Dams are important water infrastructure whose main purposes can be compromised by sedimentation. This causes loss of storage volume, affecting river sediment fluxes and morphology. However, sediment management strategies can be implemented to reduce these impacts. Our goal is to characterize and quantify key processes of an idealized and reduced physical model of water injection dredging, applicable as a sediment management technique. Three sets of experiments were conducted, varying the following parameters: (a) jet discharge; (b) jet angle; (c) bed angle. The spatio-temporal evolution of the main physical processes (scour hole formation, sediment suspension development, and downstream deposition) was analyzed using images of profiles acquired during the experiments. We identified two distinct transport modes depending on how the jet flow connects with the turbidity current, each associated with different stages of scour hole development. In our experiments, the bed-perpendicular component of the exit velocity (momentum) of the jet is the primary driver of the morphological evolution. We demonstrate self-similarity in the longitudinal profiles of the scour hole and downstream deposit. Finally, we discuss practical implications of this study, such as the net displacement of the material, scaling, and limitations. This research contributes to the development of innovative sediment management strategies for water reservoirs and other hydraulic structures.

Current riverine plastic monitoring best practices mainly consider surface observations, thus neglecting the underlying distribution of plastics in the water column. Bias on plastic budgets estimations hinders advances on modelling and prediction of plastics fate. Here, we experimentally disclose the structure of plastics transport in surface water flows by investigating how thousands of samples of plastics commonly found in fluvial environments travel in turbulent river flows. We show for the first time that surface tension plays a key role in the transport of plastics since its effects can be of the same magnitude as buoyancy and turbulence, therefore holding a part of the dispersed buoyant plastics captive by the water surface. We investigate two types of transport; surfaced plastics (surface tension-turbulence-buoyancy dominated), in contact with the free surface, and suspended plastics (turbulence-buoyancy dominated). We prove that this duality in transport modes is a major source of error in the estimation of plastic budgets, which can be underestimated by 90 % following current, well-established monitoring protocols if sampling is conducted solely in the water surface. Based on our empirical findings, we optimize physics-driven monitoring strategies for plastic fluxes in rivers, thereby achieving over a ten-fold reduction of the bias and uncertainty of riverine plastic pollution estimates.