SEDIMENT YIELD FROM COASTAL LANDSLIDES BASED ON HISTORICAL DIGITAL TERRAIN MODELING: BIG SUR, CALIFORNIA

Along the Big Sur coastline in central California, the Coast Range descends steeply into the Pacific Ocean, creating one of the most extreme coastal slopes in the coterminous United States. Weak rocks and steep topography provide ideal conditions for frequent large landslides that contribute a substantial portion of material to the littoral sediment budget. Little is known about the sediment budget in this area, including the amount, rate and frequency of sediment input to the system from coastal landslides. Adjacent to the Big Sur coastline is the Monterey Bay National Marine Sanctuary (MBNMS), a protected area of coastal waters and home to a variety of aquatic species.

The California Department of Transportation (Caltrans) is exploring solutions to the difficulties they face with keeping slide-prone coastal Highway 1 open and safe while minimizing impacts to the MBNMS below the road. As a contribution to developing a highway management plan, and in order to advance the fundamental understanding of landslide and coastal processes along this stretch of coastline, a technique is developed to quantify the historical volume of sediment that enters the littoral system from coastal slope failures.

The technique uses aerial photography and digital photogrammetry to produce 3-dimensional stereo models from which digital terrain models (DTMs) are extracted. The terrain models are created from both recent and historical photography to produce both a 52-year mean lowering rate and the sediment yield from coastal landslides in terms of cubic meters per kilometer of coastline per year. Data for seven approximately 5-km-long sections of coastline along the Big Sur coast were processed and digital terrain models subtracted to determine the total sediment volume loss. The mean lowering rates vary significantly along the coastline, from as low as 2 mm/yr (+ 0.3 mm/yr) to nearly 80 mm/yr (+ 6 mm/yr). This variation is largely attributed to differing lithologies, and thus strengths, of the slope forming materials. Compressive strength data collected within the seven study sections correlate reasonably well with the volumetric loss data, although there is some scattering of the data. This scattering is most likely a result of the dramatic variation in the strength of the material within a particular rock type.