Deep erosion pits were created in several places, affecting water levels and thus the reliability of flood forecasts. The eroded sand was mostly deposited back on the banks in the immediate vicinity in packages up to 3 metres thick. The findings were published in Nature and underline risks for future flooding.
Deep pits and shifting sand dunes under water
During the floods caused by extreme rainfall from depression 'Bernd', water discharge near Maastricht rose from 50 to over 3,300 cubic metres per second in just two days. The enormous force of the water caused unexpected changes in the river bottom. In the so-called bottle necks - narrow parts of the river where the water flows at an accelerated rate - heavy soil erosion occurred. The researchers found as many as 16 deep potholes in the river bottom between Meers and Berg, some more than 15 metres deep.
These pits were created as the water not only stirred sand and gravel, but also exposed deeper layers of fine sand and washed them away. In a few days, the Meuse moved as much sediment as it normally does in 10 years. This became clear from measurements of thickness in the field immediately after the high water and measurements of the composition of sediment samples at the VU Sediment Laboratory.
Consequences for safety and water management
The changes to the river bottom were not without consequences. Some villages along the Meuse were only centimeters away from flooding. Moreover, in some places the actual water level turned out to be up to half a metre higher than predicted. One of the causes: the rapid and unexpected changes in the river bottom during high water. This has direct implications for the reliability of flood models, on which important safety decisions are based.
Incomplete river widening increases risks
Since the floods of the 1990s, much work has been done to widen the Meuse to give room for high water. However, the widening has not been carried out uniformly everywhere. Where the river remains locally narrowed, new bottlenecks arise where water flows through at extreme speeds - up to 20 kilometres per hour. Such speeds are exceptional for a Dutch river and create great underwater forces.
Knowledge of subsurface essential
According to co-researcher earth scientist Willem Toonen (VU), the study shows how important it is to better map the subsurface of rivers. "From geological research we know relatively much about the subsurface of the Meuse, but in many rivers we don't really know how the deeper soil layers are constructed. However, precisely these layers can quickly wash away during high water, with major consequences for erosion, water levels and infrastructure." Rijkswaterstaat therefore wants to investigate how the bottom of the Meuse can be mapped more accurately to better assess risks.
Relevance for future river management
Due to climate change, extreme precipitation and high water levels are likely to become more frequent in the future. This study shows that unexpected subsurface changes, such as deep erosion pits, are a serious factor in water safety. The insights from this study are therefore important for the management of not only the Meuse, but also other rivers worldwide.
"This is a good example of how knowledge of the subsurface and long-term processes in river systems is crucial for dealing with the consequences of climate change," Toonen said.