State of the Bay Report 2010-Final - Anchor Environmental

State of the Bay 2010: Saldanha Bay and Langebaan Lagoon
Anchor Environmental
comprised of sand particles but that mud now made up a noticeable, albeit small, component at
most sites in the Bay (Figure 6.1). 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 jetty
was dredged (indicated by arrows in Figure 6.1), 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
jetty, the Yacht Club basin and the Mussel Farm area (Figure 6.1). 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 indicated almost complete recovery of
sediments over the 1999 situation to a majority percentage of sand in five of the six sites examined
for this study (Figure 6.1). 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 to the maintenance dredging that took place at the Mossgas and multipurpose
terminals at the end of 2007/beginning of 2008 (see §4.3.1 for more details on this). 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. There has been a
progressive increase in the mud content of sediments adjacent to the Multi-purpose Quay from
2000-2008, and this is having a noticeable effect on the health of the macrobenthic community in
this area. The 2008 benthic macrofauna survey revealed that benthic health at both the Yacht Club
basin and adjacent to the Multi-purpose Quay is alos severely compromised, with benthic organisms
being virtually absent from the former (see §8 for more details on this).
In summary, the natural, pre-development state of sediment in Saldanha Bay comprised
predominantly sand particles, however, increasing development and human impact in the Bay has
reduced the overall wave energy and altered the current circulation patterns. As a result, mud
(cohesive sediment) has been accumulating progressively in surface sediments in the Bay. Dredging
of Small Bay in 1997/98 also re-suspended large amounts of mud from the deeper lying sediments,
which then settled out in the surface sediments. These fine sediments have, however, gradually
been washed out of the Bay or have been reburied through the course of time. The situation in
2008/2009 reflected that of the reduced wave action and current velocities with slightly higher than
normal amounts of mud in the sediments. Any future dredging or other such large-scale disturbance
to the sediment in Saldanha Bay are likely to result in similar increases in the mud proportion as was
evident in 1999, with accompanying increase in metal content (refer to §6.3.2 for more details on
this).