Weir impacts on sediment transport and geomorphic response to their removal in the Bocq River (Belgium)

Bedload transport is an important component of the ecological equilibrium of rivers. Artificial features such as dams or weirs interrupt the continuity of sediment transport, causing river habitat degradation. These barriers are numerous in the Bocq catchment (233 km²), with an average of one barrier every 1.8 km. They consist of old weirs (1-3 m high) related to ancient uses (watermill, iron factory, irrigation system).
Since 2009, the Bocq catchment has been subjected to a large-scale restoration project (EU LIFE+ project Walphy) including weir removal, resulting in the re-establishment of bedload continuity, and thus in an improvement in river ecological quality.
Before undertaking sustainable rehabilitation work, an assessment of hydromorphological and sedimentological conditions was conducted on several weirs, based on the identification of impacted areas (upstream influence on water level and bedload transport) and on the evaluation of the trapping efficiency of bedload material. The latter rests on the characterisation of the weir structure and the sediment storage quantification. Particle size measurements and the use of Passive Integrated Transponder (PIT) tags have demonstrated different scenarios of bedload trapping, from absence to partial trapping.
The Spontin weir removal was combined with an appropriate assessment of the hydromorphological and geomorphological impacts. The surveys carried out pre- and post-removal have shown an improvement of hydromorphological conditions on the upstream reach, with shallow, fast-flowing habitats, more suitable for salmonid species. Geomorphic responses to the weir removal were quick in relation to the 11-year flood that followed. The release of the trapped sediment was studied using topographic surveys and PIT-tagged pebbles. We thus highlighted the development of a new point bar and a moderate stream bed aggradation downstream.