Debris-flow-dominated sediment transport through a channel network after wildfire

Field studies that investigate sediment transport between debris-flow-producing headwaters and rivers are uncommon, particularly in forested settings, where debris flows are infrequent and opportunities for collecting data are limited. This study quantifies the volume and composition of sediment deposited in the arterial channel network of a 14-km(2) catchment (Washington Creek) that connects small, burned and debris-flow-producing headwaters (<1 km(2)) with the Ovens River in SE Australia. We construct a sediment budget by combining new data on deposition with a sediment delivery model for post-fire debris flows. Data on deposits were plotted alongside the slope-area curve to examine links between processes, catchment morphometry and geomorphic process domains. The results show that large deposits are concentrated in the proximity of three major channel junctions, which correspond to breaks in channel slope. Hyperconcentrated flows are more prominent towards the catchment outlet, where the slope-area curve indicates a transition from debris flow to fluvial domains. This shift corresponds to a change in efficiency of the flow, determined from the ratio of median grain size to channel slope. Our sediment budget suggests a total sediment efflux from Washington Creek catchment of 61 x 10(3) m(3). There are similar contributions from hillslopes (43 +/- 14 x 10(3) m(3)), first to third stream order channel (35 +/- 12 x 10(3) m(3)) and the arterial fourth to fifth stream order channel (31 +/- 17 x 10(3) m(3)) to the total volume of erosion. Deposition (39 +/- 17 x 10(3) m(3)) within the arterial channel was higher than erosion (31 +/- 17 x 10(3) m(3)), which means a net sediment gain of about 8 x 10(3) m(3) in the arterial channel. The ratio of total deposition to total erosion was 0.44. For fines <63 mu m, this ratio was much smaller (0.11), which means that fines are preferentially exported. This has important implications for suspended sediment and water quality in downstream rivers. (c) 2019 John Wiley & Sons, Ltd.