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International audience; Simulated free surface transients in periodic urban layouts have been reported to be self-similar in the space-time domain when averaged on the scale of the building period. Such self-similarity is incompatible with the head loss model formulae used in most porosity-based shallow water models. Verifying it experimentally is thus of salient importance. New dam-break flow laboratory experiments are reported, where two different configurations of idealized periodic buildings layouts are explored. A space-time analysis of the experimental water level fields validates the self-similar character of the flow. Simulating the experiment using the two-dimensional shallow water model also yields self-similar period-averaged flow solutions. Then, the Single Porosity (SP), Integral Porosity (IP) and Dual Integral Porosity (DIP) models are applied. Although all three models behave in a similar fashion when the storage and connectivity porosities are close to each other, the DIP model is the one that upscales best the refined 2D solution.

We propose a finite-volume model that aims at improving the ability of 2D numerical models to accurately predict the morphological evolution of sandy beds when subjected to transient flows like dam-breaks. This model solves shallow water and Exner equations with a weakly coupled approach while the fluxes at the interfaces of the cells are calculated thanks to a lateralized HLLC flux scheme. Besides describing the model, we ran it for four different test cases: a steady flow on an inclined bed leading to aggradation or degradation, a dam-break leading to high interaction between the flow and the bed, a dam-break with a symmetrical enlargement close to the gate and a dam-break in a channel with a 90° bend. The gathered results are discussed and compared to an existing fully coupled approach based on HLLC fluxes. Although both models equally perform regarding water levels, the weakly coupled model looks to better predict the bed evolution for the four test cases. In particular, its results are not affected by an excessive numerical diffusion encountered by the coupled model. Moreover, it usually better estimates the amplitudes of the maximum deposits and scours. It is also more stable when subject to high bed–flow interaction.

A Laser Profilometry Technique was upgraded to detect underwater bed level evolution with a laser and a single camera for highly transient laboratory experiments. While being very simple, the proposed procedure achieves a similar performance as that met by more complex and expensive methods. This technique was applied to a dam-break test case over a mobile bed in a channel with a 90° bend, for which water and bed levels were both continuously recorded at five different places. Photogrammetry was then used to reconstruct the final topography, whose main features could be explained thanks to the analysis of only a few linear bed profiles appropriately chosen. This cheap, simple, flexible and yet accurate method thus proves to be an interesting alternative to more complex methods for morphodynamics investigations in the case of fast transient flows.

A new coupled finite-volume scheme based on the Augmented Roe solver adapted to simulate morphological evolution of arbitrary cross-sections is presented. In pure hydrodynamic conditions, the Augmented Roe scheme has proven to provide accurate results and a constant discharge in steady-flow conditions. Here, this scheme is extended to solve the one-dimensional Saint-Venant–Exner system of equations written for arbitrary cross-sections. Therefore, new eigenvalues and source-term calculations are proposed to account for the irregular shape of the cross-sections. The performances of the proposed scheme are assessed by comparison with three different one-dimensional numerical models aimed at simulating morphological changes, with coupled or uncoupled approaches, and based on HLL or Roe-based flux calculations. Numerous test cases were examined, including water at rest, steady flows and transient flows for which experimental results exist. The results show that the proposed scheme provides stable and accurate results for a wider range of situations than is available with other classical models.

Knowledge of the velocity distribution is key for understanding and modelling any type of flow. For steady flows over fixed beds, theoretical and experimental approaches evidenced a logarithmic law...

Transient flows differ from steady flows in the sense that their dynamic variables, e.g. velocity, change with time. This imposes several constraints both on the measurement techniques to use as well on the data processing. The dam-break flow, as an example of severe transient flow, is used in this paper as test case for the application of different statistical methods identified in the literature: the common statistical tools (e.g. time average, standard deviation), the application of a Fourier-series-based method and the ensemble statistics approach. Data were collected using a fast acquisition camera and particle image velocimetry was used to obtain quantitative information about the velocity field of a set of seven dam-break flows. From the analysis made in this paper it was possible to identify in which conditions each statistical method can be applied to the dam-break flow.

