State of the Bay Report 2011-Final.pdf - Anchor Environmental

State of Saldanha Bay & Langebaan Lagoon 2011
Activities & discharges
3.3 Discharges and activities affecting environmental health
3.3.1 Dredging and port expansion
Dredging of the seabed is performed worldwide in order to expand and deepen existing
harbours/ports or to maintain navigation channels and harbour entrances (Erftemeijer and Lewis
2006), and dredging has thus been touted as one of the most common anthropogenic disturbance of
the marine environment (Bonvicini Pagliai et al. 1985). The potential impacts of dredging on the
marine environmental can stem from both the removal of substratum from the seafloor and the
disposal of dredged sediments, and include:
Direct destruction of benthic fauna populations due to substrate removal
Burial of organisms due to disposal of dredged sediments
Alterations in sediment composition which changes nature and diversity of benthic
communities (e.g. decline in species density, abundance and biomass)
Enhanced sedimentation
Changes in bathymetry which alters current velocities and wave action
Increase in concentration of suspended matter and turbidity due to suspension of
sediments. The re-suspension of sediments may give rise to:
o Decrease in water transparency
o Release in nutrients and hence eutrophication
o Release of toxic metals and hydrocarbons due to changes in physical/chemical
equilibria
o Decrease in oxygen concentrations in the water column
o Bioaccumulation of toxic pollutants
o Transport of fine sediments to adjacent areas, and hence transport of pollutants
o Decreased primary production due to decreased light penetration to water column
(Erftemeijer and Lewis 2006, Bonvicini Pagliai et al. 1985, OSPAR Commission 2004, National
Ports Authority 2007).
Aside from dredging itself, dredged material may be suspended during transport to the
surface, overflow from barges or leaking pipelines, during transport to dump sites and during
disposal of dredged material (Jensen and Mogensen 2000 in Erftemeijer and Lewis 2006).
Saldanha Bay is South Africa’s largest and deepest natural port and as a result has
undergone extensive harbour development and has been subjected to several bouts of dredging and
marine blasting. Saldanha is perfectly situated for the shipment of large quantities of iron ore from
the Sishen mines in the Northern Cape. However, before the first shipment could be loaded the port
had to be protected from strong wave activity. To remedy this, the first major development
occurred in 1973 whereby Marcus Island was joined to the mainland via the construction of a
causeway. Further development involved the construction of the General Maintenance Quay and
the Rock Quay over the period 1974 to 1976. During this process 25 million m 3 of sediment were
dredged from the Bay to facilitate the entrance of large ore carriers, and the resulting dredged
material was used to construct the harbour wall (Moldan 1978). A Multi-Purpose Terminal was
added to the iron ore terminal in 1980 and the Small Craft Harbour was built in 1984. These
developments all required extensive dredging and submarine blasting which significantly impacted
sediment composition and benthic community structure. Since this time three further dredging
operations have been implemented in Saldanha Bay.
The first of these was associated with the expansion of the Multi-Purpose Terminal in
1996/7 when 2 million m 3 of material was removed from an area approximately 500 000 m 2 in
extent on the Small Bay side of the ore terminal. The dredge spoil was disposed of on land in a
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