PhD defence: Sediment Dynamics in Estuarine Tidal Flats in Transition

To preserve the important intertidal sandbanks and also the salt marshes off our coasts for the future, we need more turbid water. That is one of the striking conclusions from the thesis that coastal morphologist Tim Grandjean of NIOZ in Yerseke will defend at Utrecht University. "These natural areas outside our dikes are essential for nature as well as for coastal defense. But because of the way we are now building in the Delta and the hinterland, coastal defense is endangered in the long term," Grandjean warns.

Satellite measurements

For his research, Grandjean linked decades of satellite measurements of tidal grounds around the world with data on turbidity of the water, which is also measured by satellites. In the long run, a very clear relationship emerged: intertidal grounds only grow when the water is turbid enough. "After all, turbid water means: more sand and silt particles in the water, which can allow the coast to grow," the researcher explains.

Expensive sensors

In addition to those ‘old’ data, Grandjean also collected new information on the dynamics of intertidal grounds. To do so, he used technology developed in NIOZ's own workshops. "The height of intertidal grounds can be measured with very expensive sensors or with labor-intensive, manual measurements. But this way, we could never collect data in many different places at the same time. The technicians at NIOZ managed to develop sensors that were cheap enough to record the height of the bottom to the millimeter, several times a day, at almost 200 locations along the Western and Eastern Scheldt for several years now."

Heavy dynamics

Those precise measurements show that at high tide, an intertidal zone can easily vary one or two centimeters in height within a single day. "For you or me, a centimeter more or less under our boots may not matter, but if you are a worm or a mussel, or a seed of eelgrass, that is indeed too much dynamics," Grandjean says.

Jetties work

Grandjean also looked at the grounds between jetties that were constructed by the province of Zeeland, to improve the quality of tidal nature along part of the banks of the Westerschelde. "Of course, we already knew that these jetties, also along rivers, can protect the coast, but our measurements show that benthic life along the Westerschelde also benefits from these measures. The dynamics decrease enough in time and space, for benthic animals to settle better."

Coastal defenses must improve

The protection of the coasts of the Wester- and Oosterschelde is by no means luxury, according to Grandjean. Due to the construction of the storm surge barrier in the Oosterschelde in the 1980s, this area has suffered enormous ‘sand starvation’. Less sand is entering from the North Sea, or in Grandjean's words: the water has become too clear, causing the intertidal banks to erode. In the Westerschelde, too, many valuable coastal areas are disappearing because of the dredging that is needed to keep the port of Antwerp accessible.

"In the long run, this is not sustainable," Grandjean believes. "When too much intertidal area disappears outside the dikes of Zeeland, the safety of complete coast is at stake. So not only for nature, but also for safety, we need to start protecting the areas outside the dikes much better. The water has to become murkier, so to speak, so we may also have to reopen the Oosterschelde storm surge barrier, for example. The only alternative is that we raise the dikes further, but in doing so we lose nature as well."