Investigating the influence of weirs and small dams on coarse sediment connectivity in low-energy rivers: A case study of the orne and vire rivers in Normandy, France

Abstract

The removal of weirs has become emblematic of efforts to restore the good ecological status of rivers in France (Water Framework Directive -WFD- in 2006; the French Eel national management plan in 2007); however, it is increasingly facing opposition for the recreational and cultural considerations it brings (Germaine and Lespez, 2017). Previous studies have highlighted that bedload can pass through or over these weirs, although they acknowledged the difficulty in applying their conclusions to all geographic contexts. In this context, we aimed to determine the influence of these structures on the bedload connectivity in low-energy, highly human modified rivers. In this way, the objective of this study is threefold: generate knowledge about the mobility of a potential bedload in low-energy, highly anthropized contexts, better understand the influence of weirs and dams on these bedlaod dynamics, and identify the associated morphological changes. We monitored the bedload fraction of the Orne and Vire rivers (France) for 4 years using 848 pebbles equipped with passive transponders injected upstream of 2 dams and 4 weirs of different size and 2 reference sites where a weir or dam had been removed. While tracers' mobility was low (∼3–5 m yr−1 for T < 2 yrs), it was highly influenced by the presence of transverse structure (shorter distances and frequency of mobilization), even small ones. After 4 years of monitoring, no tracers crossed any of the weirs. This reduction in mobility was also due to decrease of shear stress within the reservoir as well as the physical barrier created by the weir. This interruption in bedload continuity resulted in a downstream coarsening of bed material downstream of the weirs but not at sites that had been restored for several years. Ultimately, the results of this study contribute to the scientific debate by focusing on two river reaches that have been greatly influenced by human activities for centuries.