Design Study for the creation of lotic habitats at the weir complexes in the river Meuse

Abstract

Water levels in the river Meuse drop during periods of low river discharges, making it unnavigable for shipping. To maintain navigability in the Dutch part of the river Meuse, seven weir complexes were constructed in the river. These complexes regulate the river and maintain target water levels to allow for shipping throughout the entire year. The complexes were constructed in the early 20th century and are all reaching the end of their technical lifetime. Therefore, they require replacement or renovation. This provides the opportunity to explore ecosystem restoration at these complexes.
The seven weir complexes are located at Borgharen, Linne, Roermond, Belfeld, Sambeek, Grave, and Lith. Each complex consists of weirs, locks, and fish ladders. These complexes act as barriers to fish migration, the river’s sediment transport, and reduce the lotic habitats in the river (in Dutch: ‘Stromende habitats’). The reduction of lotic habitats leads to a decline in species that depend on these environments.
The objective of this report is to study the possibility of creating an optimized ecological route at conceptual level for the weir complexes in the Dutch part of the river Meuse to create environmental conditions for the formation of lotic habitats. This optimized ecological route is referred to as an ecological channel. The channel was designed to support specific endangered river species, referred to as the target river species.
The channel was initially designed for weir complex Sambeek, which serves as the case study location. This complex was selected as it has the most available space, which provides more flexibility for the channel’s design. Subsequently, an assessment was conducted to determine whether the channel could be applied to the other complex locations. To form lotic habitats, the channel must meet certain environmental conditions that are based on the needs of the target river species. These conditions must be achieved during the critical reproductive months of these species. The environmental conditions primarily consist of varying flow conditions, which are achieved by varying inflow rates, indicating the need of an intake structure.
The ecological channel was designed through an iterative process, as its dimensions and flow conditions have interdependent relationships. These parameters had to be iteratively adjusted until a suitable combination was found that met the required conditions. To streamline the process and reduce the number of possible combinations, the design of the channel’s intake structure and the channel’s dimensions were done separately.
The final ecological channel design includes an intake structure consisting of a flap gate and vertical-slot fish passage. An impression of the final channel design at weir complex Sambeek is shown in the figure on the following page. The channel design meets the required environmental conditions for habitat formation for river discharges up to 500 m3/s for weir complex Sambeek, Linne, Roermond, and Grave, and for discharges up to 250 m3/s at complex Borgharen, Belfeld, and Lith. Both discharge ranges include the critical reproductive months of the target river species, as was required. The final design shows that the required environmental conditions for lotic habitat formation can be achieved at the weir complexes in the Dutch part of river Meuse, potentially leading to an increase in the populations of the target river species.
The channel design may not accurately represent reality due to uncertainties in the estimations and limitations of the channel’s boundary conditions, available space, and simplifications of its hydraulic processes. In addition, even if the required environmental conditions are achieved, it does not guarantee that the river species will utilize the channel, as their behaviours can be unpredictable, and their response may not be as anticipated. To develop a more realistic and detailed design, it is recommended to construct a hydraulic model and conduct further research on the behaviours of the river species.