Sounding out erosion on the Mekong river banks: insights from combined terrestrial laser scanning, multibeam echo sounding and acoustic Doppler profiling

Friday, 18 December 2015: 16:15
2003 (Moscone West)
Jim Best1, Christopher R Hackney2, Julian Leyland2, Stephen E Darby3, Daniel R Parsons4, Rolf E Aalto5 and Andrew Paul Nicholas6, (1)Univ. Illinois at Urbana Champaign, Geology, Geography & GIS, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, Champaign, IL, United States, (2)University of Southampton, Geography and Environment, Southampton, United Kingdom, (3)University of Southampton, Southampton, SO14, United Kingdom, (4)University of Hull, Hull, United Kingdom, (5)University of Exeter, Geography, Exeter, EX4, United Kingdom, (6)University of Exeter, Exeter, United Kingdom
Knowledge of bank erosion processes and rates along very large rivers remains incomplete, primarily due to the difficulties of obtaining morphological and flow data close to the bank across various flow stages. Moreover, obtaining such process information through the entire flow and bank depth has also proved challenging. Here, we present data from a series of high spatial resolution topographic (Terrestrial Laser Scanner and Multibeam Echo Sounder) and flow (Acoustic Doppler Current Profiler) surveys undertaken on the Mekong River, Cambodia, which reveal the temporal and spatial evolution of a series of embayments on the outer bank of a large meander. These techniques yield unique data that reveal how the flow field responds to the morphology of the outer bank and subaqueous slump blocks. Specifically, we show that in the early stage of embayment growth, deposited slump blocks induce flow upwelling and bank-directed flow that enhances bank erosion. Our data also suggest that as the initial erosion process continues, a threshold embayment size is reached. Below this threshold, flow separation acts to enhance embayment growth along with the fluid dynamic effects of slump blocks, but above the threshold size, the separation zone in the embayments acts as a protective layer, thus slowing erosion. This field data allows proposition of a new conceptual model of embayment evolution.