Spatial Distribution of Mass Movement In the Paonia-McClure Pass Area, Colorado, USA

Mass movement is a dynamic process that can be activated by earthquakes, rapid snowmelt, intense rainstorms or gravity. Whereas mass movement plays a major role in the evolution of a hillslope by modifying slope morphology and transporting material from the slope to the valley, it also poses as a potential natural hazard. Prediction of the location and volume of transported mass on potentially unstable slopes is an important issue in the assessment of mass movement hazards and hillslope evolution. A promising approach is to examine the relationships of area, volume, length, height and width of existing movements through ratio quantification.

We mapped 683 movement features in the Paonia to McClure Pass area of western Colorado from aerial photographs and field surveys. The area covers ~600 km2. The total area of movement was classified as debris flows (29%), rockslides (26%), debris slides (23%), soil slides (15%) and highway and forest road influenced landslides (7%). Of the 683 features only 540 were studied in detail; landslides associated with Highway 133 and forest roads were not studied.

The frequency of large landslides occurring is small compared to the frequency of small landslides based on area and volume parameters. Area-volume, length-area, length-volume, height-area, and height-volume of the landslides are related by a power function. The length-area relationships show the highest correlation coefficient for the rockslides and the poorest correlation coefficient for the soil slides. Debris flows and debris slides correlation coefficients are intermediate. Length-width ratios for the landslides were also calculated. Ratios range from 0.3 to 14. The average length-width ratios for debris flows, rockslides, debris slides and soil slides are 4.0, 2.5, 2.7, and 2.0, respectively, suggesting soil slides have the highest tendency to extend laterally.

Knowing these relationships serve as the first steps in constructing landslide hazard zone maps for an area.