source: anuga_work/production/hobart_2006/report/modelling_methodology.tex @ 3721

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hobart testing and report making

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2Geoscience Australia aims to define the economic and social threat posed to urban communities
3by a range of rapid onset natural hazards. Through the integration of natural hazard research, defining national exposure and
4estimating socio-economic vulnerabilities, predictions of the likely impacts of events can be made.
5By modelling the likely impacts on urban communities as accurately as possible and
6building these estimates into land use planning and emergency
7management, communities will be better prepared to respond to
8natural disasters when they occur.
9
10
11%GA bases its risk modelling on the process of understanding the hazard and a community's
12%vulnerability in order to determine the impact of a particular hazard event.
13%The resultant risk relies on an assessment of the likelihood of the event.
14%An overall risk assessment for a particular hazard would then rely on scaling
15%each event's impact by its likelihood.
16
17To develop a tsunami risk assessment,
18the tsunami hazard itself must first be understood. These events are generally modelled by converting
19the energy released by a subduction earthquake into a vertical displacement of the ocean surface.
20%Tsunami hazard models have been available for some time.
21The resulting wave is
22then propagated across a sometimes vast stretch of ocean towards the
23area of interest.
24%using a relatively coarse model
25%based on bathymetries with a typical resolution of two arc minutes.
26The hazard itself is then reported as a maximum wave height at a fixed contour line near the coastline,
27(e.g. 50m). This is how the preliminary tsunami hazard assessment was reported by GA
28to FESA in September 2005 \cite{BC:FESA} for a suite of Mw 9 earthquakes
29evenly spaced along the Sunda Arc subduction zone.
30The assessment used the Method of Splitting Tsunamis (MOST)
31\cite{VT:MOST} model.
32%The maximal wave height at a fixed contour line near the coastline
33%(e.g. 50m) is then reported as the hazard to communities ashore.
34%Models such as Method of Splitting Tsunamis (MOST) \cite{VT:MOST} and the
35%URS Corporation's
36%Probabilistic Tsunami Hazard Analysis 
37%\cite{somerville:urs} follow this paradigm.
38
39While MOST is suitable for generating and propagating the tsunami wave from its source, it is not adequate to
40model the wave's impact on communities ashore. 
41To capture the \emph{impact} of a tsunami to a coastal community,
42the model must be capable of capturing more detail about the wave,
43particularly how it is affected by the local bathymetry, as well as the
44local topography as the wave moves onshore.
45%the details of how waves are reflected and otherwise
46%shaped by the local bathymetries as well as the dynamics of the
47%runup process onto the topography in question.
48It is well known that local bathymetric and topographic effects are
49critical in determining the severity of a hydrological disaster
50\cite{matsuyama:1999}. To model the impact of the tsunami wave on the
51coastal community, we use ANUGA \cite{ON:modsim}. In order to capture the
52details of the wave and its interactions, a much finer resolution is
53required than that of the hazard model. As a result, ANUGA simulations concentrate
54on specific coastal communities. MOST by contrast uses a
55coarser resolution and covers often vast areas. To develop the impact
56from an earthquake event from a distant source, we adopt a hybrid approach of
57modelling the event itself with MOST and modelling the impact with ANUGA.
58In this way, the output from MOST serves as an input to ANUGA.
59In modelling terms, the MOST output is a boundary condition for ANUGA.
60
61\bigskip %FIXME (Ole): Should this be a subsection even?
62The risk of a given tsunami scenario cannot be determined until the
63likelihood of the tsunami is known. GA is currently building a
64complete probabilistic hazard map which is due for completion
65in late 2006.
66
67The paleotsunami investigations are looking for evidence of tsunami impact
68to Tasmania from the Puysegur Trench. The largest known event is a Mw 7.2, but
69it is believed that the trench could support events up to Mw 8.7.
70Two tsunami sources are used in this study; a Mw 8.5 and 8.7 occurring
71in the same location but with different slip parameters.
72Figure \ref{fig:mw87} and Figure \ref{fig:mw85} 
73show the maximum wave height of a tsunami initiated
74by a Mw 8.7 anad Mw 8.5 event respectively, off
75the Puysegur Trench. These event provide the source and
76boundary condition to the
77inundation model presented in Section \ref{sec:anuga}. These events
78were agreed by the Steering Group in September 2005.
79
80\begin{figure}[h]
81
82  %\centerline{ \includegraphics[width=140mm, height=100mm]
83%{../report_figures/mw87.jpg}}
84
85  \caption{Maximum wave height (in cms) for a Mw 8.7 event off the
86coast of Java}
87  \label{fig:mw87}
88\end{figure}
89
90\begin{figure}[h]
91
92  %\centerline{ \includegraphics[width=140mm, height=100mm]
93%{../report_figures/mw85.jpg}}
94
95  \caption{Maximum wave height (in cms) for a Mw 8.5 event off the
96coast of Java}
97  \label{fig:mw85}
98\end{figure}
99
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