Deltares Study: Lack of Space for Mud Deposition Causes Turbidity

Deltares, Delft University of Technology and Utrecht University have published a paper which shows that turbidity in the Ems-Dollard estuary is to a large extent attributable to a reduction in the space where sediment can settle.

This is the first time this mechanism has been demonstrated worldwide.

This study drew on geological information, field measurements and models.

The Ems-Dollard is one of the last large open estuaries in the Wadden Sea area and in Northwest Europe. The study provides possible approaches to counteracting increasing turbidity in the estuary, the brackish transitional zone between the river and the sea.

This is important because the estuaries are the arteries on which life in delta areas depends: virtually all major ports and cities are located in estuaries.

Lack of space

The reduction of the space where mud can settle is mainly caused by natural accretion and the reclamation of large sections of the Ems-Dollard estuary.

Millions of tons of mud settled here annually. A tidally-dominated estuary like the Ems-Dollard can be thought of as a huge pump that, on balance, sucks mud landward, where it settles in calmer waters.

Due to the loss of these settlement areas in the Ems-Dollard estuary, mud levels in the water had to increase. Initially, this process was – albeit unintentionally – slowed down because mud was removed from the estuary for a time in Germany, where dredged material was deposited on land.

When that dredging stopped, the water slowly became more turbid in a process that is not unique to the Ems-Dollard and is actually found in many other estuaries.

Further research

The finding of the study – that turbidity in the Ems-Dollard estuary is to a large extent the result of the reduction in the area where mud can settle – threw new light on the long-standing debate about the effects of dredging and dumping in the estuary.

The Ministry of Infrastructure and the Environment, and the Groningen provincial authority, have decided in consultation with all other stakeholders to conduct further research with the aim of identifying possible ways of reducing sediment levels in water, strengthening connectivity in the estuary, and improving the hydromorphological integrity of the system.

This research was made possible by studies financed by Rijkswaterstaat as part of the Water Framework Directive project ‘Sediment Transport in the Ems-Dollard’ and the studies for the Delta program financed by the Ministry of Economic Affairs.

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