The bathymetry of the Aliwal Shoal, Scottburgh, South Africa
C . B o s m a n a * , R . U k e n a a n d A . M . S m i t h b
This paper presents the first accurate geo-referenced bathymetric map of the Aliwal Shoal and illustrates its value as an essential tool for a wide range of applications useful to the broader scientific community. The dense coverage of echo-sounding data in the survey area permitted the construction of a detailed bathymetric contour map and 3-D model. It showed that the Aliwal Shoal forms part of a much larger offshore reef complex than was previously realized. Morphologically, the Aliwal Shoal is dominated by three distinct features, namely, the Crown, Spur and Ridge. The high- resolution bathymetric map presented here enabled the accurate positioning of the boundaries of the Restricted Zones of the newly proclaimed Aliwal Shoal Marine Protected Area. This geo- referenced GIS-compatible map will form the base map for future studies, ranging from the mapping of biological seafloor habitats to ocean current modelling, thereby also performing the function of an environmental planning and management tool.
The Aliwal Shoal (Fig. 1) is one of the best-preserved examples of the many submerged reefs, parallel to the coast, documented on the southeastern continental shelf.1–6 These reefs consist primarily of aeolianite with subordinate beachrock and are thought to represent Late Pleistocene palaeocoastlines.3,4 In 1884, the SS Nebo sank after striking the reef; in 1974, the MV Produce similarly sank, coming to rest on the seafloor northeast of the Aliwal Shoal.7 In 1963, the Aimée Lykes grounded on the reef but made it to Durban harbour. Rock samples removed from the damaged hull confirmed an aeolianite origin8 and foramini- fera provided a Pliocene–Recent age,9 revised later as Late Pleis- tocene.10 Several researchers have included the Aliwal area in regional studies2,11–13 but none has dealt specifically with this reef and it has remained for the most part unmapped (Fig. 2). In this paper we present the first detailed geo-referenced map of the
aJoint Council for Geoscience/University of KwaZulu-Natal Marine Geoscience Unit, School of Geological Sciences, Howard College Campus, Durban 4041, South Africa.
bASC Research, Durban. E-mail: firstname.lastname@example.org *Author for correspondence. E-mail: email@example.com
Fig. 1. Locality map of the study area which falls within the ‘Controlled Zone’ of the newly proclaimed Aliwal Shoal Marine Protected Area (MPA). The Restricted Zones within the Controlled Zone are also shown.
Aliwal Shoal and illustrate its usefulness as a scientific and envi- ronmental management tool.
Echo-sounding data were collected on two separate sea cruises, in March and June 2001. The survey grid comprised northwest–southeast and northeast–southwest tracklines, set 120 m apart (Fig. 3). In all, 266 line-kilometres were surveyed. Depth measurements for the bathymetry map were recorded using an Odom EchoTrac Model 3100 single-beam digital hydro- graphic echo-sounder and a 200-kHz narrow-beam (10°) Odom transducer. The accuracy of the echo-sounder was 0.01% of total water depth, dependent on the accuracy of the value of the velocity of sound. Navigation and position fixes were provided by a Fugro OmniStar 12-channel differential Global Positioning
Fig. 2. Maps of the Aliwal Shoal. (a) Bathymetry (10-m intervals) and (b) geological map showing unconsolidated sediment thickness (isopachs in 10-m intervals) and aeolianite (black area). After Martin and Flemming. 12