Year

2013

Degree Name

Bachelor of Environmental Science (Honours)

ANZSRC / FoR Code

0406 PHYSICAL GEOGRAPHY AND ENVIRONMENTAL GEOSCIENCE, 040601 Geomorphology and Regolith and Landscape Evolution

Department

School of Earth & Environmental Science

Advisor(s)

Tim Cohen

Abstract

The process-based classification of upland drainage networks and the response of bedrock channels to extreme events has attracted significant attention in the recent literature. The acknowledgement of the importance of headwater reaches to catchment-scale ecological and geomorphological processes highlights the need for process-based studies in such settings and an examination of how such settings respond to big flood events. A catastrophic flood event in the Lockyer Valley in 2011 resulted in significant geomorphic changes across the catchment, particularly in bedrock-confined reaches. This study assesses the post-flood channel morphology in three reaches and examines the response of such settings to the catastrophic 2011 flood and subsequent flooding in 2013. Field-survey data, multi-temporal LiDAR analysis, flood frequency analysis and sediment entrainment threshold calculations were used to investigate the reach-scale morphological response of steep, bedrock-confined channels.

The results indicate that the three study reaches underwent catastrophic stripping during the 2011 flood with large-scale destruction of channel units and in-channel vegetation to create a highly disorganised channel morphology. The flood frequency analysis demonstrates that the 2011 flood is the largest on record and represents a ~50-yr recurrence interval (ARI). The extent of geomorphic change due to this extreme event increased with stream order (catchment area), evidenced through volumetric analysis of alluvium eroded. However, net erosion per unit area was the greatest in the steepest (smallest catchment area) reach. The longitudinal profiles and channel cross-sections in all three reaches show extensive channel lowering and widening with erosion to bedrock occurring along much of the valley floor. Channel cross sections expanded by up to 220% and longitudinal profiles experienced significant reductions in morphological variance with the loss of vertical variability. It is estimated that this event, with an estimated discharge range of 415-897 m 3/s and an estimated unit stream power range of 616-1077 W/m2, mobilised the entire grain-size population from sand to boulders up to 1670mm in diameter. Channel recovery through vertical accretion of sedimentary material and channel narrowing has occurred to some extent following the 2011 event, most likely the function of a ~6-yr ARI event in 2013. However, modern channel morphologies do not reflect existing classification frameworks for mountain stream morphology due to the disturbance of sediment supply and transport capacity conditions during this event. It is hypothesised that rare, large magnitude floods dominate the episodic morphological evolution of such settings with the recovery of in-channel units (e.g. pools and riffles) dependent on the subsequent frequency of bedload transporting events.

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