Debris flow dominated alluvial fans in the Victorian high country indicate that landscape denudation through the Holocene has been dominated by post-bushfire runoff events

Abstract

Bushfires play a major role in shaping the Australian landscape. Whilst the role of fire in shaping and changing vegetation assemblages is relatively well understood, there is still debate about the significance of fire in driving landscape denudation, relative to other processes, such as major rainfall and flood events. Studies of post-fire landscape impact of recent bushfires indicate that the response is sensitive to the frequency and magnitude of extreme bushfires and intense rainstorms, with the greatest response occurring when storms occur in the post-fire period before vegetation has recovered. Where storm events occur shortly after a major bushfire, hillslope erosion is enhanced, due to debris flows and erosion of both primary hillslope sediment and sediment stored in hillslope channel networks. We excavated nine trenches, in five alluvial fans at the base of hillslopes on the floodplain of the Nariel valley, northeast Victoria. This area was burnt by the 1939 and 2003 bushfires, although some of the fans were unburnt in 2003. The trenches were up to 3.5 m deep, and in four cases intersected the underlying floodplain sediment at the base of the trench, indicating that they provide a full record of sedimentation for that sector of the fan. Fan stratigraphy consisted of sub-horizontal (parallel to the fan surface) units 0.3–0.5 m thick, with occasional units 1–1.2m thick, and cross-cutting channelized units. Debris flow deposits accounted for 80–90 % of the observed sediments, with water-laid gravels and soil units forming the remainder. Most soil layers were burnt, and most (but not all) debris flow units contained charcoal. A typical stratigraphy consisted of 6–8 debris flow units per fan, with four units containing a fire signature or overlying a burnt soil layer. Radiocarbon dating of the fireassociated units is underway: preliminary results will be reported at the AQUA meeting