Green Infrastructure Optimization and Planning around San Francisco Bay


Projects > Stormwater Management > Implementation Planning

Investigating Economic Benefits of Countywide vs. Jurisdictional Implementation Strategies for A San Francisco Bay TMDL

End Client: City/County Association of Governments (C/CAG) for San Mateo County, CA

Paradigm supported the C/CAG forSan Mateo County, CA in developing a countywide watershed and stormwater management modeling system, which was also applied for Reasonable Assurance Analysis(RAA) and development of a Green Infrastructure (GI) Plan. The existing-condition hydrology and water quality watershed model integrated insights from decades of local research, monitoring, and modeling conducted by several agencies. With a San Francisco Bay TMDL as the primary policy driver, one of the interesting research questions investigated was: Can we quantify the potential cost savings of a countywide implementation approach vs. jurisdictional-scale implementation to achieve the same water quality goal?

The modeled baseline scenario was a continuous simulation of runoff volume, sediment load, and sediment-associated PCB load for water year 2002 (10/1/2001 –9/30/2002), the critical-condition year identified in the Bay TMDL. The modeled baseline also included: (1) existing facilities, (2) code-required GI for projected future new and redevelopment areas, and (3) three large identified regional projects that provide stormwater capture, infiltration, and treatment from multiple jurisdictions. The Municipal Regional Stormwater Permit (MRP, Order No. R2-2015-0049) required that a portion of the TMDL allocation (17.6%) be met through implementation of GI by 2040. The optimization objective was to identify how much additional GI (expressed as low-, medium-, and high-priority green streets opportunity) was required to achieve water quality objectives while minimizing overall lifecycle costs to do so.

There are many possible ways to achieve a 17.6% load reduction for San Mateo County; however, collective stakeholder interests are best served if implementation responsibility is equitably allocated. Cost-benefit optimization for this study was formulated with two different sets of constraints. The “Proportional” approach stipulates that each jurisdiction must individually achieve ≥17.6% load reduction. On the other hand, the “Targeted” approach achieves the 17.6% load reduction countywide by allowing the management burden to vary freely across jurisdictional boundaries. We hypothesized that the targeted approach could provide significant cost savings over the proportional approach, especially where pollutant sources are spatially concentrated. Four modeling scenarios were configured for this analysis as follows:

Scenarios 1 and 2, with a cohesive sediment load reduction objective, represent the most conservative approaches. Those scenarios assume that given the uncertainties about PCB source locations, targeting an overall 17.6% load reduction of cohesive sediment in general (silts and clays) achieves the PCB load reduction objective for GI. Scenarios 1 and 2 are proportional and targeted, respectively.

Scenarios 3 and 4 assume that PCB sources are spatially distributed according to locally derived spatial distributions of potential PCB hot spots. Optimization process targets individual subwatersheds with the highest likelihood of having PCBs. These two scenarios highlight the potential cost savings (relative to Scenarios 1 and 2) that could be realized if PCB sources are identified and managed. Scenarios 3 and 4 are proportional and targeted, respectively.