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

• Main Authors: M. Baggs Sargood, T. J. Cohen, C. J. Thompson, J. Croke
• Format: Article
• Language: English
• Published: Copernicus Publications 2015-06-01
• Series: Earth Surface Dynamics
• Online Access: http://www.earth-surf-dynam.net/3/265/2015/esurf-3-265-2015.pdf
• ISSN: 2196-6311; 2196-632X

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 (<i>R</i>² = 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.