See Which Bay Area Schools Are at Risk From Rising Seas

Fifty-two Bay Area public schools sit so close to the bay’s high-tide line that they’re already at risk of being inundated with ocean and ground water, and their risk will grow as seas continue to rise, according to an analysis by KQED and Climate Central.

Some schools already cope with routine floods, like Marin County’s Redwood High School, which has installed a pump to keep its parking lot dry during high tides, and Mill Valley Middle School, where the drop-off zone floods multiple times a year.

“The biggest risk to these schools is that they’re in areas with high groundwater, and they get flooding when it’s a combination of a high tide and a heavy rain,” said Kristina Hill, director of UC Berkeley’s Institute of Urban and Regional Development.

The high-tide line in the San Francisco Bay has risen by 8 inches as heat trapped by pollution has melted ice and heated and expanded the world’s oceans over the last century. As that creep accelerates scientists expect floods to become more regular because about 250 square miles of the Bay Area sits just above — and in some cases below — that line, on former wetlands that were drained and piled up with rocks, rubble and soil for development.

By 2050, moderate projections show seas rising an additional 7 inches (PDF), at which point floods will become even more common at most of the schools, our analysis found. By 2100, the projections show the bay rising 3 feet above today’s levels, at which point the number of flood-risky schools will multiply.

We aimed to identify Bay Area schools with flood risk today, and those likely to have problems by the middle of the century. We looked at two types of floods, both of which will become more common with sea level rise: coastal floods, where high tides and storm surges push the ocean up onto dry land; and groundwater floods, where saltwater saturates the soil under the shore, driving up the layer of freshwater floating above it so much that it emerges and inundates properties.

The KQED and Climate Central analysis found:

• Fifty-two schools are already at risk of groundwater floods linked to sea level rise.
• Seven of them also carry the risk of coastal floods, compounding the likelihood of inundation. By 2050, 16 will carry both risks.
• By 2050, another school — Design Tech High in Redwood City — will be at risk of flooding.
• The bulk of the flood-risky schools are located in San Mateo, Alameda and Marin counties.

(Map produced by Matthew Green/KQED) 

Data sources and methodology

To identify flood-risky schools, KQED and Climate Central pulled location data for more than 1,700 public schools in the Bay Area’s nine counties from the California State Geoportal. Using QGIS, we mapped those schools onto flood risk data from the U.S. Geological Survey’s Our Coast Our Future hazard map, focusing on the risk as it stood in 2010, the model’s baseline, and with a moderate sea level rise projected (PDF) by 2050.

We added a radius of 150 meters for each school, to capture flooding impacts outside the single point location coordinates provided by the state. That’s about a 17.5 acre circle, about the size of the median school campus of 10 acres, according to a database of California school sizes.

Our analysis likely underestimates the real-life flood risk, according to flood experts we spoke with, because it doesn’t capture other factors, like intensifying rainfall and localized land subsidence.

For the coastal flood analysis, we considered the risk of inundation during a 100-year-storm, which state law requires all schools to be constructed to withstand.

We did not include four schools in Foster City and seven schools in San Mateo from the results, because those cities have recently built flood-protective levees not accounted for by the database.

Groundwater floods

Unlike coastal inundation, groundwater floods depend on underground, out-of-sight factors like topography, soil composition and the water table. In the absence of a detailed subsurface geology map for the region, the model uses a range of plausible geologies based on the research that does exist. We limited our results to campuses that the USGS model indicates already have emergent water, at or above the surface of the land.

The model requires an assumption of soil permeability: In highly permeable soil, water drains quickly, limiting the likelihood of a flood. In the least permeable soil, groundwater drains slowly, increasing that likelihood. Research by UC Berkeley’s Hill on groundwater floods in the Bay Area has shown that the least-permeable assumption best matches up with real-life flood risk.

To zero in on schools whose groundwater flood risk is related to sea level rise, we consulted with Hill and Patrick Barnard, research director for USGS’ Climate Impacts and Coastal Processes Team. We constrained our results to campuses under 10 feet of elevation, which we determined with this USGS tool, and within a half-mile of the bay or a tidally influenced river, which we determined using Google Maps’ distance calculator and by researching rivers’ head of tide.

Barriers like levees don’t protect against rising groundwater, so in this case, we did not eliminate the schools in Foster City or San Mateo.

This story was produced by KQED and Climate Central, a nonadvocacy science and news group.