Silt dynamics in the Wadden Sea. Where mud matters.

Published: 19 March 2021

The current silt dynamics in the Wadden Sea are markedly influenced by human activities and interventions, such as the dredging of waterways and the use of silt for land reclamation. In this context, ‘silt’ is taken to mean sediment with a grain size of less than 63 micrometres. For the trilateral Wadden Sea (the overall Wadden Sea area shared by the Netherlands, Germany and Denmark) there is a lack of solid insight into silt management. This inspired the Wadden Academy and Programme towards a Rich Wadden Sea (PRW) to ask Deltares to launch a study into this. Where does the silt come from, what happens to it in the Wadden Sea and what are the potential consequences of human intervention? The report Where Mud Matters is now complete. It provides the first silt balance assessment for the trilateral Wadden Sea. The report was presented to Bernd Scherer, chair of the Trilateral Programming Committee on Wadden Sea Research, during the 2nd Sediment Solutions webinar.

Nature and economy

For centuries, the introduction and deposition of silt has contributed significantly to the development of tidal basins and estuaries in the Wadden Sea. Silt fosters the growth of mudflats and salt marshes and has various effects – positive and negative – on the Wadden Sea’s food web. Although the Wadden Sea World Heritage Site is an important nature reserve, the area also provides important economic services (in the form of fishing, tourism and shipping). Because of these economic services, it is necessary to intervene in sedimentary processes such as deepening waterways, land reclamation and building flood defences.

Silt transport and deposition

The total amount of silt transported to and deposited in the trilateral Wadden Sea has now been quantified for the first time. The transport is estimated to be 12.1 to 16.5 million tonnes per annum. Most of the silt comes from the Dover Strait, via the so-called Continental Stream in the North Sea. Silt is mainly deposited on the upper mudflats that are connected to the mainland, on the salt marshes and in the sheltered quays. Sediment extraction also occurs, entailing sediment being extracted from the system and used on land. The total amount of silt deposition and extraction is estimated to be 10.8 to 11.3 million tonnes per annum. This implies that silt transport currently exceeds the quantity deposited or extracted, though not by much.

Effect of rising sea level

It is difficult to predict at present how the rise in sea level will affect sedimentation in the long run. In the short term, the development of tidal basins will probably keep pace with the rise in sea level. The flow of silt runs from west to east and so as the sea level rises more silt will gradually be deposited in the western Wadden Sea. However, the total amount of silt within the system is limited. Hence this could have consequences for the availability of silt in the eastern Wadden Sea. The Wadden Sea could be partially submerged if the rise in sea level is more pronounced. This process could be accelerated by human interventions geared towards reduced silt availability. For the time being, however, these predictions for the trilateral Wadden Sea are limited due to insufficient insight into the natural processes and, therefore, a lack of knowledge about how the system responds to human silt balance interventions.