Researchers retrieve an acoustic current profile meter from a tidal sandbank in the Ems-Dollard estuary, which they use to measure the currents due to the wind and tides, as well as the waves and the sand concentrations. Photo: Reinier Schrijvershof
Intensive use by humans and rising sea levels are making river deltas change shape. This has far-reaching consequences for shipping, biodiversity, the risk of flooding and the availability of freshwater. Professor Ton Hoitink and his team are trying to get a better understanding of how these mechanisms influence one another and use that to come up with sustainable designs for waterways.
River deltas – triangular landforms intersected by river branches – connect the river to the sea in an area where sedimentation of sand, silt and clay takes place. Climate change and intensive use by humans have disturbed the equilibrium in these deltas and the animals and plants that live there, says Ton Hoitink, professor of Environmental Fluid Mechanics at Wageningen University & Research and affiliated with the Wageningen Institute for Environment and Climate Research (WIMEK).
Rising sea levels are changing the tides – the ebb and flood flow – in the delta, which can lead to problems such as waterways silting up or river banks eroding. Often this also increases the risk of flooding. Drier summers also reduce the availability of freshwater as saline seawater then enters the delta more easily. That has consequences not only for drinking water supplies but also for the use of freshwater in agriculture and industry.
The intensive use of the rivers by humans also means less and less sand is available, explains Hoitink. “With the construction of dams, dykes and barrages, there is less sedimentation of sand in the delta. At the same time, more sand is needed because of rising sea levels. Eventually, this process will have an impact on the construction of houses. Sand is second only to water in importance among the raw materials used by humankind.” Sand is used not only in building materials but also as an ingredient in electronics and cosmetics.
Nature itself
Hoitink’s research aims to encompass all the changes affecting rivers and therefore allow better prediction of the impact of rising sea levels and a changing climate. He does this by analysing measurements in the field, by building models and by studying sediment flows in a laboratory specially designed for the purpose. “A lot is known about the transport of sand by river flows and mud transport, for example in ports, but there is a gap in our knowledge about the transport of mixtures of sand and mud. In the lab, we study sediment transport and river bed formation with sand-and-silt mixtures under controlled conditions.” The researchers’ goal is to use that knowledge to develop predictive models and reduce the uncertainty in the current river models.
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Our holy grail is to have models that can calculate exactly when tipping points occur
Hond-Paap, one of the largest remaining tidal sandbanks in the Netherlands, regulates the exchange of sand and silt between the land and the water. Photo: Reinier Schrijvershof
Hoitink, who received a Vici grant for his research on river deltas, says, “We want to know what nature itself does in response to all these developments. In the Oude Maas and Dordtse Kil tidal rivers, deep erosion pits are developing due to the changing watercourse. Those pits form a threat to the waterway infrastructure because they can make the river banks unstable. At present, we can only explain these developments in retrospect. For us, the holy grail is to have models that can calculate beforehand exactly when tipping points occur, for example leading to the development of deep erosion pits. If you can do that, you will be able to take preventive measures.”
An ecological disaster
In addition to the Dutch delta, Hoitink is also looking at river deltas and estuaries in Bangladesh, China, Canada and Germany. For example, Hoitink’s team and German colleagues are studying the Ems, a river that flows into the Wadden Sea and is incredibly turbid due to the continuous inflow of mud. “That’s an ecological disaster because nothing can live in that water. Our research shows how this is connected to the efforts to keep the waterway deep enough for shipping.”
The analysis of all the international data shows that rises in sea levels do not lead to proportionate increases in extreme water levels in rivers. So it is not the case that if the sea level increases by one metre, the peak water levels in the rivers increase by the same amount. This is relevant information, says Hoitink: “For example, when building dykes in the future.”
At the grid nodes of this model, water levels and flow speeds in the Maas are calculated for a range of depths. This is used to see whether peak water levels in this area rise as fast as sea levels.
Maintaining the current river system in the Netherlands poses a formidable challenge, says Hoitink. “Many rivers demand a lot of maintenance, which takes vast amounts of time and money. We find more and more interventions are needed, such as filling erosion pits with rubble and carrying out sand replenishment, to keep the system we have at the moment intact. The Netherlands can afford to do this. The rivers and the Port of Rotterdam are so important to the economy that we are prepared to make this investment. But that doesn’t apply to countries such as Bangladesh, which don’t have the money for strong coastal defences.”
That is why he is uneasy about presenting the Dutch water system as something iconic, which is the reputation it has internationally. “At international conferences, I always say our approach is based on deep pockets. But time will tell whether that is the best solution in the long term. At the moment, we’re mainly fighting the symptoms rather than tackling the crux of the problem.”
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Rivers need more room to flood their banks as we prepare for rising sea levels
Replacing the Delta Works will be a gigantic project. Photo: Shutterstock
Hoitink advocates a system that encompasses more sustainable solutions. “What I have in mind is a delta that doesn’t need so much upkeep. If we take the example of Rotterdam, a solution that relocates the port activities towards the sea would prevent a lot of the problems, such as the high maintenance costs for dredging, and saline intrusion – which threatens the availability of freshwater. Agriculture could also make way for fisheries, which become possible if the river mouth is more natural.”
Replacing the Delta Works
Hoitink’s research can also help in deciding how best to replace the Delta Works that were built after the North Sea Flood of 1953. Those structures will have to be replaced in a few decades. “That will be a gigantic project. Our research gives an idea of where we should be heading to make sure the solutions are sustainable.” For example, more room should be created for the sea and rivers to flood their banks, says Hoitink. “This is necessary to prepare more broadly for rising sea levels.”
As regards the future, Hoitink hopes his predictive models will become just as accurate as the models of the meteorologists working in the same building on the Wageningen campus. “That scientific field has advanced so much in the past decade that they can now predict rain showers quite accurately. I’d like to see that in our field too.”
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