The paper discusses the rapid damping of pressure peaks in a water-hammer phenomenon after the end of a complete valve-closure maneuver. This effect is due to flow characteristics not considered when one-dimensional models are employed. Such an effect is linked to the cross-sectional velocity profiles, and therefore to the intrinsic two-dimensionality of the flow field. Applying a 2-D model, recently proposed in the literature, to expand the limited experimental data available with numerical results, useful information on the evolution of the velocity profiles during a transient has been obtained. Starting from an in-depth inspection of the terms in the momentum equation, an additional term is introduced to model the effects of the flow-field two-dimensionality in a 1-D formulation. Finally, the adequacy of a relationship previously proposed by the writers to evaluate the additional term is specifically showed for fast transients in the field of low-Reynolds-number flows when no cavitation occurs, even if its validity has been proven elsewhere for rather different conditions.

In this paper, the results of a benchmark test launched within the framework of the NSF–PIRE project “Modelling of Flood Hazards and Geomorphic Impacts of Levee Breach and Dam Failure” are presented. Experiments of two-dimensional dam-break flows over a sand bed were conducted at Université catholique de Louvain, Belgium. The water level evolution at eight gauging points was measured as well as the final bed topography. Intense scour occurred close to the failed dam, while significant deposition was observed further downstream. From these experiments, a benchmark was proposed to the scientific community, consisting of blind test simulations, that is, without any prior knowledge of the measurements. Twelve different teams of modellers from eight countries participated in the study. Here, the numerical models used in this test are briefly presented. The results are commented upon, in view of evaluating the modelling capabilities and identifying the challenges that may open pathways for further research.

A new approach is proposed for calibrating hydraulic network models. This procedure uses a nonlinear optimization algorithm along with a standard, off-the-shelf, network solver. Pipe roughness is adjusted until simulation results agree with the values observed in the field. To achieve realistic calibrated roughness values, the objective is to minimize the sum of the squares of the differences between the calibrated and initial pipe roughness estimates, under a set of constraints determined from a sensitivity matrix. The algorithm is applied to a sample network, and the resulting calibrated network is tested under different loading conditions.

Dike overtopping is an important issue in both scientific and practical aspects of sediment embankment erosion. This research is concerned with the validation of a one-dimensional numerical code applied to an unsteady two-phase flow over an initially trapezoidal-shaped sediment dike, including the transition from sub- to supercritical flows and the flow propagation over a steep slope. The approach is based on the shallow-water equations coupled with a sediment erosion code. The numerical results are compared with recent laboratory experiments, resulting in a reasonable prediction of the temporal free surface and bed evolution. The remaining model inaccuracies mainly originate from overflow initiation with extremely small flow depths, streamline curvature effects with the related underestimation of flow velocities close to the sediment surface, and the changing saturation in the dike body not accounted for in the model. Despite these deficiencies, the computational model will be helpful, particularly in flood risk assessment and management.

The impact of a dam-break wave on an erodible embankment with a steep slope is studied in the paper using both experimental and numerical approaches. The laboratory experiments have been specifically designed and performed for a range of the storage water levels and the embankment elevations and slopes. The simulations were carried out using a recently-developed two-phase depth-integrated model, supplemented with a geofailure operator to account for the possible occurrence of geotechnical collapses. A comparison between numerical and experimental results indicates that the two-phase model reproduces the experimental free surface elevation well, with or without the geofailure operator. On the other hand, especially for high embankment slopes, this operator appears to be important for predicting the observed morphological evolution. The results also show that, due to the geotechnical collapses, water and sediment velocities may have opposite signs. While the models based on equal direction of the liquid and the solid motions cannot reproduce this effect, the proposed two-phase approach easily accounts for such a peculiarity of the investigated process.