Post-fire debris flows in southeast Australia: initiation, magnitude and landscape controls

Abstract

© 2013 Dr. Petter NymanSurface runoff and sediment availability can increase after wildfire, potentially resulting in extreme erosion, flash floods and debris flows. These hydro-geomorphic events supply large amounts of sediment to streams and can represent a hazard to water supply systems, infrastructure and communities. This thesis combines observations, measurements and models to quantify and represent the post-fire processes that result in hazardous catchment responses. The processes that constitute risk to water quality and infrastructure were identified through field surveys of burnt catchments in the eastern upland of Victoria (southeast Australia) where impacts had occurred. The survey established that the majority of high-impact events after wildfire were linked to runoff-generated debris flows, a process previously undocumented in the region. The debris flows were initiated through progressive sediment bulking, and occurred in response to short duration and high intensity rainfall events, within one year after wildfire. Debris flows were confined to dry sclerophyll forests that had been subject to crown fire. Wet forest types displayed comparatively subdued responses, a pattern attributed to the relatively high infiltration capacity in these systems. Infiltration and sediment availability were isolated as the key hillslope components that were sensitive to burning and which strongly influenced catchment processes and debris flow susceptibility. The aims of subsequent work were therefore to develop models of infiltration and sediment availability as controls on hillslope response and use these to quantify changes in key parameters during recovery from wildfire. Infiltration was modelled as function of surface storage (H), matrix flow (Kmat) and macropore flow (Kmac). Macropore flow was found to be the main parameter driving the temporal trends in infiltration capacity during recovery from wildfire. Water repellency was ubiquitous in headwater recovering from wildfire, although the strength diminished during prolonged wet weather conditions, a dependency which could be modelled as a function of monthly weather patterns. Sediment availability was highly variable with soil depth, a feature which contrasts with assumptions underlying commonly used erosion models, typically developed in agricultural systems. The majority of erosion following wildfire was found to occur in a shallow layer of highly erodible material which could be represented through dnc, a parameter describing the depth of non-cohesive soil. This depth of available soil decreased exponentially during recovery. The models of sediment availability and infiltration were effective at capturing both spatial variability and recovery processes and form a basis on which to model debris flow initiation and magnitude in variable landscapes during recovery from wildfire