Autumn 2000

11

How high was the storm surge from

Tropical Cyclone Mahina?

North Queensland, 1899

by Jonathan Nott, James Cook University,

Cairns, & Matthew Hayne, Australia

Geological Survey Organisation, Canberra

Introduction

Tropical Cyclone Mahina was the most
intense tropical cyclone to cross the
Australian coast in historical times. Its
central pressure was recorded by baro-
meter at 27 inches (914hPa) as the eye
approached the coast at Bathurst Bay
(

Figure 1

) at approximately 4.30 am on

the 22

nd

 March 1899 (Whittingham 1958).

Over 300 persons lost their lives when a
fleet of pearling luggers and schooners
were wrecked and sunk by the pheno-
menal seas. One of the most interesting
aspects of this event was the eyewitness
report of a 43 foot (13m) surge at Ninian
Bay adjacent to Barrow Point 30 km south
of Bathurst Bay which extended inland
for 2–3 miles (3–5 km). Constable Kenny,
camped on a ridge fully 40 foot above sea
level, was inundated to his waist by a ‘tidal
wave’ (storm surge and associated
ephemeral sea level rise) at his camp site
some 0.5 miles (800m) inland at approxi-
mately 5 am (Anonymous 1899). This
account suggests this surge was the largest
ever recorded in Australia.

There is considerable departure bet-

ween the size of this reported surge and
the results of numerical models that
determine the magnitude of surges based
upon the storm characteristics and the
local bathymetry. In this instance a
number of numerical simulations of the
Cyclone Mahina storm surge were under-
taken using Hubbert et al. (1991) Australian
real-time system for forecasting tropical
storm surges (Callaghan 1993). No appre-
ciable surge (

<

 3m) was produced by the

model. The Jelesnianski-Trajer nomogram
technique, still widely used for forecasting
surge heights by the Australian Bureau of
Meteorology, also produced a surge height
of 2-3m for Cyclone Mahina.

Because of these discrepancies, and

because predictions of surge inundation
during modern cyclone events are based
upon these forecasting methods, it is
important to verify the magnitude of the
surge during Cyclone Mahina. Virtually
all marine inundations either transport
marine materials landward or leave
erosional indicators in the coastal land-
scape. By recognising these ‘fingerprints’
it is possible to determine the magnitude

resulting from breaking waves. Wave run-
up is the extent or height to which broken
waves swash onto the land.  Such inun-
dations typically leave distinct markers
in the coastal landscape. These include
sedimentary deposits or debris lines of
both ephemeral and longer-lived (century
to millennia scale) materials. The former
includes seaweed, coastal grasses and
marine faunal carcases and the latter
fragments of corals, shells and coarse-
grained marine sands. Erosional indi-
cators can also be produced during the
inundation; these include small wave
eroded scarps or terraces within un-
consolidated coastal deposits such as
raised gravel beaches or ridges as well as
the general disturbance of terrestrial
materials. The heights of these features
can be surveyed to mean sea level or some
other datum and materials within the
deposit dated, usually using carbon dating,
to give an approximate age of the event.
In this manner a frequency/ magnitude
record of pre-historic marine inundation
events can be developed.

At Ninian Bay we surveyed four separate

transects from the modern beach to
approximately 30m above sea level. These

of a past inundation event and its
approximate age. We undertook a field
survey of the area from Cape Melville to
Ninian Bay to ascertain the heights of past
storm surges in this region and determine
if there is physical evidence of the
extreme magnitude surge reported to
have occurred during Cyclone Mahina.
The results of this survey are presented
here.

Heights of past surges in the
Bathurst Bay region

Storm related marine inundation is the
term adopted here to include storm surge,
wave set up and wave run-up. Here we
refer storm surge to mean a rise above
normal water level resulting from a
reduction in atmospheric pressure
combined with the effects of surface wind
stress.  Wave set-up is a quasi-steady
super-elevation of the sea-surface due to
the onshore mass transport of water

Figure 1: Location map

Approximate path of Tropical

Cyclone Mahina

Australian Journal of Emergency Management

12

transects were located at the eastern and
western ends of Ninian Bay and extended
over the rocky headlands at these locations.
The other transects were located within
the central portion of the bay and extended
onto a 20m high aeolian sand ridge that
runs parallel to the shore approximately 1
km inland. Other transects were also
undertaken along the crest and seaward
flank of this sand ridge for approximately
500m towards its western end. Further
transects, perpendicular to the coast, were
also surveyed on the rocky headlands and
promontories both bounding and within
Ninian Bay. A similar procedure was
undertaken at Cape Melville in the vicinity
of the Outridge monument and along the
coast west of Cape Melville for approxi-
mately 5 km. Along each of these transects
a thorough search was undertaken for
marine inundation deposits.

