Time to adjust the sedimentation rates in the Hand-on-the-Tap-principle?

The Wadden Sea region is a unique nature reserve, as it changes over time by tides and currents. Needless to say, investigations whether this area could drown due to the combined effects of sea level rise and subsidence caused by mining activities are justified. Given the recent discussion 'whether or not to extract gas under the Wadden Sea region from the FrisianTernaard field', concerns on drowning recently popped up again. Weighing the decisive processes, in addition to sea level rise and subsidence a third factor needs to be considered: the amount of sand and mud that is deposited in the Wadden Sea region.

Compensatory accumulation potential

As long as sand and mud fill up the space created by seafloor subsidence and rising sea level, the tidal flats will not drown. This is called the compensatory accumulation potential. This potential plays an important role in the Hand-on-the-Tap Principle; this principle is a method to assess whether mining activities in the Wadden Sea should be allowed during the next decades. When the combined effect of sea level rise and seafloor subsidence would exceed the compensatory sediment accumulation capacity, the tidal flats could become in danger of drowning. In that case it will be decided to reduce or stop the gas production or salt extraction ('hand on the tap').

The Wadden Sea region is continuously being monitored. This shows that even in the Pinkegat tidal basin -the part of the Wadden Sea with the most subsidence due to gas and salt extraction- any effect of the subsidence by mining activities is immediately nullified by the deposition of sand or mud. Consequently, at the surface no subsidence is observed. Recent research shows that in the next decades the actual compensatory sediment accumulation of the eastern Wadden Sea region will be sufficient to compensate for both subsidence and sea level rise. Even if sea level rises according to the scenario’s for climate change as published by the KNMI in October 2021.

Is the Dutch Wadden Sea system drowning?

The Dutch Wadden Sea system can only drown if the water level rises faster than can be compensated by the actual sedimentation rate. The latest insights indicate this could happen in a couple of centuries (if by that time no mitigating measures are taken). Currently -as well as during the next decades- the eastern Wadden Sea region is moving in the direction of becoming land: the tidal flats are shallowing.

The Wadden Sea system is complex and influenced by multiple processes. In this complex system, subsidence induced by gas extraction plays a minor role. Until now, no indications of subsidence of the seabed are observed at all. In the Hand-on-the-Tap-principle, the compensatory accumulation potentials of the different tidal basins of the Wadden Sea region are documented. These values are estimates, based on the past. However, due to human interventions, the present and future situation of the region has changed significantly. Therefore, TNO argues that the currently used values for the compensatory accumulation capacity in the Hand-on-the-Tap Principle should be adjusted.

Want to know more? The extended version of this article with the associated (scientific) references can be found via this link.