Linkages to Ensure Sustainability – Merging Flood Resiliency Planning, Green Infrastructure, and Water Quality Improvement
The San Mateo County Department of Public Works (County) recently initiated a Flood Resiliency Program to work with cities to address growing concerns with climate change and sea level rise, coupled with ongoing challenges managing flood control and water quality in each of the County’s watersheds. One of the first collaborative projects of this program is the development of a Flood Management Plan for Bayfront Canal, an area adjacent to San Francisco Bay impacted by stormflows from Redwood City, Menlo Park, Atherton, Woodside, and County Unincorporated Areas. The Canal experiences flooding on a regular basis, impacting several residential and commercial areas. As a parallel effort, the City/County Association of Governments of San Mateo County (C/CAG) is working with the County and cities to prepare models and perform a Reasonable Assurance Analysis (RAA) to evaluate the amount and cost of green infrastructure (GI) needed to provide necessary reductions of PCBs and mercury to meet requirements of Total Maximum Daily Loads (TMDLs) and the Municipal Regional Stormwater Permit (MRP). The FMP seeks to evaluate multi-benefit solutions to solving the flooding issues in the Canal, including the co-benefits of GI to reduce flood flows in addition to water quality benefits. An important tool in the evaluation of management scenarios for the FMP is a system of models that can assess watershed hydrology, hydraulics of the drainage system, and impacts of various flood control designs. This presentation will demonstrate the results of the technical analysis supporting the FMP, which combines traditional hydraulic modeling with new advanced approaches for representing GI and associated co-benefits for water quality and flood mitigation.
The first task of this effort was to develop a baseline hydrology model that predicts runoff volume and peak discharge values, which are key inputs for flood modeling analysis. Runoff timeseries were generated for subwatersheds using the calibrated hydrology and pollutant loading model used to support the RAA, based on the Loading Simulation Program C++ (LSPC). The LSPC results were also compared to the HEC-HMS hydrology model used to support the FMP.
The second task focused on the analysis of runoff volume and peak flow reductions to the Canal. To support the RAA, a model was developed to represent various types of GI implemented at increasing levels throughout the watershed. Based on the System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN), the model simulates the benefits of GI including the reduction of stormwater volumes and loads of sediment, PCBs, and mercury. To meet requirements of the MRP and TMDL for PCBs, a GI implementation scenario was identified that can provide 17.6% cohesive sediment load reduction. This scenario was evaluated for the FMP, in addition to another scenario that considered approximately double the amount of GI. Four design storm hydrographs were analyzed for flood control benefits of each GI implementation scenario, ranging from 2-year to 25-year storm events. The SUSTAIN model evaluated the reduction of these peak flows for input to a separate XP-SWMM model used to evaluate the extent of flooding in the Canal and neighboring flood control channels.
Results of the analysis demonstrated that smaller storms resulted in higher flow volume and peak flow reductions, as GI are most effective in treating runoff from small storms or early stages of larger storms. The benefits of GI implementation will be assessed in combination with other more traditional flood control designs to identify the most comprehensive, cost-effective, and sustainable projects to address the flooding in the Canal. The consideration of the dual benefits of GI to address pollutant load reductions and flood control mitigation provides further justification for the investment, and greater assurance that funding will be considered for GI through separate/parallel planning efforts.