1 | The main features of the |
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2 | tsunami wave and resultant inundation ashore is described in this section. |
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3 | We have |
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4 | chosen a number of locations to illustrate the features |
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5 | of the tsunami as it approaches and impacts Onslow. |
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6 | These locations have been chosen as we believe they would |
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7 | either be critical |
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8 | in an emergency situation, (e.g. the hospital and power station) or |
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9 | effect recovery efforts, (e.g. the airport and docks). These locations |
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10 | are described in Table \ref{table:locations} and shown in |
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11 | Figure \ref{fig:points}. The water's stage and speed |
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12 | at each of these locations are shown |
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13 | as a function of time in the series of graphs shown in |
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14 | Appendix \ref{sec:timeseries}. It is assumed that the earthquake is |
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15 | generated at the beginning of the simulation, i.e. time = 0 minutes. |
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16 | Stage is defined as the absolute |
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17 | water level (in metres) relative to AHD |
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18 | \footnote{For an offshore location such as Beadon Bay West, |
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19 | the initial water level will be that of the tidal scenario. In the |
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20 | case of MSL, this water level will be 0. As the tsunami wave moves |
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21 | through this point, the water height may grow and thus the stage will |
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22 | represent the amplitude of the wave. For an onshore location such as the |
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23 | Light Tower, the actual water depth will be the difference between |
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24 | the stage and the elevation at that point. Therefore, at the beginning |
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25 | of the simulation, there will be no water onshore and therefore |
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26 | the stage and the elevation will be identical.}. Both stage and speed |
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27 | (in metres/second) for |
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28 | each scenario (HAT, MSL and LAT) are shown |
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29 | on consistent scales to allow comparison between point locations. |
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30 | As a useful benchmark, Table \ref{table:speedexamples} |
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31 | describes typical examples for a range of speeds found in the |
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32 | simulations. |
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33 | |
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34 | \begin{table}[h] |
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35 | \label{table:speedexamples} |
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36 | \caption{Examples of a range of velocities.} |
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37 | \begin{center} |
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38 | \begin{tabular}{|l|l|}\hline |
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39 | {\bf Velocity (m/s)} & {\bf Example} \\ \hline |
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40 | 1 & leisurely stroll pace\\ \hline |
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41 | 1.5 & average walking pace \\ \hline |
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42 | %2 & 100m Olympic male freestyle \\ \hline |
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43 | %3 & mackeral \\ \hline |
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44 | 4 & average person can maintain running for 1000m \\ \hline |
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45 | %5 & blue whale \\ \hline |
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46 | 10 & 100m Olympic male sprinter \\ \hline |
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47 | 16 & car travelling in urban zones (60 km/hr) \\ \hline |
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48 | \end{tabular} |
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49 | \end{center} |
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50 | \end{table} |
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51 | |
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52 | A tsunami wave typically has a small amplitude and typically travels at 100's of kilometres per hour. |
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53 | The low amplitude complicates the ability to detect |
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54 | the wave. As the water depth decreases, |
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55 | the speed of the wave |
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56 | decreases and the amplitude grows. Another important feature of tsunamis |
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57 | is drawdown. This means that the water is seen to retreat from the beaches |
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58 | before a tsunami wave |
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59 | impacts that location. Other features |
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60 | include reflections (where the wave is redirected due to the |
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61 | influence |
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62 | of the coast) and shoaling (where the wave's amplitude is amplified |
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63 | close to the coast due to wave interactions). |
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64 | These features are seen in these scenarios, and are consistent |
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65 | for HAT, MSL and LAT. |
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66 | There is a small wave, followed |
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67 | by a large drawdown and then a large secondary wave. |
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68 | |
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69 | These |
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70 | features are illustrated in Figure \ref{fig:gaugeBeadonBayeast} |
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71 | where a small wave can be seen at around 200 mins. For the HAT |
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72 | case (shown in blue), the amplitude |
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73 | of the wave at this location is around 0.8 m\footnote{In this |
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74 | scenario, the initial water level is 1.5 m, which means that |
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75 | the actual amplitude is the difference between the stage value |
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76 | and the initial water level; 2.3 - 1.5}. |
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77 | The drawdown of around 4.3 m (i.e. 2.3 - -2) then occurs at around 230 mins |
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78 | (i.e. 3.8 hours after the event has been generated), before |
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79 | the second wave arrives |
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80 | with an amplitude of around 3.6 m (i.e. 4.1 - 1.5). A further wave |
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81 | is then evident a short time later (around 255 mins) |
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82 | which further increases the amplitude to around 5 m (i.e. 6.6 - 1.5). |
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83 | These features are replicated at each of the offshore points (those |
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84 | points with negative elevation as shown in Table \ref{table:locations}). |
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85 | |
