ESTABLISHING A LONG-TERM RECORD OF COASTAL EROSION ALONG SONOMA COAST STATE BEACHES, CALIFORNIA

Coastal erosion is exacerbated by natural and human-driven perturbations of the climate system. Natural climatic cycles that affect the west coast of North America, such as the El Nino Southern Oscillation, increase the erosive energy of waves. Coastal erosion is also accelerated by rising sea levels associated with climate change. In northern California, erosion rates are also affected by uplift of emergent portions of the tectonically active coastline. However, while many studies investigate the erosion of southern California beaches, few detailed coastal erosion studies in northern California exist.

Our project establishes a record of intra- and inter-annual erosion processes at three Sonoma Coast State Beaches: Salmon Creek, Horseshoe Cove, and Goat Rock. Salmon Creek is a river-cut, estuarine spit-bar complex. Horseshoe Cove and Goat Rock are both wave-cut, erosional shore zone landforms controlled by mass wasting and gravity processes, but differ in terms of breaking wave energy. We initiated our study by collecting baseline beach profile information at the study sites listed above in Fall 2005. Subsequent beach profile data were collected from these locations at least once a year. Beach profiles include the location of berms, dunes, and other prominent beach features. Initially, profiles were created with traditional surveying tools. In Fall 2007, a total station was utilized to collect beach profile data. Results of these surveys indicate significant intra- and inter-annual variations in the shape of the beach, the location of dunes, and the presence of vegetation. However, the shape and location of the sea cliff remains relatively constant year to year, suggesting that the majority of coastal erosion may occur during significant episodic events (such as El Niño), rather than during yearly changes to the beach. These results are consistent with previous studies of southern California beaches and highlight the importance of establishing a baseline and long-term record of beach erosion at northern California sites. Studies such as these will provide a basis for understanding the response of beaches in this area to different stresses and allow prediction of what type of events will have drastic effects upon the structure of the beaches in the area.