Lack of space for mud deposition causes turbidity in Ems-Dollard estuary

Published: 30 May 2016

Deltares, Delft University of Technology and Utrecht University have published a paper in Marine Geology (2016) showing 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. The 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 our major ports and cities are located in estuaries. Higher turbidity in an estuary reduces the amount of light reaching the single-celled algae at the base of the food web. High turbidity can also cause problems for fish and sight predators. That is why an improved understanding of the Ems-Dollard constitutes a major step forward for the more sustainable management of estuaries around the world, believes Deltares.

Lack of space

Dollard vanuit de luchtThe 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.

About space in the Ems in the past

Slibvang DollardIn the past, the Ems estuary had a great deal of room for mud to settle. In the early 16th century, the Dollard was formed after the flooding of a large peatland area which had become vulnerable  due to drainage for agriculture which made the land subside below sea level. This enormous basin measuring 350 km2 was the main sediment catchment area for the Ems estuary. Every year, the equivalent of 24-36 million wheelbarrows of mud were deposited here: about two or three million tonnes. The area slowly rose above sea level and it was gradually reclaimed again. It is precisely this history of impoldering in combination with the vertical thickness which is derived from geological information that makes it possible to calculate sedimentation patterns fairly accurately.
As impoldering continued, the size of the Dollard area was reduced. Permanent mud deposition was blocked and so the mud started to accumulate in the water column. Impoldering, with the exception of the small Breebaart polder, came to an end in 1924.

The deepening of shipping channels, particularly in the tidal river Ems, also contributed to increasing turbidity. Sediment concentrations in the water and near to the bed rose extremely rapidly in this tidal river, probably as a result of deepening operations in the 1990s. Studies will be conducted in the future to determine the role played by this factor.

Possibilities for estuaries worldwide

Groningen © Rijkswaterstaat - foto: Joop van Houdt

Groningen
© Rijkswaterstaat – foto: Joop van Houdt

The results of the study can be used to determine what needs to be done to achieve the objectives for the Ems-Dollard estuary. For example, to reduce turbidity, it is preferable if the dredged material from channel and port maintenance is not returned to the water. If possible, it should be stabilised or removed from the estuary and put to beneficial use elsewhere.
The study opens up new avenues for more sustainable management in estuaries throughout the world, where mud concentrations are rising and large sections of land are being reclaimed. At the same time, land subsidence is affecting large areas behind dikes due to groundwater abstraction and settlement. It may be possible to raise these areas by depositing material from maintenance dredging. This will make estuary waters clearer. Enlarging estuaries so that new sedimentation areas are created may also help to lower water levels and contribute to flood risk management and withdraw mud from the estuary. Thus creating appealing natural areas reconciles both human activities and natural values.

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 programme financed by the Ministry of Economic Affairs.

 

References
D.S. van Maren; A.P. Oost; Z.B. Wang & P.C. Vos (2016). The effect of land reclamations and sediment extraction on the suspended sediment concentration in the Ems Estuary, Marine Geology

M. Taal, B. van Maren, D.S., A.G. Brinkman, C. Schmidt (2015). Slib en primaire productie in het Eems-estuarium, een samenvatting van vier jaar meten, modelleren en kennis bundelen en verwerven. Deltares, Imares & RWS.

Ministry of Infrastructure and the Environment & Groningen provincial authority (2015). Economie en Ecologie Eems-Dollard in balans; Eindrapport MIRT-onderzoek