Getting to the bottom of it

| Rense Kuipers

PhD candidate Lieke Lokin does not have to wander far from the office to find out how the subject of her research manifests itself in the outside world. They can even be found in the babbling brooks on campus: river dunes. She tries to get to the bottom of what goes on under the surface of rivers.

Photo by: RIKKERT HARINK

Lokin regularly catches herself in the act of professional deformation. During a walk on the campus, she peeks over the railing of a bridge to see what the bottom of a stream looks like. Or she peers out of the window when her train to Twente crosses the IJssel river. 'Rivers have become an obsession, while I once chose to study civil engineering because I wanted to design skyscrapers. Nowadays, I always keep an eye on the water: how high is the level, is the current stronger, are the flood plains flooded? I can also spend hours looking at timelapses on Google Earth, how rivers have changed course over the years. Somehow, I still find it fascinating, how they meander through the landscape like veins. And rivers are of life importance. There is a reason why a large portion of the world's population lives along the water.'

 'Rivers are of life importance'

In her research, the PhD candidate from the ET department Marine and Fluvial Systems focuses on what happens at the bottom of a river. Because where there is a current, sediment moves. And that is also how dunes form - and break down - in a riverbed, explains Lokin. 'The rule of thumb is that river dunes grow and become steeper at high water - when a river flows faster. At low water, they flatten out, but that doesn't mean they disappear completely. How do these dunes move and how can we explain this with the physics we know? That was the starting point of my research.'

How these ripples form in the river bed can have far-reaching consequences, Lokin explains. ‘If the water level is low, the dunes are literally hurdles for ships. The highest dune in fact determines the maximum draught for ships - and in the worst case, whether they will run aground. At high water, especially when the water level is extremely high, you want to drain the water as best you can, to prevent flooding. If the water flows faster, then river dunes become higher and the flow is more difficult. After all, a river flows best with a surface that is as flat as possible. If you don't intervene by dredging in the right places, you end up with a vicious circle.'

And then there is the big C-word: climate change. In that light, the height of river dunes is no small matter. 'The expectation is that extremes will increase in size and frequency. We use water levels as an important parameter in the design, construction or raising of dikes to protect us from flooding. The better and more certain we can predict, the better we can make decisions. Not only the societal, but also the economic consequences can be enormous. If you want to raise a dike by just 10 centimetres, it can easily cost multiple millions.'

'Not only the societal, but also the economic consequences can be enormous'

The best way to predict how rivers will flow (or overflow) in the future is with the help of data, as Lokin knows. In this respect, she has access to a gold mine through the overarching project her research is part of, called Rivers2Morrow. Every fortnight, a boat with measuring equipment sails over the Waal from Lobith to Rotterdam, charting the bottom. ‘It is quite unique that we collect so much data so consistently. It even allows me to zoom in on a small section of the river. And this real-life data offers other insights than a laboratory setting. In the lab, you mainly have the problem of scalability. Just try to simulate the behaviour of sand grains on a scale of 1 to 100.'

Still, usable soil data from such a murky waterway is not easy to come by. ‘That is mainly because of the abundance of data. I have to make it readable - and at the same time take into account the presence of other morphological phenomena on the river bed,' explains Lokin. And then there are the dynamics of the river dunes, which make the research all the more difficult. 'They can be as long as a football field and as high as two metres. Depending on the current, they can move up to twenty metres per day. By gaining better and better insight into the behaviour of the dunes, we are trying to optimise an existing model.'

'I can place my part of the puzzle'

Optimising should work, perfecting will be a bridge too far. The subject is simply too unfathomable for that, Lokin explains. We can process and reproduce reasonably reliable data, while the physics behind it is correct to a certain extent. A theoretical physicist would say that we are making very large assumptions, but we cannot avoid simplifying and making assumptions. We do take that uncertainty into account. I know that this subject is too complex to capture in my four-year PhD project. But I can place my part of the puzzle.'