SAM Structural Stormwater Controls (SSC) Science Review & Synthesis Study

OCI recently completed important research to inform recommendations for Structural Stormwater Control (SSC) requirements for the Municipal Stormwater Permit.

In 2018, municipal stormwater permittees (MS4) in Western Washington requested a stakeholder group work on policy and technical issues related to future SSC requirements. They had questions about where the Washington State Department of Ecology (Ecology) got their permit requirements and how they came up with the basis for the requirements. In response to this request, Ecology in 2020 selected the City of Tacoma and OCI team to collect and summarize existing scientific findings on SSCs. OCI was the prime for the consultant team with subconsultants from Parametrics, Aspect Consulting, GeoSyntec, and Cascadia.

What are Structural Stormwater Controls (SSC)?
SSC requirements aim at retrofitting existing unmanaged and/or inadequately managed developed areas to reduce adverse impacts to the watershed hydrology and pollutant discharges from storm sewer systems not adequately addressed in other parts of the permit-required stormwater management program.

Using Science to Inform Policy
The goal of this project was to develop an objective science-based white paper to inform Permittees on the selection of future SSC projects and provide the scientific basis to inform future policy discussions, expected in the Fall of 2021.

OCI’s work focused on developing responses to two categories of research questions: (1) The intended stormwater-related benefits of SSC project/activity types; and (2) The selection and application of SSC project/activity types. The research questions were answered using information gathered from a detailed literature review, including the permit history and events relevant to the SSC requirements, journal articles, reports, permit documents, statistical analysis of data collected, interview responses from Permittees and Ecology, and input from the consultant team and a technical advisory committee (TAC).

Study Outcomes and Next Steps
OCI and the team published a white paper that provides responses to the research questions along with a summary of the information collected during the study. A copy of this document along with OCI presentations about the study is located on the Washington Stormwater Center website (link below). A key aspect of this study is that the TAC arrived at a consensus that the contents of this white paper reflect the status of relevant research. Ecology is in the process of forming a policy advisory committee to discuss and provide recommendations regarding the SSC requirements for the next permit based on the information provided in OCI’s white paper.

The published white paper and presentations are available on the Washington Stormwater Center website: municipal/smap/