Beach and Cliff Erosion Processes at Solana Beach, California, 1984-1990by Allen H. Harker, California Dept of Boating and, Waterways, La Jolla, United States,
Reinhard E. Flick, California Dept of Boating and, Waterways, La Jolla, United States,
Abstract: Observations of back beach elevation changes, profiles and cliff erosion over a 7-year period from 1984-1990 provide a detailed local view of these processes and rates at a typical Southern California site, located about 35 km north of downtown San Diego. The 250 m long study area is located at the northern end of Solana Beach where cliff-top private homes were built over 20 years ago. The area is similar to long stretches of Southern California coastline and knowledge of the details of the processes contributing to erosion here may have application elsewhere in this highly urbanized and intensively used coastal region. The prevalence of mostly calm winters and average surf during the study period, compared with the time from about 1978 to 1983, has contributed to net beach accretion. Cliff erosion in the study area is caused by a combination of marine and subaerial factors and controlled by the local micro-geology. Ocean wave undermining and scour at cliff face faults, cracks and joints leads to development of caves that collapse over time and cause slumps of the unconsolidated overlying strata. The contribution of beach-sand sized material from landslides and cave collapse is on the order of 1 m3/m per year, which is only 3% of the typical seasonal on-offshore profile change of about 35 m3/m. Less catastrophic, but still episodic, cliff face erosion may contribute annualized sand volumes of from 3 to 100 m3/m of beach per year at isolated locations and over short periods. These findings underline the uncertainty attached to estimates of sand contributions associated with cliff erosion on this coast. Nevertheless, the implication is that the contribution of sand from cliff erosion to the local beach sediment budget is negligible. Beach profiles taken before and after construction of a 45 meter long, 7 meter high seawall in summer 1988, and at a control range, suggest that the wall has yet had no measurable effect on the seasonal beach sand level changes.
Subject Headings: Erosion | Cliffs | Beaches | Sand (hydraulic) | Coastal management | Beach profiles | Ocean waves | Sandy soils | North America | California | United States | San Diego
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