“Shallow Groundwater Response to Sea-Level Rise”

This is a 67-page report prepared in 2022 for Alameda, Marin, San Francisco, and San Mateo Counties by the Pathways Climate Institute, the San Francisco Estuary Institute, and the leader of the Institute for Urban and Regional Development at UC Berkeley. Of the eight lead authors, there are three members with PhD degrees and four with Masters degrees. In addition to their degrees, two authors are also licensed Professional Engineers. The study focused on the San Francisco Bay side of each county and does not include the Pacific coastline of Marin, San Francisco, or San Mateo Counties.

Although the report is fairly specific to the Bay Area, in looking at the selections of what is covered and what was chosen to report on, many correlations to our situation here in Arcata can easily be found. 

Significant quotes:

• Low-lying inland areas could flood from below by emergent groundwater long before coastal floodwaters overtop the shoreline.
• The significance of rising groundwater and groundwater inundation may create the need to re-evaluate sea-level rise driven flooding in some communities to develop effective flood risk reduction strategies.
• Rising groundwater can destabilize foundations, flood basements and other underground structure, and increase infiltration into sewers.
• Above-ground highways, roadways, and railways are also at risk; a higher than anticipated groundwater table can exert detrimental effects on roadway and railway bases and subgrades.
• Contaminated Sites:  The interaction of rising groundwater and contaminated sites could pose challenges for public health and the environment.
• The California Toxic Tides study significantly underestimates the number of sites located in low-lying coastal communities that contain legacy contamination from past military, industrial, manufacturing, or other purposes and does not consider sites that will be impacted by groundwater rise.
  Current remediation regulations consider a static climate, meaning they do not consider a rising groundwater table.
• Liquefaction: The areas most at risk of liquefaction are generally located along the Bay shoreline and Bay tributaries in former floodplains, marshplains, wetlands, mudflats, and open water areas that were filled for development. These same areas are at risk of rising groundwater, and as the groundwater table rises, the liquefaction risk is likely to increase.
• The assessment does not consider potential increases in future extreme precipitation that are likely to occur as the climate changes.

Adaptation

From the report:

• As communities adapt to sea-level rise, adaptation plans must also consider rising groundwater tables. Traditional levees and floodwalls designed to keep coastal floodwaters out may not provide protection from rising groundwater, leaving communities at risk of flooding from below. 
• Foundations and other belowground structures that could be subject to corrosion, infiltration, and increased buoyancy should be monitored much more frequently to ensure building safety as groundwater rises. Design standards and building codes already exist for infrastructure, roadways, foundations, and structures constructed in areas with a high groundwater table. However, all new infrastructure and/or rehabilitation projects in at-risk areas should design for a groundwater table that is higher than the highest annual groundwater table.
• Bolder strategies, such as creating floating neighborhoods that can adapt to rising sea level and fluctuating conditions while providing wildlife habitat and ecosystem services can also be explored.