In order to validate the numerical methods aimed at the simulation of fast transient flows involving sediment transport and morphological changes, data are required. However, field data are scarce, or, if existing, are often inaccurate or incomplete, due to the difficulty of taking reliable measurements in such difficult flow conditions. Laboratory experiments constitute a good alternative to obtain validation data for numerical models. When performing simplified experiments, a limited number of well-identified flow features can be highlighted if appropriate measurements are taken. Advances in experimental techniques in the last decades have significantly enlarged the field of possible data acquisition, especially thanks to the development of non-intrusive techniques such as digital imagery. Non-intrusive techniques are of paramount importance when considering sediment transport because a measurement device interacting with the flow would also modify the observed morphological features. In this paper, several imaging-based techniques are presented for water-level and bed evolution measurements. The key features and advantages are discussed but also the drawbacks of those techniques. The discussion is illustrated by different examples that have resulted in data sets commonly used by scientists all over the world to test their numerical simulation tools.

Mathematical models used to describe sediment transport are updated to include more physical characteristics resulting in more accurate numerical predictions. Hence, the present work studies the influence of gravity effects in the bed load transport over steep sloping beds. For this purpose the 2D gravity projections on global coordinates have been included not only in the sediment fluxes, which is the classical approach, but also in the water fluxes and in the pressure/friction terms. Additionally, a set of experiments involving steep slopes has been designed and carried out. Comparisons between the numerical and the experimental results are reported. The gravity projections included in the hydrodynamic and morphodynamic models lead to more accurate predictions in two situations: (i) at the initial stages of experiments where the bed slopes angles are higher; and (ii) under large water and solid discharges, where the interaction between the bed and the water is stronger.

Modelling flow through an urban area is challenging because of the significant difference between the scale of individual streets and the whole district scale. This paper presents idealized experiments where buildings are represented by square blocks. The experiments were designed to highlight the effect of the street width and orientation to the main flow direction. In addition, a porosity model is developed for representing the urban district as a whole without considering the detailed geometry of the streets, but accounting for the main effects on the flow. The model introduces a tensor of drag coefficients aimed at representing directional effects. The drag coefficients are calibrated and validated using the experimental results. © 2016 International Association for Hydro-Environment Engineering and Research.

International audience; 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.; Les résultats d'un test comparatif de modèles numériques sont présentés. Le test a été effectué dans le cadre du projet NSF-PIRE sur les impacts de crues extrêmes et les expériences de rupture de barrage 2D sur fond sableux utilisées ont été effectuées à l'université Catholique de louvain en Belgique. Les niveaux d'eau en 8 points ainsi que les topographies initiale et finale ont été enregistrés. Des simulations en aveugle de cette érosion intense ont été effectuées par plusieurs équipes de chercheurs. Les résultats sont commentés en terme de capacité et voies de recherche.

The small-scale spatial variability of eddy viscosity which is characteristic for the turbulent shear stress in compound open-channel flows was studied and investigated in this paper. Different options including a constant value, zero-equation, one-equation, two-equation, and Smagorinsky turbulence models for parameterizing the eddy viscosity were developed in the framework of the discontinuous Galerkin finite-element SLIM model and applied for presenting the complex velocity profile in two different experimental data sets of laboratory flumes. A very good qualitative agreement was achieved between numerical results and measurement data for both velocity and flow depth of all experimental data sets in general. In addition, the calculation results showed that the turbulent Smagorinsky empiricism allowed a better presentation of non-uniform velocity in the floodplain and transition regions between plain and main channels than the others in all calculated cases. This empiricism predicted a very close variation of eddy viscosity in comparison with the results calculated by the depth-averaged Reynolds’ stress and the lateral gradient of longitudinal velocity. The eddy viscosity varies significantly in the channel section; in particular the small values often occurred around the middle location of floodplains and the central location of the main channel while the large values appeared in the transition regions, presenting different minimum and maximum values of eddy viscosity in each flow region. The effects of eddy viscosity variation on lateral distribution of velocity profile were also investigated and discussed.

