BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

5 DESCRIPTION OF THE AFFECTED ENVIRONMENT
Carter et al. (2003) and Steffani et al. (2003) have provided environmental descriptions
for Table Bay and the Port of Cape Town based on the available information for the
region. The descriptions provided below are taken from these authors supplemented by
survey data obtained subsequent to these syntheses. The environmental description is
partitioned into a regional overview and site specific information on the proposed dredge
areas in the Ben Schoeman Dock Basin and two candidate dredge spoil dump sites in
outer Table Bay.
5.1 Regional Overview
5.1.1 Physical features
Table Bay is a log spiral bay anchored by rocky headlands at Mouille Point in the south
and Blouberg in the north and contains Robben Island and the important Port of Cape
Town. Maximum water depth in the centre of the bay attains 35m, increasing to 70–80m
outside of a line between Mouille Point and the western shores of Robben Island (Figure
1).
The seabed is mainly covered by thin layers of sand but has areas of partly exposed bed
rock. Fine sand is generally confined to the eastern nearshore region between Blouberg
and the harbour. However, a tongue of fine sediments extends from the nearshore zone
seaward to a depth of approximately 25 m between Table View and Rietvlei. Smaller
pockets of fine sand are found at the bay entrance and on the eastern shore of Robben
Island. Medium sand covers the remaining areas of Table Bay (Figure 2; Woodborne
1983; CSIR 1997). The major sources of the sand in Table Bay are seasonal (mainly
winter) inputs from the Diep and Salt rivers and local erosion of Malmesbury shales
(Quick and Roberts 1993). There is no substantial sediment supply to the bay from
longshore transport from the south. Sediment is transported out of Table Bay by local
wave and storm driven transport and the overall residence time for surficial sediments is
estimated at 2-3 years (CSIR 1997).
The shoreline of Table Bay (Figure 3) from Blouberg to Mouille Point consists of 3 km of
rocky shore (at Blouberg and at Mouille Point), 13 km sandy beach (between Blouberg
and Table Bay harbour) and 4 km of artificial shore protection and breakwaters
comprising the Port of Cape Town (CMS 1998). Robben Island has a total shoreline of
9km, of which 91% is rocky (CMS 1998).
5.1.2 Oceanography
Table Bay is located within the southern Benguela upwelling system and its circulation
and water properties are characteristic of this region. Surface currents are mainly wind
driven with velocity generally decreasing with depth. Quick and Roberts (1993) quote
typical velocities as being 20 – 30 cm/s at the surface and near bottom flows being
generally less than 5cm/s. Summer circulation in Table Bay is mainly clockwise.
Nearshore currents are wave driven; waves approach the coast obliquely (200° - 260°
predominantly) generating northward flow. Due to generally low current velocities water
volume periods in Table Bay are generally long with an average period of 4 days (96
hours) (Quick and Roberts 1993). This particularly applies to the bottom waters where
van Ieperen (1971, cited in Quick and Roberts 1993) noted that currents were
7