Fate and transport of seacliff failure sediment in Southern California

Citation:
Olsen, MJ, Johnstone E, Driscoll N, Kuester F, Ashford SA.  2016.  Fate and transport of seacliff failure sediment in Southern California. Journal of Coastal Research. :185-199.

Date Published:

2016/12

Keywords:

airborne, cliff erosion, LIDAR, runup, seacliff erosion, sediment transport, Terrestrial Laser Scanning

Abstract:

Continual erosion and collapse of unstable seacliffs along the economically important coastline of San Diego County, California, threatens existing development and public safety. Frequent time-series mapping of the seacliffs and beaches provides valuable insight into the processes responsible for cliff erosion and into the reworking and transport of the failed material. High-resolution terrestrial laser scan (TLS) data provide quantitative data for analyzing seacliff morphology, capturing patterns over time and across a wide range of spatial scales. Through an ongoing "rapid response" program operational since spring 2007, eleven substantial seacliff failure sites were mapped pre-collapse, immediately post-collapse, and repeatedly after the collapse to constrain processes causing cliff failure and estimate the rate at which failed material is reworked. Comparison of the TLS data with water levels and climate data highlights the contributing mechanisms to the seacliff failures and the rapid reworking of the failed material. Failure sites were categorized based on the frequency of wave contact (i.e., total water level) compared with the beach elevation to assess differences in the rates of sediment reworking. For example, unconsolidated failed material on the beach was reworked quickly by waves at sites where waves reached the failure on a nearly daily basis. Conversely, other failure masses with less wave contact were only reworked during storm events producing larger waves. At sites where the failure material consisted of large boulders, there are feedback mechanisms at play where the failed material protects the cliff toe by stabilizing talus deposits, akin to riprap engineering techniques. Failures due to wave undercutting and notching were observed to migrate laterally at these sites. This lateral progression of failures might explain the long-term linear retreat of the seacliffs in the region, which minimizes the development of embayments and promontories.

Notes:

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Website

DOI:

10.2112/si76-016