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The present work experimentally investigates the dynamics of unsteady gravity currents produced by lock-release of a saline mixture into a fresh water tank. Seven different experimental runs were performed by varying the density of the saline mixture in the lock and the bed roughness. Experiments were conducted in a Perspex flume, of horizontal bed and rectangular cross section, and recorded with a CCD camera. An image analysis technique was applied to visualize and characterize the current allowing thus the understanding of its general dynamics and, more specifically, of the current head dynamics. The temporal evolution of both head length and mass shows repeated stretching and breaking cycles: during the stretching phase, the head length and mass grow until reaching a limit, then the head becomes unstable and breaks. In the instants of break, the head aspect ratio shows a limit of 0.2 and the mass of 123Environ Fluid Mech the head is of the order of the initial mass in the lock. The average period of the herein called breaking events is seen to increase with bed roughness and the spatial periodicity of these events is seen to be approximately constant between runs. The rate of growth of the mass at the head is taken as a measure to assess entrainment and it is observed to occur at all stages of the current development. Entrainment rate at the head decreases in time suggesting this as a phenomenon ruled by local buoyancy and the similarity between runs shows independence from the initial reduced gravity and bed roughness.

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Gravity currents produced by full-depth lock-release of saline water into a fresh water tank are studied focusing on the influence of the initial density of the saline mixture in the lock and the bed roughness on gravity current kinematics. Temporal evolution of the current front position and front velocity are analysed and related to different phases of the current. Time-space evolution of current depth-averaged density and current height are assessed as well. Roughness of the channel bed plays an important role in the current kinematics, particularly in decreasing the front velocity due to extra drag at the bed. The analysis of Froude numbers, estimated with the initial and local reduced gravity and established with different length scales of the current, allow for the definition of the important variables and current dynamics of each phase of the current development.

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The main objective of the present study is to identify the impacts of bed mobility on the vertical profile of the mean longitudinal velocity and on resistance in flows over water‐worked beds of poorly sorted mixtures of sand and gravel. Water‐worked beds with sediment transport are explicitly distinguished from immobile beds with imposed sediment feed. Flows with different equilibrium sediment transport rates are generated in a laboratory flume. The initial bed mixtures featured combinations of sand and gravel modes. Data collection included instantaneous velocities measured with Laser Doppler Annemometry. Wall similarity, in the sense of Townsend (1976), is assumed. The parameters of the formulae are discussed within three scenarios comprising different definitions ofu* and ks combined with different conceptions of the Von Kármán constant (κ flow independent or flow dependent). It is shown that the parameters of the formulae that express the velocity profile vary with the Shields number and with the initial bed composition. The variation is independent of the adopted scenario, except in what concerns the formulation of hydraulic smoothening in the presence of sand sizes, which depends on the definition of ks.

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.

Confluences with low discharge and momentum ratios, where narrow steep tributaries with high sediment load join a wide low-gradient main channel that provides the main discharge, are often observed in high mountain regions such as in the upperRhone river catchment in Switzerland. Few existing studies have examined the hydro-morphodynamics of this type of river confluence while considering sediment discharge in both confluent channels. This paper presents the evolution of the bed morphology and hydrodynamics as observed in an experimental facility with a movable bed. For that purpose, one experiment was carried out in a laboratory confluence with low discharge and momentum ratios, where constant sediment rates were supplied to both flumes. During the experiment, bed topography and water surface elevations were systematically recorded. When the bed topography reached a steady state (so-called equilibrium) and the outgoing sediment rate approximated the incoming rate, flow velocity was measured at 12 different points distributed throughout the confluence, and the grain size distribution of the bed surface was analyzed. Typical morphodynamic features of discordant confluences such as a bank-attached bar and a flow deflection zone are identified in this study. Nevertheless, the presence of a marked scour hole in the discordant confluence and distinct flow regimes for the tributary and main channel, differ from results obtained in previous studies. Strong acceleration of the flow along the outer bank of the main channel is responsible for the scour hole. This erosion is facilitated by the sediment discharge into the confluence from the main channel which inhibits bed armoring in this region. The supercritical flow regime observed in the tributary is the hydrodynamic response to the imposed sediment rate in the tributary.