Short Virtual Presentation & Digital Poster 10th Australian Stream Management Conference 2021

Worn down and wasting away: the impact of rainsplash erosion in alluvial gullies (#60)

James S Daley 1 , Andrew Brooks 1 , John Spencer 1 , Robin Thwaites 1 , Justin Stout 1
  1. Griffith University, Southport, QLD, Australia

Accurately quantifying gully sediment yields has become increasingly important for achieving Great Barrier Reef water quality targets and more recently in determining cost structures for environmental crediting. Gullies are generally perceived to be dominated by overland flow and associated erosion processes such as mass failure. But downwearing caused by rain splash is at present an unknown variable in gully sediment yields. Across four sites in the Burdekin River Basin, 251 erosion plates were installed to monitor this process over the duration of the 2019-2020 wet season, a below average rainfall year. Erosion plates were placed at specific locations within the gullies that did not receive overland flow (i.e. isolating the process just to that driven by direct rainsplash erosion). All sites were coupled with rain gauges and soil material layers were sampled at each site to provide data soil properties. The majority (70%) of erosion plates formed pedestals 5-50 mm high over the wet season, while a further 28 had toppled over. Only 22 plates formed minimal or no pedestal. These pedestals provide a proxy for downwearing of the surrounding surfaces and suggests downwearing as a diffuse erosion source is quite important, yielding on average 360 ± 53 t/yr of fine sediment in the study gullies, previously unaccounted for in other short-term measurements or predictions. Rainsplash sediment yields account for 25-70% of total gully yields, suggesting common alluvial gully treatment options focused on catchment management and headwall retreat that don’t consider surface treatment may be of limited success.

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