The dynamics of a three-dimensional gravity current is investigated by both laboratory experiments and numerical simulations. The experiments take place in a rectangular tank, which is divided into two square reservoirs with a wall containing a sliding gate of width b. The two reservoirs are filled to the same height H, one with salt water and the other with fresh water. The gravity current starts its evolution as soon as the sliding gate is manually opened. Experiments are conducted with either smooth or rough surface on the bottom of the tank. The bottom roughness is created by gluing sediment material of different diameters to the surface. Five diameter values for the surface roughness and two salinity conditions for the fluid are investigated. The mathematical model is based on shallow-water theory together with the single-layer approximation, so that the model is strictly hyperbolic and can be put into conservative form. Consequently, a finite-volume-based numerical algorithm can be applied. The Godunov formulation is used together with Roe’s approximate Riemann solver. Comparisons between the numerical and experimental results show satisfactory agreement. The behavior of the gravity current is quite unusual and cannot be interpreted using the usual model framework adopted for two-dimensional and axisymmetric gravity currents. Two main phases are apparent in the gravity current evolution; during the first phase the front velocity increases, and during the second phase the front velocity decreases and the dimensionless results, relative to the different densities, collapse onto the same curve. A systematic discrepancy is seen between the numerical and experimental results, mainly during the first phase of the gravity current evolution. This discrepancy is attributed to the limits of the mathematical formulation, in particular, the neglect of entrainment in the mathematical model. An interesting result arises from the influence of the bottom surface roughness; it both reduces the front velocity during the second phase of motion and attenuates the differences between the experimental and numerical front velocities during the first phase of motion.

Landslides falling into water bodies can generate impulsive waves, which are a type of tsunamis. The propagating wave may be highly destructive for hydraulic structures, civil infrastructure and people living along the shorelines. A facility to study this phenomenon was set up in the laboratory of the Technical University of Catalonia. The set-up consists of a new device releasing granular material at high velocity into a wave basin. A system employing laser sheets, high-speed and high-definition cameras was designed to accurately measure the high velocity and geometry of the sliding mass as well as the produced water displacement in time and space. The analysis of experimental data helped to develop empirical relationships linking the landslide parameters with the produced wave amplitude, propagation features and energy, which are useful tools for the hazard assessment. The empirical relationships were successfully tested in the case of the 2007 event that occurred in Chehalis Lake (Canada).

This paper reports the results of an experimental series carried out to study flow and sediment distribution in a 90° channel diversion. Each experiment is performed letting the system evolve until an equilibrium state is achieved. The obtained results confirm that sediment tends to move into the lateral branch as suggested in previous studies. The experiments also show a dependence of the phenomenon on the total water discharge and the bed topography evolution in the diversion area. These results have not been previously reported. Over a threshold value of momentum flux ratio, a depression is formed in the main branch that acts as a blocking mechanism for sediment, capturing and directing it towards the lateral branch. This study highlights the importance of interrelations at fluvial diversions between the evolution of bed topography, flow characteristics and sediment distribution.

An experimental study of local scour around rectangular bridge piers founded on two circular cylindrical piles, where the piles are exposed as a result of scouring, is presented. The problem is the interaction of two widths (pier and piles) if the interface is set at different elevations with respect to the bed. Clear water tests were made with a velocity close to the threshold of movement for the uniform sand used. From the analysis of the test results, a width-weighting method is suggested so that the closer to the bed the base of the pier is, the greater the influence on the scour. This method has proven to give good predictions for the local scour at piled bridge piers.