lettering unizeit

Movement on the seabed

Sandy coasts and the Wadden Sea are under threat by climate change and the rise in sea-levels, and this is a situation that cannot be stopped by dykes or sand filling. So it is important that the dynamics of sediment transportation be thoroughly researched and developments forecast.

waves in the sand
© Pixabay

The sand waves that the current leaves in the sediment are clear to see when the sea withdraws as the tide goes out. The patterns in the sand are not only pretty, but also of scientific interest.

Wave-like patterns in the sand appeal to Professor Christian Winter. While footsteps left in shallow waters on the beach are quickly washed away, sand waves in deeper water remain there for longer. There is even a technical name for them: ripple marks, or ripples for short. These ripples, and larger forms on the seabed or river beds are the result of water flow, and in turn determine the current. "

The fine profile patterns of the sediment make the bed rough, and therefore affect flow behaviour and thus also the transport behaviour of sediment," explains Professor Winter, who heads the Coastal Geology and Sedimentology working group at the Institute of Geosciences and supports the marine priority research area Kiel Marine Science (KMS). Professor Winter is not alone with his passion for shapes on the bottom of the sea. He quotes the standard saying of geomorphologists: "When there is a pattern, there is a story."

In order to research these stories in detail, the equipment used by the scientist – who recently made the move from the University of Bremen to Kiel – includes an instrument, a so-called “lander”, among other things. "It's an autonomous measuring frame with weights on the legs and so stands very firmly on the bottom of the sea," explains Professor Winter. Various sensors measure the flow, turbulence and turbidity there, as well as the sediment concentration and grain size distribution. The lander's features also include cameras and lasers for high-resolution measuring of the seabed that records even tiny structures down to the drill holes made by worms. "We go out on a research vessel and, for instance, position the lander on the bottom before a storm event, and afterwards bring it back up again. We use these measurements to help us understand better what processes cause the sediment to be transported, and where it is transported to."

However, these investigations are not limited to the physical processes, but also cover the interaction with microorganisms and other life forms on the seabed. Including, for instance, starfish, worms, shells and crabs. Shell banks strengthen the sediment, while worms, crabs and similar creatures tend to mobilise it. 

Professor Winter: "The first thing we use the measurements on the seabed for is to establish what processes are at work in the tiny section that we are able to see with the lander. We use this information to refine our numeric models. The models are required in order to research the large-scale and long-term effects of these processes." An understanding of sediment dynamics is required, for instance, in order to predict the consequences of rising sea levels for the Wadden Sea, or what causes the siltation of rivers.

Professor Winter and his colleagues are investigating sediment-dynamic processes in the Weser estuary as part of a co-operation project with the Federal Waterways Engineering and Research Institute in Hamburg. "For instance, we want to find out how mud is transported and carried into the river course. This fine sediment can change the consistency of the seabed and the flow dynamic." 

Too much mud can also become a serious problem in estuaries, as it has in the Ems. "The Ems is a well-known example of how too much human intervention can affect a natural system. In this case, regular dredging has strengthened tidal dominance. The tide brings more sediment into the river than the ebb takes out. Which means that more and more mud accumulates. The consequence of this is that there is no longer any life along large stretches of the Ems because the water has become so cloudy."

On the other hand, though, human intervention is required if the Wadden Sea and the coastline are to be preserved, as has been laid down in the "Wadden Sea Strategy 2100" by the state of Schleswig-Holstein. "The Wadden Sea will respond directly to a rise in sea level. We want to know whether it can grow along with a rise quickly enough or not, whether sufficient sediment is available, and when and how that will be shifted." Professor Winter's working group is developing the technical solutions to protect the Wadden Sea, although not as a matter of priority. "We're focusing on the fundamental processes and the prognostic capacity of our models." These will then form a basis from which sustainable coastal protection measures can be adopted. 

Author: Kerstin Nees