INVESTIGATING CONTRIBUTION OF SEA LEVEL INDUCED GROUNDWATER SHOALING TO URBAN FLOODING IN IMPERIAL BEACH, CALIFORNIA (Invited Presentation)

The Southern California coastal city of Imperial Beach is surrounded by the Pacific Ocean to the west, the San Diego Bay to the north, and the Tijuana Estuary to the south and experiences frequent coastal flooding during elevated sea level and wave events. The City of Imperial Beach Public Works Department has also observed zones of frequent flooding in the interior of the city, especially in particularly low-lying areas. As part of an NSF-funded collaboration between San Diego State University and Scripps Institution of Oceanography, we are investigating the causes of this inland flooding, possible links to groundwater shoaling of the shallow, unconfined aquifer beneath the city, and whether sea level rise will exacerbate urban flooding. In December 2021, we installed four shallow groundwater monitoring wells near frequently flooded areas: (1) on a street between the ocean and the estuary, (2) just inland of the estuary, (3) in the Public Works parking lot just South of San Diego Bay, and (4) near the city center in a low-lying neighborhood. Each of these wells is equipped with an RBR Solo logging pressure (i.e., water depth) at 1Hz, and we have done repeated Conductivity-Temperature-Depth (CTD) casts to track the salinity structure of the well and surrounding groundwater. Soil samples taken during well installation show that much of the city is built on dense clay with relatively low hydraulic conductivity, which is consistent with our water table measurements showing effective dampening of tidal signals near the estuary and bay. Preliminary results will be presented and future work will be discussed regarding the potential contribution of sea level induced groundwater shoaling to urban flooding.