Yves Zech

Two-dimensional shallow-water model with porosity for urban flood modelling

Floodplains with urban areas have significant effects on inundation flows. Large-scale modelling of such zones thus requires a special treatment to involve those effects. This paper presents a shallow-water model with porosity to account for the reduction in storage and in the exchange sections due to presence of buildings and other structures on the floodplains. The introduction of the porosity in the shallow-water equations modifies the expressions for the fluxes and source terms. Furthermore, it implies the addition of a specific source term. The equations are solved by means of a finite-volume scheme with a modified HLLC Riemann solver and upwind treatment of the source terms. The possibilities of the proposed approach are demonstrated by an application to a large-scale experiment that was part of the European IMPACT project, which represents the severe flooding of the Italian Toce valley. This demonstrates the key advantage of the method, as it allows an accurate representation of the flow without detailed meshing of the urbanized area.

Small-scale laboratory dam-break waves on movable beds

This paper documents a series of small-scale laboratory experiments of dam-break waves propagating over loose granular beds, realised in a dedicated flume equipped with a fast downward-moving gate. The flood wave generates intense geomorphic action as bed sediments are eroded and entrained by the flow. Two distinct bed materials, sand and Polyvinyl Chloride (PVC), and a range of initial bed configurations are explored. The sediment bed is either flat or features an upward or downward discontinuity of level across the gate. The case of an initial non-zero water depth downstream is also investigated. The flow is imaged through the sidewalls with fast digital cameras. Imaging techniques are used to identify characteristic flow regions and to derive full velocity fields by tracking the movements of individual bed sediments as well as neutrally-buoyant tracers. The data set described in the paper is provided in digital form in the electronic supplement to this special issue.

Undular bores and secondary waves -Experiments and hybrid finite-volume modelling

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.

Numerical and experimental water transients in sewer pipes

The paper deals with transient transcritical flow in closed sewer pipe. Among the various shock-capturing schemes used for solving hyperbolic systems of conservation laws, an upwind scheme is adopted in such a way that automatic description of hydraulic jumps and bores becomes possible by a steep variation of hydraulic variables over a few grid points. The Pavia Flux Predictor scheme (P.F.P.) was selected, because of its simplicity, robustness and physical consistency. To validate the numerical model, experiments were carried out for a steep slope circular pipe. An application to flush flows at sewer heads is also presented in a context of parsimonious water consumption.

Experimental study of dam-break flow against an isolated obstacle

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 to characterise the flow. At five different locations, the water level evolution was measured by means of water level gauges while the velocity was measured by means of an acoustic Doppler velocimeter. Then, the surface velocity field was obtained using digital imaging techniques.

Dam-break induced sediment movement: Experimental approaches and numerical modelling

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 Université 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 research.

Dam-break flow through an idealised city

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.

Dam-break flows over mobile beds: Experiments and benchmark tests for numerical models

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.

Dam-break flow on mobile bed in abruptly widening channel: experimental data

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.

Dam-break induced morphological changes in a channel with uniform sediments: measurements by a laser-sheet imaging technique

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.