Disappearing Beaches: Modeling Shoreline Change in Southern California

This article is part of the May 2017 issue of the Sound Waves newsletter.

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Southern California could lose up to two-thirds of its beaches by 2100, if sea level rises 3 to 6 feet (0.9 to 1.8 meters) and human intervention is limited. That is according to a study conducted at the U.S. Geological Survey (USGS) and recently published in the Journal of Geophysical Research–Earth Surface.

Lead author Sean Vitousek and his USGS collaborators—Patrick Barnard, Patrick Limber, Li Erikson, and Blake Cole (now with Hyperloop One)—developed a new mathematical model, CoSMoS-COAST (Coastal Storm Modeling System–Coastal One-line Assimilated Simulation Tool), to forecast changes in shoreline shape and position. They applied it in Southern California to see where shorelines would be by the end of this century. The results indicate that 31 to 67 percent of Southern California beaches could erode completely—up to existing coastal infrastructure or sea cliffs.

“Beaches are perhaps the most iconic feature of California, and the potential for losing this identity is real. The effect of California losing its beaches is not just a matter of affecting the tourism economy. Losing the protecting swath of beach sand between us and the pounding surf exposes critical infrastructure, businesses, and homes to damage. Beaches are natural resources, and it is likely that human-management efforts must increase in order to preserve them,” said Vitousek, who was a post-doctoral fellow at the USGS when he conducted this study. He is now a professor in the Department of Civil and Materials Engineering at the University of Illinois at Chicago.

Although a majority (72 percent) of beaches in Southern California show historical trends of growing larger (due to large artificial beach nourishments since the 1930s), predictions indicate that erosion caused by accelerated sea-level rise will shrink nearly all the beaches.

“Beaches in Southern California are a crucial feature of the economy, and the first line of defense against coastal-storm impacts for the 18 million residents in the region. This study indicates that we will have to perform massive and costly interventions to preserve these beaches in the future under the erosive pressures of anticipated sea-level rise, or risk losing many of the economic and protective benefits beaches provide,” said USGS geologist and coauthor Barnard.

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The CoSMoS-COAST model is part of the larger Coastal Storm Modeling System (CoSMoS), which provides detailed forecasts of flooding due to storms of various intensities over a range of future sea levels. CoSMoS developers initially focused on forecasting flooding of present-day shorelines. But chronic erosion, driven by accelerating rates of sea-level rise, will gradually alter today’s shorelines and move many of them farther inland. The scientists realized that using present-day beach profiles in flood-prediction models could lead to an underestimation of the future hazard. CoSMoS-COAST enables them to predict the position and shape of a future coastline before running CoSMoS to forecast the flooding likely to occur there.

CoSMoS-COAST takes into account important factors that influence the shape and position of a shoreline, including transport of sediment by longshore currents (the process that causes sand to pile up on one side of a jetty), transport of sediment across the shoreline by tides and waves (the process that moves sand from beaches to offshore sandbars during winter storms), and landward migration of the shoreline due to sea-level rise. It also incorporates measured shoreline data to calibrate predictions and improve reliability. The model can be applied to virtually any coastal setting, including the complex mixture of dunes, bluffs, cliffs, estuaries, river mouths, and urban infrastructure that make up Southern California beaches.

Although shoreline change is very hard to predict, scientists are confident in CoSMoS-COAST’s predictive capability applied to the forecast period (2010–2100) because of how accurately the model is able to reproduce historical shoreline change between 1995 and 2010. This flexible and multifaceted model can help managers assess and adjust their beach-nourishment efforts to keep pace with potential long-term erosion.

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“The public already has to overcome obstacles in getting to the beach, from limited public transportation to illegally blocked pathways,” said California Coastal Commission Executive Director John Ainsworth. “The prospect of losing so many of our beaches in Southern California to sea-level rise is frankly unacceptable. The beaches are our public parks and economic heart and soul of our coastal communities. We must do everything we can to ensure that as much of the iconic California coast is preserved for future generations as possible.”

The full report, “A model integrating longshore and cross-shore processes for predicting long-term shoreline response to climate change,” was posted online March 15, 2017, by the American Geophysical Union in their Journal of Geophysical Research–Earth Surface.

The news was picked up by numerous media outlets; examples include the Huffington Post (http://www.huffingtonpost.com/entry/california-beach-erosion_us_58d97a32e4b0f805b3227bfb), Scientific American (https://www.scientificamerican.com/article/sunken-pleasure-california-will-need-mountains-of-sand-to-save-its-beaches/), NBC Los Angeles (http://www.nbclosangeles.com/news/local/Study-Predicts-Significant-Southern-California-Beach-Erosion-417269103.html), KRON4 San Francisco (http://kron4.com/2017/03/28/study-predicts-significant-southern-california-beach-erosion/), Smithsonian Magazine (http://www.smithsonianmag.com/smart-news/california-may-lose-popular-surfing-spots-rising-seas-180963001/), Surfer Magazine (https://www.surfer.com/environmental-news/geophysicists-say-goodbye-to-your-beaches-socal/#z5wl2JDp60RxepTT.97), Southern California Public Radio (KPCC, http://www.scpr.org/programs/airtalk/2017/03/29/55871/unless-we-do-something-about-it-many-socal-beaches/), the Orange County Register (http://www.ocregister.com/articles/beaches-747708-scientists-sand.html), the International Business Times (http://www.ibtimes.com/south-california-beaches-rising-sea-level-could-erode-most-shoreline-2100-usgs-report-2516093), the Hans India (http://www.thehansindia.com/posts/index/Young-Hans/2017-03-30/Study-predicts-major-Southern-California-beaches-erosion/289935), and the MeteoWeb in Italy (http://www.meteoweb.eu/2017/03/clima-le-spiagge-della-california-del-sud-a-rischio-erosione-entro-il-2100/878743/).