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Seacliff Retreat and Coarse Sediment Yields in Southern California

by Craig H. Everts, Moffatt and Nichol, Engineers, Costa Mesa, United States,



Document Type: Proceeding Paper

Part of: Coastal Sediments

Abstract:

Seacliffs are erosional slopes. They form when a receding shoreline impinges on coastal terraces, hills, or mountains. Wave-caused erosion at the base of a seacliff controls its long-term retreat rate. At higher elevations, various non-marine processes subsequently reduce the wave-steepened slope. An empirical method is described to estimate mean, decades-long seacliff retreat rates, and the rate at which coarse, beach-type sediment is liberated in southern California. Beach width is considered a key variable control because the beach above MSL must be removed during early stages of a storm before waves can attack the seacliff. Thus, beaches protect seacliffs and seacliffs nourish beaches. About 5 percent of the total sand volume reaching beaches in the Oceanside Littoral Cell (8% excluding artificial beachfill) today comes from seacliffs. Seacliffs are likely to become a more significant source if sea level rise accelerates. Seacliff contributions will increase as beaches become narrower while river and other natural contributions will be unaffected. Most beaches will adjust to a new dynamic width such that increased contributions of seacliff material will be in balance with the reduced beach width caused by sea level rise. Seacliff protective devices will affect this balance, resulting in narrower beaches. Beach berms less than 20-m wide provide a low level of protection against wave attack. Seacliff retreat declines greatly when the berm is between 20 and 30-m wide. Near-complete protection is provided by a 60-m wide berm.

Subject Headings: Beaches | Coastal management | Beach protection and nourishment | Sediment | Water waves | Sea water | Ocean waves | California | United States

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