Wildfire impacts on the processes that generate debris flows in burned watersheds

Abstract

Every year, and in many countries worldwide, wildfires cause significant damage and economic losses due to both the direct effects of the fires and the subsequent accelerated runoff, erosion, and debris flow. Wildfires can have profound effects on the hydrologic response of watersheds by changing the infiltration characteristics and erodibility of the soil, which leads to decreased rainfall infiltration, significantly increased overland flow and runoff in channels, and movement of soil. Debris-flow activity is among the most destructive consequences of these changes, often causing extensive damage to human infrastructure. Data from the Mediterranean area and Western United States of America help identify the primary processes that result in debris flows in recently burned areas. Two primary processes for the initiation of fire-related debris flows have been so far identified: (1) runoff-dominated erosion by surface overland flow; and (2) infiltration-triggered failure and mobilization of a discrete landslide mass. The first process is frequently documented immediately post-fire and leads to the generation of debris flows through progressive bulking of storm runoff with sediment eroded from the hillslopes and channels. As sediment is incorporated into water, runoff can convert to debris flow. The conversion to debris flow may be observed at a position within a drainage network that appears to be controlled by threshold values of upslope contributing area and its gradient. At these locations, sufficient eroded material has been incorporated, relative to the volume of contributing surface runoff, to generate debris flows. Debris flows have also been generated from burned basins in response to increased runoff by water cascading over a steep, bedrock cliff, and incorporating material from readily erodible colluvium or channel bed. Post-fire debris flows have also been generated by infiltration-triggered landslide failures which then mobilize into debris flows. However, only 12% of documented cases exhibited this process. When they do occur, the landslide failures range in thickness from a few tens of centimeters to more than 6 m, and generally involve the soil and colluvium-mantled hillslopes. Surficial landslide failures in burned areas most frequently occur in response to prolonged periods of storm rainfall, or prolonged rainfall in combination with rapid snowmelt or rain-on-snow events. © 2011 Springer Science+Business Media B.V

Control del arrastre de sedimentos post-incendio: efectividad de fajinas en un bosque seco montañoso (Salta, Argentina)

“…Los incendios forestales pueden modificar propiedades clave del suelo y alterar los procesos hidrológicos que allí ocurren. Por ejemplo, las temperaturas elevadas y la pérdida de la cobertura vegetal reducen la capacidad de infiltración (Shakesby and Doerr 2006;Moody et al 2013;Moody and Ebel 2014) y aumentan la escorrentía y la erosión del suelo (Tálamo et al 2016), con el consiguiente riesgo de arrastre de sedimentos a las viviendas ubicadas en el pie de monte de los sectores incendiados (Geertsema and Highland 2011;Parise and Cannon 2011). Por lo tanto, cobra importancia el diseño y la implementación de técnicas de control de erosión del suelo en las laderas recientemente quemadas a fin de minimizar las posibles consecuencias negativas de la alteración de los procesos hidrológicos.…”

Improvement in quantifying debris flow risk for post-wildfire emergency response

“…The scientific advance which improved defining debris flow risk was the recognition that nearly all post-fire debris flows are initiated through runoff-dominated erosion by surface overland flow rather than infiltrationtriggered failure mobilizing discrete landslide masses (Cannon 2001;Cannon et al 2003;Cannon and Gartner 2005;Parise and Cannon 2012). During storm events, the surface runoff progressively incorporates sediment from the hillslopes and channels into the water.…”

“…During storm events, the surface runoff progressively incorporates sediment from the hillslopes and channels into the water. At some point within the channel, storm flow reaches a proportion of sediment within the water which transforms it to a debris flow (Parise and Cannon 2012;. In many instances, material eroded from within the channels has provided the majority of the sediment resulting in debris flow occurrence Wagner et al 2013).…”

Debris flow run-out simulation and analysis using a dynamic model

“…The latter is quite common in areas with scarce vegetation or that have been recently affected by forest fires, thus becoming more susceptible to erosion (e.g., Cannon et al, 2001a;Cannon and Gartner, 2005;Cannon et al, 2008;Santi et al, 2008;Baum and Godt, 2010;Cannon et al, 2011;Kean et al, 2013;Staley et al, 2013b;Staley et al, 2014). 25 The consumption of vegetation, ash deposition, changes in physical properties of soils and rocks and the presence of waterrepellent soils are typical consequences of fire (Cannon and Gartner, 2005;Cannon et al, 2010;Parise and Cannon, 2012).…”