City Leading Bay Area in Studying Impacts of Sea Level Rise Locally

The report includes several maps like the above that depicts increasing impacts of groundwater rise. This map shows flood impacts with 48 inches of rising seas.

City Leading Bay Area in Studying Impacts of Sea Level Rise Locally

The Alameda City Council unanimously accepted a pioneering report on the effects of sea level rise on groundwater last month. The report, “The Response of the Shallow Groundwater Layer and Contaminants to Sea Level Rise” finds that rising groundwater levels are a hidden threat related to sea level rise.

According to the city’s press release, “communities are developing climate adaptation plans to protect communities from flooding. However, these plans often neglect an important potential flood hazard — emergent groundwater.”

The process is described as a “slow but chronic threat” that, as waters rise, damages buried infrastructure, floods below-grade structures and emerges aboveground, even before coastal floodwaters crest the shoreline.

The study explores the links between sea level rise, precipitation and the elevation of the shallow groundwater surface. A suite of potential adaptation strategies to address rising seas and rising groundwater are presented.

The city’s Climate Action and Resiliency Plan (CARP) identified emergent groundwater as a potential future hazard and recommended additional analyses to better characterize the response of this aspect of sea level rise. Alameda has several areas where industrial uses contaminated shallow groundwater. In areas with high contaminant concentrations, emergent groundwater may cause unacceptable exposure levels to humans, pets and wildlife.

Known contaminated lands are examined in the study, which acknowledges that many of the sites are currently in the cleanup process.

The study uses well data collected for the California State Water Resources Control Board to develop its estimated impacts. It provides analysis of long-term groundwater trends and highlights its response to large precipitation events, with the surface rising by five feet or more during wet winters.

To estimate how high the present-day groundwater surface could rise in relation to the ground, the well data collected during wet winters (between 2000 and the present) was analyzed. In areas with limited monitoring well data, soil boring logs supplemented the well data.

The report details the impact of seven sea level rise scenarios (i.e., 12, 24, 36, 48, 52, 66 and 108 inches) and areas with emergent groundwater were mapped in each scenario. The report showed that areas at risk of flooding increased by up to 25 percent when considering both threats, and some areas were flooded by emergent groundwater long before coastal floodwaters crested the shoreline.

The report concludes that emergent groundwater flooding is expected to have consequences for Alameda residents. During wet winters, emergent groundwater flooding will likely be sporadic and localized. Over time, building foundations will be increasingly susceptible to scour and soil erosion resulting in foundation subsidence and structural damage.

The report suggests adaptation measures could include: modifying lagoon operations, increasing stormwater pumping capabilities and wetproofing below-grade utilities. Longer-term measures include: filling low-lying neighborhoods, raising structures and managed retreat.

See the full report at https://tinyurl.com/yxpoet3p.