ESTUARIES IN SOUTH AFRICA

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GROEN RIVER ESTUARY: Hydrodynamics and mouth dynamics

	Hydrodynamics
	The Namaqualand coastline is under the influence of the northward- flowing Benguela current and the characteristics upwelling of cold nutrient-rich water of sub-Antarctic and South Atlantic origin. Upwelling is enhanced during summer, by the prevalence of southerly to south-easterly winds, which tend to move inshore surface water away from the coast, thereby making room for the deeper-lying cooler water (Heydorn and Tinley 1980). (Ref 1)
	The influence of the Benguela Current, combined with predominantly southerly winds, is largely responsible for the cool climate at the coast and the prevalence of fog during the nights and in the mornings. (Ref 1)
	The prevailing swells are generated in the South Atlantic and approach the West Coast, predominantly from a south-westerly direction. They reach a height of over 1,6m for 50 percent of the time in the nearshore area, as measured at Oranjemund, approximately 280km to the north of the Groen (J. Rossouw pers.comm.). Wave frequency at Buchu Bay, approximately 253 km to the north of the Groen is 10-15 seconds for 95 percent of the waves (Ashby, Harper and Van Schaik 1973). (Ref 1)
	The wave and swell patterns of the West Coast play a major role in the longshore distribution of marine sediments, erosion and deposition phenomena and hence also in beach and dune formation (Tankard and Rogers 1978). (Ref 1)
	At the time of the ECRU survey, there were several rotating cells of water in the arcuate bay at the mouth. These cells gave rise to drift currents moving northwards and southwards from the middle of the bay close and parallel to the beach. These drifts fed into rip currents moving seawards past the rocky promontories on the northern and southern sides of the bay as well as on either side of the reef in the centre of the bay situated about 200 m off the beach. (Ref 1)
	Mouth dynamics
	The mouth of the Groen is situated approximately 31km to the south of the Bitter River mouth, 66km south of Hondeklipbaai and 267km to the south of the Orange River mouth. (Ref 1)