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

State of Saldanha Bay & Langebaan Lagoon 2011
Sediments
the construction of an offshore structure, such as a groyne, to change the wave and current
dynamics suspected to be underlying cause of the erosion. A positive Record of Decision (ROD) for
the construction of the gabion wall was issued by the Department of Environmental Affairs and
Development Planning, Western Cape (DEADP), and construction was initiated during 2010 (J. Kotze
– Langebaan Ratepayers Association, pers. comm. 2011).
A study undertaken (WSP 2010) to investigate the methodology involved in determining
setback lines within the coastal zone, demonstrated that the recommended erosion setback line for
Paradise Beach is well behind the first line of properties on the beachfront. They calculated the total
setback line by estimating short term erosion, the erosion distance predicted to occur in 100 years
and the predicted sea-level rise (distance calculated by assuming a 1 metre vertical rise in sea level
together with average beach slope). Using five points of reference on the beach, the total setback
distance recommended ranged from 92 m to 120 m from the high water mark.
5.2 Monitoring of sediment particle size composition in the Bay
The particle size composition of the sediments occurring Saldanha Bay and Langebaan Lagoon are
strongly influenced by the wave energy and current circulation patterns in the system. Courser or
heavier sand and gravel particles are found in areas with high wave energy and strong currents as
the movement of water in these areas suspends fine particles (mud and silt) and flushes these out of
the area. Disturbances to the wave action and current patterns which reduce the movement of
water can result in the deposition of mud in some areas. Since 1975, industrial developments in
Saldanha Bay (Marcus Island causeway, iron ore terminal, multi-purpose Terminal and establishment
of a yacht harbour) have resulted in some level of obstruction to the natural patterns of wave action
and current circulation prevailing in the Bay. The extent to which changes in wave exposure and
current patterns has impacted on sediment deposition and consequently on benthic macrofauna
(animals living in the sediments), has been an issue of concern for many years. The quantity and
distribution of different sediment grain particle sizes (gravel, sand and mud) through Saldanha Bay
prescribes the status of biological communities and the extent of possible organic loading that may
occur in Saldanha Bay.
Contaminants, such as metals and organic toxic pollutants, are predominantly associated
with fine sediment particles (mud or cohesive sediments). This is due to the fact that fine grained
particles have a larger surface area for the adsorption and binding of pollutants. Higher proportions
of mud, relative to sand or gravel, can thus lead to high organic loading and trace metal
contamination. It follows then that with a disturbance to natural wave action and current patterns,
an increase in the proportion of mud in the sediments of Saldanha Bay, could result in higher organic
loading and dangerous levels of metals occurring (assuming that these pollutants continue to be
introduced to the system). Furthermore disturbance to the sediment (e.g. dredging) can lead to resuspension
of the mud component from underlying sediments, along with the associated organic
pollutants and metals. It may take several months or years following a dredging event before the
mud component that has settled on surface layers is scoured out of the Bay by prevailing wave and
tidal action. Changes in sediment particle size in Saldanha Bay are of particular interest here and are
summarised in this section.
5.2.1 Historical data
The earliest studies reporting on the sediments of Saldanha Bay and Langebaan Lagoon were
conducted by Flemming (1977) prior to large scale development of the area. Flemming (1977),
however, did not report on the distribution of the mud component of the sediments in Saldanha Bay
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