SEA CLIFF RETREAT, CAMPUS AND ISLA VISTA BEACHES, SANTA BARBARA, CALIFORNIA

Regional retreat rates for southern Santa Barbara County coastal bluffs between Point Conception and Rincon Point (Carpinteria, California) range from 1.5 to 29 in/yr as calculated in GIS from 1998 and 2010 NOAA LiDAR data. Retreat rates range from 3-9 in/year along the Isla Vista bluffs, judging from two cadastral surveys in 1965 and 1994, and 12 in/yr from qualitative estimates over four decades. The average retreat rate along Lagoon Road on the University of California, Santa Barbara campus is 9.5 in/ year, as calculated from 1997 LiDAR and 2012 Dynamic Laser Scanning (DSL) data. Direct measurements of cliff top retreat range from 3 to 9 in/yr ± 3 in between 2001 to 2015.

The sea cliffs comprise weak, highly fractured diatomaceous clay shale of the late Miocene Sisquoc Formation in both Isla Vista and Campus Beach study sites. Sea cliff erosion has already imperiled residences atop the Isla Vista bluffs at least since 1960, whereas at an average retreat rate of five inches/year, the edge of the Campus Beach sea cliff has only 15 more feet to go in 36 years before it will meet the east margin of Lagoon Road, a principal campus thoroughfare. Average rates belie the episodic nature of the retreat, however, and mask the effects of intensified wave attack during brief and unusual storms including El Niños. To date, measurements have not bracketed specific events, but that is one goal of several focused studies now underway to bracket the 2015-16 El Niño.

Qualitative observations say cliff retreat has increased over the last three decades, evidently because the supply of beach sand, which otherwise buffers the cliffs from direct wave attack, has steadily diminished since 1957 when Bradbury Dam was constructed across the Santa Ynez River. Regrettably, quantitative measurements of sea cliff retreat did not commence early enough establish a compelling correlation between seacliff retreat and decrease of sand supply.