Coastal Landslide Material Loss Rates Associated with Severe Climatic Events

Deep-seated landslides along the California coast deliver large amounts of material to the nearshore littoral environment. Landslide movement, a combined result of slope base undercutting by waves and ground saturation, is highly episodic. Movement occurs primarily during periods of high rainfall and large waves, such as those associated with El Niño events. This analysis applies remote-sensing techniques to quantify the volumetric net loss rates at three specific landslide sites along the Big Sur coast over three approximately decadal time periods, two of which contained the largest El Niño events of the twentieth century. High-resolution historical terrain models were compared in order to provide surface- elevation change data for each landslide complex. To determine the material influx to the littoral system, the landslide complexes were divided into upper and lower slopes, and the surface-elevation change was converted to a volume loss. Some material lost from the upper slope was deposited at the slope base, not into the littoral system. We describe a method to calculate the net loss that omits the deposition volumes from the upper slope. Loss rates were found to be substantially higher during the periods in which El Niño events occurred. This is especially true during the period of the 1997–1998 El Niño, when 75% of the total material volume was lost, and loss rates were as much as sixteen times higher than during non–El Niño periods