Most of the recent developments concerning efficient numerical schemes to solve the shallow-water equations in view of real world flood modelling purposes concern the two-dimensional form of the equations or the one-dimensional form written for rectangular, unit-width channels. Extension of these efficient schemes to the one-dimensional cross-sectional averaged shallow-water equations is not straightforward, especially when complex natural topographies are considered. This paper presents different formulations of numerical schemes based on the HLL (Harten-Lax-van Leer) solver, and the adaptation of the topographical source term treatment when cross-sections of arbitrary shape are considered. Coupled and uncoupled formulations of the equations are considered, in combination with centred and lateralised source term treatment. These schemes are compared to a numerical solver of Lax Friedrichs type based on a staggered grid. The proposed schemes are first tested against two theoretical benchmark tests and then applied to the Brembo River, an Italian alpine river, firstly simulating a steady-state condition and secondly reproducing the 2002 flood wave propagation. © IWA Publishing 2013.

Experimental data related to the analysis of reservoir geometry effects on dam-break flows are provided. Four different reservoir plan shapes were considered, namely a long reservoir, a wide, a trapezoidal shaped and one with a 90° bend. Time variations of the water level were measured with ultrasonic sensors and a high-speed camera. Velocity components were recorded using an acoustic Doppler velocimeter. It was observed that 2D effects are enhanced in the wide and trapezoidal reservoirs and that the long and 90° bend reservoirs produce similar results, with a mainly 1D flow. Finally, discharge estimates based on the water level and velocity data were compared with predictions by empirical formula. Due to the simplified geometry considered, existing formulae are only indicative and cannot be directly applied to practice.

The City of New Orleans is surrounded by an intricate network of floodwalls and dikes built to protect it from flooding from the Mississippi River and Lake Pontchartrain. In 2005, following Hurricane Katrina, New Orleans experienced one of the most devastating urban floods in recent history caused by several dike breaches. This work presents an idealized case study of a part of this event on a scale model. It concerns the flow through one of these breaches and flooding in the surrounding urban area. The scale model reproduces part of the remaining dike, the complex morphology caused by the breach and a number of houses around the area. The flow field computed by a two-dimensional depth-averaged finite-volume numerical model is compared with measurements on the scale model. As the transient part of the flood is quite short, the numerical and experimental results were compared under steady-state flow, showing satisfactory agreement.

Embankment breaching processes are very complex and involve mixed-regime free-surface flow with overfalls and hydraulic jumps, pressurized pipe flow, strong vertical and lateral erosion, discrete mass failure, and headcut migration. The failure mode and mechanism are affected by upstream and downstream water conditions, embankment configurations, and soil properties and state. Great progress has been made to investigate embankment breaching processes through laboratory and field experiments and real-world case studies. However, most laboratory experiments were for smallscale homogeneous embankments, only a few outdoor experiments were conducted at large scales (up to several meters in height) and/or were of composite construction, and only limited data sets for historical embankment failures were sufficiently documented. A number of parametric, simplified physically-based, and detailed multidimensional physically-based embankment breach models have been established in the past decades, but prediction with these models involves significant uncertainties. The biggest limitation of the existing breach models is quantifying erosion rates or erodibility of cohesive soils and sediment entrainment under embankment break/breaching flows. It is important to conduct more large-scale laboratory experiments and field case studies to improve existing embankment breach models or develop new ones. These models should also be enhanced by incorporating better physical insights, by using more efficient computational technologies, and integrating them into more robust flood forecasting and risk assessment systems with comprehensive relevant databases

This research presents experiments of dam-break flow on mobile bed in an abruptly expanding channel, the wider section being twice as wide as the narrow. The bed material consisted of coarse uniform sand. The flow near the one-sided flume expansion induces a two-dimensional morphological evolution. Important scour at the corner of the abrupt expansion was observed, while sediment deposition occurred in the wider channel reach. Measurements of the flow evolution and the final bed topography were obtained using ultrasonic sensors and digital imaging techniques, both having the advantage of being non-intrusive. The available experimental data consist of temporal water level evolutions at nine gauging points, water front propagations, velocity fields at given times at the free surface and topographical surveys of the final bed elevation at various cross-sections. This data set is available upon request to the scientific community for the validation of numerical models.

