This is an English synthesis of the full summary of the day in Dutch prepared by Bert-Jan van Weeren, STOWA.
What is the impact of wood in streams on backwater? Does it increase flood proneness? How do regional rivers interact hydrologically with the main rivers? Why do some re-meandered streams change their planform and others not? Answering these questions can lead to better stream restoration projects.
RiverCare researchers from the Wageningen University, Jasper Candel and Tjitske Geertsema focused their research on the above questions. To communicate this knowledge, STOWA organized a field visit last February 21st, 2019. Fifty participants among specialists and managers involved in river restoration projects at water boards visited one of the research locations (the Dommel River, which is a tributary of the Meuse River).
At the Dommel (see left-bottom photo), Jasper sampled a core of five meter depth. This sample showed the deposition over the last ten thousand years. The history of the deposition showed that the Dommel bend is stable, because the Dommel deposits fine erosion resistant material in the outer bank. The River is not able to remove these deposits and therefore fixes itself in its own deposits.
In this context, he also referred to the Drentsche Aa, which has an angular meander pattern that results from a peat-filled deposit. The conclusion of Jasper therefore was to look at the history of the (stream) deposits to better understand how it may develop. Thereby, project managers can determine in advance how much land they would have to allocate for a river restoration project. The value of this research was demonstrated with a similar case in a restoration project led by the water board Dommel.
After Jasper’ s explanation, Tjitske explained her research of discharge and water level interventions for water retention in the regional system. At first, she showed that simultaneous occurrence of discharge peaks in the Dommel/Aa and Meuse River are common. Thereby, she investigated the discharge and rainfall regimes of the Dommel, Aa and Meuse Rivers and how these regimes interact at the confluence. As second part, she developed a simple model with the most important parameters determining the backwater effects due to the insertion of wood in streams. This model could help to determine the optimal location for wood in streams, such that backwater predominantly occur for low discharges but not for high discharges. Finally, she studied the backwater effects due to sharp bends, which were particularly relevant for the water board Rijn and Ijssel . Her findings show that secondary flow patterns are changing in time and no backwater effects are apparent in sharp bends.
After the presentations, Chairman of the day Peter Voorn of Natuurmonumenten stimulated the discussion. The field day ended with the call of Rob Ruijtenberg from STOWA to take these new findings into account for future application.
Last modified: 26/09/2019