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South African Journal of Science 101, May/June 2005

Research Letters

Fig. 3. dGPS tracklines showing data coverage in the survey area. Also shown are the Produce and Crown Restricted Zones of the MPA and the 25-m isobath, which was used to delineated the Crown Area Restricted Zone.

System (dGPS) capable of single-second update rate and sub-metre accuracy. Using an average survey speed of 2.5 knots, this translated into a bathymetric data point every 1.29 m along each trackline.

In total, 101 433 data points were used to create the bathymetric model. All depth values were tidally corrected to the mean sea-level (MSL) datum (local tidal range is 2 m). Data were gridded in Surfer version 7 software using point kriging to produce seafloor contours as used in earlier reef surveys on the southeastern continental shelf.14 Grid sizes and search patterns were optimized to represent the reef most accurately using a 20 m × 20 m grid with an octant search pattern and a search radius of 180 m. Digital terrain models were generated using ER Mapper 6.4 and Surfer 8 software to provide three-dimen- sional (3-D) models of the continental shelf (Fig. 4b) in addition to a 2-D contour plot (Fig. 4a), providing visualization of the reef morphology from any orientation.

Aliwal Shoal morphology

The Aliwal Shoal forms part of a much larger offshore reef complex than was previously realized. The seaward expression of the reef is a narrow ridge with a curvilinear sinusoidal north- east–southwest (coast-parallel) trend (Fig. 4) that has an abrupt northern edge called the Crown. The landward expression has a linear margin from which a narrow ridge, just over 1 km long (the Spur), projects shoreward. The shallowest part (6 m below MSL) is located in the northern extremity of the Crown, with a gradual increase in depth with distance southwards. The steep- est gradient recorded, on the northern edge of the Crown, is 18°. Other gradients vary from 6.4° on the seaward side to 8° on the landward side.

Fig. 4. (a) Bathymetric contour map of the study area showing the main morpholog- ical features and the boundaries of the Restricted Zones of the MPA. Contour interval is 2 m. (b) 3-D bathymetric model of the study area; view is from the south, sunshading from 150° with a 75° azimuth.

bedform, interpreted as submerged spit-bar12 and reinterpreted as a sandridge15 (Fig. 2). The landward side of the shoal, between the Sandridge and the Ridge, is a sand-filled depression, roughly divided into two basins by the Spur. The depression slopes towards the Sandridge (Fig. 4). The Spur consists in part of a northwards-migrating, 2–3-m-high subaqueous dune.

The average depth of the Crown is 12.5 m and it contains almost all of the SCUBA-diving locations used by recreational divers, the best known being the Main Pinnacle Ridge and North-Eastern Pinnacles (Fig. 4). The southern Ridge is deeper with an average depth of 19.5 m. This portion and the southern continuation of the Crown ridge are characterized by several other isolated pinnacles. The total area covered by the Aliwal Shoal is in excess of 5.56 km2 and extends westwards to the coast beyond the Ridge.

The Aliwal Shoal averages 380 m in width in the north, narrows slightly in the central region but widens to more than 2 km in the south, where it bends shorewards (Fig. 2). This southern plateau region is called the Ridge.8 Northwest of the shoal is a large

Data application

This dataset provides the first detailed and useful information on the nature of the seafloor in the Aliwal Shoal area and enabled the accurate positioning of the newly proclaimed Restricted

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