Towards spatially distributed quantitative assessm

Authors: John Davis Jakeman Ole M Nielsen Kristy Van Putten Richard Mleczko David Burbidge Nick Horspool

Publish Date: 2010/07/11

Volume: 60, Issue: 5, Pages: 1115-1138

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Abstract

This paper presents a framework and data for spatially distributed assessment of tsunami inundation models Our associated validation test is based upon the 2004 Indian Ocean tsunami which affords a uniquely large amount of observational data for events of this kind Specifically we use eyewitness accounts to assess onshore flow depths and speeds as well as a detailed inundation survey of Patong City Thailand to compare modelled and observed inundation Model predictions matched well the detailed inundation survey as well as altimetry data from the JASON satellite eyewitness accounts of wave front arrival times and onshore flow speeds Important buildings and other structures were incorporated into the underlying elevation model and are shown to have a large influence on inundation extentThis project was undertaken at Geoscience Australia and the Department of Mathematics The Australian National University The authors would like to thank Niran Chaimanee from the CCOP for providing the post 2004 tsunami survey data building footprints satellite image and the elevation data for Patong City Prapasri Asawakun from the Suranaree University of Technology and Parida Kuneepong for supporting this work Drew Whitehouse from the Australian National University for preparing the animation of the simulated impact Rick von Feldt for locating the Novotel from the video footage and for commenting on the model from and eyewitness point of view and Alex Apotsos for his extensive and extremely constructive comments and suggestions This paper is published with the permission of the Chief Executive Officer Geoscience AustraliaThe URSGA model described below was used to simulate the propagation of the 2004 Indian Ocean tsunami across the open ocean based on a discrete representation of the initial deformation of the seafloor For the models shown here the uplift is assumed to be instantaneous and creates an initial displacement of the ocean surface of the same size and amplitude as the coseismic seafloor deformation URSGA is well suited to modelling propagation over large domains and is used to propagate the tsunami until it reaches shallow water typically the 100mdepth contourURSGA is a hydrodynamic code that models the propagation of the tsunami in deep water using a finite difference method on a staggered grid It solves the depth integrated nonlinear shallow water equations in spherical coordinates with friction and Coriolis terms The code is based on Satake 1995 with significant modifications made by the URS corporation Thio et al 2008 and Geoscience Australia Burbidge et al 2008 URSGA is not publicly availableThe utility of the URSGA model decreases with water depth unless an intricate sequence of nested grids is employed In comparison ANUGA described below is designed to produce robust and accurate predictions of inundation but is less suitable for earthquake source modelling and large study areas because it is based on projected spatial coordinates Consequently the Geoscience Australia tsunami modelling methodology is based on a hybrid approach using models like URSGA for tsunami propagation up to an offshore depth contour typically 100 m The wave signal and the velocity field are then used as a timevarying boundary condition for the ANUGA inundation simulation on boundary segments that are in the direct path of the incoming tsunami refer to Section 32 All other boundaries are transmissiveANUGA is a free and open source hydrodynamic inundation tool that solves the conserved form of the depthintegrated nonlinear shallow water wave equations using a finitevolume scheme on an unstructured triangular mesh The scheme first presented by Zoppou and Roberts 1999 is a highresolution Godunovtype method that uses the rotational invariance property of the shallow water equations to transform the twodimensional problem into local onedimensional problems These local Riemann problems are then solved using the semidiscrete centralupwind scheme of Kurganov et al 2001 for solving onedimensional conservation equations The numerical scheme is presented in detail in Roberts and Zoppou Zoppou and Roberts 2000 Roberts and Zoppou 2000 and Nielsen et al 2005 An important capability of the finitevolume scheme is that discontinuities in all conserved quantities are allowed at every edge in the mesh This means that the tool is well suited to adequately resolving hydraulic jumps transcritical flows and the process of wetting and drying Consequently ANUGA is suitable for simulating water flow onto a beach or dry land and around structures such as buildings ANUGA has been validated against the wave tank simulation of the 1993 Okushiri Island tsunami Nielsen et al 2005 Roberts et al 2006 and dam break experiments Baldock et al 2007 ANUGA has also been used in an interdisciplinary framework to develop mitigation strategies in the fields of spatial planning and coping capacity Taubenböck et al 2009 More information on ANUGA and how to obtain it are available from https//datamininganueduau/anugaThe coupling of URSGA and ANUGA is subject to numerical errors These errors include a discretisation error resulting from the linear interpolation of the URSGA solution onto the boundary of the ANUGA domain an inability to capture flows such as ocean currents from ANUGA to URSGA and the inability to model the propagation of the tsunami through any transmissive boundaries The magnitude of these errors is extremely problem dependent as difficult to estimate and thus not discussed further

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