Cyclone Yasi impacts - Geography

Severe Tropical Cyclone Yasi: impacts on nearshore reefs of the central-northern Great Barrier
Reef, Australia
1. Cyclone Yasi: storm chronology and landfall path
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Cyclone Yasi was a very large (~700 km wide) Category 5 cyclone that impacted the central-northern region of
Queensland and large areas of the Great Barrier Reef shelf in early February 2011.
It was among the most powerful recorded cyclones to have hit the Queensland coast. Previous cyclones of a
comparable intensity include Cyclone Mahina (1899) in Princess Charlotte Bay ~350 km to the north, and the 1918
cyclones at Mackay and Innisfail.
Cyclone Yasi began developing as a tropical low northwest of Fiji on 29th January 2011 and started tracking in a
general westward direction (see: http://www.bom.gov.au/cyclone/history/yasi.shtml). The system intensifying to a
cyclone and was named Yasi on the 30th January, maintaining a westward track and rapidly intensifying before
being upgraded to Category 4 at 7pm on 1st February. During this time, Yasi started to take a more west to
southwest path and began to accelerate towards the Queensland coast (Fig. 1). Yasi further intensified and at 4am
on 2nd February was upgraded to a Category 5 system.
The eye of the storm (~35 km wide) passed over the area between Mission Beach and Tully, some 140 km south of
Cairns (Fig. 1) between midnight and 1am on Thursday 3rd February.
Instrumentation that survived the event recorded a central pressure of 929hPa. In Mission Beach, close to where
Yasi made landfall, wind gusts were estimated to have reached 290 km/h. The peak storm surge in this area was
estimated at ~ 7 m and to have inundated ~ 300 m inland. Fortunately the storm surge coincided with a low tide,
but nonetheless water levels rose ~2.3 m above the Highest Astronomical Tide (HAT) level.
Very high rainfall also occurred during the event, the largest rainfall totals were near to, and to the south of, the
cyclone and were generally in the order of 200-300 mm in the 24 hours up to the landfall period.
Fig. 1. Satellite image of Cyclone Yasi over the
Queensland coast (http://www.bom.gov.au/cyclone/
history/yasi.shtml)
Fig. 2. Map showing the path of Cyclone Yasi and cyclone
intensity (scale 1-5, numbered circles) as it approached the
Queensland coast (based on maps at: http://www.bom.
gov.au/cyclone/ history/yasi.shtml)
This fact sheet summarises the key impacts of Cyclone Yasi on four nearshore reefs along the Queensland coast of
Australia. Three of these reefs are located close to (within 5-10 km) of the landfall path – these reefs are King Reef,
Dunk Island and Lugger Shoal. A fourth reef, more distal (~150 km south) was also examined (Fig. 3). Each of these
reefs differs not only in terms of geographic position with respect to Yasi’s path, but also in terms of their
geomorphology, their evolutionary state, and in terms of their pre-Yasi ecological condition (in terms of live coral cover
and coral distribution across the reefs).
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Fig. 3. Map showing the location of the four reef sites discussed in this
fact sheet.
2. Impacts on selected inner-shelf reefs within the Great Barrier Reef
2.1 King Reef
2.1.1 Location and Pre-Yasi Characteristics
 King Reef (17° 46.2’ S, 146° 072’ E) is the largest mainland-attached fringing reef on the Queensland coast and
covers an area of ~ 3 km2 (Fig. 4).
 The reef structure is relatively old (senescent) – significant vertical and lateral accretion ceased at this site by
~4500 yBP (Roche et al. 2010) – and is characterised by an expansive, relatively planar reef flat surface. The
substrate across the main areas of the reef flat were, pre-Cyclone Yasi, dominated by a mixed carbonateterrigenoclastic sediment veneer with abundant abraded coral rubble clasts and rhodoliths. Isolated Goniastrea
colonies colonised this surface (Fig. 5).
 In contrast, along the seaward edge of the reef an expansive field of Porites microatolls was present, with intermicroatoll substrates exhibiting high live coral cover (~30%) and dominated by Montipora digitata, Porites rus and
Porites lobata, Echinopora sp. and Acropora pulchra (Fig. 6).
Fig. 4. Location of King Reef and aerial shot looking landward across King Reef towards Kurramine Beach
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Fig. 5. Views across the expansive, planar reef flat at King Reef pre-Yasi (Aug 2009) showing sediment infilled relict reef flat surface
and isolated Goniastrea colonies.
