Getting a grip on mud

For a few months, the serene calm enjoyed by the people of Kapellebank has been being disturbed. The elderly living near this bay of the Scheldt estuarine normally spend their days contemplating the past and watching ships pass by whilst sitting on the bench. But now they have an extra distraction; about a dozen of people dressed in raincoats venturing into the mudflats several times a week during low tide. Everybody knows you shouldn’t walk there. Whenever one of the strangers sinks in the sludge and stumbles a salvo of laughter arises from the bystanders

Hydraulic engineer Dr. Bram van Prooijen (CEG faculty) can’t be bothered by these bystanders. He is the leader of a research project that focusses on the interaction between currents and mud. “On the contrary”, he says, “the audience actually proved helpful as they could tell us about the morphology of the bay in passed times.” The morphology apparently has hardly changed for decades, making it an ideal area for a long time series study.

Van Prooijen’s project is a collaboration between TU Delft, the State Key Laboratory of Estuarine and Coastal Research in Shanghai, oceanic research institute NIOZ and water research institute Delft IHE. The researchers are especially interested in the way so-called eddies (the swirling of a fluid caused when the fluid flows past an obstacle or in this case a river bank), eat away at the mudflats in the estuary of the Scheldt river.

At many places in the estuarine erosion of the mudflats is a big problem, leading to loss of foraging territory for birds. The erosion is accelerated indirectly as a result of the dredging of the sea-lane.

To better understand the processes at play and to find ways to counter the erosion the researchers are taking heaps of measurements. On each excusrsion, they bring with them a whole arsenal of tools, including a drone that films the eddies from 130 meters altitude, drifters to measure currents, a jet ski to reassemble the drifters and sensors that measure sediment transport which they mount on steel frames at low tide.

“It is very difficult to simulate the behavior of mud”, says Van Prooijen. “Mud particles stick together much more than sand grains do. Mud can be very hard yet also very soft. It is really complicated material.”

In the Yangtze delta eroding mudflats is also a topic of concern. Especially so after the installation of the Three Gorges Dam which spans the Yangtze River. Since the installation of this dam, less sediment is transported to the Yangtze delta, making Shanghai more vulnerable to floods. Hence the participation of the State Key Laboratory of Estuarine and Coastal Research in this research project.

“One thing that makes this research project interesting for us Chinese is the fact that the erosion processes in which we are interested all occur in a very small study area”, says Qin Zhu, whilst pointing at the extremities of the bay, about 600 meters apart from each other, and the water line, which at low tide is only a few hundred meters. In the Yangtze delta the researchers have to walk at least six kilometres to the waterline, which is undoable with heavy equipment.