Can coarse bedload pass through weirs?

Restoring active bedload transfer in human-impacted rivers has received increasing attention in recent years, notably in response to the European Water Framework Directive (WFD), which requires that the continuity of rivers not be disturbed by anthropogenic features such as dams or weirs. The Bocq River (233 km2), a moderate-gradient stream in Wallonia, Belgium, has a hydraulic resource that was formerly largely exploited with 74 weirs (up to 2.3 m high) along 43 km. We examined the effects of seven old abandoned weirs on bedload transport for three different types of weirs (defined by the presence and position of the sluice gate system). First, the volume estimates of bedload stored in reservoirs indicated that, despite their old age, the reservoirs were not completely filled (between 25 and 50% filled compared to the reservoir volume capacity) and did not evolve very much since 1990. Second, the grain size analysis of bed material upstream, downstream and within the reservoirs, and the direct measurements of sediment transport (slag particles and PIT-tagged pebbles) demonstrated that bedload continues to be transported out of the reservoir, even though the selective trapping of coarser elements was observed within the reservoir. Particles in the range of the median can pass over the crest of weirs, but the coarser elements tend to remain in the reservoirs. This trapping effect is mitigated when the weir has open or collapsed flushing gates that facilitate bedload transfer. This indicates that weirs act as leaky barriers that allow bedload to pass through, although the individual geomorphic setting plays a primary role in determining the local sediment continuity. These findings suggest that river connectivity is less impacted than initially thought and is likely to increase over time as old weirs gradually fall into disrepair. This needs to be acknowledged when planning barrier removal projects.