Simplified and detailed two-dimensional modeling approaches to transient flows in urban areas, based on finite-volume solution of the shallow water equations, are compared. Through the example of a dam-break flow in a simplified urban district for which accurate laboratory data exist, various methods are compared: (1) the solution of the two-dimensional shallow water equations with a detailed meshing of each street; (2) the use of a porosity concept to represent the reduction of water-storage and conveyance in the urban area; and (3) the representation of urban areas as zones with higher friction coefficient. Accuracy and adequacy of each method are assessed through comparison with the experiments. Among the simplified models, the porosity approach seems to be the most adequate as head losses at the entrance and the exit of the city are considered.

Inertia effects are seldom considered in morphological modeling, and most of the transport models were developed from laboratory experiments in steady uniform flow conditions. This paper considers first the hysteresis effects in transient flows between discharge, velocity, and bottom shear stress. These effects can be taken into account as far as the complete shallow-water equations are used. Secondly, inertia effects linked to the sediment response to acting forces are considered. Three types of models are investigated: (i) sediment movement instantaneously adapting to hydrodynamic changes, (ii) spatial or temporal lag laws to give space or time to the sediments to progressively reach the transport capacity, and (iii) a two-layer model, able to account for the inertia of the sediment layer. Finally, three examples are presented: a scour hole downstream of an apron, a jump over a mobile bed, and a dam-break wave. Inertia effects appear significant in the modeling, especially the latter case.

This study relates to sudden transient flow of the dam-break wave type in an idealised city in order to investigate the effects of flow depth and velocity on such a city. Particularly, the study aims at assessing the ability of a finite-volume numerical model, commonly used is inundation modelling, to reproduce fast transient flow including the multiple interactions with obstacles. Experiments were conducted involving two different configurations: (1) a square city layout of 5 × 5 buildings aligned with the approach flow direction, and (2) a square city layout of 5 × 5 buildings not aligned with the approach flow direction. Data were recorded using water-level gauges and digital-imaging technique. These form a complete data set available to validate numerical models aimed at transient flow modelling in complex geometries.

This study presents a novel, simple, but rather accurate approximation of the eigenvalues of the system formed by the Saint-Venant-Exner equations, based on the comparison between eigenvalues for the complete system and eigenvalues for the water phase only. Moreover, a strategy is proposed to compute efficiently the intercell fluxes by properly adapting a Harten, Lax, and van Leer scheme for each equation. Two examples of transient transcritical flows are developed: the erosive migration of a knickpoint induced by an increase in the bed slope, and the evolution of a hydraulic jump over a mobile bed.

SLIM is an unstructured mesh, finite element model of environmental and geophysical fluid flows, which is being improved to simulate fine-grained sediment transport in riverine and marine water systems. A 2D depth-averaged version of the model is applied to the Mahakam Delta (Borneo, Indonesia), the adjacent ocean, and three lakes in the central part of the Mahakam River catchment. The 2D code is coupled to a 1D section-averaged model for the Mahakam River and four tributaries. The coupled 2D/1D model is mainly aimed at simulating fine-grained sediment transport in the riverine and marine water continuum of the Mahakam River system. Using the observations of suspended sediment concentration (SSC) at five locations in the computational domain, the modelling parameters are first determined in a calibration step, for a givenperiod of time. A validation step is then performed using data related to another period of time. It is concluded that the coupled 2D/1D modelreproduces very well the observed suspended sediment distribution within the delta. The spatial distribution of sediment concentration in the deltaand its temporal variation are also discussed.

The IMPACT project addressed the assessment and reduction of risks from extreme flooding caused by natural events or the failure of dams and flood defence structures. It was funded by the European Commission (EC), started in November 2001 and lasted for 3 years. This paper presents a dam-break flow experiment in a channel with a triangular bottom sill (the bump), that was part of the IMPACT benchmarking programme (WP3: flood propagation). This test case highlights some key issues linked to the propagation of dam-break waves on dry bed in the presence of a bed slope. Two types of measurement devices were used to obtain the evolution of the water depth. First, three water level gauges were used to obtain the time evolution of the water level at three different locations around the bump. Then, using high speed digital cameras, the flow was filmed through the glassy walls of the channel, resulting in a continuous experimental surface profile by combining the images acquired at different locations along the ch...

