The Eastern Scheldt Storm Surge Barrier, completed in 1986, was built to protect the Netherlands from flooding. The barrier has resulted in changes in the environment: specific fauna and flora are found around the concrete structures of the barrier; currents and tides have changed; sand and stone deposits have formed around the pillars; and changes in sand movements have resulted in the lowering of tidal flats.
The aim of this paper is to describe how the Eastern Scheldt Storm Surge Barrier and the existing and proposed tidal energy turbines within it affect harbor porpoises and harbor seals. Both species show individual differences in behavior and in responses to structures and sounds, and the conclusions drawn in this paper may not be valid for all animals in the population. In addition, sick animals may behave differently, or may be transported through the barrier by the currents. No consideration is given to effects on fish, the prey of porpoises and seals; any effect on fish may in turn affect their predators. Some of the information in this paper is applicable to nearby areas for which tidal energy turbines are being considered (Grevelingendam and Brouwersdam), but some is uniquely applicable to the biology and physics of the Eastern Scheldt Storm Surge Barrier.
The storm surge barrier restricts the movements of harbor porpoises, as they prefer not to enter narrow spaces such as the open gates. In addition, the strong turbulence caused by currents during the flood and ebb tides reduce the echolocation ability of porpoises to orient themselves and forage close to the barrier. The few harbor porpoises that cross the barrier probably do so at slack tides (most likely at high slack tide in the deeper central gates of the 3 sections of the barrier). At high slack tide, the turbines are turning very slowly or not at all, so healthy harbor porpoise are not expected to collide with the blades.
In the scenario of large-scale roll-out of turbines in multiple gates of the barrier, the turbines are expected to add to the barrier function, as they may partially block the gates. However, this added barrier function will affect a maximum of 30% of the total no. of gates of the barrier, if one array turbines is placed in every third gate (in 17 of the 62 gates). Lifting the turbines out of the water at slack tide will increase the likelihood of harbor porpoises crossing the barrier.
The storm surge barrier does not restrict the movements of harbor seals; they have been observed to cross the barrier even when the current is strong and turbulent. Turbines are unlikely to reduce the willingness of most harbor seals to cross the barrier. Harbor seals are not expected to collide with the blades, as they can use several sensory systems to detect the moving turbine blades, and have sufficient space below the turbines to cross the barrier.
Suggestions for research include tracking individual animals (either with tags, or via detectors in the gates), to see if, and under which conditions, the species cross the barrier. Underwater sound recordings of the active turbines will improve the prediction of their effect on the behavior of the seals and porpoises.
The attached position paper also appears as an appendix in Oosterscheldekering Tidal Turbines Monitoring Annual Report 2018.