Fig. 6. View across the seaward margins of King Reef pre-Yasi (August 2009) showing extensive live Porites microatolls and high
live coral cover on the inter-microatoll reef surface.
2.1.2 Ecological Impacts of Cyclone Yasi at King Reef
Across the main reef flat Cyclone Yasi had little apparent effect on the ecology of the reef (live coral cover was already
very low i.e., <5%). Goniastrea and Turbinaria corals that colonised this surface remained mostly intact (Fig. 7), and
impacts were restricted to isolated broken or over-turned corals. Along the seaward reef flat margins more extensive
ecological impacts occurred. Most Porites microatolls/bommies survived intact (despite evidence of isolated
breakage/titling), but high in-situ mortality of Acropora sp., Montipora sp. and Echinopora sp. occurred (Figs. 8, 9). These
dead substrates are now largely covered with filamentous and turf algae and the inferred cause of mortality was
freshwater bleaching during the extensive rainfall that accompanied Yasi’s track.
Fig. 7. View post-Yasi (Aug. 2011) showing unaltered structure of
the main reef flat and Goniastrea colonies still in-situ.
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Fig. 8. Left - View across Acropora stands pre-Yasi (Aug 2009) and, right, same areas of the reef post-Yasi (Aug. 2011).
Fig. 9. Dead in-situ coral substrate along the seaward margin of King Reef, mostly in-situ and covered with filamentous algae.
2.1.3 Geomorphic Impacts of Cyclone Yasi at King Reef
Cyclone Yasi had little or no impact on the large scale geomorphology of King Reef. There is little evidence for major
reworking of the reef flat and no evidence for major scouring or erosion. The only clear evidence of the cyclones
passage at this site are occasional coral blocks thrown up onto the reef flat (Fig. 10), but these are not abundant and
there is no storm rubble ridge produced in this locality. Most of the numerous Porites microatolls also remain undamaged
and alive, although some have been fractured in-situ and/or have been partly tilted (Fig. 10).
Fig. 10. Isolated coral blocks deposited on the seaward reef flat and rare examples of fractured microatolls.
2.2
Lugger Shoal
2.2.1 Location and Pre-Yasi Characteristics
 Lugger Shoal (17°57.5′ S, 146°6.5′ E) is a small (~450m by 150m) ‘L-shaped’ shoal reef located close to shore
within a muddy, headland protected, embayment (Fig. 11). It is a young reef, have started accreting only ~ 800 yBP
and has reached sea level quickly and is already in a ‘late mature’ evolutionary state (Perry et al. 2009).
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Fig. 11. Location of Lugger Shoal and aerial shot looking south across Lugger Shoal towards Tam O’Shanter Point.
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Prior to Cyclone Yasi the dominant feature of the reef flat were large Porites bommies, with the inter-bommie areas
infilled with a mixed sediment-coral rubble matrix. This substrate was veneered by fine-grained (mixed carbonateterrigenoclastic) sediment and colonised by a wide range of coral species, including: Acropora sp. Turbinaria
frondens, Goniastrea aspera, Favia sp., Favites sp. Galaxea fascicularis and Platgyra sp (Fig. 12).
Fig. 12. Left - view landwards across Lugger Shoal (pre-Yasi) showing the dominant Porites bommies that provide much of the basic
structure of the reef. Right – Goniastrea colonies colonising the reef flat surface.
2.2.2
Ecological and Geomorphic Impacts of Cyclone Yasi at Lugger Shoal
Post-Yasi investigations at this site were hampered somewhat by tide state and wave conditions, but based on the
observations that could be made there had been little major impact on the reef – either ecologically or
geomorphologically. The basic structure of the reef – dominated by large Porites colonies – was mostly intact, although a
few large bommies, especially towards the rear of the reef flat, had been either partially fractured or toppled (Fig. 13).
The living tissue cover of these colonies remains, however, intact. There was also no obvious change in the topographic
relief on the reef flat in terms of the depth to substrate surface in the intra-bommie areas, and no storm ridge or coral
block deposition was found either on the reef flat or as onshore deposits. Colonies of Goniastrea and Galaxea that
previously colonised the inter-bommie substrates also seem to have survived largely intact (Fig. 13), with limited
evidence of any fracturing or toppling, although other previously common taxa, such as Turbinaria frondens were
noticeably absent and are assumed to have been removed during the cyclone.
