The nearshore dynamics of Matroos Bay : field and theoretical investigations
Master Thesis
1977
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University of Cape Town
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Abstract
Field and theoretical investigations were made of the nearshore circulation of Matroos Bay, a small embayment on the south-western coast of Africa. The field study revealed two dominant circulation types, denoted "anticlockwise" and "clockwise". The anticlockwise circulation type was characterized by longshore currents flowing from the back of the bay to the northern headland where a rip current was located. The clockwise circulation type was characterized by longshore currents flowing from the northern headland to the back of the bay where a rip current was situated. Anticlockwise circulation types were primarily found to occur during periods of low waves, the clockwise circulation during periods of high waves. Winds and coastal currents were found not to have a major influence on the circulation within the bay. The theoretical study was based on a model of longshore currents in a semi-circular bay (O'Rourke and Le Blond, 1972). This study showed that a change of the height of the wave could lead to a change in the direction of flow of the longshore current. In agreement with the field study, a low wave height was found to result in an anticlockwise circulation type, a high wave height resulting in a clockwise circulation type. The change from one circulation type to the other depended on the relative strength of two opposing mechanisms. One mechanism arose from the obliquity of the wave approach to the shore. This mechanism generated longshore currents towards the back of the bay and was dominant during high waves. The other mechanism resulted from the non-uniformity of the wave height around the bay and generated longshore currents flowing from the back of the bay. Coastal currents were measured as part of the field study. A fair correlation was-observed between the direction of the coastal current and that of the wind, particularly if the winds were moderate to strong. Temperature and salinity measurements were also made. These properties showed seasonal variations in accord with the principles of upwelling. Surface water was found to have a salinity similar to that found at a depth of 400 to 600 metres in the South Atlantic Ocean. The water was upwelled from a greater depth during the summer than during the winter. Heating of the water was found to be greater in summer than in winter whilst dilution of the water, due to rainfall, was found to be negligible, even during winter. The interchange of water between the bay and the coastal circulation was found to be dependent on the circulation type.
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Bibliography: pages 113-119.
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Reference:
Gunn, B. 1977. The nearshore dynamics of Matroos Bay : field and theoretical investigations. University of Cape Town.