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

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State of Saldanha Bay & Langebaan Lagoon 2011 Sediments dredged to accommodate an expanded the SANDF Boat park. The former programme seems to have had a minimal impact of the Bay while the latter was potential more significant and is discussed in detail in §5.2.2 and 5.4.2. The percentage mud in the Bay sediments was reduced at most sites in Small Bay between 2008 and 2009, between 2009 and 2010, and again between 2010 and 2011. This bay-wide progressive reduction in mud content suggests a shift in the balance between the rate at which fine sediments are suspended and deposited and the rate at which currents and wave activities flush fine sediments from the Bay. Unfortunately no early historical data is available for grain size distribution in Langebaan Lagoon, and only the recent results from the 2004, 2008, 2009, 2010 and 2011 surveys could be included in this report. During these surveys, the sediments in Langebaan Lagoon were principally composed of medium to fine grained sands with a very small percentage of mud. This is most likely due to the strong tidal currents experienced in the Lagoon. In summary, the natural, pre-development state of sediment in Saldanha Bay comprised predominantly sand particles; however, developments and activities in the bay (causeway, ore terminal, Yacht Club Harbour and mussel rafts) reduced the overall wave energy and altered the current circulation patterns. This compromised the capacity of the system to flush the bay of fine particles and led to the progressive accumulation of mud (cohesive sediment) in surface sediments in the Bay, followed in more decent times by a reduction in the mud fraction. Dredge events, which re-suspended large amounts of mud from the deeper lying sediments, seem to be a dominant contributor to the elevated mud content in the Bay and results of surveys have shown a general pattern of an increase in mud content following dredge events followed by a recovery in subsequent years. 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 § 5.4 for more details on this). 5.2.2 Sediment Particle size results for 2011 Sediment samples were collected from a total of 30 sites in April 2011 to be tested for particle size composition, particulate organic carbon and nitrogen and trace metals. Ten of the sites were in Small Bay, seven in Big Bay, two in Donkergat, two in Salamander Bay and nine in Langebaan Lagoon (Figure 5.12). Results from the 2011 survey are presented in Figure 5.14 and Table 5.1. These results indicate that fine muddy sediments made up a small proportion of the particle size composition throughout the Bay and Lagoon in 2011. Areas prone to the accumulation of muddy sediments include the Yacht Club Basin, which is a very sheltered site, and deeper sites along the ore terminal and in the middle of Big Bay. These spatial variations are most likely a reflection of the flushing capacity at these sites. Flushing of sediments is influenced by the current strength and the depth at the sites. Langebaan Lagoon has extremely low to negligible mud content in the sediments throughout. The deposition of fine grained particles in Langebaan Lagoon is most likely prevented by the strong tidal currents and the shallow nature of the Lagoon. A comparison to a similar image based on the results from the 2010 survey (Figure 5.13) reveals that the percentage composition of mud in the sediments has reduced to varying degrees at all sites throughout the system between 2010 and 2011. This result is an indication of the ongoing recovery of the system since the last dredge event. Over time more and more fine sediment is continuously being removed from the system. It is likely that the alteration to the current patterns and wave energy caused by the infrastructure in Saldanha Bay has reduced the rate of recovery at certain positions in the system. This recovery rate has been significantly impaired at the Yacht Club 92 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Sediments Basin where the rate of removal of fine sediments is possibly lower than that experienced at other sites in Small Bay. Figure 5.11 shows a decreasing trend in the proportion of fine sediments at five Small Bay Sites and one Big Bay site between 1999 and 2004 (dredge events in 1997/8) and again between 2008 and 2011 (dredge events in 2007/8 and 2009/10), which further supports the premise that dredging events are the primary contributor of fine sediments in Saldanha Bay. 93 ANCHOR e n v i r o n m en t a l
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Afrolittorina knysnaensis (no/0.5 m
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Similarity Iron Ore Jetty Iron Ore
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Lynch Point State of Saldanha Bay &
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Number of species State of Saldanha
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