
Multi-channel Concept
A tamed river
Natural rivers are always in motion. Driven by the seasons, they move through the landscape, varying in width and depth, changing their course, and relocating sediments. This stands in stark contrast to the tamed rivers we have created with groynes and dikes. These interventions have brought us much efficiency but also have their drawbacks. One of these is that the river can only expend its energy in one direction: downward. The bed level of the Rhine has eroded at some locations by almost two metres since 1950.
The Multi-channel concept
This causes problems for shipping, which increasingly encounters obstacles on the riverbed. Furthermore, it negatively impacts agriculture, nature, and freshwater supply, which face problematic droughts. The incising river draws groundwater deeper and deeper. Yet there is an alternative. In many places, we have seen how areas can be restored by allowing the river more space. Its energy can disperse in more directions, and its dynamic, living character returns; see Room for Living Rivers. The Multi-channel Concept takes this principle a step further. In this concept, bypasses are created so that the flow can be two-dimensional.
The idea is inspired by the longitudinal dams near Tiel. Here, the river is narrowed so that the water level rises, creating a better navigable depth. Instead of just a narrow longitudinal stone dam, the Multi-channel Concept creates a wider, sandy island with a natural shoreline in the former groyne area. Behind this, a permanently flowing secondary channel is created, receiving about 5-10% of the river water, the same amount as the dam near Tiel. This side channel results in a decrease in the river's erosive energy, contributing to addressing the eroding bed level challenge.
The Multi-channel concept as NBS
The secondary channels thus form a natural alternative to longitudinal dams and can be seen as a Nature-based Solution: a natural solution to a societal problem. Due to this concept, erosion is stopped, which results in the prevention of enhanced drought issues in the floodplains, benefiting both agriculture and nature. Moreover, the secondary channel itself restores dynamic river nature that we have not seen in the Dutch river system for a long time. Underwater and along the banks, essential habitats are formed for all kinds of species, from microscopic life to enigmatic fish species like barbel, common nase, and common dace.
For more information, see Development of Middenwaal and https://media.stroming.nl/middenwaal/