Landslides, Debris Flows, and Rock Falls

Pictured above: Airplane photo of scars (pale stripes) from soil slips that developed into short-distance run-out debris flows. These debris flows, located near Santa Monica Mountains NRA, originated during long-duration and intense winter storms, producing more rainfall than is average.

Rainfall triggered landslides and debris flows

Major storm events or especially rainy seasons can trigger landslides and fast-moving debris flows. Very focused geologic mapping in the California Coast Ranges (Santa Monica Mountains National Recreation Area, Point Reyes National Seashore, Golden Gate National Recreational Area) and the Appalachian Mountains (Shenandoah National Park, Blue Ridge Parkway, Delaware Water Gap National Recreation Area, Great Smoky Mountains National Park) provide managers with information on the distribution of landslides and landslide susceptibility.

Pictured above: Excerpt from USGS Map showing potential debris flow and flood hazards in and adjacent to Shenandoah National Park, Virginia. USGS Open File Report 97-438, Morgan and others.

In 2000, USGS completed a comprehensive digital data base of geologic information for the San Francisco Bay region, including the areas of Golden Gate National Recreation Area and Point Reyes National Seashore. These maps and data include: rock and stratigraphic (bedrock layers) relations, hillside materials and their properties, such hazards as landslides and areas susceptible to debris flow tracks, slope, aspect (direction of slope face), curvature, and certain land-use information. The Santa Monica Mountains are notorious for the large number and great variety of landslides; a soil-slip susceptibility map is underway. The USGS is developing a 1:24,000-scale digital geologic-map database of the Santa Monica Mountains National Recreation Area with emphasis on the nature and distribution of landslides.

Ground failure related to debris flows in the Appalachians Highlands costs, on average, millions of dollars annually. Debris flows pose a hazard to trail hikers and motorists in the parks. Whenever several days of heavy rain have occurred, debris flows can be expected in Appalachian parks such as Shenandoah National Park and Great Smoky Mountains National Park. The USGS provides information to land and emergency management agencies to help them mitigate loss of life and property. Recent and planned products include digital maps, databases, and susceptibility analyses, technical papers, and fact sheets as well as conferences, and workshops.

Pictured above: The USGS rock-fall potential map (Wieczorek and others) for the Yosemite valley floor has been used by the NPS for site planning.

Rock Fall Hazards

Rock falls are a special hazard in some parks. On July 11, 1996, a huge rock weighing 200 tons broke away from Granite Point, near a popular trail head and concession stand in Yosemite NP. The rock disintegrated when it landed, creating an air blast that was so powerful that it flattened as many as 2,000 trees in the area. One person was killed at the concession stand, and 14 people were seriously injured. The dust kicked up from the pulverized granite blocked out the sun and coated tents and recreational vehicles, not unlike ash fall from a volcano.

Fire and Flood

The connection between forest fires and major sedimentation events has been recognized for some time, particularly in southern California where the concept of "fire-flood sequences" was first defined (e.g., Kotak and Kraebel, 1935). Recent research on the connection between fire and increased land-slide or debris-flow hazards have been conducted recently in the Santa Monica Mountains NRA, Bandelier National Monument, and El Morro National Monument, and post-burn hazard assessments.

Pictured above: A debris flow generated on hill slopes during the June 26, 1996, rainstorm. The path of the flow is lined by levees up to 15-cm high. The levees, or banks, consist of pine needles, pumice and charcoal fragments, and some silty matrix. Note that the flow was deflected around the burnt log in the center of the photograph. From Cannon, Open File Report 97-136.

For example, conditions at Bandelier National Monument immediately after the 1996 Dome Fire were evaluated by the Burned Area Emergency Rehabilitation (BAER) Team; their report identified the potential for debris flows in Capulin Canyon within the Monument, and recommended further. USGS landslide specialist Sue Cannon evaluated the site. A moderate potential for debris- and hyper-concentrated flow activity was identified for the two major tributary canyons to Capulin Canyon based on evidence of both summer of 1996 and possible historic significant debris-flow activity.