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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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Volume 3, issue 2 | Copyright
Earth Surf. Dynam., 3, 265-279, 2015
https://doi.org/10.5194/esurf-3-265-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 05 Jun 2015

Research article | 05 Jun 2015

Hitting rock bottom: morphological responses of bedrock-confined streams to a catastrophic flood

M. Baggs Sargood1, T. J. Cohen1, C. J. Thompson2, and J. Croke4,3 M. Baggs Sargood et al.
  • 1GeoQuEST Research Centre – School of Earth and Environmental Sciences, University of Wollongong, NSW, 2522, Australia
  • 2Australian Rivers Institute, Griffith University, 170 Kessels Road, Nathan, QLD 4111, Australia
  • 3School of Geography, Planning and Environmental Management, University of Queensland, St Lucia, Brisbane 4072, Australia
  • 4Department of Science, Information Technology, Innovation and the Arts, Queensland Government, Boggo Road, Dutton Park, Brisbane 4102, Australia

Abstract. The role of extreme events in shaping the Earth's surface is one that has held the interests of Earth scientists for centuries. A catastrophic flood in a tectonically quiescent setting in eastern Australia in 2011 provides valuable insight into how semi-alluvial channels respond to such events. Field survey data (3 reaches) and desktop analyses (10 reaches) with catchment areas ranging from 0.5 to 168 km2 show that the predicted discharge for the 2011 event ranged from 415 to 933 m3 s−1, with unit stream power estimates of up to 1077 W m−2. Estimated entrainment relationships predict the mobility of the entire grain-size population, and field data suggest the localised mobility of boulders up to 4.8 m in diameter. Analysis of repeat lidar data demonstrates that all reaches (field and desktop) were areas of net degradation via extensive scouring of coarse-grained alluvium with a strong positive relationship between catchment area and normalised erosion (R2 = 0.72–0.74). The extensive scouring in the 2011 flood decreased thalweg variance significantly removing previous step pools and other coarse-grained in-channel units, forming lengths of plane-bed (cobble) reach morphology. This was also accompanied by the exposure of planar bedrock surfaces, marginal bedrock straths and bedrock steps. Post-flood field data indicate a slight increase in thalweg variance as a result of the smaller 2013 flood rebuilding the alluvial overprint with pool-riffle formation. However, the current form and distribution of channel morphological units does not conform to previous classifications of bedrock or headwater river systems. This variation in post-flood form indicates that in semi-alluvial systems extreme events are significant for re-setting the morphology of in-channel units and for exposing the underlying lithology to ongoing erosion.

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We document the responses of bedrock-confined rivers to an extreme flood which occurred in southeast Queensland, Australia, in 2011. Through a combination of field- and desktop-based analyses we show that widespread removal of coarse-grained mantle occurred, with boulders up to 4m in diameter being locally mobilised. We show that normalised erosion in this extreme event is scaled to basin area and that this large flood has exposed bedrock steps and straths exposing them to ongoing erosion.
We document the responses of bedrock-confined rivers to an extreme flood which occurred in...
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