This paper describes benchmark testing of six two-dimensional (2D) hydraulic models (DIVAST, DIVASTTVD, TUFLOW, JFLOW, TRENT and LISFLOOD-FP) in terms of their ability to simulate surface flows in a densely urbanised area. The models are applied to a 1·0 km × 0·4 km urban catchment within the city of Glasgow, Scotland, UK, and are used to simulate a flood event that occurred at this site on 30 July 2002. An identical numerical grid describing the underlying topography is constructed for each model, using a combination of airborne laser altimetry (LiDAR) fused with digital map data, and used to run a benchmark simulation. Two numerical experiments were then conducted to test the response of each model to topographic error and uncertainty over friction parameterisation. While all the models tested produce plausible results, subtle differences between particular groups of codes give considerable insight into both the practice and science of urban hydraulic modelling. In particular, the results show that the terrain data available from modern LiDAR systems are sufficiently accurate and resolved for simulating urban flows, but such data need to be fused with digital map data of building topology and land use to gain maximum benefit from the information contained therein. When such terrain data are available, uncertainty in friction parameters becomes a more dominant factor than topographic error for typical problems. The simulations also show that flows in urban environments are characterised by numerous transitions to supercritical flow and numerical shocks. However, the effects of these are localised and they do not appear to affect overall wave propagation. In contrast, inertia terms are shown to be important in this particular case, but the specific characteristics of the test site may mean that this does not hold more generally.

/pdf/benchmarking-2d-hydraulic-models-for-urban-flooding-4epkxyg0fl.pdf

Benchmarking 2D hydraulic models for urban flooding

http://www.hach.ulg.ac.be/cms/system/files/Mignot_2006.pdf

Modeling floods in a dense urban area using 2D shallow water equations

Recent developments in flood modeling have led to the concept of coupled (sewer/surface) hydraulic models. In this paper two coupled models are examined; a one-dimensional (1D) sewer model coupled with a 1D surface network model (1D/1D) and a 1D sewer model coupled with a two-dimensional (2D) surface flow model (1D/2D). Flow over the terrain is better modeled by 2D models, whereas in confined channels 1D models provide a good approximation with less computational effort. This paper presents a comparison of the simulation results of 1D/1D model and a 1D/2D model. The methodology adopted for setting up the models is outlined and explained in detail as well as the 1D/1D modeling techniques used for reproducing the 1D/2D model results. The surface flow comparison clarifies the limitations of the 1D/1D model and indicates that the definition of the surface pathways, the linking elements sewer/surface, and inclusion of virtual manholes are key factors for setting up a more accurate 1D/1D model.

Comparison of 1D/1D and 1D/2D Coupled (Sewer/Surface) Hydraulic Models for Urban Flood Simulation

Flooding is a major hazard in both rural and urban areas worldwide, but it is in urban areas that the impacts are most severe. An investigation of the ability of high-resolution TerraSAR-X synthetic aperture radar (SAR) data to detect flooded regions in urban areas is described. The study uses a TerraSAR-X image of a one-in-150-year flood near Tewkesbury, U.K., in 2007, for which contemporaneous aerial photography exists for validation. The German Aerospace Center (DLR) SAR end-to-end simulator (SETES) was used in conjunction with airborne scanning laser altimetry (LiDAR) data to estimate regions of the image in which water would not be visible due to shadow or layover caused by buildings and taller vegetation. A semiautomatic algorithm for the detection of floodwater in urban areas is described, together with its validation using aerial photographs. Of the urban water pixels that are visible to TerraSAR-X, 76% were correctly detected, with an associated false positive rate of 25%. If all the urban water pixels were considered, including those in shadow and layover regions, these figures fell to 58% and 19%, respectively. The algorithm is aimed at producing urban flood extents with which to calibrate and validate urban flood inundation models, and these findings indicate that TerraSAR-X is capable of providing useful data for this purpose.

Flood Detection in Urban Areas Using TerraSAR-X

Two-dimensional, high-resolution modeling of urban dam-break flooding: A case study of Baldwin Hills, California

The paper discusses the use of a code solving two-dimensional (2D) shallow-water equations in an urban area. Two application examples are described: a laboratory model representing low-density housing, and the city of Nimes (France), which suffered a catastrophic flood in October 1988. From these examples, the paper shows that such 2D models can provide general flow dynamics. Moreover, they can integrate description of some details (such as obstacles and storage areas), which can contribute to an understanding of some flow features and thus provide accurate results locally. For engineering use, the limits are computational time and number of cells: the two examples described show that a grid adapted to urban topography features is a way to reduce these disadvantages while providing reliable results with a convenient numerical stability, even if the size of the cells varies considerably.

Urban flood modelling using computational fluid dynamics

Lors des crues extremes en ville, une forte part des ecoulements reste en surface. Pour simuler ces inondations, deux modeles sont presentes : le logiciel REM2 U unidimensionnel a pour objectif de simuler la propagation des debits de crue dans l'ensemble d'un reseau de rues alors que le logiciel Rubar 20 bidimensionnel vise a fournir plus d'information sur ces ecoulements. Des calculs avec ces deux logiciels ont ete menes sur la crue d'octobre 1988 dans un quartier de Nimes. Lors de cet evenement, les hauteurs d'eau maximales ont depasse deux metres en certains points et les vitesses 2 m/s ce qui entrainait des passages en regime torrentiel. A partir des donnees rassemblees sur les sections en travers des rues, des maillages de calcul limites au reseau de rues ont ete construits pour les deux logiciels afin de permettre un calcul detaille. La comparaison des resultats avec les laisses de crue montre des situations tres contrastees d'un point a un autre pour une hauteur d'eau maximale moyenne sur l'ensemble de la zone inondee correctement simulee. L'ecart sur cette hauteur est, en moyenne, de 1 m ce qui provient des incertitudes sur les observations, sur la topographie et sur les conditions aux limites, des approximations lors de la modelisation et de particularites locales non decrites. Entre les deux logiciels, l'evolution des hauteurs et des vitesses est generalement tres proche bien que, comme pour la comparaison avec les laisses de crue, des differences locales importantes sont observees.

/pdf/estimation-des-niveaux-d-inondation-pour-une-crue-eclair-en-3nb59rwdib.pdf

Estimation des niveaux d'inondation pour une crue éclair en milieu urbain : comparaison de deux modèles hydrodynamiques sur la crue de Nîmes d'octobre 1988

S. Haider

Papers

Modeling floods in a dense urban area using 2D shallow water equations

Urban flood modelling using computational fluid dynamics

Estimation des niveaux d'inondation pour une crue éclair en milieu urbain : comparaison de deux modèles hydrodynamiques sur la crue de Nîmes d'octobre 1988