Sandra Soares-Frazão

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.

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.

Flash flood flow experiment in a simplified urban district

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.

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.

Experiments of dam-break wave over a triangular bottom sill

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 channel. An automated procedure was designed to measure the water depth from the digital images. This procedure has been successfully validated by comparison with the gauge records. Finally, all those measurements form a data set that can be used for the validation of numerical models.

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.

Simplified versus Detailed Two-Dimensional Approaches to Transient Flow Modeling in Urban Areas

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.