Abstract
AbstractThis methods paper details the first attempt at monitoring bank erosion, flow and suspended sediment at a site during flooding on the Mekong River induced by the passage of tropical cyclones. We deployed integrated mobile laser scanning (MLS) and multibeam echo sounding (MBES), alongside acoustic Doppler current profiling (aDcp), to directly measure changes in river bank and bed at high (~0.05 m) spatial resolution, in conjunction with measurements of flow and suspended sediment dynamics. We outline the methodological steps used to collect and process this complex point cloud data, and detail the procedures used to process and calibrate the aDcp flow and sediment flux data. A comparison with conventional remote sensing methods of estimating bank erosion, using aerial images and Landsat imagery, reveals that traditional techniques are error prone at the high temporal resolutions required to quantify the patterns and volumes of bank erosion induced by the passage of individual flood events. Our analysis reveals the importance of cyclone‐driven flood events in causing high rates of erosion and suspended sediment transport, with a c. twofold increase in bank erosion volumes and a fourfold increase in suspended sediment volumes in the cyclone‐affected wet season. Copyright © 2016 John Wiley & Sons, Ltd.
Highlights
The world’s largest rivers, defined as rivers with a mean annual discharge >1000 mÀ3 sÀ1 by Latrubesse (2008), deliver ~19 billion tonnes of sediment to their sink zones every year (Milliman and Farnsworth, 2011)
Despite the postulated importance of cyclone-driven flows on bank erosion and suspended sediment transport (Darby et al, 2013), to our knowledge there have been no direct measurements of bank retreat before, during and after cyclone driven flood events, with previous studies limited to analysis of remotely sensed imagery and event-based modelling (Buraas et al, 2014; Nagata et al, 2014; Magilligan et al, 2015)
The biggest drawback of terrestrial laser scanning (TLS) compared with multibeam echo sounding (MBES) is that it traditionally requires a static setup, recently efforts have sought to develop mobile laser scanning (MLS) for the rapid, high resolution characterisation of much larger terrestrial areas (Alho et al, 2009; Vaaja et al, 2011; Kasvi et al, 2013), building on earlier lower resolution airborne laser scanning systems (Thoma et al, 2005)
Summary
The world’s largest rivers, defined as rivers with a mean annual discharge >1000 mÀ3 sÀ1 by Latrubesse (2008), deliver ~19 billion tonnes of sediment to their sink zones every year (Milliman and Farnsworth, 2011). Five of the 15 largest rivers on Earth can be found in Asia: the Ganges-Brahmaputra, Yangtze, Mekong (the focus of the present study), Pearl and Irrawaddy These ‘Asian mega-rivers’ play a key role in global biogeochemical cycles, accounting for a disproportionately large amount (~14%) of the total global sediment flux delivered to the oceans (Milliman and Syvitski, 1992). In spite of their global importance, little is known about the processes that are responsible for driving sediment transfer and exchange through large river systems. It is not clear whether, and under what circumstances, large flow events might lead to either a net sequestration of sediment via floodplain deposition or net mobilisation of stored sediments through bank erosion and lateral channel migration (Gomez et al, 1995; Carroll et al, 2004; Kale, 2007; Sambrook Smith et al, 2010)
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