Monitoring of Coastal Bluffs Using 3-D Laser Scanning and Conventional Mapping

The coastal bluffs located south of San Francisco, California attract continuous attention due to the size and nature of their episodic failures both from large winter storms and from seismic shaking. Observations made by numerous researchers coupled with recent failures have led to a renewed interest in establishing proper set-back distances for future construction. However, data describing the size, volume, and geological setting of the bluff failures has not been collected as yet in this area. This is a limiting factor to developing models capable of analyzing these failures. To measure the size and volumetric change of these coastal bluff failures, we utilize a three-dimensional laser scanning system setup on the beach facing the bluff and capable of collecting 6000 data points per second of bluff face. The data is then post-processed to form a digital terrain model (DTM). The high resolution of the constructed DTM allows for the capture of small features of the evolving bluff face. We aim to scan the bluffs at least four times during the coming winter storm season so that scans can be overlaid with one another. This will allow for the measurement of volumetric change in the bluff as well as provide understanding of the failure morphology over time and due to the related failure mechanism (rainfall, groundwater seepage, wave erosion, etc.). In addition we are using a GPS-based digital mapping system that allows for the collection of pertinent information including lithology, joint structure, and failure planes. Active portions of the bluffs are currently being mapped both on aerial photographs and oblique photos taken from the beach level. Mapped oblique photos are then overlain on the DTM created using the 3-D laser.