1 | The following information is required to undertake the |
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2 | inundation modelling; |
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3 | |
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4 | \begin{itemize} |
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5 | \item onshore and offshore elevation data (topographic and bathymetric data, |
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6 | see Section \ref{sec:data}), |
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7 | \item initial conditions, such as initial water levels (e.g. determined by tides), |
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8 | \item boundary conditions (the tsunami source as described in |
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9 | Section \ref{sec:methodology}), and |
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10 | \item computational requirements relating to the mesh construction. |
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11 | \end{itemize} |
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12 | |
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13 | The initial conditions used for this scenario are MSL, HAT and LAT which were |
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14 | defined in Section \ref{sec:data}. Figure \ref{fig:IC} shows the MSL, HAT and |
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15 | LAT contours to illustrate the water level for each of these inial conditions. |
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16 | The dynamics of |
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17 | tidal effects (that is, the changes in water height over time for |
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18 | the entire study area) are not currently modelled. |
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19 | Sea floor friction will generally provide resistance to the water flow |
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20 | and thus reduce the impact somewhat. However, limited |
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21 | research has been carried out to determine |
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22 | the friction coefficients, and |
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23 | thus it has not been incorporated |
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24 | in the scenario. The |
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25 | results are therefore likely to be over estimates. |
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26 | |
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27 | \begin{figure}[h] |
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28 | |
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29 | \centerline{\includegraphics[width=\paperwidth]{../report_figures/onslow_dli_contour.jpg}} |
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30 | |
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31 | \caption{Onslow region showing the initial conditions used for the study; |
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32 | -1.5m AHD (LAT), 0m AHD (MSL) and 1.5m AHD (HAT) contour lines.} |
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33 | \label{fig:IC} |
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34 | \end{figure} |
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35 | |
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36 | |
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37 | To set up a model for the tsunami scenario, a study area is first |
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38 | determined. Preliminary investigations have indicated that the |
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39 | output from MOST or URS should be input to ANUGA |
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40 | at the 100m water depth. Historical run-up heights are |
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41 | of the order of 10 m and we would expect that a tsunami wave |
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42 | would penetrate no higher for this scenario, hence we have |
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43 | bounded our study region at 10m elevation. |
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44 | Current computation requirements define a coastline |
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45 | extent of around 100 km. Therefore, the study area of around 6300 km$^2$ |
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46 | covers approximately 100 km of |
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47 | coastline and extends offshore to the 100m contour line and inshore to |
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48 | approximately 10m elevation. |
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49 | |
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50 | The finite volume technique relies on the construction of a triangular mesh which covers the study region. |
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51 | This mesh can be altered to suit the needs of the scenario in question. The mesh can be refined in areas of |
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52 | interest, particularly in the coastal region where complex behaviour is likely to occur. |
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53 | In setting up the model, the user defines the area of the triangular cells in each region of interest\footnote{Note that the cell |
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54 | area will be the maximum cell area within the defined region and that each |
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55 | cell in the region does not necessarily have the same area.}. |
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56 | The cell areas should not be too small as this will cause unrealisticly long computational time, |
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57 | and not too great as this may inadequately capture important behaviour. |
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58 | %There are no gains in choosing the area to be less than the supporting data. |
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59 | Figure \ref{fig:onslow_area} shows the study area with regions of difference cell areas. The total number |
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60 | of cells is 401939. |
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61 | Lateral accuracy refers to the distance at which we are confident in stating a region is inundated. |
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62 | Figure \ref{fig:onslow_area} shows the maximum triangular cell area and lateral accuracy for each region. |
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63 | Therefore we can only be confident in the calculated inundation extent in the Onslow town centre to within 30 m. |
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64 | |
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65 | |
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66 | \begin{figure}[hbt] |
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67 | |
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68 | \centerline{ \includegraphics[scale=0.125]{../report_figures/onslow_resolution_zones.jpg}} |
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69 | |
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70 | \caption{Study area for Onslow scenario highlighting four regions of increased refinement. |
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71 | Region 1: Surrounds Onslow town centre with a cell area of 500 m$^2$ (lateral accuracy 30 m). |
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72 | Region 2: Surrounds the coastal region with a cell area of 2500 m$^2$ (lateral accuracy 70 m). |
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73 | Region 3: Water depths to the 50m contour line (approximately) with a cell area of 20000 m$^2$ (lateral accuracy 200 m). |
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74 | Region 4: Water depths to the boundary (approximately 100m contour line) with a cell area of 100000 m$^2$ (lateral accuracy 445 m). |
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75 | } |
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76 | \label{fig:onslow_area} |
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77 | \end{figure} |
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78 | |
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79 | |
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80 | The final item to be addressed to complete the model setup is the |
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81 | definition of the boundary condition. As |
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82 | discussed in Section \ref{sec:methodology}, a series of events corresponding |
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83 | to a range of return periods are selected as the tsunami sources. |
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84 | The resultant tsunami wave is made up of a series |
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85 | of waves with different amplitudes which is affected by the energy |
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86 | and style of the event as well as the bathymetry whilst it travels |
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87 | from its source to Onslow. The amplitude and velocity of each of these |
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88 | waves are then provided to ANUGA as boundary conditions and propagated |
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89 | inshore. |
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90 | |
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91 | The tsunami generation and propagation modelling uses the URS paradigm. |
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92 | The URS output is saved every 2.5 seconds and with a spatial resolution |
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93 | of two arc minutes. |
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94 | |
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95 | Sea floor friction will generally provide resistance to the water flow |
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96 | and thus reduce the impact somewhat. Initial experimental results |
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97 | indicate that the friction coefficient shoudl be 0.017. |
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98 | The following table summarises the modelling parameters; |
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99 | |
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100 | \begin{table} |
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101 | \begin{center} |
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102 | \caption{Parameters used in ANUGA for the Onslow scenario.} |
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103 | \begin{tabular}{|l|l|l|}\hline |
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104 | Mesh & & \\ \hline |
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105 | & resolution in Region 1 & 500 m$^2$ \\ \hline |
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106 | & resolution in Region 2 & 2500 m$^2$ \\ \hline |
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107 | & resolution in Region 3 & 20000 m$^2$ \\ \hline |
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108 | & remaining resolution & 100 000 m$^2$ \\ \hline |
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109 | Model parameters & & \\ \hline |
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110 | & friction & 0.017 \\ \hline |
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111 | & minimum stored height & 0.1 m \\ \hline |
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112 | \end{tabular} |
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113 | \end{center} |
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114 | \end{table} |
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