The Effects of Coastal Protection Structures on Beaches Along Northern Monterey Bay, California

As a result of the severe coastal wave damage experienced over the past decade along the California coast, a large number of shoreline protection structures have either been built or proposed. As the numbers of structures and their coastal frontage has increased, concern with the effects of the structures on the adjacent beaches has arisen. A one-year study along the central California coast was carried out in an effort to resolve some of the critical questions regarding the impact of these structures on beaches. Based on precise, biweekly, wading-depth surveys in the vicinity of four protective structures, a number of consistent seasonal beach changes were observed: (1). With the arrival of winter waves, the summer berm in front of the four seawalls was eroded before the berm in front of the adjacent unprotected beaches. (2). Where a sloping permeable revetment is adjacent to an impermeable vertical seawall, the berm in front of the vertical seawall was eroded sooner. (3). Once the berm on the unprotected beach retreated landward beyond the seawall, beach face profiles in front of the seawall and adjacent beach were usually indistinguishable. (4). Accelerated erosion occurred at the downdrift ends of the seawalls and extended as far as 150 m downcoast. This excess scour is believed to result from a combination of wave reflection and sand impoundment upcoast. (5) Rebuilding of the nearshore beach profile occurred in a uniform manner with no obvious differences between seawall-backed and adjacent unprotected beaches.