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Groundwater Monitoring and Modeling for Sea Level Rise Impacts

Solution Overview

San Francisco's Port, Department of Building Inspection, and Office of Resilience and Capital Planning are conducting groundwater data collection and modeling efforts to better understand the impacts of rising groundwater at the shoreline, including liquefaction risks.[7] This approach addresses a critical climate adaptation challenge, as rising groundwater levels in the San Francisco Bay Area could impact twice as much land area as coastal flooding alone over the next century, putting more than 5,200 state- and federally-managed contaminated sites at risk.[6] Higher sea levels will also increase the elevation of the groundwater table, increasing the susceptibility of some soils to liquefaction during an earthquake.[7]

Technical Components

The groundwater data collection and modeling initiative builds upon extensive technical foundations established through regional research efforts. Pathways Climate Institute LLC and the San Francisco Estuary Institute gathered and analyzed multiple data sets and collaborated with city and county partners to analyze and map the existing highest annual shallow groundwater table and its likely response to future sea-level rise.[4] The project produced web maps showing existing depth to groundwater and future conditions under various sea-level rise scenarios.[4] The effort produced existing and future condition depth to groundwater GIS data in geodatabase format.[4] Pathways developed GIS layers of existing shallow groundwater surface that represents the water table at its highest elevation during wet winters, when emergent groundwater is first likely to occur.[5] The existing groundwater surface was modified in response to sea level rise scenarios to represent a range of future conditions, with sea level rise scenarios selected to pair with sea level rise inundation mapping presented in the Adapting to Rising Tides Shoreline Flood Explorer.[5] The USGS model MODFLOW was used to estimate the depth of the present-day coastal water table and the future response of groundwater levels to a range of sea-level rise scenarios for the entire California coast.[2] The extremely high resolution of the model allows it to capture the influences of topography and drainage networks on groundwater levels that could help pinpoint areas where ponding at land surface may occur, or where the water table intersects shallow, sub-surface infrastructure, such as basements, parking garages, buried utilities, road beds, and other infrastructure.[2] A unique aspect of the MODFLOW model work is that it covers the State of California at high resolution by stitching together overlapping model domains.[2] The Bay Area groundwater risk maps were based in part on a 2019 analysis led by Hill of more than 11,000 measurements of groundwater levels in the Bay Area.[6] The groundwater measurements showed places where weird stuff is happening that models wouldn't predict, such as finding extra high groundwater levels on the uphill side of a compacted roadbed, or places where groundwater is surprisingly low because a leaking sewer pipe is allowing groundwater to drain away.[6] A team of geologists and hydrologists assessed the impacts of sea-level rise on liquefaction by computing changes in liquefaction potential index for 347 boring sites around the San Francisco Bay for groundwater table models developed for current and increased sea-levels of up to 5 meters for the magnitude 7.0 HayWired earthquake scenario.[3]

Implementation Details

The groundwater data collection and modeling work emerged from stakeholder engagement for the releases of the USGS HayWired earthquake scenario and the Coastal Storm Modeling System projects, in which local planners and engineers asked where, why, and by how much liquefaction hazards may change due to sea-level rise in the future.[3] Pathways Climate Institute and the San Francisco Estuary Institute collaborated to extend their groundwater research to better support regional adaptation planning, funded by pro bono contributions from Pathways and a California Resilience Challenge Grant submitted on behalf of four Bay Area counties: Alameda, San Mateo, San Francisco, and Marin County.[5] The initial effort covered four counties (Alameda, Marin, San Francisco, San Mateo) and was funded by the Bay Area Council's California Resilience Challenge.[4] The mapping focused on the San Francisco Bay side of each county and did not include the Pacific coastline of Marin, San Francisco, nor San Mateo Counties.[4] This project included digitizing hundreds of soil borings to fill data gaps and improve the assessment, extensive coordination with city and county staff to ensure the end products meet their needs, and coordination with at-risk communities on messaging and outreach strategies.[5] City and county representatives provided essential support by gathering data and reviewing depth-to-groundwater maps.[4] This project included a Technical Advisory Committee with academics Dr. Kristina Hill (Professor at UC Berkeley) and Dr. Kevin Befus (Assistant Professor at University of Arkansas) and scientist Dr. Patrick Barnard (USGS) who are pushing the science of groundwater's response to sea level rise forward.[5] A research team led by UC Berkeley professor Kristina Hill published a report and a series of searchable maps earlier in 2023 highlighting Bay Area regions that face the highest risk of flooding due to rising groundwater.[6] Hill and collaborators at the Pathways Climate Institute and the San Francisco Estuary Institute published the groundwater risk report and maps.[6] U.S. Geological Survey scientists studied the impacts of sea-level rise on liquefaction severity around the San Francisco Bay Area, California for the magnitude 7.0 HayWired earthquake scenario along the Hayward Fault.[3]

