LANDSLIDE WARNING CAPABILITIES IN THE UNITED STATES

Numerous landslide-related fatalities in the late 1960s stimulated research into early warning strategies for precipitation-induced landslides, including the development of rainfall intensity and duration thresholds. In 1985, the U.S. Geological Survey and National Weather Service began operating an experimental landslide warning system in the San Francisco Bay area. Two decades later, despite technological advances, landslide-warning capabilities in the U.S. generally remain underdeveloped. Empirical landslide warning thresholds now exist for several areas but the rainfall amounts and durations range over three orders of magnitude across the U.S. and over an order of magnitude across individual counties. Antecedent moisture is also a significant factor in areas that have distinct wet and dry seasons. Instrumental monitoring has revealed that soil moisture and pore pressure response to rainfall (and therefore slope stability) is complex and not adequately predicted by simple moisture balance indices. Ongoing research indicates that field monitoring in combination with numerical modeling would provide more reliable forecasts of landslides than presently possible using empirical thresholds. The state of knowledge and resources available to issue alerts of precipitation-induced landslides vary across the U.S. State and federal agencies currently issue alerts for potential shallow, rapidly moving landslides and debris flows in a few areas along the Pacific coast and for areas affected by Atlantic hurricanes, but most lack resources needed to provide continuous (24x7) support. The state of Utah provides alerts for potential deep-seated landslides on the basis of ground-water monitoring, precipitation, and activity of many pre-existing landslides. Many recent alerts of potential landslides have been fairly general and more specific information about affected areas, probability of landslide occurrence, and expected timing would make these alerts more useful. An approach to providing this information that is technically feasible (given adequate resources) is dynamic modeling that incorporates real-time subsurface monitoring data and quantitative rainfall forecasts into storm-specific landslide susceptibility maps.