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. RÉSUMÉLes zones urbanisées présentes dans les plaines d'inondation des rivières peuvent avoir des conséquences importantes sur l'écoulement lors de crues. Une modélisation à grande échelle de ces zones implique donc un traitement adapté afin de reproduire leurs effets. Cet article présente un modèle utilisant des équations de Saint-Venant modifiées, prenant en compte la porosité des zones urbanisées afin de représenter la réduction de capacité de stockage et de passage induite par la présence des bâtiments et autres structures dans la plaine d'inondation. L'introduction de la porosité dans les équations de Saint-Venant implique une modification des expressions des flux et des termes sources. De plus, elle requiert l'adjonction d'un terme source supplémentaire. Ces équations modifiées sont résolues par un schéma de volumes finis avec un solveur de Riemann de type HLLC et un traitement décentré amont des termes sources. Les possibilités de cette nouvelle approche sont démontrées par une application à un cas test expérimental à grande échelle issu du projet européen IMPACT. Ce cas test consiste en une inondation sévère de la vallée italienne du Toce. Cet exemple illustre l'avantage clé de la méthode proposée, à savoir une représentation précise de l'écoulement sans devoir recourir à un maillage détaillé de la zone urbanisée.
Flood risk analysis increasingly involves the integration of a full range of loading conditions as well as multiple defence system states, overlaid by uncertainty analysis. This type of analysis involves the simulation of many thousands of flood events. To keep model runtimes to practical levels an efficient yet robust flood inundation model is required. To accommodate this need a rapid flood spreading model (RFSM) has been developed that utilises the availability of good quality topography data and advanced GIS techniques. This paper describes recent improvements to the RFSM that have focused on incorporating additional physical processes within the spreading algorithm (multiple spilling and friction). This improved model is applied to a number of different sites with comparisons made to a more complex hydrodynamic model. The findings of this comparison demonstrate a good degree of similarity between the RFSM and more complex models, with a significantly reduced runtime overhead.
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