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

State of Saldanha Bay & Langebaan Lagoon 2011
Sediments
and Langebaan Lagoon as, at that time, they were considered to have an “overall low content”. The
mud component in Saldanha Bay prior to development (1977) was thus considered to be negligible
and the sediments comprised predominantly sand particles (size range from 1 mm to 60 µm, Figure
5.11).
Due to concern in the deteriorating water quality in Saldanha Bay, however, sediment
samples were collected again in 1989 and 1990, these data are presented in this report (Jackson and
McGibbon 1991). At the time of the Jackson and McGibbon study, the iron ore terminal had been
established dividing the Bay into Small Bay and Big Bay, the multi-purpose quay had been added to
the ore terminal, various holiday complexes had been established on the periphery of the Bay and
the mariculture industry had begun farming mussels in the sheltered waters of Small Bay. The 1989
and 1990 studies revealed that sediments occurring in both Small Bay and Big Bay were still primarily
comprised of sand particles but that mud now made up a noticeable, albeit small, component at
most sites in the Bay (Figure 5.11). The Jackson and McGibbon (1991) study concluded that an
increase in organic loading in the Bay had indeed occurred although this was not strongly reflected
in the sediment analysis conducted at the time.
The next study on sediment particle size in Saldanha Bay occurred nearly a decade later, in
1999. However, immediately preceding this (in 1997/98) an extensive area adjacent to the ore
terminal was dredged (indicated by arrows in Figure 5.11), resulting in a massive disturbance to the
sediments of the Bay. The 1999 study clearly shows a substantial increase in the percentage of mud
particles making up the sediment composition, specifically at the Multi-purpose Quay, Channel end
of the ore terminal, the Yacht Club Basin and the Mussel Farm area (Figure 5.11). Two sites least
affected by the dredging event were the North Channel site in Small Bay and the site in Big Bay. The
North Channel site is located in shallow water where the influence of strong wave action and current
velocities are expected to have facilitated in flushing out the fine sediment particles (mud) that are
likely to have arisen from dredging activities. Big Bay remained largely unaffected by the dredging
event that occurred in Small Bay and is presumably mediated to some extent by the scouring action
of oceanic waves prevalent at this site.
Subsequent studies conducted in 2000 and 2001 indicated that the mud content of the
sediment remained high but that there was an unexplained influx of coarse sediment (gravel) in
2000 followed by what appears to be some recovery over the 1999 situation. The 2000 results are
somewhat anomalous and may be related to an unidentified processing error that arose when the
samples were analysed. Sampling conducted in 2004 shows almost complete recovery of sediments
over the 1999 situation to a majority percentage of sand in five of the six sites examined for this
report (Figure 5.11). The only site where a substantial mud component remains is at the Multipurpose
Quay. The shipping channel adjacent to the Quay is the deepest section of Small Bay
(artificially maintained to allow passage of vessels) and is expected to concentrate the denser
(heavier) mud component of sediment occurring in the Bay.
The survey conducted in 2008 revealed that there had been an increase in the percentage of
mud at all sites, most notably in the Yacht Club Basin and at the Multi-purpose Quay. This was
probably due the maintenance dredging that took place at the Mossgas and multipurpose quays at
the end of 2007/beginning of 2008 (see §3.3.1). The Yacht Club basin and the Small Bay side of the
Multi-purpose quay are sheltered sites with reduced wave energy and are subject to long term
deposition of fine grained particles. The 2008 benthic macrofauna survey revealed that benthic
health at both the Yacht Club basin and adjacent to the Multi-purpose Quay is severely
compromised, with benthic organisms being virtually absent from the former (see (see §7.2– BMF
for more details on this).
Smaller dredging programmes were also undertaken in the Bay 2009/10, when 7 300 m 3 of
material was removed from an area of approximately 3 000 m 2 between Caisson 3 and 4 near the
base of the Iron ore terminal on the Saldanha side, and a 275 m 2 area in Salamander Bay was
91
ANCHOR
e n v i r o n m en t a l