Groundwater Adds to Flooding Risks in Bay Area

by Judith Schumacher-Jennings

As sea levels rise, the water beneath our feet will be rising too.  Rising oceans will punish shorelines hit by increasingly powerful storms.  Salt water will inundate rivers further upstream. The rising seas will also push groundwater closer to the surface, exacerbating flooding throughout coastal regions, like the Bay Area.

With all the dire warnings about rising sea levels, it’s probably easy to envision how rising seas will overtop existing shorelines.  Watching television coverage of Hurricanes Florence and Michael in 2018, it’s also easy to understand how more powerful storms increase storm surges and create devastation farther and farther inland.

Those are the obvious impacts of rising oceans.

Scientists are also observing increased urban riverine flooding due to extreme precipitation events, such as an atmospheric river or in years of El Nino. Forecasting models show both phenomena will occur more frequently in the future.

New Research on Groundwater

Compounding these issues, scientists now report the amount of water underground will contribute to flooding.  New research shows the groundwater table must be considered to create a more accurate understanding of the potential for flooding.

Ellen Plane and Abby Mohan presented the new research at the 2018 Bay Delta Science Conference in Sacramento. The Nov. 8, 2018 edition of Maven’s Notebook, a website devoted to California water issues, had extensive coverage of the presentations. Ellen Plane is from the College of Environmental Design at UC Berkeley and Abby Mohan is a marine geographer and Geographic Information System (GIS) analyst at Silvestrum Climate Associates. Link to Maven’s Notebook: https://mavensnotebook.com

Groundwater Levels Are Up and Down

Shallow groundwater depth varies with large rainfall events, periods of drought and tidal influences near shorelines. In addition, groundwater levels vary seasonally with the water table at its highest during the rainy season and falling during the drier months.

A saturated water table heightens flooding risks

As sea levels rise, the saltier bay water along the coastline will push the fresher groundwater layer up creating a rise in the groundwater table. The groundwater will likely rise to the surface and pond, creating emergent flooding. This will particularly occur during wet years. Saturated ground will create emergent ponds in areas that extend well beyond the areas where sea level rise is likely to flood.

In New Orleans and in the Delta with similar soil to the Bay Area, pumping has had little success in mitigating flooding because it causes subsidence (land to sink), which could be even more pronounced in areas of fill.

Emergent Groundwater Flooding

Groundwater will displace impervious surfaces, pushing the surface up, cracking and breaking it apart as the water finds its way out. If the groundwater table were well below a concrete parking lot there would be no issue. But if the groundwater table lies immediately underneath the concrete, water would find a way to get into the cracks and cause infrastructure instability and flooding.

Additionally, rising groundwater will cause inflow and infiltration into wastewater pipes, causing backups. Underground systems need redesign to be more waterproof and more resilient, not only to sea level rise, but to rising groundwater.

Effective Planning Must Consider Groundwater

New building regulations to address the potential of rising groundwater could result in a requirement for additional FEMA flood insurance, adding to the cost of housing. As sea levels rise and intrude farther inland, the interface between the salt water and fresh water will rise and push the fresh water lens upward, especially in a shallow unconfined coastal aquifer areas. This rise will potentially cause emergence and inundation in unexpected areas, not directly connected to salt water.

The East Oakland area is not directly connected to the bay. Yet the area could experience ponding from groundwater emergence alone. In the East Palo Alto area there is a lot of high-value development, such as the Google campus. Adaptation planning is already taking place, but its focus must not be limited to preventing direct inundation.  Such a singular focus would not address the ponding from groundwater behind the levee. It could become a cost-prohibitive and ineffective project if the additional threat from groundwater is not taken into account.

The Bay Area Is Very Vulnerable
San Francisco Bay Infill Development         Photo by Paul Moderacki

A lot of development around the Bay Area is built on artificial unconsolidated fill. Marin County, especially around San Rafael, has high groundwater and direct inundation threats from sea level rise. As a result there would be a higher liquefaction risk during a seismic event.

There is a housing crisis around the Bay Area, especially in Silicon Valley. When locating new developments, planning must consider emergent groundwater. The combination of direct inundation and groundwater emergence on coastal development will mean increased flooding.

There is a lot of contamination in the soil around the Bay Area. As sea levels rise, those contaminants are going to become mobilized and cause public health threats around the Bay.

Therefore we need to start shifting how we think about sea level rise planning to include this additional threat of rising groundwater. Adaptation and resilience measures must consider all three flooding components.

Watch a nine minute time lapse of the 2018 Atlantic Hurricane season: https://www.youtube.com/watch?time_continue=547&v=3j1_gxTnJok

Courtesy of the Rossmoor News, November 28, 2018. Email Judith Schumacher-Jennings at sjmadrone@sonic.net

print

Leave a Reply

Your email address will not be published.