1 | The calculated run-up height and resulting inundation ashore is determined by |
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2 | the input topographic and bathymetric elevation, the |
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3 | initial and boundary conditions, as well as the cell area of the computational |
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4 | mesh. |
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5 | Ideally, the data should adequately capture all complex features |
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6 | of the underlying bathymetry and topography. Any limitations |
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7 | in the resolution and accuracy of the data will introduce |
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8 | errors to the inundation maps, in addition to the range of approximations |
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9 | made within the model. |
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10 | |
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11 | In this study, we used the Australian Height Datum (AHD) |
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12 | as the vertical datum. Mean Sea Level (MSL) is approximately equal to |
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13 | 0m AHD with the Highest Astronomical Tide (HAT) |
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14 | and Lowest Astronomical Tide (LAT) defined as 1.5m AHD |
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15 | and -1.5m AHD respectively for Onslow \cite{antt:06}. |
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16 | These values are tidal |
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17 | predictions based on continous tidal observations from Standard Ports |
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18 | over a period of |
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19 | at least one year, with the Australian Hydrographic Service |
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20 | recommending this be extended to three years to capture |
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21 | changes to the mean sea level. Onslow is listed as |
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22 | a Standard Port. As an aside, current work at GA is |
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23 | extracting information from LANDSAT imagery to reconstruct the |
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24 | tidal variations for various WA locations. Future modelling of |
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25 | these areas will incorporate this information. |
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26 | |
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27 | Data for this study have been sourced from a number of agencies. With |
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28 | respect to the onshore data, the Defence Imagery and Geospatial |
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29 | Organisation (DIGO) supplied the Digital Terrain Elevation |
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30 | Data Level 2 (DTED) which has been authorised for Australian Tsunami |
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31 | Warning System use only. The resolution of this data is 1 second |
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32 | (about 30 metres), and has been produced from 1:50 000 contours, elevations and |
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33 | drainage. In addition, the Department of Land Information (DLI) has provided a |
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34 | 20m Digital Elevation Model (DEM) and orthophotography |
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35 | covering the NW Shelf. The DTED Level 2 data is ``bare earth'' while the |
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36 | DLI data is distorted by vegetation and buildings. |
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37 | |
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38 | Figure \ref{fig:contours_compare}(a) shows the contour lines for |
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39 | HAT, MSL and LAT for Onslow using the DTED data where it is evident |
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40 | that the extent of the tidal inundation is exaggerated. In particular, |
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41 | parts of Onslow town appears to be inundated at HAT before a tsunami has |
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42 | even been generated. This is due to |
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43 | short comings with the digital elevation model (DEM) created from |
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44 | the DTED data. |
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45 | Figure \ref{fig:contours_compare}(b) shows |
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46 | the contour lines for HAT, MSL and LAT for Onslow using the WA DLI data. |
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47 | It is obvious that there are significant differences in each DEM with |
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48 | total station survey information and the knowledge |
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49 | of the HAT contour line pointing to increased confidence in the WA DLI |
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50 | data over the DTED data for use in the inundation modelling. |
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51 | The impact difference based on these two onshore data sets |
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52 | will be discussed in Section \ref{sec:issues}. |
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53 | |
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54 | |
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55 | \begin{figure}[p] |
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56 | \center{(a)} |
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57 | \centerline{ \includegraphics[width=150mm, height=100mm] |
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58 | {../report_figures/onslow_dted_contour.jpg}} |
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59 | |
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60 | % \caption{Onslow region showing the -1.5m AHD (LAT), 0m AHD (MSL) |
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61 | %and -1.5m AHD (LAT) contour lines using the DTED Level 2 data.} |
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62 | % \label{fig:contours_dted} |
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63 | %\end{figure} |
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64 | |
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65 | %\begin{figure}[hbt] |
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66 | \center{(b)} |
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67 | \centerline{ \includegraphics[width=150mm, height=100mm] |
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68 | {../report_figures/onslow_dli_contour.jpg}} |
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69 | |
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70 | \caption{Onslow region showing the -1.5m AHD (LAT), 0m AHD (MSL) |
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71 | and 1.5m AHD (HAT) contour lines using the (a) DTED Level 2 data and |
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72 | the (b) WA DLI data.} |
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73 | % \label{fig:contours_dli} |
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74 | \label{fig:contours_compare} |
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75 | \end{figure} |
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76 | |
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77 | With respect to the offshore data, the Department of Planning and |
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78 | Infrastructure (DPI) have provided state digital fairsheet data around |
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79 | Onslow. This data cover only a very small geographic area. (Note, |
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80 | similar data have been provided by DPI for Pt Hedland and Broome.) |
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81 | The Australian Hydrographic Office (AHO) has supplied extensive |
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82 | fairsheet data which has also been utilised. In contrast to the onshore data, the offshore data is a series of survey points which is typically not supplied on a fixed grid. In addition, offshore data typically does not have the coverage of the onshore data, and often the offshore data will have gaps where surveys have not been conducted. |
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83 | The coastline has been generated by |
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84 | using the aerial photography, two detailed surveys provided |
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85 | by WA DPI and a number of total station surveys of Onslow. |
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86 | The WA DLI data surrounding the coast are error prone and |
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87 | have been clipped at the derived coastline. |
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88 | Appendix \ref{sec:metadata} provides more details and the supporting metadata |
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89 | for this study, including images of the data extent. |
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90 | Table \ref{table:data} summarises the available data. |
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91 | |
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92 | \begin{table} |
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93 | \caption{Available data for the North West shelf tsunami inundation studies.} |
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94 | \label{table:data} |
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95 | \begin{center} |
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96 | \begin{tabular}{|l|l|}\hline |
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97 | Data & Detail \\ \hline |
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98 | DIGO DTED Level 2 & Onshore, 1 second $\approx$ 30m \\ \hline |
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99 | DLI & Onshore, 20m DEM and orthophotography \\ \hline |
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100 | DPI & Offshore, fairsheet data around Onslow \\ \hline |
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101 | AHO & Offshore, fairsheet data for North West Shelf region \\ \hline |
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102 | \end{tabular} |
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103 | \end{center} |
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104 | \end{table} |
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105 | |
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106 | |
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107 | \pagebreak |
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108 | |
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