The benthic environments of Sydney Harbour, New South Wales

Summary

Sydney Harbour has been used as a test bed for numerous seabed sensors during the international hydrographic conference, Shallow Survey 2003. To examine the relationships between acoustic backscatter and benthic habitats we combined some of the Shallow Survey 2003 data sets with sediment, underwater video and new high-resolution multibeam sonar data. The new data sets show details of the seabed such as dredged areas, while acoustic backscatter imagery helped to delineate various seabed habitats.

Sydney Harbour

Sydney Harbour is a drowned river valley that forms the seaward section of the most urbanised estuary in Australia. The seabed morphology of Sydney Harbour is complex and irregular with a series of deep ‘holes' up to 45 m deep, and shoals where the water depth can be less than 3 m. The main channel is bordered by numerous elongate shallow embayments and bay-head beaches that sit between sandstone headlands.

A focus area for the benthic habitat mapping project was the Sow and Pigs Reef in the outer harbour. This area was selected to compare and integrate the data provided by various benthic mapping tools because of its known geomorphic and biological diversity. Surface sediments, underwater video and high-resolution multibeam bathymetric data were collected during this study.

The bathymetry of Sydney Harbour , which includes a series of deep ‘holes'.

Figure 1. The bathymetry of Sydney Harbour, which includes a series of deep ‘holes'. The dashed line indicates the position of the landward margin of the flood tide delta. To the west (left) of this line the bottom sediments become dominated by terrestrial mud.

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Outer Harbour

The Sow and Pigs Reef is bordered by the Western and Eastern Channels, which were dredged for navigation and are clearly defined in the acoustic image below (Figure 2). The Western Channel was last dredged in 1967-68 and the relatively straight sides of the channel and smoother channel floor reflect the more efficient dredging technology employed, compared to the previous dredging that formed deep ‘potholes'. The dredged areas in the southern part of the Eastern Channel were last dredged between 1926 and 1931, and the potholes left by this operation are still clearly visible. There is another dredged area just offshore off Bradleys Head, where the original dredging and subsequent slumping are evident in the bathymetric image (Figure 2). A large shipwreck just off Bradleys Head, the SS Currajong, is also visible in the image.

A topographic model of the bed of Sydney Harbour produced with GeoSwath data from the Shallow Survey 2003 Common Dataset.

Figure 2. A topographic model of the bed of Sydney Harbour produced with GeoAcoustics GeoSwath acoustic data from the Shallow Survey 2003 Common Dataset. A) Dredge cuttings and subsequent slumps, and a ship wreck off Bradleys Head. B) Marine sand moving into the Eastern Channel, infilling the ‘potholes'. C) Close up of the Western Channel showing bedforms produced by sand that moves under the strong tidal currents.

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Seabed Sediments

The outer harbour seabed comprises well-sorted and rounded marine quartz sand transported into the harbour from the inner shelf by tidal currents. These tidal delta sands become finer, with an increase in mud and shell, at the delta front (near Sow and Pigs Reef). High-resolution acoustic images of this area show the tidal delta is still growing, with sandy bedforms moving into the dredged areas from a seaward direction. Upstream of the tidal delta there are extensive muddy deposits that are dominated by fine sediment, typically dark grey to black, shelly, organic-rich sandy mud.

Acoustic Backscatter

Acoustic backscatter intensity data is also recorded by the swath mapper, providing textural information about the seabed which helps to delineate habitats. Backscatter data for the Sow and Pigs Reef area is shown, which was collected with a Reson 8125 system. The deeper areas feature low acoustic backscatter values, probably due to a smooth muddy seabed. The Deviation Cut dredged area has highly variable acoustic backscatter values, likely due to the patterned seabed texture. In contrast, the shallower sandy areas within Watsons Bay have consistent intermediate backscatter, while areas of high backscatter correspond to seagrass patches and gravely sediments that are common in the shallower zones.

Acoustic backscatter signatures can also be compared with underwater video transects to gain further sediment and habitat information. In general, high backscatter is related to coarse gravely sands, areas of sparse seagrasses (mainly Halophila sp.) and sandy areas that have a rippled surface. However, these features cannot be distinguished by their backscatter alone. Areas of low backscatter occur in deeper water and are related to better-sorted sands and muddy sands where there are few discernible bedforms.

3D multibeam image and acoustic backscatter intensity for a section of the Eastern Channel (Reson 8125 data).

Figure 3. 3D multibeam image and acoustic backscatter intensity for a section of the Eastern Channel (Reson 8125 data). The enlarged area, B, includes the backscatter intensity, site-specific seabed types based on classified underwater video, and a broad-scale indication of sediment type.

View the 3D model of Sydney Harbour

More information on Sydney Harbour (Port Jackson)

Further information from the coastal CRC.

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