The shoreline and seaside cliffs in California are eroding at increasing rates, especially in the short term, and a variety of causes—from El Nĩno to geology—may be involved.
About 40% of California beaches are eroding in the long term—defined as 120 years—and that number increases to 66% in the short term—or last 25 years—according to a new study by the U.S. Geological Survey (USGS) that’s highlighted in the Journal of Coastal Research.
In their article on the USGS analysis, “Rates and Trends of Coastal Change in California and the Regional Behavior of the Beach and Cliff System,” Cheryl J. Hapke, Dave Reid, and Bruce Richmond examine the spatial relationship between shoreline change and cliff retreat on a regional scale.
The USGS research found that higher rates of cliff retreat correspond with lower rates of shoreline erosion on high-relief coasts, while low- to moderate-relief coastal areas typically are areas with high shoreline change and high cliff retreat. This reflects a strong relationship between the features and geology of the coast and how they affect beaches or cliffs.
For the research, the California coast was divided into three areas: Northern, Central, and Southern. Overall, Northern California coasts are a bit more rugged and steep with high-energy waves, while the more urbanized Southern California coast is home to lower-energy waves.
“In general, the cliff-retreat rates are highest in Central and Northern California, with the exception of the San Diego region in Southern California. Rates of shoreline change are highest in Central California,” the authors write. “There is a distinct trend of decreasing coastal cliff-retreat rates from north to south, with relatively uniform cliff retreat in the southern part of the state.”
Central California is the most diverse region with developed terraces and coastal bluffs, plus pocket and linear beaches, according to the authors: “Numerous seawalls and revetments exist along this stretch of coast, especially in more heavily developed areas. These structures, built in response to cliff erosion … act to reduce the rate of cliff retreat, although they may have negative impacts.”
And while it has the longest stretches of linear beaches, “the Southern California coastline … is the most heavily affected by human development and engineering structures in the state,” according to the article. “Many of the portions of the coast that are backed by cliffs have coastal protection structures, which have likely affected the rates of cliff retreat and thus contribute to Southern California having the lowest average retreat rates in the state.”
Cliff-retreat rates appear to be most influenced by geology, with erosion rates higher in focused headland areas where wave energy typically is high. Other factors that may affect erosion include seasonal weather patterns such as El Nĩno, which brings above-average rainfall and larger waves; coastline areas with large, deep-seated landslides; and ocean facilities such as harbors and ports that affect sediment flow.
The researchers found the highest long-term accretion rates near coastal engineering structures, beach nourishment sites, and areas with high sediment supply from large rivers. The increase in long- and short-term erosion “implies that erosion hazards have increased in California, especially from the 1950s-70s to the late 1990s.”
With recent environmental changes such as global warming and rising sea levels, the USGS study serves as an important resource for future management of coastlines, according to the article.
“As interest and concern about impacts of global sea-level rise on the world’s coastline continue to increase, a fundamental understanding of past behavior of coastal systems … is critical to future planning and management,” write Hapke, Reid, and Richmond.
The Journal of Coastal Research is an official publication of The Coastal Education and Research Foundation. To learn more about the society, please visit: http://www.cerf-jcr.org/.
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