Setting the New Standard for Strategic TMDL Compliance: Rapid, High Resolution Watershed Master Planning
The City of San Diego (City) recently led the completion of a number of Water Quality Improvement Plans (WQIPs), pursuant to the San Diego Regional Municipal Separate Storm Sewer System (MS4) Permit and the applicable total maximum daily load (TMDL) requirements. While the WQIPs prescribe the type and extent of implementation for various water quality strategies, they do not currently contain sufficient detail to identify specific project-by-project structural priorities to meet interim and final MS4 and TMDL milestones. To provide the high-resolution data needed to drive systematic and cost-effective implementation of green infrastructure (GI) projects, the City has developed a comprehensive and dynamic Watershed Master Plan (WMP) in the Chollas Creek Watershed that quantifies progress towards water quality goals and incorporates synergies with other municipal programs. The WMP has the capability to dynamically assess the cost-based water quality benefits of specific GI projects against one another and incorporates a robust prioritization logic that realizes the complex nature of implementing retrofit GI facilities within a highly urbanized environment. Integral components involved in the development of the WMP include: (1) generation and processing of seamless, high-resolution datasets, (2) incorporation of previously established models for the watershed (i.e., TMDL, WQIP, etc.), (3) identification of all feasible structural best management practices (BMP) locations (i.e., distributed low impact development (LID), green streets and regional multi-use treatment areas), and (4) developing a framework to assess the benefits associated with the thousands of potential BMP implementation scenario permutations. The models developed for the WMP included innovative updates to the EPA SUSTAIN model to allow for BMP-specific performance data and refined costing information to be incorporated. The model also employed a multi-tiered optimization approach that assesses BMP implementation at different spatial scales (i.e., catch basin, sub-regional, in-stream, etc.) and accounts for the interdependency of the stormdrain system at the watershed scale. The model results were incorporated into a prioritization framework that included the ability to re-rank projects when additional critical City-specified decision variables (i.e., synergies with capital improvement projects (CIPs), road pavement moratoriums, disadvantaged community grant funding opportunities, etc.) were considered. The resulting prioritized projects were incorporated in a web-based GIS interface that can be utilized in conjunction with other City maps and datasets as part of both the adaptive management and asset management processes. The WMP demonstrated that rapid watershed-scale assessment of retrofit GI opportunities is a critical linkage between the complex, watershed-scale regulatory requirements in San Diego and the identification of specific GI projects that will most efficiently achieve progress towards compliance within a municipality. The WMP highlights the interdependent nature of selecting BMPs to meet water quality requirements, demonstrating that saying “no” to one project inherently means saying “yes” to another potentially more costly and less effective project. The multi-beneficial nature of the WMP will also allow for more efficient use of funds and resources, coordination with other capital improvement projects, and a dynamic spatial BMP identification and tracking database. Ultimately, the output of this project gives the City a project-by-project roadmap that is prioritized to implement high-impact and high-efficiency BMPs first, leaving less desirable projects for later implementation. This approach ensures that the City is achieving the most rapid pace towards compliance possible and may ultimately set the stage for “BMP practicability” discussions with key stakeholders in the years to come.