BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

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area to be dredged (inferred from CMS 1995a). The biofouling community should show a similar pattern. Figure 20: Predicted worst case exceedance (days) of 20mg/l suspended sediment concentration at mid-depth in the Port of Cape Town during cutter suction dredger operations. (Winter, Cutter Suction Dredger; copied from Figure B.7.3a, Van Ballegooyen et al., 2006) In view of the predicted turbidity plume distributions and biological features the potential environmental impacts of elevated suspended sediment concentrations for the harbour area are assessed as: Nature of impact – Generation of suspended sediment plumes in the dredge area over the dredging period and potential sublethal or lethal impacts on biological organisms and/or communities inhabiting harbour sediments and structures Extent – Local, water quality modelling predicts that the thresholds defined by EMBECOM (2004) will not be exceeded at any point outside of the immediate dredge area. Duration – Short-term, potential effects should be limited to the duration of the dredging activity Intensity – Low, adverse effects are experienced generally at suspended sediment concentrations higher (>100mg/l) than those predicted for the dredging or for longer exposure periods (>3 days) than are expected to be generated. Probability – Possible, elevated suspended sediment concentrations are a typical byproduct of dredging activities and could impact the filter feeding component (mainly barnacles) of the fauna Status of impact – Negative Degree of confidence – High Significance – Very Low, due to the small extent, short duration and low intensity of the impact Mitigation – Not considered as necessary due to the very low significance. 57
Impact Assessment Table No Mitigation Extent Intensity Duration Consequence Probability Significance Status Confidence Local Low Shortterm Very Low Possible VERY LOW -ve High 1 1 3 1 Mitigation n/a n/a n/a n/a n/a n/a n/a n/a 6.2.1.3 Settlement of material suspended during dredging and alteration of sediment characteristics and effects on sediment biota and ecological processes The harbour is by design a quiet water area and it is likely that the majority of the fines in dredge hopper overwash and those suspended by dredge head turbulence will resettle in the harbour. This may lead to inundation of sediment biota adjacent to the dredge areas possibly altering community structure and/or disrupting ecological processes. As stated above the harbour sediment community is relatively impoverished and may be kept that way by pollution and physical disturbance (e.g. sediment suspension) by shipping. Hall (1994) has shown that benthic macrofauna can survive short term inundation and Maurer et al. (1980 1981, 1982) demonstrated that a bivalve mollusc (Mercenaria mercenaria) and gastropod (Nucula proxima) could migrate vertically up to 16cm when inundated with sand; the crustacean amphipod Parahaustorius longimerus and xanthid crab Neopanope sayi could migrate through 7 – 30cm and polychaetes Scoloplos fragilis and Nereis succinea could manage up to 30cm. Therefore given the generally slow sedimentation rates of fine particles (
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BEN SCHOEMAN DOCK BERTH DEEPENING S
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the test organisms. The EC 5 is usu
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Turbidity Upwelling Vulnerable Wrac
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Another important issue is the poss
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6.2.1.3 Settlement of material susp
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2 METHODOLOGY For the assessment tw
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The proposed deepening of the Ben S
Page 15 and 16: 4.3 Cumulative effects Table Bay re
Page 17 and 18: undetectable in 80% of the measurem
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: Accordingly the potential environme
Page 69 and 70: are summarised in the table and it
Page 71 and 72: Figure 21b: Predicted number of day
Page 73 and 74: considered to be low. Further, the
Page 75 and 76: A A B B B Figure 23a and b: Predict
Page 77 and 78: dependent on these include the harm
Page 79 and 80: structures. The apparent lack of su
Page 81 and 82: Diving birds such as cape cormorant
Page 83 and 84: Figure 24: The predicted worst case
Page 85 and 86: Figure 25 depicts the simulation mo
Page 87 and 88: 6.2.3.2 Alteration of benthic biolo
Page 89 and 90: Impact Assessment Table No Mitigati
Page 91 and 92: Nature of impact - Turbid plumes of
Page 93 and 94: Figure 26b: Predicted worst case ex
Page 95 and 96: Figure 27b: Predicted worst case ex
Page 97 and 98: suspended sediment concentrations i
Page 99 and 100: Figure 28a: Dredge spoil dump site
Page 101 and 102: 6.2.3.6 Elevated water column turbi
Page 103 and 104: The evaluation with mitigation is:
Page 105 and 106: Figure 29a: Predicted exceedance in
Page 107 and 108: 6.3 Cumulative Impacts The proposed
Page 109 and 110: Impact summary Table (cont.) Impact
Page 111 and 112: Impact summary Table (cont.) Impact
Page 113 and 114: mitigated by ensuring that the Cutt
Page 115 and 116: 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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