Sustainable Planning for Climate Change Resiliency – Assessing the Benefits of Green Streets to Mitigate Future Stormwater Impacts
The City/County Association of Governments of San Mateo County (C/CAG), in cooperation with its 21 member agencies, is leading the development of a Sustainable Streets Master Plan (SSMP) for all of San Mateo County. Funded by a Caltrans climate change adaptation planning grant, the goal of the SSMP is to improve water quality and promote resilience to the impacts of climate change, while creating approaches to serve as models that can be used by other entities throughout California. The SSMP identifies and prioritizes opportunities for sustainable streets throughout the county that incorporate green infrastructure with complete street designs. These sustainable streets provide multiple benefits beyond water quality and climate change resiliency, including the integration of stormwater and transportation planning efforts (e.g., bicycle and pedestrian plans), reduced flooding, and improved aesthetics and public safety.
A comprehensive hydrologic, water quality, and BMP modeling system was developed by C/CAG to support member agencies with green infrastructure (GI) planning and the demonstration of pollutant load reductions that meet future goals set by the Municipal Regional Stormwater Permit (MRP) and TMDLs. To support the SSMP, this modeling system was used to simulate future projections of climate change impacts, and the benefits of all GI projects to mitigate associated increases of stormflows. In order to predict future precipitation associated with climate change, an ensemble of 20 climate change projections (i.e., 10 models × 2 future pathways) from Cal-Adapt was considered. Cal-Adapt synthesizes climate change projections and research from California’s scientific community, with funding and advisory oversight by the California Energy Commission. The analysis considered two future projection scenarios that represent best- and worst-case projections of future carbon emissions. Both scenarios were used to represent spatially variable precipitation throughout the county, and the comparison of current and future modeled impacts on multiple storms typically used for flood management and BMP design, including the 2, 5, 10, 25, 50, and 100 year storms.
The above current and future condition scenarios were used to assess the benefits of GI in terms of mitigating the increased flows associated with climate change. First, all GI planned to address MRP and TMDL goals within each municipal jurisdiction were modeled to quantify the stormwater capture based on future climate change precipitation scenarios. These results were evaluated for hundreds of watersheds throughout the county to provide an understanding of the spatial variability of climate change impacts and GI benefits. Next, the modeling system was used to quantify only the change in runoff from the roadway network, and the isolated benefits of green streets to mitigate the change in runoff. This analysis supported the assessment of benefits of green streets included in the SSMP, and the reduced strain on the storm drain systems that directly receive much of the roadway runoff. Results of both scenarios demonstrated a significant benefit of GI for reducing climate change impacts for smaller and more frequent storms (i.e., 2, 5 and 10-year storms). For larger and less frequent storms, GI had less impact, but still demonstrate benefits. GI was shown to have a greater benefit for mitigating climate change impacts on the roadway network, including 100% mitigation of runoff increases for the 2-year storm, and similar significant benefits for larger storms.
The goal of the presentation is to provide guidance to audience members in terms of how to quantify the resiliency of municipal or watershed BMP or GI implementation plans in the face of the potential impacts of climate change. These benefits will add to sustainability conversations and provide justification for funding of BMP and GI implementation, especially structural management practices that provide stormwat
John Riverson is an engineer and principal at Paradigm Environmental with 21 years of experience in water modeling, stormwater management, data analytics, and systems design and application.
Matthew Fabry serves as Manager for the San Mateo Countywide Water Pollution Prevention Program, a program of the City/County Association of Governments of San Mateo County, which assists the 21 San Mateo municipalities with stormwater compliance issues. He is currently Chair of the Bay Area Stormwater Management Agencies Association and has over 25 years of experience in water quality and stormwater management. Matthew has worked in municipal, regulatory, and consultant capacities, holds degrees in environmental engineering and music, and is a registered civil engineer in the State of California.