No evidence for marine inundation

events was discovered above approxi-
mately 3–5m HAT. Below this level
abundant deposits of marine shell, coral,
pumice and wave transported sands were
evident. At the eastern end of Ninian Bay
these deposits extend up to the base of
the granite slopes 2–3m HAT. No marine
carbonates or sands were found amongst
the granites or within the narrow valleys
that extend into the backing range of hills.
Weathered material derived from the
granites (Gruss) and littering the gentle
granite slopes above 3m HAT also showed
no signs of disturbance. A similar
situation occurs at the western end of
Ninian Bay. Here shells and fragments of
shells were found within crevasses of the
folded strata of the headland up to a height
of approximately 4–5m HAT. Above this
height no other marine carbonates or
sands were located except within a small
midden (shells deposited by Aborigines
after eating the contents) towards the
crest of the headland. The sands that
comprise the bulk of this midden are fine-
grained and hence appear to have an
aeolian origin.

Shells on the sand ridge behind Ninian

Bay as well as those behind a small lagoon
at the rear of the main barrier were found
at a height of approximately 10–15m HAT.
These shells displayed clear anthro-
pological associations having a con-
centrated spatial distribution, a limited
number of species (predominantly of the
genus Anadara) of uniformily large size
and no indication of littoral scaring
suggesting they were alive at the time of
deposition. The absence of coral, pumice
or wave emplaced sand that discount the
possibility of midden associations was
found at any site.

A similar situation was observed at

Cape Melville. Here small blocks and
fragments of coral along with marine
shells and coarse-grained marine sands
occur to a height of approximately 3 m
HAT. These deposits are in the form of a
series of beach ridges that extend parallel
to the coast for hundreds of metres to
over one kilometre. These beach ridges
were no doubt deposited during cyclonic
events and represent the elevation to
which the most extreme storm related
marine inundations have reached in
recent geological time. No other frag-
ments of shell, coral or wave emplaced
sands were found extending to elevations
above the level of these beach ridges.
Reports of large coral blocks located close
to and at higher elevations than the
Outridge memorial located approximately
600m inland were not substantiated. Only
locally derived terrestrial sediments from
the granite hills behind surround this
memorial.

Discussion

The heights of marine deposits at Cape
Melville and Ninian Bay suggests that the
majority, if not all, storm related marine
inundations have only extended to a
maximum elevation of 3–5m HAT in
recent geological history (up to 6,000 yrs
B.P.). This result stands in stark contrast
to the report of the 13m inundation at
Ninian Bay in 1899. A number of reasons
may account for this apparent dis-
crepancy. First, the 13m inundation may
not have deposited any material above
3–5m HAT, especially in areas fronted by
extensive stands of vegetation. Often
storm surges, by themselves, represent a
reasonably passive rise in sea level and
their transporting capacity may be
minimal because of their inherent low
forward velocity. Waves on top of the surge
and other associated currents are usually
responsible for transporting marine
materials landward. In this case, and at
locations between 0.5 and 1 km inland
from the coast, current velocities within
the water column may have only been
sufficient to have transported materials
up to approximately 3–5m HAT. Above
this elevation vegetation and other
obstacles may have inhibited the fluid
flow such that the water column was
largely devoid of suspended materials
except for light organics such as leaves
and other flotsam. Such materials may
have decomposed over the last century
and hence no longer mark the elevation
to which the inundation extended.

This explanation, however, is unlikely

to account for the lack of wave trans-

ported material above 5 m on headlands
and rocky promontories; at these loca-
tions there is little to inhibit the velocity
of waves on top of a surge. Ninian Bay
was in the left forward quadrant of
Cyclone Mahina. The phenomenal seas
of a severe category 5 cyclone in this
quadrant could be expected to transport
at least small fragments of shell and sand
up to elevations well in excess of the height
of the actual storm surge on headlands.
This is especially so considering that on
steeply fronted coasts wave run-up can
be as much as 5 times wave height at the
shore (Camfield 1980, Synolakis 1987). At
these locations wave transported deposits
could be expected to remain within
crevasses and cracks in the rock for
considerable time even under continued
tropical rainfall conditions.

