Development and dynamics of river estuaries
It sounds like magic: being able to predict the unpredictable course of a river. But this is exactly what physical geographer Maarten Kleinhans, Professor of Process sedimentology of river-dominated systems in the Faculty of Geosciences does. Remarkably, he usually manages to keep his feet dry. In his lab, he and his research group recreate meandering rivers, in order to study effects of natural environmental factors as well as human intervention on the pattern and course of rivers. The new insights gained from the experiments form the basis for better computer simulations. The resulting models predict the present and future of natural and affected rivers. The Science and Technology Foundation (STW) awarded him a Vici grant and the European Research Council awarded him a Consolidator Grant for his new ideas about estuaries: river mouths with ebb and flood.
Author: Youetta Visser
“Estuaries are dynamic places,” explains Kleinhans. “The tides and the outflow of the river have combined effects that we hardly understand.” Most river estuaries are ecologically valuable nurseries for shellfish and foraging grounds for birds. But most major ports, that must remain navigable, are also located in estuaries. Kleinhans says: “Human impacts on these natural systems are significant, and impacts of deterioration of estuaries impacts humans. Millions of people over the world live around river estuaries, from Antwerp to Brazil, India and China. We aim for ideas and predictions that help to prevent floods and protect habitats whilst maintaining access to ports.” Of course, Kleinhans also pays regular visits to river estuaries himself. “Among other things, the Vici and ERC grant award made it possible to expand our research and investigate the ecological characteristics of the underlying landscape and to analyse the impact of human intervention on the river.”
Experiments in ‘the Metronome’
Various factors play a role in the formation and dynamics of estuaries, including the buried landscapes accumulated over the last few thousand years, natural land accretion processes, the amount of river inflow, and the species and density of vegetation and shellfish. In order to study this interplay, Kleinhans invented a tidal machine: a flume of 20 metres by 3 metres that gently tilts back and forth every minute, causing the tidal flows of ebb and flood. He calls it ‘the Metronome’. “Over the course of a day, patterns form in the sand. Thus natural river estuaries develop for the first time ever on the small scale of the laboratory.”
In the Metronome, the research team take measurements using cameras and a laser to record effects of a single factor changed in isolation. Examples include dredging and dumping, as well as adding mud and life plants. The researchers combine these experiments with computer simulations that enable many more variables to be included, for example, the impact of different types of valleys or rising sea levels. Satellite images of rivers are used to supplement the research data. “My team of researchers includes all kinds of different experts: an ecologist, a geologist, a satellite image specialist, modellers, physicists, etc. We are all working together to get to the bottom of river estuaries, so to speak.”
Mudflats and salt marshes
Kleinhans is making numerous discoveries in these areas. “We are recognising patterns in the experiments and models that we also see in nature. This has enabled us to build experiments and computer simulations of mudflats along the estuaries for the first time. We are now also trying to get the salt marshes associated with mudflats to develop.” Despite the cloak of muddy waters, nature is gradually revealing its secrets. “River channels that divide around a sand bar are usually short-lived. But with reversing ebb and flood currents something extraordinary happens: one of the channels is primarily active during the ebb and another during the flood. Now that we have seen this develop in the experiments right before our very eyes, the challenge is to explain it.”
High tide in Utrecht
At the transitions between the ebb and flow gullies in nature, shallow shoals develop that cause problems for shipping but are valuable habitats for flora and fauna. “The institutions, companies and other stakeholders who work on the Westerschelde (Western Scheldt) are therefore taking a close look at our work.” To build bridges between these parties and the ongoing research, Kleinhans organises the annual Christiaan Brunings Lecture. He is curious about the effects of ‘grasping roots and sticky substances’. “We already know that river banks are less likely to collapse if they built up from mud and floodplain vegetation. Does it also work like this in river estuaries with mudflats and salt marshes? And is it possible to modify dredging and dumping strategies such that habitats improve?” Behind him, as the Metronome tilts, the tide rises again in Utrecht. But Kleinhans already knew that.