BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

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Intensity: This predicts whether the impact is destructive or innocuous and is described as low (where environmental functions and/or processes are negligibly altered), medium (where the environment continues to function but in a modified manner), or high (where environmental functions and/or processes are severely altered). Probability: Here the likelihood of the impact occurring is assessed as improbable (90% chance that the impact will occur). The status of the identified impacts and the degree of confidence in the predictions are also given as follows: Status of impact: Classification of the impact as positive (a benefit), negative (a cost), or neutral. Confidence: The degree of confidence that can be invested in the predictions based on the available information and specialist knowledge. When impacts are identified as negative with a significance rating of medium or higher, or precaution should be applied, mitigation objectives are set (i.e. ways of reducing negative impacts), and attainable mitigation actions recommended. In the case where mitigation is not feasible or necessary, this will be stated as such. Monitoring procedures and review programmes to assess the effectiveness of mitigation are recommended. 6.2 Assessment of potential environmental Impacts In this assessment the risk factors and associated effects listed in section 4 are matched against the environmental attributes and features described in section 5. This allows identification of potential impacts and their evaluation according to the criteria set out in section 6.1. The potential impacts are partitioned by location where the effects (stressors) originate, i.e. the dredge area within Ben Schoeman Dock, the candidate dredge spoil disposal sites, and adjacent environments. The impacts are further divided by activity; i.e. dredging and blasting in the dredge area and spoil dumping at the disposal sites. 6.2.1 Harbour dredge area: dredging 6.2.1.1 Removal of biological communities in the dredge target areas During the proposed dredging approximately 1.2 million cubic metres of sediment will be dredged from the port. These sediments will have marine biota associated with them which will be removed along with the dredge spoil. There is a marked gradient within the port in biofouling organisms on harbour structures, biomass declining rapidly with distance from the harbour entrance. There are no reported surveys on spatial distributions of benthos in the sediments but spot sampling in Duncan Dock, Victoria Basin and Alfred Dock showed a complete absence of fauna (CMS 1995a). This was attributed to anoxic sediments and hydrocarbon pollution. Such a severe situation is not expected for Ben Schoeman Dock Basin sediments, as indicated by AVS and hydrocarbon concentrations, but it is likely that the soft sediment benthos is impoverished. 55
Accordingly the potential environmental impact is assessed as: Nature of impact –Sediments and their associated organisms will be physically removed from the seabed along the path of the dredge head during dredging Extent – Local, the affected area will be ~ 0.8 km 2 , and totally contained within the harbour. Duration – Unknown, recolonization can be rapid (1-3 years, Ellis 1996, Newell et al. 1998) but other stress agents unconnected with the proposed dredging can affect this Intensity – Medium, the majority of the benthic organisms is likely to die or be removed from the dredge areas but overall numbers should be low Probability – Definite Status of impact – Negative Degree of confidence – High Significance – Very Low, due to the local extent of the impact, the fact that it will occur in an 'artificial' habitat, that the area is probably already impoverished, and that there is no demonstrable ecological dependency of Table Bay on the port. Mitigation – Not considered to be necessary (or feasible) Impact Assessment Table No Mitigation Extent Intensity Duration Consequence Probability Significance Status Confidence Local Medium Shortterm Very Low Definite VERY LOW -ve High 1 2 4 1 Mitigation n/a n/a n/a n/a n/a n/a n/a n/a 6.2.1.2 Effects of turbid plumes generated by dredging on organisms inhabiting harbour sediments and structures Dredging activities characteristically generate turbid plumes of suspended sediment. High suspended solid concentrations can exert deleterious effects on organisms through light attenuation (phytoplankton and algae), interference with filter feeding (zooplankton, mussels, oysters, barnacles), damage to gills and respiratory processes (mussels, oysters, abalone, fish), reduction of visibility reducing foraging success (cormorants, penguins, predatory fish) but also assisting predator avoidance in fish (Clark 1997a), etc. EMBECOM (2004) reviewed the available scientific literature on the effects of varying suspended sediment concentrations on a range of biota and showed that total suspended sediment concentrations
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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
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
Page 59 and 60: 2 Cluster Dendrogram and MDS ordina
Page 61 and 62: the well developed ripples in the c
Page 63: 6 IMPACT IDENTIFICATION AND ASSESSM
Page 67 and 68: Impact Assessment Table No Mitigati
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
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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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