The relationship between incident wave energy and seacliff erosion rates: San Diego County, California

The coastline of San Diego County, California, is characterized by steep se acliffs cut into 5 to 115 m high uplifted marine terraces. Over the past fe w decades, rapid population growth in the area has promoted a substantial i ncrease in cliff-top development, despite a limited understanding of the lo ng-term cliff erosion rates and their controlling factors. Wave erosion at the base of the seacliff is usually assumed to be a basic driving mechanism of coastal cliff retreat. We investigated the influence of waves on seacli ff erosion by comparing high-resolution, long-term seacliff erosion rates t o wave parameters (height, energy, and power or energy flux) in 10 m of wat er, the break-point, and at the cliff toe. Seacliff erosion rates range fro m 3.0 cm/yr in well-lithified Cretaceous sandstone to 43.0 cm/yr in unlithi fied Pleistocene sands. The wave parameters were calculated using the Calif ornia Data Information Program (CDIP) Southern California Refraction-Diffra ction Model (SCRDM), an empirical relationship for breaking wave height, an d a new term we define as relative power at the cliff toe. Directional wave data from offshore South-Central California were used to initialize the mo del. The distribution of wave power in 10 m of water and at the breakpoint and cliff toe appears to be inversely related to historical seacliff erosio n rates at our study sites. As a result, our findings suggest that waves, w hile an important mechanism of seacliff erosion, are secondary to material properties in the overall retreat of San Diego seacliffs. Along the San Die go coastline, material strength appears to largely determine seacliff stabi lity and the rate and manner of retreat.