Artificial water storage, originated by the construction of dams, is essential for the sustainable health and welfare of civilizations since it supplies water for human consumption, irrigation and energy production. Furthermore, dam reservoirs are used for recreation, navigation and they provide safety in the downstream valleys against extreme flood events and droughts. All reservoirs are subjected to sedimentation which, without adequate prevention and mitigation counter-measures, threatens their sustainability. As well as the evident loss of storage capacity, the adequate and safe operation of water intakes and bottom outlets belonging to the vital outlet structures can be affected by the deposition of sediments in the reservoir. This Vision Paper first addresses the state-of-the-art and the main scientific advances in terms of prevention and mitigation measures against sedimentation in reservoirs. Then, the main research challenges, which result from the assessment of what are the remaining open questions, are pointed out. The emerging research methodologies to study sedimentation processes are also discussed.

Field observations in a wide range of environments have shown that sediment availability is a major control on the suspended sediment observations in streams. Here we examine, via laboratory experiments, how the amount of proximal in‐channel fine sediment storage relative to the upstream fine sediment distal supply influences the observations of suspended sediment concentrations in streams. Experiments under idealized conditions in a laboratory flume with different ratios of proximal and distal sediment supplies were conducted under a varying flow regime. In addition, the role of the sediment particle size of the supplied sediment on suspended sediment observations was explored. The combinations of proximal and distal sediment supply result in multiple responses of the channel bed and sediment quantity within the channel bed, and the responses adjust through aggradation and degradation. The signature of sediment concentration observed at the upstream section of the channel, given by the distal supply, differs from the downstream observations of the total conveyed sediment (distal and proximal), as shown by an in‐phase analysis of sediment concentration‐discharge plots. Furthermore, we show that nonuniform sediment mixtures may result in a change in the direction of the hysteresis observed between sediment concentration and discharge (i.e., from a clockwise hysteresis to a counterclockwise hysteresis). We also demonstrate that the ratio between sediment distal supply and proximal sediment availability modulates the magnitude of the aggradation/degradation processes in the channel reach and thus the joint observations of sediment concentration and discharge.

Overbank sedimentation is predominantly due to fine sediments transported under suspension that become trapped and settle in floodplains when high‐flow conditions occur in rivers. In a compound channel, the processes of exchanging water and fine sediments between the main channel and floodplains regulate the geomorphological evolution and are crucial for the maintenance of the ecosystem functions of the floodplains. These hydrodynamic and morphodynamic processes depend on variables such as the flow‐depth ratio between the water depth in the main channel and the water depth in the floodplain, the width ratio between the width of the main channel and the width of the floodplain, and the floodplain land cover characterized by the type of roughness. This paper examines, by means of laboratory experiments, how these variables are interlinked and how the deposition of sediments in the compound channel is jointly determined by them. The combination of these compound channel characteristics modulates the production of vertically axised large turbulent vortical structures in the mixing interface. Such vortical structures determine the water mass exchange between the main channel and the floodplain, conditioning in turn the transport of sediment particles conveyed in the water, and, therefore, the resulting overbank sedimentation. The existence and pattern of sedimentation are conditioned by both the hydrodynamic variables (the flow‐depth ratio and the width ratio) and the floodplain land cover simulated in terms of smooth walls, meadow‐type roughness, sparse‐wood‐type roughness, and dense‐wood‐type roughness.