Characterising the Geomorphic Response of a Tropical Mega-River to an Extreme, Cyclone Induced, Flood Event.

Friday, 19 December 2014
Christopher R Hackney1, Julian Leyland2, Stephen E Darby1, Daniel R Parsons3, Rolf E Aalto4, Andrew Paul Nicholas5 and Jim Best6, (1)University of Southampton, Southampton, United Kingdom, (2)University of Southampton, Southampton, SO14, United Kingdom, (3)University of Hull, Hull, HU6, United Kingdom, (4)University of Exeter, Geography, Exeter, United Kingdom, (5)University of Exeter, Exeter, United Kingdom, (6)University of Illinois at Urbana Champaign, Geography, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, Urbana, IL, United States
Extreme events have the ability to induce extensive geomorphic change in fluvial systems as a result of elevated discharge levels, increased sediment transport capacity and associated changes in sheer stresses along channel boundaries. Understanding how rapid rises in water levels change flow structures and channel boundary roughness is key to understanding the relative significance of large events in terms of driving local and system wide geomorphic change. However, capturing the fluvial process dynamics in operation during such events is technically and logistically difficult, especially in the world’s largest rivers.

During September 2013, on the peak of the monsoon, a series of tropical cyclones induced a large flood event within the Mekong basin. At the peak of the flood wave, discharge measured ~60000 m3/s; the 11th largest flood on record. Pre and post event high resolution topographic surveys of parts of the bed and bank were captured using a combination of contiguous multibeam echo sounding (MBES) and terrestrial laser scanning (TLS) during the event. Simultaneously detailed measurements of cross sectional and near bank flow structure were acquired using an acoustic Doppler current profiler (aDcp). Together, these unique datasets can be used to characterise and assess the geomorphic impact of a cyclone induced extreme flood event on the Mekong. We show how flow structures in the near bank region evolve with stage during the extreme event and how the associated geomorphic response is modulated by the distinctive process dynamics of a mega-river.