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

The main features of the
tsunami wave and resultant inundation ashore is described in this section. 
We have
chosen a number of locations to illustrate the features
of the tsunami as it approaches and impacts Onslow.
These locations have been chosen as we believe they would 
either be critical
in an emergency situation, (e.g. the hospital and power station) or
effect recovery efforts, (e.g. the airport and docks). These locations
are described in Table \ref{table:locations} and shown in 
Figure \ref{fig:points}. The water's stage and speed 
at each of these locations are shown
as a function of time in the series of graphs shown in
Appendix \ref{sec:timeseries}. It is assumed that the earthquake is
generated at the beginning of the simulation, i.e. time = 0 minutes.
Stage is defined as the absolute
water level (in metres) relative to AHD 
\footnote{For an offshore location such as Beadon Bay West,
the initial water level will be that of the tidal scenario. In the 
case of MSL, this water level will be 0. As the tsunami wave moves
through this point, the water height may grow and thus the stage will 
represent the amplitude of the wave. For an onshore location such as the 
Light Tower, the actual water depth will be the difference between
the stage and the elevation at that point. Therefore, at the beginning
of the simulation, there will be no water onshore and therefore
the stage and the elevation will be identical.}. Both stage and speed 
(in metres/second) for
each scenario (HAT, MSL and LAT) are shown
on consistent scales to allow comparison between point locations.
As a useful benchmark, Table \ref{table:speedexamples}
describes typical examples for a range of speeds found in the 
simulations. 

\begin{table}[h]
\label{table:speedexamples}
\caption{Examples of a range of velocities.}
\begin{center}
\begin{tabular}{|l|l|}\hline
{\bf Velocity (m/s)} & {\bf Example} \\ \hline
1 & leisurely stroll pace\\ \hline
1.5 & average walking pace \\ \hline
%2 & 100m Olympic male freestyle \\ \hline
%3 & mackeral \\ \hline
4 & average person can maintain running for 1000m \\ \hline
%5 & blue whale \\ \hline
10 & 100m Olympic male sprinter \\ \hline
16 & car travelling in urban zones (60 km/hr) \\ \hline
\end{tabular}
\end{center}
\end{table}

A tsunami wave typically has a small amplitude and typically travels at 100's of kilometres per hour. 
The low amplitude complicates the ability to detect
the wave. As the water depth decreases, 
the speed of the wave 
decreases and the amplitude grows. Another important feature of tsunamis 
is drawdown. This means that the water is seen to retreat from the beaches
before a tsunami wave 
impacts that location. Other features 
include reflections (where the wave is redirected due to the 
influence 
of the coast) and shoaling (where the wave's amplitude is amplified
close to the coast due to wave interactions).
These features are seen in these scenarios, and are consistent
for HAT, MSL and LAT. 
There is a small wave, followed
by a large drawdown and then a large secondary wave. 

These 
features are illustrated in Figure \ref{fig:gaugeBeadonBayeast}
where a small wave can be seen at around 200 mins. For the HAT
case (shown in blue), the amplitude
of the wave at this location is around 0.8 m\footnote{In this
scenario, the initial water level is 1.5 m, which means that
the actual amplitude is the difference between the stage value
and the initial water level; 2.3 - 1.5}.
The drawdown of around 4.3 m (i.e. 2.3 - -2) then occurs at around 230 mins
(i.e. 3.8 hours after the event has been generated), before
the second wave arrives 
with an amplitude of around 3.6 m (i.e. 4.1 - 1.5). A further wave
is then evident a short time later (around 255 mins) 
which further increases the amplitude to around 5 m (i.e. 6.6 - 1.5).
These features are replicated at each of the offshore points (those
points with negative elevation as shown in Table \ref{table:locations}).

The wave amplitude is typically greater
for those locations which are in the shallowest water. For example,
the maximum wave amplitude at the Beadon Bay East location 
(Figure \ref{fig:gaugeBeadonBayeast}) is over
4.5m where the water depth would normally be 3.56 m. In the 
Beadon Bay West location (Figure \ref{fig:gaugeBeadonBaywest})
where the water depth would normally be 4.62 m, 
the maximum wave amplitude is much less (around 3 m). The wave amplitude
at the West of Groyne location (Figure \ref{fig:gaugeWestofGroyne})
is not greater than that seen
at the Beadon Bay East location, even though the water depth is 
much less, at 2.11m. This is probably due to its proximity
to the groyne\footnote{A groyne is a man made structure to combat
coastal erosion.}
which has impeded the tsunami wave to some degree. However, the 
maximum speed found amongst the locations is at the West of Groyne
point which is in the shallowest water.

The speed of the tsunami sharply increases as it moves onshore. There
is minimal inundation found at the locations chosen, with the Bindi Bindi
community receiving the greatest inundation for all tidal scenarios.
At HAT, the community would receive over 1 m of inundation with 
the water moving through the community at approximately 16 m/s. Referring
to Table \ref{table:speedexamples}, a person in this location could
not outrun this water movement. A small amount of water is found
at the hospital (10 cm). Whilst this seems minimal, the water is moving 
at around 6 m/s which could dislodge some items if the water was able to enter the hospital.
 
The geography of the Onslow area has played a role in offering
some protection to the Onslow community. The tsunami wave is 
travelling from the north west of the area. Most of
the inundation along the coast is that which is open to this
direction.  
The sand dunes west of Onslow 
appear to have halted this tsunami wave
(see Figure \ref{fig:MSL_max_inundation}) with limited
inundation found on the town's side of the dunes.
The inundation within the community has occurred due to the
wave reflecting from the beach area west of the creek and
returning towards the Onslow town itself.  
There are also sand dunes east of the creek which have also
halted inundation beyond them. 
Currently, we do not model changes 
to the bathymetry or topography due to effects of the water flow. 
Therefore, we do not know whether these sand dunes would withstand the
transmitted energy of the tsunami wave. 

Water features such as rivers, creeks and estuaries also play a role
in the inundation extent.
The tsunami wave penetrates the creek east of Onslow with a wave height 
over 2 m at the mouth 
(Figure \ref{fig:gaugeBeadonCreekmouth}) for the HAT scenario.
Inundation exceeds 1 m at the Beadon Creek south of dock location (Figure
\ref{fig:gaugeBeadonCreeksouthofdock}) suggesting that the wave's 
energy dissipates as inundation overflows from the creek. A large
tidal flat region surrounds the southern parts of the creek and
it is evident that the inundation is essentially caught in this
area.

As expected, there is greater inundation at HAT with increased
extent. The major road
into Onslow, the Onslow Mount Stuart Rd, remains free of inundation for
all tidal scenarios with a small amount of inundation evident at HAT at
the intersection with Beadon Creek Rd. Beadon Creek Rd services the wharf in the
creek which becomes increasingly inundated as the tide height
increases. The only road sufficiently inundated at LAT is Beadon
Creek Rd near the entry to the wharf. This road during the HAT 
scenario would be impassable as the water depths are consistently
over 1 m with a maximum water depth of around 2 m found close to 
the wharf. 

There is significant inundation of at
least 2 m on the foreshore of Onslow for MSL and HAT.
The inundation extends further as the tidal heights increase.  
At HAT, the inundation reaches the southern boundaries of 
the road infrastructure in the Onslow town centre.  
The airport remains
free of inundation for each tidal scenario. Section \ref{sec:impact}
details the impact estimates to the residential infrastructure.