News+Stories: You originally studied at TU Graz and have now returned after spending some time abroad. Have you settled in well?
Stefan Haun: (laughing) Yes, very well. I already know some of my colleagues from my studies and projects – so it's not all new and I was very well received.
You have been head of the Institute of Hydraulic Engineering and Water Resources Management since the beginning of the year. Do you have any plans for the Institute's development in the coming years?
Stefan Haun: Until now, the Institute has focused on structural hydraulic engineering. In other words, dams and electricity generation from hydropower. This will continue to be a very important topic, but I would like to expand the areas of research. I mainly work in the field of sediment hydraulics and the renaturalisation of river landscapes. I would now like to intensify these areas.
What path led you to the water, so to speak?
Stefan Haun: I myself studied Civil Engineering at Graz University of Technology (TU Graz). If I were starting today, I would certainly have opted for the Master's programme in Geotechnical and Hydraulic Engineering again. At that time, I was particularly interested in tunnelling and rock mechanics. But then the very last lecture ‘triggered’ me, as it were. It was about river engineering and sediment hydraulics. That's when I changed my focus.
Was it the right decision?
Stefan Haun: Definitely. Because of this topic, I stayed in academia and didn't take the traditional path of a civil engineer on a construction site. During my doctorate in Norway, I worked on the sedimentation of reservoirs.
What’s that?
Stefan Haun: Small and large particles are deposited in reservoirs due to the reduced movement of the water. We estimate that around one per cent of the world's available reservoir volume is silted up and lost every year. This is countered by dredging and reservoir flushing.
Reservoir flushing is a bit controversial when you consider the ecology...
Stefan Haun: That's true. But if you plan, implement and monitor this process properly, it can also be done in an ecologically responsible way.
So how is this done?
Stefan Haun: Basically exactly how you imagine. The water from the reservoir is drained. This increases the bed shear stress at the bottom of the reservoir and sediments are eroded by the water. We do this above all during floods, because the bed shear stresses are particularly high, which means that a lot of sediment is carried along and the dilution effect is also great. Flushing changes the concentration of suspended matter and the oxygen content in the lower reaches of the river and can damage the habitat. That's why we have to plan and monitor carefully.
It's not about individual stones or particles, but about many, very different things that are also under water and therefore difficult to observe. How can I model this process?
Stefan Haun: It requires extremely complex mathematical models that do not yet exist. Cohesive and rolling sediments, for example, behave completely differently, resulting in exciting effects. What's more, every reservoir and every body of water is different and there is no one-size-fits-all solution. Currently, we often work in rivers with empirical studies and formulas derived from them – including bedload transport formulas from Switzerland, where river landscapes are very similar to those in Austria. In simple terms, researchers placed particles in a channel, turned on the water and saw when and how many stones moved. If you take these formulae and apply them to reality, you suddenly have 100 to 1000 times as many particles interacting with each other. They also ‘hide’ behind each other or stick together. When one particle begins to move, many particles suddenly start to move. The particles also vary in weight and are therefore transported in the water in different ways. Biological and chemical processes are often added to this. It is therefore difficult to predict how the deposits in the reservoir will behave as a system.
I imagine that it is very difficult to verify such calculations in reality. The object under investigation is completely under water and if you drain the water, the soil has already changed as a result.
Stefan Haun: Yes, exactly – every measurement in the river or when draining the reservoir changes the system. In one project, we extracted undisturbed sediment cores from a depth of 40 metres. We used a kind of plumb line with a perspex tube that closed when lifted. We were able to extract and analyse sediments from the bed as well as deep water. But bedload measurements at the river bed are not trivial either. We are currently drafting a leaflet for practical use.
But these are all very localised phenomena; don't we have to think bigger here?
Stefan Haun: It is very important to me that we not only look at the reservoir space, but at the entire river system. This has to be done in an interdisciplinary way and is incredibly important in order to understand the big picture, including ecology and buildings. We analyse hydrology, the composition of surfaces, soil erosion, water bodies and so on. This holistic view is increasingly needed in hydraulic engineering.
Of course, this also includes the damming of our rivers that has taken place in recent decades.
Stefan Haun: Yes. A great many construction misdeeds were committed, river courses were straightened, banks were built up and many rivers were literally blocked. But nobody knew any better back then. Although this has given us a lot of land for agriculture, among other things, it has also made the river much faster and in some cases created flood problems for ourselves. In the course of renaturalisation, we are trying to build this back again and let the river do what it wants. We may give it an initial spark, but after that it is allowed to develop naturally. However, we have to monitor it closely to prevent it from getting too close to cultivated areas or even settlements, where we then have to intervene again. In addition, the risk of flooding is also reduced if the river has room to spread away from inhabited areas – in other words, we create natural retention areas where it can safely overflow its banks during floods. The big problem here is the availability of land for such projects. And of course it is not easy to get fields or meadows that are flooded during high water. There is still a lot of work for us to do here in the interests of the common good.