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Fig. 13. Tilted Porites bommie towards the rear of Lugger Shoal, and (right) Goniastrea colonies that survived intact
2.3
Dunk Island
2.3.1 Location and Pre-Yasi Characteristics
 Dunk Island (146° 09’ E, 17° 56’ S; Fig. 1) is an inner-shelf high island located ~5 km offshore from the mainland
Queensland coast. Our datasets derive from a fringing reef developed within an embayment on the north-west
corner of Dunk Island, termed ‘Resort Reef’ (Perry and Smithers 2010) (Fig. 14).
 Reef development in the embayment occurred over two discrete phases, the first between ~ 7000-4500 yBP and
the second since ~1500 yBP. Both are in ‘senile’ evolutionary states.
 The north east reef flat is at ~0.8-1.0 m above LAT and is a ‘relict’ (non accretionary) structure) and covered in
siliciclastic intertidal sands/muds and lithic clasts.
 The reef flat in the south west of the area is geomorphologically lower (~0.4-0.5m above LAT), is younger but also
in a late mature/senile evolutionary state and, in its pre-Yasi state, was veneered by muddy-sands.
Fig. 14. Location of Dunk Island and aerial view looking across the exposed reef flats at ‘Resort Reef’
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In its pre-Yasi state the surface of the northeast (older) reef flat was covered in sediment and no living corals were
observed, although the tops of numerous dead Porites microatolls were visible.
On the south west (younger) reef flat numerous small (<0.3 m in size) living Goniastrea aspera colonised this
surface and along the seaward margin there was colonisation of Acropora sp., Montipora sp., Galaxea sp. and
Favia sp. and a discontinuous zone of large Porites ‘bommies’.
2.3.2 Ecological Impacts of Cyclone Yasi at Dunk Island
Despite the intensity of the cyclone and the proximity to the landfall path ecological impacts across Resort Reef were
generally limited. It is clear, for example, that the numerous small Goniastrea sp. colonies which previously colonised the
lower elevation (younger) reef flat surface mostly survived intact, as did many seemingly fragile colonies of Turbinaria
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sp., Acropora sp. and Montipora sp. (Fig. 15). Only isolated overturned or fractured colonies were observed (Fig. 16). To
the seaward of the reef flats the field of large Porites sp. bommies also survived with no evidence of toppling or
breakage, but there is clear evidence that these colonies suffered either partial or complete mortality on their upper
surfaces – mortality consistent with freshwater bleaching (Fig. 16).
Fig. 15. Goniastrea (left) and Turbinaria (right) colonies on the younger, lower elevation reef flat which survived intact.
Fig. 16. Isolated overturned corals (left) and (right) dead (freshwater bleached) upper surfaces of Porites sp. bommies
2.3.3 Geomorphic Impacts of Cyclone Yasi at Dunk Island
The main geomorphic impacts of Cyclone Yasi on Resort Reef occurred through the landward transport and stripping of
sediment - especially from the toe of beach area. This resulted in a much narrower beach and increased exposure of the
underlying mid-Holocene reef flat (Fig. 17), and significant onshore sediment accumulation. At the time of survey this
sediment was still being cleared from the resort itself (Fig. 18). However, the basic geomorphic structure of the reef flats
appear to have changed very little, with no evidence for major reef flat erosion and only localised coral rubble deposition.
Pre Yasi
Post Yasi
Fig. 17. Pre- (August 2009) and post (Aug 2011) images showing changes in the nearshore geomorphology of the beach and high
intertidal zone at Resort Reef.
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Fig. 18. Photos showing the effects of significant onshore movement of beach sediment, now (right) having been bulldozed out of
the damaged resort infrastructure.
2.4
Paluma Shoals (South Shoal)
2.4.1 Location and Pre-Yasi Characteristics
 Paluma Shoals (19°5.43’ S, 146°33.5’ E) is located ~150km south of the main route of Cyclone Yasi and comprises
of a series of reef platforms alongside an erosional shoreline (Fig. 19). The focus of the post-Yasi assessment was
the South Shoal (extending ~750m alongshore and with a reef flat ~300m wide).
 The reef dates to ~1,200 cal yBP and reached sea level in the last 100-200 years (Perry et al. 2008). It is presently
in a ‘late mature’ stage.