Another possibility is that Constable

Kenny’s interpretation of the surge may
have been confused and then reported in
the Outridge document. Constable Kenny
may not have been camped as high above
sea level as has been presumed. He
arrived at the camp site at night (6pm)
(Anonymous 1899) and during the may-
hem of the cyclone before day break the
next morning he may not have fully
appreciated the exact elevation of his
camp. It is also possible that Kenny’s camp
was inundated by wave run-up and not
strictly surge although this may seem
unlikely if he was camped 0.5 mile inland
as suggested in the Outridge report
(Anonymous 1899). Alternatively, Kenny’s
camp may have been inundated by a peak
in terrestrial flood waters or a com-
bination of marine surge and terrestrial
run off which may have given the appea-
rance of a ‘tidal wave’.

The significance of wave run-up and at

times its apparent confusion with the
surge component of marine inundation
is not to be underestimated. The Outridge
report (Anonymous 1899) for example
was not the only document to comment
upon the height of the surge. Whittingham
(1959) reports a verbal account of 13
‘porpoises’ were found after the storm
some 50 feet (15m) above sea level, up a
cliff, on Flinders Island near Bathurst Bay.
This at first would appear to support the
claim of a 43 foot (13m) surge at Ninian
Bay. However, due to run-up, trans-
portation and deposition of animals and
debris can occur well above the depth of
water at the shore. A 3m wave at the shore
for example, supported by at least 3.5m of
surge, is possible at this location and
under these conditions according to wave
forecasting curves (Komar 1976). Waves
of this magnitude could theoretically

Autumn 2000

13

transport objects (of a certain size and
mass) up to 21m above normal sea level
(3.5m surge +3m wave +15m run-up).
This is especially the case where waves
impinge upon a steep coastal topography
for here the run-up values are significantly
greater than on gently sloping shores
(Camfield 1980, Synolakis 1987). If
‘porpoises’ were deposited at 50 feet
(15m) a.s.l. on a cliffed section of Flinders
Island then it would only require a 2–3m
wave and near equivalent surge for this
to be achieved.

Conclusion

There is little doubt that severe category
5 Cyclone Mahina struck the Bathurst
Bay region on the morning of 22

nd

 March

1899. However, the absence of marine
deposits above 3–5 m HAT suggests that
marine inundations have not exceeded
this elevation in this region over recent
geological history. These results agree

R

fereed

closely with the results of numerical
models of the storm surge associated
with Cyclone Mahina. However, these
models do not take into account wave
run-up. While it is possible that wave run-
up in some locations may have reached
significantly higher elevations as Cyclone
Mahina approached and crossed the coast
the absence of marine deposits above 5m
HAT generally suggests otherwise.

References

Anonymous 1899, 

The Outridge Report —

The Pearling Disaster, 1899

, Outridge

Printing Company, Brisbane.

Callaghan J. 1993, Tropical Cyclone

Mahina, 

Australian Bureau of Meteorology

Internal Report.

Camfield F. E., 1980,

 

Tsunami Engi-

neering,

 

U.S. Army Corp Engineers, Coastal

Engineering Research Centre Special
Report 6, Washington.

Hubbert G. D., Holland G. J., Leslie L.W.

and Manton M.M., 1991, ‘A real-time
system for forecasting tropical cyclone
storm surges’, 

Weather and forecasting,

American Meteorological Society

, No. 6,

pp. 86–97.

Komar P. D. 1976, 

Beach processes and

sedimentation

, New Jersey, Prentice Hall.

Synolakis C. 1987, ‘Run-up of solitary

waves’, 

Journal of Fluid Mechanics

, No. 185,

pp. 523–545.

Whittingham H. E. 1958, ‘The Bathurst

Bay hurricane and associated storm
surge’, 

Australian Meteorological Maga-

zine

, No. 23, pp. 14–16.

Whittingham H. E. 1959, ‘Storm surges

along the Queensland coast’, 

Australian

Meteorological Magazine

, No. 27,pp. 40–41.

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