BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

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EXECUTIVE SUMMARY This specialist study assessed the potential environmental impacts of the proposed dredging and dredge spoil dumping on the marine ecology of the Table Bay region. The assessment was based on: o A project description provided by the proponent (<strong>Transnet</strong>) o The available scientific and technical literature describing the ecology of the region o o o Detailed sediment property measurements in the proposed dredge area A detailed survey of sediment properties and benthic macrofauna distributions in two candidate dredge spoil dump sites identified as part of this study in outer Table Bay, and Simulation modelling predictions of dredging induced turbidity plume behaviour provided by Van Ballegooyen et al. (2006). The study identified 25 separate impacts and rated these according to guidelines provided by SRK Consulting, the lead consultant for the study. The significance of 21 of the 25 impacts was rated as very low or insignificant and therefore did not warrant mitigation. Of the remaining four rated at low, medium or higher significance levels two were reduced to insignificant through mitigation, the other two remaining at medium significance levels. By definition these latter two should influence the decision regarding the proposed dredging and spoil disposal activities. The impacts rated at low, medium or high significance pre-mitigation were: o Importation of alien species to the harbour area by dredgers and associated ecological effects: rated as highly significant o Discharge of dredge spoil to candidate dredge spoil disposal site #2 and effects on benthos: rated as medium significance, o Discharge of dredge spoil to candidate dredge spoil disposal site #2 compromising water quality in the Table Mountain MPA and the Robben Island exclusion zone with deleterious effects on biota: rated as low significance, and o Release of alien species at the dredge spoil disposal site(s) and their establishment in Table Bay: rated as medium significance. The mitigation recommended for the potential impacts related to the importation of alien species into the dredge area or dredge spoil disposal site(s) cannot totally remove the risk although it can reduce these to a large degree. However, even at a low risk the consequences of the establishment of alien species are serious if they become invasive. Therefore, in these cases, mitigation reduces the probability of the risk occurring but not the magnitude of the potential consequences should the risk be realised. The mitigation recommended for the impacts of dredge spoil on the benthos of candidate dredge spoil disposal site #2 and possible effects in the Table Mountain MPA and the Robben Island exclusion zone is not to use this site for spoil dumping. This obviously reduces the significance rating of these impacts to insignificant. Dredge spoil dumping at dump site #1 from Cutter Suction dredger operations in winter may also generate adverse effects in the Table Mountain MPA, but these were rated as being insignificant, due to the small area that may be impacted. It is apparent that these effects may be mitigated by ensuring that the Cutter Suction phase of the dredging is conducted in the summer months. v
Another important issue is the possible effects of the proposed project on African penguins foraging in Table Bay. The evaluation of the predicted effects yielded a low significance rating but public perceptions merit serious attention being given to efforts to ameliorate any possible impacts. The need for these should be assessed from further simulation modelling analyses of the life cycles and distributions of dredge spoil dump site surface layer turbidity plumes throughout the various phases of the dredging programme. This may lead to mitigation solutions based on timing of the various phases of dredging and spoil dumping. These refinements could not be identified in the current assessments due to the robust but coarse approach of using 'worst case' scenarios and the unavailability of plume dissipation rate estimates at the time of compilation. This assessment was founded on known and predicted behaviour of dredging induced plumes, proven toxicity concentration limits, applicable water and sediment quality guidelines and measured distributions of key elements of the environment in the dredge and candidate dredge spoil dump sites. Basic to the conclusions reached on potential toxicity effects were measurements of potential remobilisation of trace metal contaminants in the sediments targeted for dredging. The remobilisation factors determined here compare with those inferable from international studies on contaminated sediments. However, surety would be improved with further elutriation tests on the targeted sediments to bolster the information available on this issue. The model simulations predicted that the dredging induced turbidity plumes would be contained within Ben Schoeman Dock and not impinge on the beneficial use areas of the V&A waterfront, or escape into the wider Table Bay. The former should be monitored through filter performance levels at the Two Oceans Aquarium and the latter through real time monitoring (instrumented buoy and telecommunication system) located near the entrance to the harbour. The buoy system would also allow real time control of the dredging operations in terms of limiting exceedances of suspended sediment concentrations above levels identified as being possibly deleterious to marine life. DEAT/MCM operate a mussel watch programme that incorporates sampling points around the Port of Cape Town. Sampling intervals in this programme have been and should be six months. During the dredging period requests should be made for monthly coverage to show short term effects, if any, of remobilised contaminants in filter feeders. This monitoring can be augmented by suspension of mussels adjacent to the selected dredge spoil dump site to confirm that released or remobilised contaminants are below any level of concern. Finally, the survey data gained on the candidate dump sites in this study needs to be utilised in tracking changes associated with dredge spoil dumping over a realistic time span to show rates of benthos recovery and provide information on dumped dredge spoil behaviour. Opportunities for this have not been created in the past and the baseline data set gained in this study will be an invaluable tool in understanding the effects and implications of the marine disposal of dredge spoil on South Africa's inner continental shelf. vi
Page 1 and 2: BEN SCHOEMAN DOCK BERTH DEEPENING S
Page 3 and 4: the test organisms. The EC 5 is usu
Page 5: Turbidity Upwelling Vulnerable Wrac
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 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
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2 Cluster Dendrogram and MDS ordina
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the well developed ripples in the c
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6 IMPACT IDENTIFICATION AND ASSESSM
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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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BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

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