Debris Flow Generation in Burned Catchments

Abstract

Wildfires affect large forested areas in many countries worldwide, producing damage and economic losses, both as direct effect of the fires and as consequent events, including erosion and sedimentation in the recently burned areas. In addition to destruction of the vegetation, and direct losses to the built-up environment, further effects may be registered as a consequence of the fire, even weeks or months after its occurrence. Wildfire can have, in fact, profound effects on the hydrologic response of watersheds, and debris-flow activity is among the most destructive consequences of these effects. The two primary processes that have been identified for the initiation of fire-related debris flows are (i) erosion and entrainment of material by surface runoff, and (ii) infiltration-triggered failure and mobilization of a discrete, shallow landslide mass. The first process is reported by far as the most frequent. Field evidence indicates that unlike landslide-triggered debris flows, those produced in recently burned catchments have no identifiable initiation source and can occur with little or no antecedent moisture. Runoff-initiated debris flows have been produced in response to storms that occur typically from a few months to three years after the fire, often in response to the first significant rainfall of the storm season. After a wildfire, and in consequence of the following rainstorms, rills typically develop on hillslopes, initiated as miniature soil slips, and involving a few mm-thin saturated layer of soil. The persistence of such features downslope, until producing a true debris flow, depends upon a series of factors, including slope steepness, presence of loose materials, and availability of stream flow water. Typically, a discreet landslide mass of significant size is lacking at the head of the flow, whilst erosion and entrainment of significant amounts of surficial material is observed within hollows and in low-order channels. It therefore seems that the main process acting consists of surface runoff from a rainfall event, eroding sediments from hillslope and channels until a position within the drainage network where sufficient material has been entrained, relative to runoff volume, for a debris flow to be generated. The present paper analyzes, with some examples, the generation of debris flows in burned catchments, with the aim to describe the processes at the origin of these dangerous phenomena.