Improving Flood Forecasts on Reefy Shorelines

Sea-level rise, a consequence of climate change, is already affecting coastlines around the world. Because more than a third of the global population lives within 100 kilometers (62 miles) of an ocean coast, the effects of sea-level rise on people will continue to worsen. 

The Coastal Processes Group at Scripps Institution of Oceanography at UC San Diego conducts a variety of research that will help predict coastal flooding under rising seas. While much of the work is done locally on Southern California coastlines, the group also has projects throughout the Pacific, including in Hawai‘i, where the United States military has a large presence.

The U.S. Department of Defense (DoD) is interested in how American military bases will become increasingly vulnerable to climate change effects. To demonstrate that, the DoD’s Strategic Environmental Research and Development Program (SERDP) undertook a project to analyze coastal flooding of military installations throughout the Pacific. Under the leadership of John Marra from NOAA, this project assessed coastal flood vulnerability of multiple bases using novel statistical methods and various sea level rise scenarios. One of the test sites was the Marine Corps Base Hawai‘i (MCBH) located at Kāne‘ohe Bay on eastern O‘ahu. The location experiences both year-round trade wind waves and highly energetic winter waves famous among surfers.

The SERDP project’s analyses at MCBH relied on airborne laser (LiDAR) surveys conducted in 2013 and 2018 by Luc Lenain and Adam Young, and nearshore experiments conducted by University of Hawai‘i at Mānoa and Scripps researchers in 2018. The airborne LiDAR surveys yielded high-resolution maps of the base’s topography and offshore bathymetry out to depths of about 15-20 meters (49-66 feet). The nearshore experiments, led by Mika Siegelman, now a Scripps postdoctoral researcher, captured the transformation of offshore waves to shallower, nearshore waters.

The group analyzed that data to determine a method for taking offshore wave measurements like wave height and wave period to predict nearshore wave characteristics. This method allowed the SERDP project to downscale Pacific Ocean-scale wave forecasts all the way to the MCBH shoreline. However, one last piece of information to assess coastal flooding was still missing: observations of how far these waves run up on the beach.

To collect observations of this wave run-up, a team from the Coastal Processes Group again partnered with researchers from the University of Hawai‘i at Mānoa in September 2022. Together we measured nearshore wave heights with pressure sensors and the extent of their associated beach runup with a ground-based LiDAR system.

Once we analyze these data, it should be possible to predict how much of the beach will be inundated using forecasts of beach slopes and offshore wave heights and periods. We can also make estimates incorporating sea level rise effects on the total water level. Not only will this contribute to the MCBH assessment, but these same techniques will be extended to other vulnerable reef-fringed shorelines.

This work will be crucial for creating accurate coastal flood forecasts and helping policymakers determine how close to the shoreline communities should develop.

Austin Barnes is a third-year PhD student in the laboratory of oceanographer Mark Merrifield