Abstract. In this paper a local case study is presented in which detailed inundation simulations have been performed to support damage analysis and risk assessment related to the 2004 tsunami in Phang Nga and Phuket, Thailand. Besides tsunami sources, bathymetry and topography, bottom roughness induced by vegetation and built environment is considered to influence inundation characteristics, such as water depths or flow velocities and therefore attracts major attention in this work. Plenty of information available on the 2004 tsunami event, high-resolution satellite imagery and extensive field measurements to derive land cover information and forest stand parameters facilitated the generation of topographic datasets, land cover maps and site-specific Manning values for the most prominent land cover classes in the study areas. The numerical models ComMIT and Mike 21 FM were used to hindcast the observed tsunami inundation and to draw conclusions on the influence of land cover on inundation patterns. Results show a strong influence of dense vegetation on flow velocities, which were reduced by up to 50 % by mangroves, while the inundation extent is influenced only to a lesser extent. In urban areas, the disregard of buildings in the model led to a significant overestimation of the inundation extent. Hence different approaches to consider buildings were used and analyzed in the model. The case study highlights the importance and quantifies the effects of considering land cover roughness in inundation simulations used for local risk assessment.
Abstract. An important part within the German-Indonesian Tsunami Early Warning System (GITEWS) project was the detailed numerical investigation of the impact of tsunamis in densely populated coastal areas of Indonesia. This work, carried out by the German Research Centre Geesthacht (GKSS), in co-operation with DHI-WASY, also provides the basis for the preparation of high resolution hazard and risk maps by the German Aerospace Center (DLR).In this paper a method is described of how to prepare very detailed roughness maps for scenario computations performed with the MIKE 21 Flow Model FM in three highly resolved (∼10 m) priority regions, namely Kuta (Bali), Padang (West-Sumatra), and Cilacap (southern coast of Java). Roughness values are assigned to 43 land use classes, e.g. different types of buildings, rural and urban subareas, by using equivalent coefficients found in literature or by performing numerical experiments.Comparisons of simulations using differentiated roughness maps with simulations using constant values (a widely used approach) are presented and it is demonstrated that roughness takes considerable influence on run-up and inundation.Out of all simulations, the results of the worst case scenarios for each of the three priority areas are discussed. Earthquakes with magnitudes of M W =8.5 or higher lead to considerable inundation in all study sites. A spatially distinguished consideration of roughness has been found to be necessary for detailed modelling onshore.
Depth-averaged models such as non-linear shallow water (NLSW) and Boussinesq based codes usually use the quadratic friction law with Manning's coefficient to describe the surface roughness of the bottom. Large roughness elements such as buildings and tree vegetation, which are too small to be resolved by the grid of the bottom topography, are mainly considered by using purely empirical Manning coefficients. This approach, however, is not physically sound and may thus result in very large uncertainties in inundation modeling. A more physically-based approach is to determine prediction formulae for the hydraulic resistance of large roughness elements, considering for example different shapes, sizes and arrangements which can then be directly implemented in such models. Such prediction formulae can be determined on the basis of systematic simulations using a validated 3D numerical model. To better understand complex flow phenomena involved in tsunami inundation, three vertical emerged cylinders have been arranged in four different configurations with four different distances between each other and subject to a solitary wave and to a bore. A validated three-dimensional two-phase Reynolds-averaged Navier-Stokes (RANS) model and the Volume of Fluid (VOF) method (OpenFOAM) has been used to assess flow velocities and water levels near the cylinders and the inline forces acting on the cylinders. The effects of side-by-side, tandem and two staggered arrangements as well as the effect of the distances between them on the flow induced by a solitary wave and a bore are discussed. The study led to an improved understanding in the near field of cylinders, which forms the basis for further studies related to larger groups of cylinders and other shapes.
In tsunami hazard assessment, usually depth-averaged flow models are applied which use the quadratic friction law with Manning's coefficients to describe the surface roughness of the bottom. Large roughness elements such as buildings and tree vegetation, which are too small to be resolved by the grid of the bottom topography, are mainly considered by using purely empirical Manning coefficients. This approach, however, is not physically sound and may thus result in very large uncertainties in inundation modeling. A more physically-based approach is to determine prediction formulae for the hydraulic resistance of large roughness elements, considering for example different shapes, sizes and types of arrangement which can then be directly implemented in depth-averaged models such as nonlinear shallow water (NLSW) models. Such prediction formulae can be determined on the basis of systematic simulations using a well-validated 3D numerical model. To better understand complex flow phenomena involved in tsunami inundation, three vertical emerged cylinders have been arranged in four different configurations with four different distances between each other and subject to a solitary wave and to a bore. A validated three-dimensional two-phase Reynolds-averaged Navier-Stokes (RANS) model with the volume of fluid (VOF) method has been used to assess flow velocities and water levels near the cylinders. In this study, the validation of the numerical model by data obtained from large-scale model tests in the Large Wave Flume (GWK) Hanover, the flume at the Leichtweiss Institute for Hydraulic Engineering and Water resources (LWI) and the wave tank of the University of Washington is presented and the effects types of cylinder arrangement and distances between the cylinders on the flow induced by a solitary wave and a bore in the near field are discussed.
a) PLIF at t = 0 s, (b) PIV at t = 0 s (c) PLIF at t = 0.5 s, (d) PIV at t = 0.5 s (e) PLIF at t = 1.0 s (f) PIV at t = 1.0 sThese images correspond to simultaneous measurements involving Planar Laser-induced Fluorescence (PLIF, left) and Particle Image Velocimetry (PIV, right) for the examination of mixing processes in a static mixer at Re = 562. For PLIF, Rhodamine 6G is injected on the centerline in front of the mixer. The PLIF images have been acquired using an intensified CCD camera during the first of two PIV laser pulses (NdYAG, 532 nm, 80 mJ per pulse), while the CCD camera used for PIV takes double images, used to calculate the two-dimensional velocity fields. Thanks to these measurements it becomes possible to characterize quantitatively the flow behind the static mixer, in particular to determine characteristic flow frequencies as well as correlations between velocity and concentrations. Portfolio Paper
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