BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

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 (Transnet)
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.
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