LANDSLIDE COVES, PLAN CURVATURE, AND LANDSLIDE PROBABILITY IN AREAS WITH COHESIVE SOILS

Damaging landslides in the Appalachian Plateau and Midcontinent highlight the need for landslide-hazard mapping and a better understanding the geomorphology of landslide terrains. Landslide coves are cirque-like or amphitheater-shaped valleys in the headwaters of first order streams and are recognized in parts of Pennsylvania, West Virginia, Ohio, and Kansas. In these areas, landslides occur in cohesive soils and include earth slides, earth flows, and earth slide-flows. Landslide coves are larger than the individual landslides and are easily recognized on 1:24,000 scale topographic maps. Plan curvature is the curvature of the contours on a topographic map where coves are concave contours and the noses between coves are convex contours. Plan curvature data were created using U.S. Geological Survey 30-m digital elevation models for an area in northeastern Kansas where landslide-inventory and -hazard mapping is on-going. Statistical analysis of the plan-curvature and landslide datasets indicates that relatively straight slopes have the highest probability for landslides in cohesive soils, and the landslide probability decreases as the slopes become either more concave or more convex. Landslide coves (concave contours) have a slightly higher probability of landslides than areas with convex contours. The relationship between landslide probability and plan curvature may result from the interplay of two factors: water migration and soil convergence. Both surface and ground water tend to migrate toward the low central areas of the landslide coves and thus, the landslide probability should increase as one travels from an area of convex curvature into a landslide cove. However, the cohesive soil involved in the landslides is also converging toward the center of the landslide cove. The convergence of cohesive soil increases the resistance to movement within the toe and limits the rate of landslide motion. Landslide coves represent the quasi-stable end-member landform in a terrain dominated by landslides in cohesive soils. When using plan curvature in statistical analysis for landslide hazards, it is important to consider the types of landslides that dominate the region.