Providing A Better Picture of The Rapidly Changing Mekong Delta

Coupled process studies and numerical simulations of channel sand dynamics in tide dominated river channels: A Mekong Delta case study

Sep 28, 2017

At the seaward end of one of the most geopolitically complex watersheds on Earth lies the Mekong Delta. The river flows through six countries that are experiencing rapid population growth and development. The first major dam on the river was built in 1993 followed by 34 more as of April 2016, and there are another 226 more dams either under construction or planned. In addition to damming and rapid land use change in the basin that will likely impact water and sediment flow down to the delta, the delta region in Vietnam itself is seeing population increases, subsidence from water withdrawal, increasing land use for aquaculture and agriculture, fishing pressure, and increasing oil and gas activities. In short, the delta is experiencing pressures and modifications comparable to what the Mississippi River saw 50 years ago. Since the Mekong River delta is in a relatively early stage of development and changes to the river dispersal system, lessons learned in other watersheds, including the Mississippi River, are applicable in the Mekong Delta to help these nations minimize the deleterious effects associated with the rapid change to the system.

As part of an international collaboration, a multi-pronged approach was used to look at the Sông Hâu distributary of the Mekong River and the adjacent delta. The Institute’s role focused on understanding hydrodynamics and sediment transport in the river channel. Working with a field observation team from the University of Washington and universities in Vietnam, the Institute occupied seven transects of the river separate studies at high and low river discharge. This information included mapping the riverbed, measuring currents, determining sediment concentration in the water and sediment grain size. This information was used to construct and calibrate numerical models developed at the Institute for outputs that provide a better understanding of how, when, and where sediments that feed the delta are moving, and potential limitations on that movement. The models were then used to develop predictions about how the delta will change in the future if changes such as reductions in sediment load and rising relative sea levels come to pass. One of the interesting results researchers found is that at low flows, the sand in the riverbed is covered in mud caused by a salt wedge moving in from the ocean, so, despite strong tidal currents, there is no downstream sand  transport taking place. Only at high flow when the mud covering is removed after the salt wedge is pushed out of the river, is the sand exposed and experiences net downstream transport when the tides are ebbing and river flow is added to it.