source: anuga_work/production/onslow_2006/report/interpretation.tex @ 3569

Last change on this file since 3569 was 3477, checked in by sexton, 19 years ago

(1) updates to Onslow and Pt Hedland reports and (2) introduction of broome scenario

File size: 8.1 KB
Line 
1The main features of the
2tsunami wave and resultant inundation ashore is described in this section.
3We have
4chosen a number of locations to illustrate the features
5of the tsunami as it approaches and impacts Onslow.
6These locations have been chosen as we believe they would
7either be critical
8in an emergency situation, (e.g. the hospital and power station) or
9effect recovery efforts, (e.g. the airport and docks). These locations
10are described in Table \ref{table:locations} and shown in
11Figure \ref{fig:points}. The water's stage and speed
12at each of these locations are shown
13as a function of time in the series of graphs shown in
14Appendix \ref{sec:timeseries}. It is assumed that the earthquake is
15generated at the beginning of the simulation, i.e. time = 0 minutes.
16Stage is defined as the absolute
17water level (in metres) relative to AHD
18\footnote{For an offshore location such as Beadon Bay West,
19the initial water level will be that of the tidal scenario. In the
20case of MSL, this water level will be 0. As the tsunami wave moves
21through this point, the water height may grow and thus the stage will
22represent the amplitude of the wave. For an onshore location such as the
23Light Tower, the actual water depth will be the difference between
24the stage and the elevation at that point. Therefore, at the beginning
25of the simulation, there will be no water onshore and therefore
26the stage and the elevation will be identical.}. Both stage and speed
27(in metres/second) for
28each scenario (HAT, MSL and LAT) are shown
29on consistent scales to allow comparison between point locations.
30As a useful benchmark, Table \ref{table:speedexamples}
31describes typical examples for a range of speeds found in the
32simulations.
33
34\begin{table}[h]
35\label{table:speedexamples}
36\caption{Examples of a range of velocities.}
37\begin{center}
38\begin{tabular}{|l|l|}\hline
39{\bf Velocity (m/s)} & {\bf Example} \\ \hline
401 & leisurely stroll pace\\ \hline
411.5 & average walking pace \\ \hline
42%2 & 100m Olympic male freestyle \\ \hline
43%3 & mackeral \\ \hline
444 & average person can maintain running for 1000m \\ \hline
45%5 & blue whale \\ \hline
4610 & 100m Olympic male sprinter \\ \hline
4716 & car travelling in urban zones (60 km/hr) \\ \hline
48\end{tabular}
49\end{center}
50\end{table}
51
52A tsunami wave typically has a small amplitude and typically travels at 100's of kilometres per hour.
53The low amplitude complicates the ability to detect
54the wave. As the water depth decreases,
55the speed of the wave
56decreases and the amplitude grows. Another important feature of tsunamis
57is drawdown. This means that the water is seen to retreat from the beaches
58before a tsunami wave
59impacts that location. Other features
60include reflections (where the wave is redirected due to the
61influence
62of the coast) and shoaling (where the wave's amplitude is amplified
63close to the coast due to wave interactions).
64These features are seen in these scenarios, and are consistent
65for HAT, MSL and LAT.
66There is a small wave, followed
67by a large drawdown and then a large secondary wave.
68
69These
70features are illustrated in Figure \ref{fig:gaugeBeadonBayeast}
71where a small wave can be seen at around 200 mins. For the HAT
72case (shown in blue), the amplitude
73of the wave at this location is around 0.8 m\footnote{In this
74scenario, the initial water level is 1.5 m, which means that
75the actual amplitude is the difference between the stage value
76and the initial water level; 2.3 - 1.5}.
77The drawdown of around 4.3 m (i.e. 2.3 - -2) then occurs at around 230 mins
78(i.e. 3.8 hours after the event has been generated), before
79the second wave arrives
80with an amplitude of around 3.6 m (i.e. 4.1 - 1.5). A further wave
81is then evident a short time later (around 255 mins)
82which further increases the amplitude to around 5 m (i.e. 6.6 - 1.5).
83These features are replicated at each of the offshore points (those
84points with negative elevation as shown in Table \ref{table:locations}).
85
86The wave amplitude is typically greater
