MONITORING DEBRIS-FLOW INITIATION PROCESSES AT CHALK CLIFFS, COLORADO, USA

Debris flows in sparsely vegetated steeplands are often initiated by processes related to surface-water runoff rather than by failure of discrete landslides. Surface-water runoff can transform into a debris flow in a variety of ways including the “fire hose” effect, destabilization of a channel bed, sediment bulking from erosion of bank material, and failure of sediment dams formed by local deposition of bedload. Several of these mechanisms may operate during a single event; however, limited direct observations of debris-flow initiation have made it difficult to identify the relative importance of these mechanisms for a given set of topographic and hydrologic conditions. As part of a long-term effort to understand debris-flow processes in a small (0.3 km²), bedrock-dominated catchment at Chalk Cliffs, central Colorado, we have begun to explore the various modes of debris-flow initiation at the site using a network of flow-monitoring stations and video cameras. The source of the debris-flow material is from colluvium adjacent to bedrock cliffs and channels loaded by dry ravel and rockfall. We present initial observations from the first three years of this effort and show that debris flows initiate both at the steep bedrock-colluvium interface and in lower gradient sections of channel that are prone to sediment deposition. Bank failures contribute to flow volumes, but the failures took place only after the bank was undercut and eroded by fully developed debris flows. Debris-flow initiation by en masse failure of the channel bed does not appear to be a primary initiation mechanism at the site. Instead, direct measurements of bed erosion rates show that the erosion occurs progressively. These initial results provide a foundation for making more detailed observations that can provide valuable constraints to develop and test physically based models of debris-flow initiation by surface water.