Characterization and analysis of a large rockfall along the Rio Chama, Archuleta County, Colorado

Abstract

The Rio Chama area experienced a natural rockfall in spring 2004. The focus of this study is to quantify and characterize the event, to investigate the possible trigger(s) and failure mechanisms, to identify possible similar events nearby, and to produce a rockfall Source Zone Identification Map of the surrounding area. An approach that included field mapping, Geographic Information System (GIS) based analysis, and a variety of remote sensing methods was used to investigate the Rio Chama Rockfall. Field mapping provided firsthand information about the extent and character of the event. GIS based analysis assisted in narrowing the time period of occurrence by providing a platform for viewing and manipulating remote sensing data. GIS was also used to organize, process, and compile data in order to produce a Source Zone Identification Map of the surrounding area to identify source areas where similar rockfalls could occur. Optical remote sensing assisted in narrowing the time window of occurrence as well as in delineating the boundary of the slide based on damaged and undamaged vegetation. The volume of the slide was estimated from mapped boundaries and estimated thicknesses. The Rio Chama Rockfall was divided into 4 zones, the Source Zone, the Rubble Zone, the Slope Zone, and the Toe. The zones are based on type of material, size of material, amount of material, type of movement, and gradient of the slope. The Rubble Zone was further divided into individual lobes based on the lobe’s relative elevation. The trigger of the Rio Chama Rockfall appears to be runoff from snowmelt and precipitation. No similar style failures were observed in the immediate area. A map of the area was produced identifying source areas susceptible similar types of failures.