Monitoring of Coastal Bluff Stability Using High Resolution 3 D Laser Scanning

The coastal bluffs located south of San Francisco form one of the more active geologic terrains along the California coast and are typically found to be in some state of failure during each years winter storm season. In certain areas, residences are located less than 10 meters from the edge of these bluffs which highlights the need to constantly monitor the stability, and likewise the detailed geometry, of specific sections of bluffs. Characterization of the geometry of these bluffs for analysis purposes can be difficult due to the steep and constantly evolving profiles of the bluffs from ongoing failures. Ground-based 3D laser scanning technology represents a new method of surface modeling for characterizing bluff failures and is an extremely useful technique for working with steep, evolving landscapes. In this paper, we present a brief overview of the laser scanning process and describe its application in monitoring the slope behavior of three coastal bluffs composed of weakly lithified sand. We then present geomorphological analyses performed using the data gathered from the 3D laser scanning program to show how the predominant triggering mechanisms can be correlated directly to the observed bluff response and linked to the degree of cementation of the coastal bluff units. Finally, we use the data to extract quantitative indicators of failure and make predictions regarding their stability and response to future storm events. In addition to making several qualitative geomorphological conclusions concerning slope behavior and failure mechanisms, we find from the laser data that bluff sections composed of weakly cemented materials typically fail when their overall slope inclination reaches the mid to high 50° range and are stable below 50°. For slopes composed of strongly cemented materials, no correlation between geometry and failure can be deduced.