Fine sediment in mixed sand-silt environments impact bedform geometry by altering sediment mobility

The geometrical characteristics of subaqueous dunes may exert a strong control on hydraulic roughness. Conventionally, the prediction of dune existence and geometry relies on phase diagrams and empirical equations tailored for uniform, cohesionless sediments. However, in deltas, estuaries, and lowland rivers, mixtures of sand, silt, and clay are prevalent, which hamper dune prediction. We study the impact of fine sand-silt mixtures on the geometry of dunes. We built a sediment recirculation facility in the Water and Sediment Laboratory at Wageningen University, capable of recirculating mixtures of sediment. The system is composed of a 15-m long tilting flume and a low reservoir where sediment is kept in suspension using a caterpillar system, from where sediment is pumped back into the flume. We systematically varied the sand and silt content for different flow rates. We measured flow velocities profiles with an acoustic Doppler velocimeter (UB-Lab 2C by Ubertone), and captured the bed geometry using a line laser scanner. The mobility of the bed is clearly influenced by the bimodal characteristics and the level of cohesion. When non-cohesive fine sand or coarse silt were introduced to medium-sand base material, we infer that the hiding-exposure effect amplified the mobility of the coarser material. This resulted in increased dune lengths, affecting dune steepness. Conversely, the addition of weakly cohesive fine silt reduced sediment mobility and suppressed dune length. As a consequence, sediment composition has an indirect influence on hydraulic roughness, which was significantly related to leeside angle. During the high flow rates, our results suggest flickering between alternative stable river bed states.