Background/Objectives

Sea-level rise (SLR) physically affects coastal areas in two ways: (1) surface inundation/flooding of near-shoreline areas by rising seawater, and (2) an increase in groundwater level due to the rising head imposed by changes in mean sea level. Both processes have the potential to impact coastal community assets and critical infrastructure, such as roads, stormwater and sanitary sewer systems, recreation areas, structures, and more. In addition, these processes have the potential to impact existing and planned remedial actions at near-shore contaminated properties. Although surface water inundation can potentially be controlled by engineered structures and nature-based solutions, rising groundwater levels present greater challenges because groundwater rise will be a permanent condition and fewer options for adaptation measures exist for groundwater management.

The State of California has been at the forefront of ensuring resilience to climate change with the formation of the Ocean Protection Council in 2004. However, the State has made it clear that responsibility for SLR preparation lies with local governments. In 2021, the Sea Level Rise Mitigation and Adaptation Act provided a broad directive for coastal communities to evaluate the impacts of SLR, and in October 2023, Senate Bill 272 provided a specific directive for coastal communities to conduct SLR planning and adaptation by 2034. In 2022, environmental regulatory agencies in California released draft guidance for conducting SLR Vulnerability Assessments (SLRVAs) and amended waste discharge requirements for coastal landfills in the San Francisco Bay Area (SF Bay Area) that requires preparation of Long-Term Flood Protection Plans  that include an evaluation of landfill resilience (containment and stability) from SLR-induced surface water inundation and groundwater rise/emergence.

We have performed site-specific SLRVAs throughout the SF Bay Area and developed phased, prioritized adaptation plans to address SLR for municipalities, sewer districts, landfills, and contaminated sites in the SF Bay Area to ensure resilience to climate change under multiple SLR scenarios. Each project has demonstrated a variety of both physical and non-structural constraints that require a unique site-specific approach.

Approach/Activities

We performed SLRVAs that have included site-specific analysis of historical water level trends, tidal influence, groundwater flow conditions, and future development plans. The response of the groundwater system to SLR was evaluated with a flux-controlled approach (i.e., linear superposition of SLR magnitudes) and modeling that considers the dissipation of the groundwater response with distance from the shore under various climatic conditions (recharge). We generated maps of predicted surface water inundation and groundwater rise and emergence to quantify vulnerability, and we have identified a suite of site-specific adaptation actions that consider local and regional adaptation planning objectives. We developed long-term flood protection plans that include phased site-specific prioritized adaptations to address SLR impacts for key assets identified at each site.

Results/Lessons Learned

Our collective experience and knowledge from these projects has revealed that groundwater rise and emergence may result in greater impact to coastal communities and contaminated sites than surface water inundation alone. Groundwater rise and emergence exacerbates surface water flooding and is a more challenging aspect of adaptation planning because there are limited approaches available to manage groundwater. Using results and information from overly conservative and inconsistent models and screening tools has the potential to unnecessarily alarm local communities, result in unsustainable remediation approaches, and cause a burden on financial resources. Understanding the magnitude of SLR that would cause an impact, regardless of the projected timeframe at which that might occur, is of great value in the adaptation planning process. Prioritizing adaptation responses is necessary to ensure uncertainties are addressed on a site-specific basis. Lastly, because the response of groundwater levels with distance from the shoreline is more dependent on future climatic conditions (i.e., recharge) than SLR, site-specific evaluations of groundwater rise and emergence are necessary to accurately predict future conditions and potential impacts on coastal sites.