BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

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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
undetectable in 80% of the measurements made over an annual cycle. This contrasts with recent estimates of comparatively short residence times for surficial sediments in Table Bay (above). It would therefore appear that the main drivers for sediment turnover are episodic winter storms (cf Hill et al. 1994), which probably also completely flush the water body of the bay. 3735000 Stormwater Outfall Small Stormwater Outfalls Industrial & Sewage Outfalls 3740000 Bloubergstrand Robben Island Blouberg Rocks Table View 3745000 Milnerton 3750000 Diep River 3755000 Mouille Point Green Point Sea Point Cape Town Harbour Salt River Clifton Beach -45 -50 Camps Bay 3760000 65000 60000 0 km 2.5 km 5 km 55000 50000 45000 Figure 1: Table Bay bathymetry, outfall pipelines and storm water discharges. 8
Page 1 and 2: BEN SCHOEMAN DOCK BERTH DEEPENING S
Page 3 and 4: the test organisms. The EC 5 is usu
Page 5 and 6: Turbidity Upwelling Vulnerable Wrac
Page 7 and 8: Another important issue is the poss
Page 9 and 10: 6.2.1.3 Settlement of material susp
Page 11 and 12: 2 METHODOLOGY For the assessment tw
Page 13 and 14: The proposed deepening of the Ben S
Page 15: 4.3 Cumulative effects Table Bay re
Page 19 and 20: 5.1.3 Biological communities The ma
Page 21 and 22: Supralittoral fringe (Littorina zon
Page 23 and 24: Niambia sp. Coleoptera Diptera Tylo
Page 25 and 26: Midlittoral zone - The intertidal o
Page 27 and 28: Fish - Nearshore and in the sandy b
Page 29 and 30: 2000 at Robben Island revealed furt
Page 31 and 32: Table 2: Common whales and dolphins
Page 33 and 34: Table 3: Site, time, mean count (co
Page 35 and 36: 150000 125000 100000 Kg 75000 50000
Page 37 and 38: sensitive because of its conservati
Page 39 and 40: Schoeman Dock occurred in 1971 (Pro
Page 41 and 42: Table 7: Hydrocarbon distributions
Page 43 and 44: care concentration ranges for cadmi
Page 45 and 46: SS mg/l Boxplot for TSS 80 60 40 20
Page 47 and 48: The mean values calculated above ar
Page 49 and 50: o o possibility of the dredge spoil
Page 51 and 52: The gravel sized component of the s
Page 53 and 54: Table 12a: London Convention Annex
Page 55 and 56: Benthos distributions in Block #1 -
Page 57 and 58: 40 40 30 Site 1A 30 Site 1B % Speci
Page 59 and 60: 2 Cluster Dendrogram and MDS ordina
Page 61 and 62: the well developed ripples in the c
Page 63 and 64: 6 IMPACT IDENTIFICATION AND ASSESSM
Page 65 and 66: Accordingly the potential environme
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Impact Assessment Table No Mitigati
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are summarised in the table and it
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Figure 21b: Predicted number of day
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considered to be low. Further, the
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A A B B B Figure 23a and b: Predict
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dependent on these include the harm
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structures. The apparent lack of su
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Diving birds such as cape cormorant
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Figure 24: The predicted worst case
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Figure 25 depicts the simulation mo
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6.2.3.2 Alteration of benthic biolo
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Impact Assessment Table No Mitigati
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Nature of impact - Turbid plumes of
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Figure 26b: Predicted worst case ex
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Figure 27b: Predicted worst case ex
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suspended sediment concentrations i
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Figure 28a: Dredge spoil dump site
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6.2.3.6 Elevated water column turbi
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The evaluation with mitigation is:
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Figure 29a: Predicted exceedance in
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6.3 Cumulative Impacts The proposed
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Impact summary Table (cont.) Impact
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Impact summary Table (cont.) Impact
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mitigated by ensuring that the Cutt
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operations at Coega Port. Prepared
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Glassom D., K. Prochazka and G.M. B
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Protekon 2006. Port of Cape Town co
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