Collaborative Research: Fate of Irrawaddy and Salween River Sediment: Relative Importance of Oceanographic and Tectonic Controls

Half a billion humans live on river deltas, and many of these areas are now considered at risk as a result of increasing land subsidence, decreasing growth rate, and accelerating sea-level rise. Sinking land in these coastal areas greatly increases the threat from storm surge and flooding. For example, Cyclone Nargis tracked across the Irrawaddy delta in Myanmar in 2008 resulting in over 138,000 fatalities, and heavy monsoon rains in this same area in 2015 displaced 200,000 people as a result of flooding in low lying areas. To understand and predict the future evolution of river deltas, it is necessary to know how sediment from the river is transported and delivered to the onshore and offshore regions of the delta, as this is the material from which deltas arise. This project will provide an understanding of the Irrawaddy-Salween (I-S) delta in Myanmar, perhaps the Earth's last remaining major delta system for which we lack a complete picture. Specifically, the project will examine the tectonic and oceanographic factors that control the delivery of sediment, and the present pattern and rates of delta growth. This results of this study will inform the future development of a sensitive deltaic setting home to >50 million people, and add to our general knowledge of deltas worldwide. The project will also involve the mentoring and training of US undergraduate and graduate students, and will engage Myanmar scientists and students, providing opportunities for training and collaboration with US counterparts.

Much of the estimated 600 Mt of river sediment carried by the I-S system annually is delivered to a wide continental shelf in the northern Andaman Sea that is influenced by strong tides, monsoon conditions, and periodic cyclones; however the fate of this material is unknown. Existing bathymetric data and sparse initial surveys hint at a mode of subaqueous delta development on the shelf which contrasts markedly from other previously studied major deltas. In particular, the shelf exhibits a dramatic bilateral (east-west) asymmetry of the surface morphology and sediment texture: a gently sloping sandy ramp off the eastern mouths of the Irrawaddy, and a muddy inner- to mid-shelf clinoform in the adjacent western Gulf of Martaban. Field- and modeling-based experiments will enable the research team to contrast the relative roles of oceanographic sorting, transport processes, and underlying tectonics in creating this unusual deltaic expression. Geophysical observations, coring and laboratory studies will detail the architecture and quantify the amounts, source, and distribution of Holocene and recent sediment accumulating on the shelf. To explore transport pathways of sediment from the fluvial source to depositional sinks, a numerical model will be used to account for the effects of river input, waves, tidal- and wind-driven currents on the dispersal of river sediment within the coastal ocean. Model runs will represent conditions during the SE and NW monsoons, and during times of high discharge. The model will also be used to explore the potential for enhanced transport during large storms using representations of the cyclone-generated wave and wind field.