Browsing by Author "Smith, HG"

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    Changes to sediment sources following wildfire in a forested upland catchment, southeastern Australia
    (Wiley-Blackwell, 2011-08-30) Smith, HG; Sheridan, GJ; Lane, PNJ; Noske, PJ; Heijnis, H
    Few investigations link post-fire changes to sediment sources and erosion processes with sediment yield response at the catchment scale. This linkage is essential if downstream impacts on sediment transport after fire are to be understood in the context of fire effects across different forest environments. In this study, we quantify changing source contributions to fine sediment (<63 mu m) exported from a eucalypt forest catchment (136 ha) burnt by wildfire. The study catchment is one of a pair of research catchments located in the East Kiewa River valley in southeastern Australia that have been the subject of a research program investigating wildfire effects on runoff, erosion, and catchment sediment/nutrient exports. This previous research provided the opportunity to couple insights gained from a range of measurement techniques with the application of fallout radionuclides (137)Cs and (210)Pb(ex) to trace sediment sources. It was found that hillslope surface erosion dominated exports throughout the 3.5-year post-fire measurement period. During this time there was a pronounced decline in the proportional surface contribution from close to 100% in the first six months to 58% in the fourth year after fire. Over the study period, hillslope surface sources accounted for 93% of the fine sediment yield from the burnt catchment. The largest decline in the hillslope contribution occurred between the first and second years after fire, which corresponded with the previously reported large decline in sediment yield, breakdown of water repellency in burnt soils, substantial reduction in hillslope erodibility, and rapid surface vegetation recovery. Coupling the information on sediment sources with hillslope process measurements indicated that only a small proportion of slopes contributed sediment to the catchment outlet, with material derived from near-channel areas dominating the post-fire catchment sediment yield response. Copyright (C) 2011 John Wiley & Sons, Ltd.
  • 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.