Bedrock Channels: Towards a Process-Based Understanding

Thursday, 18 December 2014: 5:30 PM
Daniel R Parsons1, Stephen E Darby2, Christopher R Hackney2, Julian Leyland3, Jim Best4, Andrew Paul Nicholas5, Rolf E Aalto6, C A.P.T. Horn III7 and M.R. Thy7, (1)University of Hull, Hull, HU6, United Kingdom, (2)University of Southampton, Southampton, United Kingdom, (3)University of Southampton, Southampton, SO14, United Kingdom, (4)University of Illinois at Urbana Champaign, Geography, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, Urbana, IL, United States, (5)University of Exeter, Exeter, United Kingdom, (6)University of Exeter, Geography, Exeter, United Kingdom, (7)Department of Hydrology and River Works, Phnom Pehn, Cambodia
Most previous studies on the genesis and evolution of bedforms in large rivers have focused on aggradational bedforms within alluvial sediments, with very few investigations that concern either erosive bedform evolution or bedrock channel abrasion processes. Detailed understanding of the processes within bedrock reaches of river channels is vital if an improved understanding of formation and evolution of bedrock scours and bedforms are to be elucidated. The paper presents high-resolution bathymetry and sidescan derived from multibeam sonar (MBES) and detailed flow mapping by acoustic Doppler current profiling (ADCP) to illustrate, in intricate detail, relations between morphology, flow and sediment transport processes through a bedrock reach of the Mekong River (Cambodia) during a large flood event. A 2 by 5 km reach of the Mekong river near Sambor was surveyed with a RESON 7125 MBES system revealing incredible >40 m scour features within the bedrock substrate, with sidescan imagery also revealing the routing of alluvial sediment through the scours. A series of ADCP transects were obtained, both transverse and perpendicular to the primary downstream flow, that map the flows into, around and within these scour features. The paper will conclude by looking at how advances in measurement capability have permitted the detailed processes in such channels to be investigated for the first time at this scale.