MSGID: 1:317/3 643e1ceb   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    New approach estimates long-term coastal cliff loss    
      
     Date:   
         April 17, 2023   
     Source:   
         Stanford University   
     Summary:   
         A new method for estimating cliff loss over thousands of years in   
         Del Mar, California, may help reveal some of the long-term drivers   
         of coastal cliff loss in the state.   
      
      
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   FULL STORY   
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   In parts of California's iconic mountainous coasts, breathtaking beauty is   
   punctuated by brusque signs warning spectators to stay back from unstable   
   cliffs. The dangers of coastal erosion are an all-too-familiar reality   
   for the modern residents of these communities. Now, with a new tool,   
   researchers are bringing historical perspective to the hotly debated   
   topic of how to manage these disappearing coastlines.   
      
      
   ==========================================================================   
   Using a model that incorporates measurements of the amount of time coastal   
   cliffs and their remnant deposits were exposed at the Earth's surface,   
   Stanford researchers found that the rate of cliff erosion in the past 100   
   years is similar to that of the past 2,000 years. The proof-of-concept,   
   published in the Journal of Geophysical Research: Earth Surface April   
   17, opens the possibility of using this new approach to understand the   
   long-term history of coastal cliff erosion, or retreat, in other parts   
   of the state. The work was conducted in Del Mar, California, a beach   
   town in San Diego County with infrastructure atop its coastal bluffs.   
      
   "In this particular location, these cliff erosion rates have been the   
   same for thousands of years, so we shouldn't expect them to get lower,"   
   said senior study author Jane Willenbring, an associate professor of Earth   
   and planetary sciences at the Stanford Doerr School of Sustainability. "If   
   anything, we should expect them to be higher in the future."  Del Mar is   
   among locations that are critically important for understanding cliff   
   retreat. Homes are situated up to 70 feet above its beach, in addition   
   to public infrastructure. A major railroad between Los Angeles and San   
   Diego runs atop the coastal bluffs, where cliff failures have resulted   
   in several derailments in modern history, as well as rock fall events   
   that led to closures in recent years.   
      
   "I think this study bolsters the thinking that we should do something   
   about cliff retreat sooner rather than later," said lead study author   
   Travis Clow, PhD '22.   
      
   A natural laboratory The study area was ideal for the researchers'   
   methodology because the Del Mar beach features a narrow shore platform,   
   the bedrock where tidepools are typically found. Using nine samples   
   of bedrock, the co-authors measured concentrations of the chemical   
   isotope beryllium-10 that track landform exposure to cosmic radiation   
   from space. The data were compared with cliff retreat rates from recent   
   studies based on aerial photography, showing that coastal erosion rates   
   have remained relatively constant over the past two millennia -- at   
   about 2 to 5 inches per year.   
      
   "One of the advantages of this technique is that it gives you information   
   at the time scales that are relevant for factors like sea-level rise,"   
   Willenbring said. "Our tool estimates retreat over time periods that   
   include multiple major storms or atmospheric rivers that don't happen   
   very often, but are critical in forming the coastline."  The researchers'   
   approach explores the influence of different factors, including wave   
   impacts and weathering that occur at the shore platform and the cliff   
   interface.   
      
   "It does more than just spit out a retreat rate," said Clow, who processed   
   the samples in Willenbring's lab and measured them at the Center for   
   Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory   
   (LLNL). "It also allows us to have a relative assessment of what might be   
   driving cliff retreat over longer periods of time."  When rock becomes air   
   On sandy coasts, like those spanning much of the eastern U.S., beaches   
   are shaped by waves that pull sand out to the ocean, then re-deposit it   
   on land with the coming and going of the tides. But with rocky coastlines   
   like those along California, once a cliff erodes into the ocean, it cannot   
   be replaced, Willenbring said. Instead, it's as if the rock becomes air.   
      
   Willenbring was surprised to learn through this research that over half   
   of all coastlines on Earth are eroding like California's. The scope of the   
   problem, which will be exacerbated by sea-level rise in the next century,   
   presents an opportunity for using this new technique in other areas.   
      
   "There are plenty of other places in California and the Pacific Northwest   
   where active erosion of coastal rocky cliffs is happening, and we hope   
   to use this technique in a wide variety of environments," Clow said.   
      
   Knowledge of cliff retreat in the U.S. is about 50 years behind research   
   on the impacts of erosion and storms on sandy beaches, according   
   to Willenbring -- and that makes her excited about contributing to   
   fundamental science in this field.   
      
   "No one had even looked at how the beach width correlated with the rate   
   of cliff retreat in California," Willenbring said. "There are a lot of   
   open questions about what drives coastal erosion, and now we have a new   
   tool to be able to address some of them."  Additional study co-authors   
   are from the Scripps Institution of Oceanography at the University of   
   California, San Diego; the Center for Accelerator Mass Spectrometry at   
   LLNL; and Imperial College London. The research was supported by LLNL,   
   the California Department of Parks and Recreation, and the U.S. Army   
   Corps of Engineers.   
      
       * RELATED_TOPICS   
             o Earth_&_Climate   
                   # Oceanography # Earthquakes # Floods # Landslides   
             o Fossils_&_Ruins   
                   # Early_Climate # Fossils # Origin_of_Life # Anthropology   
       * RELATED_TERMS   
             o Coastal_erosion o Cave o Geology_of_the_Capitol_Reef_area   
             o Coastal_management o San_Andreas_Fault o Ozone_depletion o   
             Mangrove o Evaporation_from_plants   
      
   ==========================================================================   
   Story Source: Materials provided by Stanford_University. Original   
   written by Danielle Torrent Tucker. Note: Content may be edited for   
   style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. T. Clow, J. K. Willenbring, A. P. Young, H. Matsumoto, A. J. Hidy,   
      J. R.   
      
         Shadrick. Late Holocene Cliff Retreat in Del Mar, CA, Revealed From   
         Shore Platform 10 Be Concentrations and Numerical Modeling. Journal   
         of Geophysical Research: Earth Surface, 2023; 128 (4) DOI: 10.1029/   
         2022JF006855   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/04/230417142448.htm   
      
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