The project was carried out in the forested headwaters of the Rulles basin in southern Belgium, within the Forêt d’Anlier (Figure 1). This large forest massif, located in Wallonia near the Luxembourg border, forms part of the upstream catchment of the River Rulles, a major tributary of the Semois and ultimately the Meuse.
The streams targeted in the project were small, high-quality headwaters flowing through the Parc naturel Haute-Sûre Forêt d’Anlier. These gravel-bed streams are of exceptional ecological value as they are inhabited by protected fish species like Cottus gobio and Lampetra planeri and also host more than 90% of Belgium’s remaining population of the freshwater pearl mussel, Margaritifera margaritifera, a species classified as Endangered by the IUCN Red List and protected under the EU Habitats Directive (Figure 2). The freshwater pearl mussel has one of the most complex life cycles among European freshwater invertebrates. Its larvae must temporarily parasitise the gills of juvenile brown trout, Salmo trutta, before detaching and settling into clean, well-oxygenated gravel beds. As a result, the conservation status of the mussel is directly dependent on the ecological integrity and connectivity of trout habitats. The ecological condition of these streams is high in terms of water quality, but until recently, their biological potential was constrained by longitudinal fragmentation caused by road infrastructure.
Across the Forêt d’Anlier, more than 40 barriers to ecological continuity were identified. Fourteen (14) of the most problematic structures were prioritised to be addressed within this project (Figure 3). These barriers consisted primarily of undersized concrete pipes and box culverts (Figure 4) installed decades ago to facilitate crossing of small streams and the issue of ecological continuity was not considered during their design. Although individually small, the cumulative impact of these culverts significantly reduced ecological connectivity across the basin.
An assessment of fish passability showed that more than 80% of these barriers were impassable to brown trout and other small-bodied species. These small structures fragmented critical reproductive habitats, particularly for brown trout that migrate upstream – sometimes over several kilometres – to reach suitable spawning areas in headwaters. The resulting reduction in juvenile trout recruitment directly compromised the reproductive success of the freshwater pearl mussel.
The culverts created also several hydraulic and morphological problems, including excessive flow velocities due to smooth concrete surfaces, insufficient water depth during low flows, perched outlets caused by downstream erosion, artificial scour pools and channel incision, and disruption of sediment transport and spawning substrate.
The project was initiated by the Parc naturel Haute-Sûre Forêt d’Anlier in cooperation with the Walloon Department of Nature and Forests and was executed in two phases. Phase 1 took place in 2024 and included the removal of eleven (11) culverts. The remaining three (3) culverts were addressed in 2025 and during the second phase of the project. The first phase was financed through a combination of Walloon regional funding and a grant from the Open Rivers Programme — marking the first Open Rivers Programme-supported barrier removal in Belgium. The remaining three crossings were completed under Wallonia’s Recovery Plan (Plan de Relance de la Wallonie), co-financed by the EU (NextGenerationEU).
Rather than reinstalling traditional pipes, the project replaced the culverts with modular, prefabricated open-span structures (bridge-like crossings). Where old pipes existed, the removal process followed a strict sequence (Video 1):
The modular system required no addition of in-stream concrete and no mortar or cement on site, eliminating the risk of pollution. Each bridge/crossing was designed to span 2 m between abutments, exceeds 1.2 times the stream width, maintains natural slope and substrate, and supports heavy vehicles, including loaded timber trucks (Figure 6).
While individual costs varied among the sites, the modular design allowed for rapid installation and reduced machinery time. These bridges/crossings are also designed to provide long-term structural durability and easy replacement of individual components in case of damage.
The presence of Margaritifera margaritifera imposed strict environmental safeguards. The species is highly sensitive to turbidity and sediment deposition. Any uncontrolled sediment release could have caused severe mortality. To mitigate those risks, all in-channel works were carried out under dry conditions. Temporary diversions were also installed where necessary and substrate reconstruction followed strict material specifications (Video 1).
Since Forêt d’Anlier is actively managed, any structure that was going to replace the existing culverts should support heavy timber transport. Designing such structures that would secure ecological continuity and also meet the operational requirements was a key challenge. The modular open-span solution resolved this constraint but required detailed topographic surveys and precise hydraulic calculations for each site.
The remote forest environment also posed a challenge as the access and the working space were limited. The selected approach with the small prefabricated elements that can be assembled on site proved advantageous in these constrained conditions.
Collectively, the fourteen (14) removals reconnected thirty-three (33) km of ecologically valuable headwaters and restored access to upstream spawning grounds for brown trout. This will subsequently lead to improved recruitment of juvenile brown trout. By improving trout population dynamics, the project directly strengthens the long-term survival prospects of Belgium’s last viable freshwater pearl mussel populations.
Fish passage will be restored for other species as well. In fact, monitoring surveys have already documented fish passaging through the new crossings, including brown trout, European bullhead and European brook lamprey. Habitats for protected fish species will also be enhanced, and the natural sedimentation will be reinstated.
The project will also enhance the climate and hydromorphological resilience, as the open-span crossings allow natural channel adjustment during floods and reduce structural failure risk. The work flow during this project has also demonstrated a replicable alternative to traditional culvert replacement methods, and has already attracted interest from cross-border water managers and conservation organisations. The initiative shows that barrier removal does not always require large dam dismantling. Strategic action on small but cumulative barriers in headwaters can deliver basin-scale ecological gains.
On a European scale, the project has contributed to EU policy goals under the Water Framework Directive and the Nature Restoration Regulation (25,000 km of free-flowing rivers by 2030 target).
Before & After photo pairs
With many thanks to Nicolas Mayon from Parc naturel Haute-Sûre Forêt d’Anlier for providing the information and the visuals presented herein.
Prepared by Foivos A. Mouchlianitis