 CYCLONE JUSTIN Jamie Oliver
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Background yclones are one of the most important natural phenomena affecting reefs in non-equatorial regions. The severity and extent of the damage which they cause can substantially alter the physical structure, biological composition and ecological relationships within reefs. In addition, cyclones can play havoc with man-made structures. This can result in further ecological damage depending on the fate of these structures and will have an important economic effect on reef tourism. This article provides a brief synopsis of the available information on the effects of cyclone Justin on the Great Barrier Reef. The information was obtained from a variety of sources. Further detailed information can be obtained from Marji Puotinen at the Cooperative Research Centre for Ecologically Sustainable Development of the Great Barrier Reef (CRC Reef Research Centre), Terry Done and Lyndon DeVantier at the Australian Institute of Marine Science (AIMS), Jenny Baer in the Impact Assessment Unit of the Great Barrier Reef Marine Park Authority (GBRMPA), and David Haynes, Water Quality Coordinator at GBRMPA. I am grateful to these individuals for providing me with much of the information presented in this summary. Cyclone Justin was an extremely large but generally weak cyclone which occurred off the Queensland coast from 7-24 March 1997. Its movement was unusual. The cyclone spent the first five days almost stationary well off the Queensland coast before moving north-east towards New Guinea. It then moved back towards Queensland, crossing the reef near Cooktown and then headed along the coast, finally weakening into a rain depression south of Townsville. Although a simple interpretation of the path of the cyclone (figure 1) might suggest that the most substantial damage would have occurred between Cairns and Townsville, a more sophisticated analysis of predicted wind energy, carried out by CRC Reef researcher Marji Puotinen, resulted in a more complicated pattern of potentially destructive effects along the Queensland coast and the Great Barrier Reef. During its initial stationary stage, the size of the cyclone and the period of its influence caused very large waves to be generated along the middle of the reef south of Townsville even though, at this stage, the cyclone was quite some distance offshore. During the second phase, when it returned from New Guinea, its influence was more direct and caused by waves and wind near the centre of the cyclone. Its path in this instance created a zone of high wind energy across the reef near Cairns. This was followed by a less destructive zone from Townsville to the Whitsundays as the cyclone weakened after moving back offshore. Thus there were two principal areas of destructive effects: the reefs offshore from Cairns-Cooktown, and the reefs between Townsville and Mackay. While the northern area was affected by relatively severe winds on one occasion, the southern area was actually affected twice: first by the waves generated while the cyclone was offshore, and then again when the cyclone passed directly through the area as it weakened into a tropical depression. |
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Damage caused by cyclone-induced wavesReports from divers indicate that some reefs off Cairns, Townsville and the Whitsunday Islands have been damaged to a moderate degree. Generally the damage appears to be less severe than that caused by some other recent cyclones (Ivor, Joy, Winifred). The Australian Institute of Marine Science, GBRMPA and the CRC Reef Research Centre carried out a joint survey in the Whitsunday Islands region. Lyndon DeVantier of AIMS collected additional data. The data from this survey are not yet completely analysed, but preliminary results indicate that damage to coral communities has been very high in some discrete locations, but almost unnoticeable in many other spots. Approximately 58% of the 72 reefs surveyed had some degree of coral breakage, but only two of those reefs (~3%) exhibited severe levels of coral breakage when compared to the damage recorded from cyclone Ivor (a severe cyclone which caused extensive recorded damage on northern reefs). This high degree of spatial variation in coral damage is due to a number of factors including small-scale variation in the speed and direction of the wind; variation in wave height and direction; and the sheltering effect provided by other reefs and other parts of the same reef. The AIMS Long-term Monitoring Program has also detected damage at two out of the five reefs that were recently surveyed off Townsville. Damage was not severe and consisted mainly of overturned table corals. The cyclone also modified some small sand cays and spits by shifting sand and altering their overall shape. At Michaelmas Cay, off Cairns, approximately 50% of the low vegetation in the middle of the cay was buried by shifting sand. This caused a reduction in the seabird populations which breed and roost on the cay. Both the vegetation and seabird numbers are now recovering. The cay, which was closed to tourists for two weeks, has been re-opened. In the Whitsunday region, there was little evidence of sand movement on most reefs, although the two reefs which exhibited severe coral damage also had areas where scouring at the edge of the reef-flat in the back reef area had created erosion channels about 0.5-1 m deep. This erosion may have also facilitated eventual recovery of the area by exposing new areas of previously buried hard, bare substrate for new coral colonies to settle and grow on.
