The impact of Cyclone Ivor on coral reefs of the northern Great Barrier Reef (1990)

In July 1990, surveys were conducted at 46 sites, covering a 300 km section of the northern Great Barrier Reef, located to the north and south of the track of Cyclone Ivor, which passed through the region in March 1990. All sites were located on reef slopes and included normally exposed (front reef) and normally sheltered (back reef) aspects, specifically with a view to determining the effects of local shelter. Sites were spaced evenly along the Ribbon Reefs (at approximately 10-20 km intervals) and opportunistically on mid-shelf reefs. One to 3 sites were chosen on each reef, representing positions of extreme exposure or extreme shelter. At each site, between 1 and 3 surveys were conducted within different depth ranges.Surveys consisted of a 15-20 minute reconnaissance of the area. Damage to reefs was categorised as: coral breakage; dislodged Porites heads (and occasionally other massive corals); scarring on standing corals caused by waterborne debris; evidence of soft corals having being ripped off; peeling and disintegration of framework; collapse of large (several metres across) slabs of reef which remained largely intact after collapse; evidence of major deposition or removal of sand. The extent of the damage was assessed on a 5 point scale; 1 = minor; 3 = moderate; 5 = severe. Observer comparisons were made of the assessments of each site.A general description of the area covered in each survey at each site, which included depth range, slope angle, and % cover of hard coral, soft coral and dead coral was recorded. The predominant corals and zonation were also recorded.Video transects were recorded at 34 of the 46 sites. Each video transect, which lasted 5 (±1) minutes, was conducted on a meandering swim (speed 0.5 m/s) in the depth range of 2 to 12 m. The camera was pointed at about 45° below horizontal, and the distance from the bottom was kept at 1-1.5 m. No attempt was made to randomise the video transects. Rather, the path was chosen to ensure damaged and/or undamaged areas were filmed more or less in proportion to their perceived relative abundance. The video-tapes were later analysed in the laboratory for an unbiased comparison with our visual estimates of bottom characteristics and damage. The tape was paused at approximately 50 random intervals. At each stop, the major type of bottom cover (either damage category, hard coral, soft coral or substratum type) visible in the frame was recorded.Survey sites were located on: Ruby Reef, Pearl Reef, Ribbon Reef No. 2, Ribbon Reef No. 4, Ribbon Reef No. 5, Ribbon Reef No. 6, Ribbon Reef No. 7, Ribbon Reef No. 8, Ribbon Reef No. 10, Cod Hole, Yonge Reef, Carter Reef, Day Reef, Hicks Reef, Hilder Reef, Jewell Reef, Reef No. 14-077, Reef No. 14-074, Reef No. 14-047, Scooterboot Reef, Davie Reef, King Island, Clack Island, Reef No. 13-125, Pipon Island, North Warden Reef, South Warden Reef, Switzer Reef, Combe Reef, Snake Reef, Waining Reef, Eyrie Reef and Helsdon Reef.
 Surveys were conducted to assess the type and extent of damage caused to outershelf and midshelf reefs by Cyclone Ivor.
 Cyclone Ivor (central pressure 965-990), passed through the northern Great Barrier Reef (GBR) region in March 1990. It was designated a Cyclone on 17th March 1990, while it was situated in the Coral Sea, and its path kept the eye in the vicinity of a 50 km section of the outer GBR for about 3 hours. The final approach was almost perpendicular to the section of the outer GBR bearing approximately 300° between Jewell Reef and Melville Passage. It passed over Waterwitch Passage on a SSW course at around 1800 hrs on 19th March, and then moved 50 km east over the next 2 hours, a course which took it back towards the outer reefs. It then retraced its path over the outer reefs, this time continuing over Cape Melville and crossing the west coast of Princess Charlotte Bay around midnight. The cyclone lost intensity once it was over land, but nevertheless continued to generate rain and strong winds for several days. During this time, while classified as a rain depression (central pressure 999-1008 hPa), it reversed its course again, and travelled back out over the coast between Port Douglas and Cairns.

Organization: Australian Ocean Data Network