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

State of Saldanha Bay & Langebaan Lagoon 2011
Sediments
5.1.2.1 Construction of the Marcus Island Causeway and the Transnet Ore Terminal
The Marcus Island causeway and the Transnet Ore Terminal are similar structures from a
sedimentary point of view. They both extend far beyond the littoral zone where waves have limited
ability to influence sediment movement. They are therefore barriers separating areas where littoral
drift could occur. In the case of the Ore Terminal, this structure separates the beach to the north
west of the terminal from Spreeuwalle to the south east. As a result, Spreeuwalle has lost access to
one of its sediment sources.
The ore terminal may also have changed the wave dynamics in the area by refracting or
reflecting some of the incoming swells. This could cause the angle of wave incidence at beaches
near the pier (such as Spreeuwalle) to have changed sufficiently for the littoral drift to reverse, or
decrease significantly.
Before the construction of the Marcus Island Causeway, waves could travel on both sides of
the island. These waves would have been refracted inward around the island cause both
constructive and destructive interference. At a point opposite the island, the shoreline would have
been subject to much larger and more powerful waves. This generally causes beach slopes to form
at less steep gradients, but also tends to remove smaller sediment particles.
5.1.2.2 Stabilisation of the Geelbek dune system
The Geelbek dunes have been subjected to continuous encroachment by vegetation over the last 70
years (Gericke 2008). When dune encroachment occurs, sediment is held in place by the roots of
plants and the wind speed at ground level is considerably reduced. This means that the dune system
become less effective as a sediment source. However it does still function as a sediment sink. This
means that sand which is taken by the north-westerly wind from Langebaan’s beaches to the
Geelbek dune systems will not return as quickly when the south-easterly wind returns. This is likely
to have resulted in a net loss of sediment from the beaches to the dunes.
5.1.2.3 Shoreline development
When sea level, wind or wave conditions change, shorelines react by changing their nature, their
shape, or their position. Many of Langebaan’s residential or holiday developments have been built
in very close proximity to the sandy shorelines. These communities are all vulnerable to beach
erosion. In some places the width of the beach has been reduced by as much as 150 m, leaving the
house built on the first set of dunes unprotected against storm damage (Gericke 2008, Figure 5.1).
5.1.3 Changes in beach and dune morphology
Gericke (2008) studied changes in the sedimentary features of the Langebaan Lagoon and Saldanha
Bay area between 1960 and 2000. It was found that the beaches in particular had changed
significantly over this period, with a large section of the North Langebaan beach having completely
disappeared between 1988 and 2000.
Gericke’s (2008) study was updated this year and includes new data from 2000 to 2012
(Gericke 2012). Five rocky outcrops were selected across the study site and a change in width for
each site and for each image measured. The mean for each image taken between 1960 and 2012
was used as a proxy for tidal applitude at the time that each photo was taken. This was simply
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