Sandra Soares Frazao and Yves Zech Postdoctoral Researcher, Fonds National de la Recherche Scientifique, Dept. of Civil Engineering, Hydraulics Unit, Univ. catholique de Louvain, Place du Levant 1, B-1348 Louvain-la-Neuve, Belgium. E-mail: soares@gce.ucl.ac.be Professor, Dept. of Civil Engineering, Hydraulics Hydraulics Unit, Univ. catholique de Louvain, Place du Levant 1, B-1348 Louvain-la-Neuve, Belgium.

This paper describes the experiments carried out on the flooding of an urban district model for the purposes of mathematical model testing and validation. A model urban district made up of concrete blocks has been placed in an instrumented physical model of a short reach of the Alpine Toce River to simulate the flooding of a populated area. Water depth history at certain locations has been recorded for several configurations of the model city, the surrounding terrain and the intensity of the flooding conditions. The urban district appears to the flood as an obstruction leading to the formation of a strong hydraulic jump in front of the buildings. When the flood wave enters the city a complex flow pattern is established made up of oblique waves and reflections. The measurements can be readily used for validation of mathematical models of flood propagation in urban areas.

The FP5 IMPACT (Investigation of Extreme Flood Processes and Uncertainty) European project (2001-2004) addressed the assessment and reduction of risks from extreme flooding caused by natural events or the failure of dams and flood defence structures. Research has been undertaken through a combination of laboratory physical modelling, field data collection, field testing, theoretical studies and numerical simulations. This special issue of the JHR aims at disseminating the fantastic database issued from those laboratory works and field observations. The present paper summarises the various types of available data and how to use the database included in the attached DVD-ROM.

During a dam-break flow, the whole valley is involved, which implies that roads, bridges and urban buildings become obstacles to the flow. The purpose of this paper is to provide data about the influence of such an obstacle on a dam-break wave, the obstacle being an idealised representation of a single building. The experimental set-up consists in a channel with a rectangular shaped obstacle, representing a building, placed immediately downstream from the dam. The building is neither centred in the channel, nor aligned with the flow direction. Flow observation shows that after the violent impact of the wave on the building, the flow is forced to change its direction to pass the building. This implies the formation of hydraulic jumps with the consequence that the water level may rise locally higher than without building. Behind the building, a wake zone is observed. Then, further downstream, the flow slowly recovers the structure it would have had without the building. Several measurement devices were used...

This paper presents an idealised experiment of dam–break flow in a channel where both the bed and the banks are made of uniform erodible material. The aim is to study the morphological changes induced by a dam–break flow, as well in the bed profile (longitudinal erosion) as in the shape of the cross sections (lateral erosion and bank failures). The experiments show good qualitative agreement with field observations of geomorphic flows, and have the advantage that they are repeatable for what concerns the general evolution of the shape of the cross sections. A non–intrusive laser sheet technique was used to measure the shape of the cross sections. From there, a complete survey of the time evolution of the channel shape could be obtained. This survey forms a data set that can be used for the validation of numerical models.

The present paper aims to present the issues and the scope of work conducted under the framework of the European Research Project IMPACT, in the field of dam-break induced geomorphic flows, at the Universite Catholique de Louvain, Belgium. Two characteristic behaviours are described: the near- and far-field responses to the dam-break wave. This paper gives an overview of the experimental work carried out in the frame of the research programme: flat- and stepped-bed cases for the near-field, bank erosion experiment for the far-field. New developments in modelling are summarized as well for the near-field and for the far-field. The validation by comparison with experimental results highlights that a two-layer model is very efficient in modelling near-field features, while an appropriate modelling of bank failure and sediment re-distribution in the cross section is the key of far-field simulation. Some practical conclusions are given for the future of dam-break wave modelling in terms of needs for additional...

Secondary free-surface undulations (Favre waves), appearing for example after the opening of a sluice gate or at the head of a bore, cannot be reproduced by numerical models based on the hydrostatic pressure assumption. The Boussinesq equations take into account the extra pressure gradients but are difficult to integrate due to the high-order derivative terms. The paper describes the physics of wave initiation and proposes a demonstration of the Boussinesq equation based on relatively wider assumptions than usually adopted. A linear stability analysis is developed in finite-difference frame to highlight some potential source of numerical instabilities. These conclusions are transposed in a new hybrid finite-volume / finite-difference scheme, which reveals a better accuracy in period and amplitude when evaluated against experiments.