Landslide Hazards Monitoring and Assessment
Landslide hazards occur in all regions of the world and include fast-moving debris flows, slow-moving landslides, and a variety of flows and slides that are often associated with other natural disasters such as earthquakes, volcanic eruptions, floods, and wildfires. It is estimated that landslide-related fatalities average from 25 to 50 per year in the United States, and the economic costs to the Nation ranges up to $2 billion per year. USGS landslide hazards investigations focus on the urban environment, landslide hazards to infrastructure, and landslides that occur as a result of other natural disasters. They are conducted in partnership with Federal, State, and local agencies.
A landslide overview map of the conterminous United States.
To reduce the risk from active landslides, the USGS develops and uses real-time landslide monitoring systems. The immediate detection of landslide activity provided by real-time systems can be crucial in saving human lives and protecting property. The USGS landslide monitoring focuses on detecting (1 )precipitation and ground-water conditions that could destabilize a hill slope, (2) acceleration of slide movement, and (3) ground vibrations associated with movement.
The USGS is assisting BLM in remote near real-time monitoring of the Bluegill landslide, also known as the Salmon Falls landslide, in southern Idaho. The landslide is mostly on public lands administered by BLM; some portions of the landslide are on State and private lands. Since summer of 1999, BLM has been monitoring this active landslide on Salmon Falls Creek, several miles from the Snake River. The landslide movement has resulted in the formation of a dam that impounds a lake approximately 2 km long and about 40 m wide in Salmon Falls Creek. This area was one of the favorite climbing areas for local rock climbers who notified BLM that fractures were opening up and large rocks were falling. Because of the concern for public safety, the slide block area and the canyon below has been closed to public access
In FY 2001, USGS installed near a real-time landslide monitoring system at the site of the Bluegill Lake landslide, as well as, providing technical assistance and training to BLM staff in data compilation, processing and interpretation.
Topographic map showing Sinking Canyon and Salmon Falls Creek (Balanced Rock 7.5 - minute Quadrangle, USGS, 1987).
In September 2003, the USGS conducted a field reconnaissance of the landslide area. This was the result of BLM’s request to the USGS to conduct a preliminary hazard assessment of the landslide, examine possible mitigation options, and identify alternatives for further study and monitoring of the landslide. The study is presented in the USGS Open File Report 2004-1054, Assessment of the Hazards Associated with the Bluegill Landslide, South-Central Idaho. The study concluded that the most significant hazard posed by the Bluegill landslide is a rapid failure of the landslide dam that could produce catastrophic flooding, and possibly debris flow from entrainment of channel debris in the flood waters. However, based on the visual reconnaissance of the landslide area and available information, the USGS does not believe the current landslide dam poses an imminent risk. The report recommends that current mitigation efforts should focus on continued monitoring of landslide movement and periodic inspections of the landslide dam. Specifically, some of the recommendations to BLM include:
- Continue efforts to utilize InSAR and LIDAR data to develop digital elevation models of the slide and to delineate slide boundaries
- Continue the in-place GPS monitoring currently being conducted by Idaho State University for an additional 1 to 2 years unless displacement ceases
- Determine, if possible, the thickness of the underlying sedimentary rocks by mapping the geology of the landslide area in detail
- Continue near real-time monitoring of movement across the headscarp of the landslide using a cell phone equipped datalogger for early alerts
Landslide hazard assessments provide the scientific basis for land-use, emergency management, and loss reduction measures. USGS scientists have identified the geologic, geomorphic, and rainfall conditions that indicate susceptibility to fire-related debris flows based on studies of recently burned basins in southern California, New Mexico, and Colorado. These scientists are collecting data to define the relationship between post-wildfire flood and debris flow peak discharges and rainfall amounts.
An aerial photograph of a debris flow, triggered by torrential rains, on hill slopes burned by the South Canyon fire. The debris flow traveled across four lanes of I-70 and nearly dammed the Colorado River.
In July 1994, the South Canyon fire burned 2,000 acres of forest and scrub on the steep slopes of Storm King Mountain near Glenwood Springs, Colorado. In September 1994, torrential rains triggered debris flows, which poured from this burned area and inundated a 3-mile stretch of Interstate 70 with tons of mud, rock, and other debris. Detailed mapping from aerial photographs taken on November 1994, along with extensive field observations and measurements demonstrated that the net result of the September rainstorm was to flush dry-ravel deposits from the side channels, transport loose, larger material from the main channels, and precipitate the erosion of unconsolidated, burned surficial soil from the hillsides. USGS research demonstrated three potential geologic hazards on Storm King Mountain: (1) continued debris flow activity resulting from accelerated incision and entrainment of channel alluvium by high volume surface water flows; (2) mobilization of colluvial material on the channel side slope and valley fill materials in the canyons by down cutting; and (3) erosional destabilization of deep-seated landslide deposits.
One of the most intensely burned areas near the summit of Storm King Mountain, prior to the debris flow. Burnt snags (dead trees) cover the landscape.
An image of deposits left by the debris flows through one of the drainages.
The focus of the USGS research is to develop tools and methodologies for the prediction of post-wildfire landslide activity and hazard delineation. Post-fire landslide hazards include fast-moving, highly destructive debris flows that can occur when wildfires are followed by high intensity rainfalls. These types of debris flows are particularly hazardous because they can occur without warning and can strip vegetation, block drainage ways, damage structures and endanger human lives. USGS research demonstrated that most burned watersheds that do produce destructive debris flows could be readily identified by a combination of geologic and topographic characteristics.
The USGS has developed strong partnerships with BLM for conducting research and performing rapid post fire assessments as a result of cooperation following the Colorado fire, as well as fires in New Mexico, Montana, and Idaho. Information from these studies will be useful in defining what areas burned by wildfire are susceptible to debris flows and in planning post wildfire recovery and erosion/flooding mitigation efforts.
