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 x 5 buildings aligned with the approach flow direction, and (2) a square city layout of 5 x 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

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

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.

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.

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.