Topographic signature of debris flow dominated channels: implications for hazard assessment

Abstract

Debris flows constitute an important transport mechanism in mountain headwaters, which results in a topographic signature that is significantly different from that of fluvial valleys. This signature is readily visible in drainage area-slope diagrams, where fluvial reaches follow a power law distribution described by the equation S=KA, where S is channel slope, A drainage area and K and n are constants. On the other hand, headwater channels with evidences of debris-flow activity clearly depart from this trend. This paper provides additional evidences for this phenomenon with data from a headwater basin on the northern slope of the Cantabrian Cordillera (Spain), where channels with evidences of debris-flow activity significantly depart from the power law function defined by reaches where fluvial processes are dominant; the transition between these two domains is analogous to those described in other humid, mountainous regions. Identification of dominant processes in headwater channels is based on field recognition of debris flow evidences. This paper also explores the implications of this topographic signature for the analysis of debris flow hazard at a regional scale. A significant percentage of the drainage network in the study area is characterized by the morphometric attributes of debris flow dominated channels. As a result, numerous infrastructures are potentially affected by this process, mainly roads and buildings. Additionally, the analysis suggests that some settlements may also be threatened by debris flows. This work shows that a morphometric analysis based on the drainage area-slope relation may be a useful criterion for a preliminary identification of channels where debris flows are likely to occur.