Debris Flow Avulsion

Abstract

Debris flows that are confined by canyons generally exhibit distributary behavior once they exit the canyons, usually creating some sort of debris fan. This distributary nature is commonly observed in fluvial processes, as avulsion out of established flow paths allows the system to methodically fill topographic lows and develop regular, fan-shaped deposits. For debris flows, avulsion represents a serious hazard, because future debris flows may occur in areas that have not experienced events in the recent past, and flows may occur at significant distances across the fan away from currently active channels. It is important to be able to identify avulsion-susceptible areas, to quantify the likelihood of avulsion, and to model and mitigate the possibility of avulsion. Map views of several debris fans showing locations of successive events were analyzed to evaluate the degree of avulsion. In addition, cross-fan sections at three locations in Colorado were interpreted stratigraphically and analyzed to calculate a modified compensation index, K_cv, a single number that indicates significant avulsion activity (K_cv near one), or low avulsion activity (K_cv near 0.5). Areas with typical debris-flow characteristics (abundant coarse clasts, thick units, large lobes, high clay content) tended to have higher compensation indices than areas with typical stream-flow characteristics (thinner, with less clay and coarse clasts). Finally, several sites are reviewed where an understanding of avulsion could help anticipate flow behavior and direct mitigation efforts.