Reef-top currents in vicinity of Heron Island boat harbour, Great Barrier Reef, Australia: 2. Specific influences of tides meteorological events and waves.

摘要

Heron Island is a small coral cay on the western end of a large platform reef in the southern Great Barrier Reef, Australia. In 1967 a boat access channel and a small mooring basin were dredged between the western reef-rim and the island. Since then reef-top currents have flowed out through this channel, removing sediment from the reef-flat and the island’s beaches. The mooring basin and access channel were enlarged in 1987. Offreef waves (hourly) and reef-top currents (at 10 minute intervals) were measured on either side of the island during a twelve month period (17 March 1996 – 18 March 1997). An overall analysis of the data has been presented in a first report (Gourlay and Hacker 2008a). This second report presents the following detailed analyses: (i) the characteristics of reef-top currents under mild conditions, including the influence of the harbour bund walls upon reef-top tide levels and the influence of tidal asymmetry upon the duration of bund wall control; (ii) the characteristics of reef-top currents during various meteorological events; (iii) the influence of waves upon reef-top currents including comparison of calculated and measured current velocities for selected conditions/events; (iv) reef-top sediment transport and sedimentation in the boat harbor during various tropical cyclones. Mild conditions are found to have occurred in only 7% of the observed tidal cycles. The physical topography of the reef-top had an increasing influence upon currents as the tide level fell below mean sea level. The wave height (Hos), at which opposing tidal currents were suppressed, increased with increasing tidal range. Direct comparison of calculated (V) and measured (v) wave-generated current velocities when the tidal current approached zero gives good agreement, indicating that v = C(Hos – Hoscr). During tropical cyclones, the increased flow through the boat harbour entrance channel transported sediment from the reef-top and deposited it offreef at the bottom of the reef-face to the southeast of the channel entrance.