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86 | The wave amplitude is typically greater |
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87 | for those locations which are in the shallowest water. For example, |
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88 | the maximum wave amplitude at the Beadon Bay East location |
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89 | (Figure \ref{fig:gaugeBeadonBayeast}) is over |
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90 | 4.5m where the water depth would normally be 3.56 m. In the |
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91 | Beadon Bay West location (Figure \ref{fig:gaugeBeadonBaywest}) |
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92 | where the water depth would normally be 4.62 m, |
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93 | the maximum wave amplitude is much less (around 3 m). The wave amplitude |
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94 | at the West of Groyne location (Figure \ref{fig:gaugeWestofGroyne}) |
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95 | is not greater than that seen |
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96 | at the Beadon Bay East location, even though the water depth is |
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97 | much less, at 2.11m. This is probably due to its proximity |
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98 | to the groyne\footnote{A groyne is a man made structure to combat |
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99 | coastal erosion.} |
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100 | which has impeded the tsunami wave to some degree. However, the |
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101 | maximum speed found amongst the locations is at the West of Groyne |
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102 | point which is in the shallowest water. |
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103 | |
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104 | The speed of the tsunami sharply increases as it moves onshore. There |
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105 | is minimal inundation found at the locations chosen, with the Bindi Bindi |
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106 | community receiving the greatest inundation for all tidal scenarios. |
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107 | At HAT, the community would receive over 1 m of inundation with |
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108 | the water moving through the community at approximately 16 m/s. Referring |
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109 | to Table \ref{table:speedexamples}, a person in this location could |
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110 | not outrun this water movement. A small amount of water is found |
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111 | at the hospital (10 cm). Whilst this seems minimal, the water is moving |
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112 | at around 6 m/s which could dislodge some items if the water was able to enter the hospital. |
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113 | |
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114 | The geography of the Onslow area has played a role in offering |
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115 | some protection to the Onslow community. The tsunami wave is |
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116 | travelling from the north west of the area. Most of |
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117 | the inundation along the coast is that which is open to this |
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118 | direction. |
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119 | The sand dunes west of Onslow |
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120 | appear to have halted this tsunami wave |
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121 | (see Figure \ref{fig:MSL_max_inundation}) with limited |
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122 | inundation found on the town's side of the dunes. |
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123 | The inundation within the community has occurred due to the |
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124 | wave reflecting from the beach area west of the creek and |
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125 | returning towards the Onslow town itself. |
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126 | There are also sand dunes east of the creek which have also |
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127 | halted inundation beyond them. |
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128 | Currently, we do not model changes |
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129 | to the bathymetry or topography due to effects of the water flow. |
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130 | Therefore, we do not know whether these sand dunes would withstand the |
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131 | transmitted energy of the tsunami wave. |
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132 | |
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133 | Water features such as rivers, creeks and estuaries also play a role |
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134 | in the inundation extent. |
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135 | The tsunami wave penetrates the creek east of Onslow with a wave height |
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136 | over 2 m at the mouth |
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137 | (Figure \ref{fig:gaugeBeadonCreekmouth}) for the HAT scenario. |
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138 | Inundation exceeds 1 m at the Beadon Creek south of dock location (Figure |
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139 | \ref{fig:gaugeBeadonCreeksouthofdock}) suggesting that the wave's |
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140 | energy dissipates as inundation overflows from the creek. A large |
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141 | tidal flat region surrounds the southern parts of the creek and |
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142 | it is evident that the inundation is essentially caught in this |
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143 | area. |
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144 | |
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145 | As expected, there is greater inundation at HAT with increased |
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146 | extent. The major road |
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147 | into Onslow, the Onslow Mount Stuart Rd, remains free of inundation for |
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148 | all tidal scenarios with a small amount of inundation evident at HAT at |
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149 | the intersection with Beadon Creek Rd. Beadon Creek Rd services the wharf in the |
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150 | creek which becomes increasingly inundated as the tide height |
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151 | increases. The only road sufficiently inundated at LAT is Beadon |
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152 | Creek Rd near the entry to the wharf. This road during the HAT |
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153 | scenario would be impassable as the water depths are consistently |
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154 | over 1 m with a maximum water depth of around 2 m found close to |
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155 | the wharf. |
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156 | |
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157 | There is significant inundation of at |
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158 | least 2 m on the foreshore of Onslow for MSL and HAT. |
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159 | The inundation extends further as the tidal heights increase. |
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160 | At HAT, the inundation reaches the southern boundaries of |
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161 | the road infrastructure in the Onslow town centre. |
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162 | The airport remains |
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163 | free of inundation for each tidal scenario. Section \ref{sec:impact} |
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164 | details the impact estimates to the residential infrastructure. |
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