarios to see if water quality standards were, or could be, met. The modeling reports for the three plans submitted so far (King County will submit their plan in spring 2018) showed that current and future stormwater impacts caused by development in these watersheds result in receiving water bodies that do not meet water quality standards, and actions beyond site and subdivision scale of stormwater management will be needed to make receiving waters healthy in urbanized watersheds. The models in all of the watersheds projected that riparian restoration (for temperature) and large amounts of additional stormwater detention and infiltration are needed to improve receiving water conditions. Other in-stream projects (not associated with managing municipal stormwater discharges) were also modeled as having near-term and cost-effective positive outcomes on receiving water bodies. The anticipated costs to recover from these impairments is tens of thousands of dollars per acre for watersheds in Snohomish and Clark Counties. The costs per acre for these typical Puget lowland and lower Columbia developing watersheds are significantly lower than for more developed basins. (City of Kirkland’s Juanita Creek Study estimated costs were approximately $300,000 per acre). While this demonstrates that current permit requirements are having a significant impact, the modeled additional effort to recover the beneficial uses are still well beyond current funding programs and approaches. We encouraged counties to look at the spectrum of strategies available, including structural retrofits, land use strategies and education and outreach. While some of these strategies could not be modeled it was acknowledged that, if properly implemented, they could accelerate the recovery of the receiving water. The basis of the modeling included a hydrologic focus since those stormwater hydrologic impacts on streams have long been acknowledged as the primary loss of salmonid habitat (highest existing beneficial use) in urban receiving waters. Accordingly, some of the common strategies included structural stormwater controls geared towards flow control. For instance, traditional detention was a widely used strategy. Low Impact Development (LID), where feasible, provided a large lift on a smaller relative footprint. If the watershed has infiltrative soils, infiltrating stormwater facilities were the most cost effective. One important strategy that the counties did not highlight in their scenarios was changing the land use designation or zoning codes established as part of the land use comprehensive planning process under the Growth Management Act. Comprehensive planning, and stormwater management are regulated under different laws and overseen by different state and local departments with separate administrative and public processes. Despite knowing that such changes could help protect water quality without the high capital project costs identified by the models, these non-structural strategies are difficult to commit to doing in a short amount of time, are difficult to predict into the future, and receiving water habitat has not to date been a priority in growth management planning. It is difficult for stormwater managers to cross this boundary of authority and responsibility as part of an exercise required by the MS4 permit. Development stormwater infrastructure requirements in western Washington result in stormwater detention and treatment infrastructure that’s intended to mitigate (hydrologic and water quality) development impacts. Due to this construct, cities and counties typically have no design for what their stormwater infrastructure will be, or how it will impacts receiving waters in the future, on a watershed scale. They rely mostly on private investment in stormwater infrastructure improvements needed to have healthy urban receiving waters. Without better long range planning, and a funding source in addition to private investments, and in-stream and buffer improvements, with a focus on what a receiving water needs, the stormwater efforts currently underway will likely not result in healthy urban receiving waters.