Benefits and Impacts

The groundwater data collection and modeling efforts provide critical insights into liquefaction risks and infrastructure vulnerabilities. The sensitive sites are artificial fill around the Bay Area margins where the water table is already shallow.[3] For the HayWired scenario ground shaking, these sites may already have a severe liquefaction hazard, and with 1 meter of sea-level rise, the hazard at more of these sites increases to severe.[3] As groundwater levels rise due to sea level rise, liquefaction zones are expected to increase in size.[7] Findings show that low-lying coastal communities and critical infrastructure are most at risk of future rising water tables, including areas in San Francisco Bay, Santa Barbara, Ventura, Port of L.A., Long Beach, Seal Beach, San Diego Bay and San Francisco and San Diego Airports.[2] According to a 2023 study by the San Francisco Estuary Institute, scientists expect rising groundwater levels to occur many years before they reach the surface in Alameda, Marin, San Francisco and San Mateo counties, based on current groundwater levels and global climate projections.[1] A new paper, Increasing threat of coastal groundwater hazards from sea-level rise in California, published in Nature Climate Change on August 17, 2020, describes a numerical model that estimates the depth of the present-day coastal water table and the future response of these groundwater levels to a range of sea-level rise scenarios for the entire California coast.[2] The work is the first to quantify the potential coastal flooding that could result from water table rise along the entire coast of California and serves as a possible model approach for building out an assessment across the entire United States coastline.[2]

Climate Adaptation Relevance

This groundwater data collection and modeling initiative directly addresses the compounding climate hazards of sea-level rise and seismic liquefaction risk at the San Francisco shoreline. The approach enables city officials to understand how rising groundwater will interact with existing infrastructure vulnerabilities and earthquake hazards, providing essential data for adaptation planning in low-lying coastal areas where the water table is already shallow.

Business Analysis

Federal legislative support for groundwater monitoring is expanding, as U.S. Reps. Kevin Mullin, D-San Mateo, and Andrew Garbarino, R-N.Y., introduced legislation in June that would set aside $5 million over the next year for the United States Geological Survey to study and map groundwater rise nationally through 2100.[1] USGS scientists would also need to recommend mitigation strategies for communities and create a public-facing website displaying their maps, and within three years, the agency would need to submit a report of its findings to Congress.[1] Regional collaboration models have proven effective, as demonstrated by the California Resilience Challenge Grant funding mechanism that supported the initial four-county assessment.[5] Last year, the San Francisco Bay Area Planning and Urban Research Association, or SPUR, partnered with local organizations to publish a study about East Palo Alto's flood risk due to groundwater rise.[1] The pro bono contributions from Pathways Climate Institute alongside public grant funding demonstrate a hybrid financing model that leverages both private sector expertise and government resources to advance critical climate adaptation research.[5]

[1]: mercurynews.com [2]: usgs.gov [3]: usgs.gov [4]: sfei.org [5]: sfei.org [6]: news.berkeley.edu [7]: sfport.com

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