Multi-channel concept Middenwaal
Groynes and summer and winter dikes narrow the rivers, channels, and floodplains. Due to the restrictions of the natural flow, the bed level has degraded. Since the end of the 19th century, our rivers have been eroding into the landscape. At some locations, the bed level of the Waal is two metres lower than it was in 1950.
This causes problems for shipping, which increasingly encounters obstacles on the riverbed that do not erode, such as cables, pipelines, and "hard layers." It also affects agriculture, nature, and freshwater supply, which suffer from problematic droughts; the incising river draws groundwater deeper and deeper.
Room for the River Due to Bypasses
Yet there is an alternative. Stroming, WWF and ARK Natuurontwikkeling developed the Multi-Channel Concept and applied it to the Middenwaal from Nijmegen to Tiel. HKV has calculated the hydraulic effects.
The Multi-Channel Concept is based on the longitudinal dams near Tiel. By slightly narrowing the navigation channel and permanently diverting some of the river water through a secondary channel, the river gains space and can disperse its energy more effectively. Due to the narrowed channel, the river’s depth does not decrease while creating a more natural bypass. Unlike the narrow longitudinal dams, the Multi-Channel Concept involves creating a wide, sandy island with a hard shore on the navigation channel side and a more natural secondary channel on the other side. This not only restores the river’s dynamic and natural flow but also helps sediment settle again in the navigation channel, preventing deep incisions and restoring the groundwater level.
Connecting Longitudinal Bypasses
By creating such secondary channels like a string of beads into the landscape, alternating on the left and right banks, the principle is applied over a large distance, creating an extensive river landscape. Ancient processes such as sedimentation and erosion once again shape the landscape, where wild riparian forests and swamps alternate with stream valley grasslands and lush meadows.
For eleven consecutive secondary channels throughout 25 km, HKV has conducted hydraulic calculations. Additional energy-distributing measures have also been included, such as lowering or removing summer dikes (link to HKV report to be included).
Conclusion
The combination of measures has the potential to limit riverbed degradation, yet further optimization is still needed. By managing the precise connections of the secondary channels and fine-tuning additional elements such as lowering summer dikes and allowing vegetation on the banks, the puzzle can be put together to prevent bed subsidence in a natural landscape.