87for those locations which are in the shallowest water. For example,
88the maximum wave amplitude at the Beadon Bay East location
89(Figure \ref{fig:gaugeBeadonBayeast}) is over
904.5m where the water depth would normally be 3.56 m. In the
91Beadon Bay West location (Figure \ref{fig:gaugeBeadonBaywest})
92where the water depth would normally be 4.62 m,
93the maximum wave amplitude is much less (around 3 m). The wave amplitude
94at the West of Groyne location (Figure \ref{fig:gaugeWestofGroyne})
95is not greater than that seen
96at the Beadon Bay East location, even though the water depth is
97much less, at 2.11m. This is probably due to its proximity
98to the groyne\footnote{A groyne is a man made structure to combat
99coastal erosion.}
100which has impeded the tsunami wave to some degree. However, the
101maximum speed found amongst the locations is at the West of Groyne
102point which is in the shallowest water.
103
104The speed of the tsunami sharply increases as it moves onshore. There
105is minimal inundation found at the locations chosen, with the Bindi Bindi
106community receiving the greatest inundation for all tidal scenarios.
107At HAT, the community would receive over 1 m of inundation with
108the water moving through the community at approximately 16 m/s. Referring
109to Table \ref{table:speedexamples}, a person in this location could
110not outrun this water movement. A small amount of water is found
111at the hospital (10 cm). Whilst this seems minimal, the water is moving
112at around 6 m/s which could dislodge some items if the water was able to enter the hospital.
113 
114The geography of the Onslow area has played a role in offering
115some protection to the Onslow community. The tsunami wave is
116travelling from the north west of the area. Most of
117the inundation along the coast is that which is open to this
118direction. 
119The sand dunes west of Onslow
120appear to have halted this tsunami wave
121(see Figure \ref{fig:MSL_max_inundation}) with limited
122inundation found on the town's side of the dunes.
123The inundation within the community has occurred due to the
124wave reflecting from the beach area west of the creek and
125returning towards the Onslow town itself. 
126There are also sand dunes east of the creek which have also
127halted inundation beyond them.
128Currently, we do not model changes
129to the bathymetry or topography due to effects of the water flow.
130Therefore, we do not know whether these sand dunes would withstand the
131transmitted energy of the tsunami wave.
132
133Water features such as rivers, creeks and estuaries also play a role
134in the inundation extent.
135The tsunami wave penetrates the creek east of Onslow with a wave height
136over 2 m at the mouth
137(Figure \ref{fig:gaugeBeadonCreekmouth}) for the HAT scenario.
138Inundation exceeds 1 m at the Beadon Creek south of dock location (Figure
139\ref{fig:gaugeBeadonCreeksouthofdock}) suggesting that the wave's
140energy dissipates as inundation overflows from the creek. A large
141tidal flat region surrounds the southern parts of the creek and
142it is evident that the inundation is essentially caught in this
143area.
144
145As expected, there is greater inundation at HAT with increased
146extent. The major road
147into Onslow, the Onslow Mount Stuart Rd, remains free of inundation for
148all tidal scenarios with a small amount of inundation evident at HAT at
149the intersection with Beadon Creek Rd. Beadon Creek Rd services the wharf in the
150creek which becomes increasingly inundated as the tide height
151increases. The only road sufficiently inundated at LAT is Beadon
152Creek Rd near the entry to the wharf. This road during the HAT
153scenario would be impassable as the water depths are consistently
154over 1 m with a maximum water depth of around 2 m found close to
155the wharf.
156
157There is significant inundation of at
158least 2 m on the foreshore of Onslow for MSL and HAT.
159The inundation extends further as the tidal heights increase. 
160At HAT, the inundation reaches the southern boundaries of
161the road infrastructure in the Onslow town centre. 
162The airport remains
163free of inundation for each tidal scenario. Section \ref{sec:impact}
164details the impact estimates to the residential infrastructure.
Note: See TracBrowser for help on using the repository browser.