Damage caused by structures on the reefAs a result of the winds and waves generated by the cyclone whilst it was offshore from Cairns, tourist pontoons at Low Isles, owned by Quicksilver Connections Ltd, and Norman Reef (figure 2), owned by Great Adventures, broke free from their moorings and were washed onto the adjacent reef-flat. Both pontoons have been subsequently towed off the reef-flat. Preliminary assessments of the damage caused during both the grounding of the pontoons and their subsequent removal have been carried out by Mike Short from the Queensland Department of Environment. At Low Isles a total area of 0.2 ha was damaged, in an area of coral and seagrass. At Norman Reef an area of about 0.25 ha was damaged on a coral dominated reef-flat. Although the damage was quite severe in spots (total removal of all corals and/or seagrass), at the scale of the entire reef, the damage caused by both groundings cannot be considered to be a major ecological impact. A more detailed survey of the scars caused by the grounding and removal of the Norman Reef pontoon has been conducted by a consultant marine biologist, but the results have not yet been analysed. Recovery of the Norman Reef scar will be monitored over the next two years.Although some other tourist pontoons suffered damage to the superstructure, the pontoons at Norman Reef and Low Isles were the only ones to break free of their moorings. Investigations are currently being conducted to determine what caused the moorings to fail during the cyclonic conditions - which they were designed to withstand - and what steps are required to ensure that such failures do not recur. Other smaller structures were also damaged at various locations. For instance, a large section of the pontoons of a floating marina in Cairns Harbour was destroyed, while at Hinchinbrook Island resort a walkway to a pontoon was removed from its supports. At Hardy Reef, a helicopter pontoon broke free, while a swim pontoon at Hamilton Island suffered a similar fate. Both were retrieved relatively undamaged. No reports of any ecological damage arising from these incidents have been received.
Effects on water qualityCyclones can have a major influence on the water quality of the Great Barrier Reef lagoon, through the effects of turbid, sediment-laden water coming from adjacent rivers in flood (figure 3). The Great Barrier Reef Marine Park Authority and AIMS sampled river flood plumes resulting from cyclone Justin rainfalls in March 1997. Floodwater samples were collected at approximately 70 nearshore sites located between Trinity Inlet (near Cairns) and Lucinda (near Ingham) and these will be analysed for chlorophyll a, phaeophytin, suspended solids, particulate and dissolved nutrients and pesticides. The extent of the flood plumes was also surveyed from a light aircraft and the plume mapped into a Geographic Information System. Water samples were also collected during the flood event from the Russell River. Samples were collected from below a pristine rainforest site, from a site below banana growing areas and from a site below cane growing areas in order to compare the impact of a variety of land uses on floodwater quality. Offshore coral communities were also surveyed before and after the flood event to investigate the impact of freshwater plumes on coral communities.Ongoing research and monitoringAlthough cyclones are a natural source of disturbance, it is important that management agencies such as GBRMPA support and encourage ongoing research into the patterns of cyclone activity on the Great Barrier Reef and their effects on reef biota. This knowledge is important in understanding both the ability of reefs to recover from different patterns of physical disturbance, and the patterns of natural stress which reef communities experience even in the absence of human activities.
There are a number of research programs which are investigating
aspects of cyclone damage on the Great Barrier Reef. Long-term
monitoring at specific sites, led by Terry Hughes at James Cook
University and Terry Done at AIMS, is building up a detailed picture of
the dynamics of coral communities and their responses to cyclonic
disturbances. The CRC Reef Research Centre is supporting Terry Done's
and Marji Puotinen's work on the development of models of wind and wave
patterns which can predict the spatially variable damage to reefs
caused by cyclones. An important by-product of this research is the
publication of an atlas of cyclones in the Great Barrier Reef. The AIMS
Long-term Monitoring Program continues to survey reefs in all sections
of the Great Barrier Reef on an annual basis. This work will enable the
detection of damage and subsequent recovery at a regional scale.
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Great Barrier Reef Marine Park Authority
PO Box 1379 TOWNSVILLE QLD 4810. Phone: (077) 500 700, Fax: (077) 726
093
E-mail:
registry@gbrmpa.gov.au
Figure 2. The pontoon at Norman Reef being towed off the reef-flat.
The white rectangle is the helicopter pontoon.
Figure 3. Flood plume from the Herbert River after cyclone Justin
Finally, GBRMPA will commission additional surveys and monitoring
projects to fill in information gaps as required or to document damage
at particular reefs of management significance.