Impact of check dam series on coarse sediment connectivity

Abstract

Check dams are frequently built in mountain streams as sequences of stair-like grade-control structures to stabilise longitudinal profiles and decrease bedload transport rates. However, their effect on sediment connectivity is still not properly understood owing to missing field data about particle mobility through check-dam sequences. In this study, we tracked tagged particles in a reach intensively managed by torrent control works (i.e., a straightened channel stabilised by a sequence of consolidation check dams and bank reinforcements) and in an untreated pool-riffle reach with similar catchment-scale characteristics. Data on water level and particle mobility were collected over two years and five resurveys. In addition, we measured the grain-size characteristics of armour and subarmour bed layers to assess potential armouring processes in relation to the presence of the check dams. The tracking of tagged particles showed clear differences in the longitudinal and channel-unit patterns of particle retention between the studied reaches after two bankfull events. The presence of the torrent control works accelerated the downstream transport of the tagged particles despite lower calculated unit stream powers in comparison with those in the unmanaged reach. We observed a close-to-normal longitudinal distribution of the tagged particles and longer travel distances in the managed reach. In contrast, the pool-riffle reach indicated high retention of recovered particles in fresh bar deposits and a left-skewed distribution of the final travel distances. In addition, the managed reach with check dams indicated strong bed armouring processes that likely made the vertical exchange of particles between sedimentary layers difficult. In contrast to the general perception of check dams as decelerators of downstream sediment fluxes, our field experiment demonstrated a specific case when sequences of consolidation check dams in small and relatively low-gradient managed channels can act as boosters of longitudinal sediment connectivity.