Browsing by Author "Nyman, P"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    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
    (Australasian Quaternary Association Inc, 2014-06-29) Marren, P; Kermode, SJ; Nyman, P
    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.
  • No Thumbnail Available
    Item
    Quantifying sources of fine sediment supplied to post-fire debris flows in forest catchments using fallout radionuclide tracers
    (Elsevier, 2012-02-15) Smith, HG; Sheridan, GJ; Nyman, P; Child, DP; Lane, PNJ; Hotchkis, MAC; Jacobsen, GE
    Fine sediment supply has been identified as an important factor contributing to the initiation of runoff-generated debris flows after fire. However, despite the significance of fines for post-fire debris flow generation, no investigations have sought to quantify sources of this material in debris flow affected catchments. In this study, we employ fallout radionuclides ((CS)-C-137, Pb-210(ex) and Pu-239,Pu-240) as tracers to measure proportional contributions of fine sediment (<10 mu m) from hillslope surface and channel bank sources to levee and terminal fan deposits formed by post-fire debris flows in two forest catchments in southeastern Australia. While Cs-137 and Pb-210(ex) have been widely used in sediment tracing studies, application of Pu as a tracer represents a recent development and was limited to only one catchment. The ranges in estimated proportional hillslope surface contributions of fine sediment to individual debris flow deposits in each catchment were 22-69% and 32-74%. The greater susceptibility of Pb-210(ex) to apparent reductions in the ash content of channel deposits relative to hillslope sources resulted in its exclusion from the final analysis. No systematic change in the proportional source contributions to debris flow deposits was observed with distance downstream from channel initiation points. Instead, spatial variability in source contributions was largely influenced by the pattern of debris flow surges forming the deposits. Linking the tracing analysis with interpretation of depositional evidence allowed reconstruction of temporal sequences in sediment source contributions to debris flow surges. Hillslope source inputs dominated most elevated channel deposits such as marginal levees that were formed under peak flow conditions. This indicated the importance of hillslope runoff and fine sediment supply for debris flow generation in both catchments. In contrast, material stored within channels that was deposited during subsequent surges was predominantly channel-derived. The results demonstrate that fallout radionuclide tracers may provide unique information on changing source contributions of fine sediment during debris flow events. (C) 2011 Elsevier B.V. All rights reserved.