Chapter 9: Influence of Rock Type and Wave Regime on Varying Rates of Shoreline Erosion, Santa Cruz Island, California

Robyn Suddeth & Nathan Burleyi

The evolution of island shoreline geometry is driven primarily by lithology and surrounding wave regime, with additional effects from internal rock structure, stream channels, groundwater seepage, and other hillslope processes on the island itself. Many of these driving factors are dynamic and interconnected, making it difficult to determine which are most dominant as island shape continues to change. This discussion combines aerial imagery and maps from previous studies, oceanographic data, and firsthand visual observation to make a simplified assessment of the role of rock type and wave regime on the shaping of Santa Cruz Island, in southern California, and to create four categories of shoreline erosions potential, ranging from low to very high. A comparison of island lithology, nearshore bathymetry, and surrounding wave regime will show that recent changes in island shape have been controlled first by the island’s geologic history and structure, which differentially exposes rock type and determines orientation to nearby land masses and ocean bathymetry, secondly by waves, and lastly by the combined effect of rock discontinuities and other erosion processes. The coastline most vulnerable to high rates of erosion on Santa Cruz Island are those places where softer sedimentary rocks meet high incoming wave energy, both on its western coast, and on the northern side of the island’s thin connecting neck. This vulnerability makes it likely that the island will slowly progress over time towards two distinct eastern and western Santa Cruz Islands.