 The pre-Yasi state of the reef was dominated by colonies of Galaxea fascicularis and Goniastrea aspera microatolls
(~2m diameter). These colonies exhibit a strongly heliotropic growth (Smithers and Larcombe 2003) and were a
highly distinctive feature of the reef flats (Fig. 20). Along the landward edge of the reef flat low relief Porites rus
pavements formed extensive flat-topped colonies. Live coral cover was measured at ~50-60% across the entire reef
flats but ~80% in the central to seaward areas.
Fig. 19. Location of Paluma Shoals and view looking northwards across the Paluma shoals reef complex
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Fig. 20. Pre-Yasi images of the South Shoal at Paluma showing extensive fields of Goniastrea corals and high live coral cover.
2.4.2 Ecological Impacts of Cyclone Yasi at Paluma Shoals
Live coral cover remains relatively high along the seaward margin of the reef flat with only localised toppling/fracturing of
Goniastrea colonies (Fig. 21). Similarly, across the central and landward areas of the reef flat, the large Goniastrea
bommies remain mostly in-situ and generally unaffected, except for minimal toppling and overturning. The major
ecological impact of Cyclone Yasi has been the widespread loss of previously abundant Acropora pulchra stands.
Porites rus remain abundant and undamaged save some localised toppling and tilting in the rear reef zone.
Fig. 21. Toppling and tilting of larger coral colonies across the reef flat at Paluma Shoals
2.4.3 Geomorphic Impacts of Cyclone Yasi at Paluma Shoals
The most obvious geomorphic impacts of Cyclone Yasi at Paluma Shoals occurred as: 1) the deposition of a patchily
developed rubble and shingle ridge along the seaward margin of the south Shoal – essentially a landward thinning
sheet-like deposit of Acropora sp., Turbinaria sp. and blocks of Galaxea sp (Fig. 22); and 2) widespread deposition of
Acropora shingle across the central and landward areas of the reef flat – these deposits forming localised sheets of
rubble in and around the remaining in-situ larger coral colonies on the reef flat (Fig. 23).
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Fig. 22. Sheet-like rubble ridge along the seaward margin of the South Shoal comprising a mix of Acropora sp., Turbinaria and
Galaxea shingle.
Fig. 23. Acropora shingle deposited around larger coral
heads (that remain in-situ) across the central and
landward areas of the reef.
Key points: Category 5 Cyclone Yasi which struck the north east Queensland coast in Feb 2011 had a major impact on
the regions coastal infrastructure, but as evident in the above summary, markedly variable impacts on the ecology and
geomorphology of the inner-shelf reefs in the region occurred. One of the key factors influencing the nature and
magnitude of disturbance has been reef evolutionary state – because this influencing the contemporary reef structure
and its topographic complexity, and thus the coral communities that colonise the reef surface. In general physical
disturbance of the reef structure and of coral communities has been limited (the exception being Paluma Shoals), but
freshwater-induced bleaching had a major impact at some sites (King Reef and Dunk Island)
Ecological Impacts. Overall the ecological impacts of Cyclone Yasi appear relatively limited. On most reefs examined
some evidence of larger colony toppling/overturning occurred. The most obvious physical destruction occurred at the
most distal site (Paluma Shoals) where widespread breakage (and deposition) of branched Acropora occurred. At King
Reef, widespread coral mortality also occurred but appears to have been caused by freshwater-induced bleaching –
these dead coral stands remaining in-situ. Ecological impacts thus appear to have been spatially highly heterogeneous
and primarily influenced as much by pre-existing reef ecological condition as by proximity to the storm path.
Geomorphic Impacts. At all but one of the reef sites examined there was only a limited geomorphic ‘signature’ of the
passage of Cyclone Yasi – this being at Paluma Shoals the most distal of the sites examined. At King Reef, Dunk Island
and Lugger Shoal there was no evidence of major reef and reef flat erosion (despite their unlithified structure), no
evidence for storm ridge deposition (presumably due to limited available coral to supply their development), and only
isolated deposition of coral blocks. At Paluma Shoals, however, a distinctive rubble and shingle ridge formed along the
seaward margin, and widespread deposition of Acropora shingle sheets occurred across the central and landward areas
of the reef flat.
Chris Perry, University of Exeter
Scott Smithers, James Cook University
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