ESTUARIES IN SOUTH AFRICA

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GAMTOOS ESTUARY: Hydrodynamics and mouth dynamics

	Hydrodynamics
	The south-westward flowing Agulhas current which flows south of the -200m isobath is the major offshore oceanic water movement off the coast opposite the Gamtoos Estuary. However, south of Algoa Bay which lies to the east of the Gamtoos, the Agulhas current swings away from the coast resulting in a zone which is then subjected to mainly tidal, wind and swell driven currents (Dr J Lutjehars, pers.comm.). Close inshore the predominant swell direction from the SW causes longshore currents which flow in a northeasterly direction. In bays such as St Francis Bay, within which the Gamtoos Estuary is situated, local eddies can cause longshore sediment transport towards the deep curve of the bay i.e in a westward direction in the case of St Francis Bay (Heydorn and Tinley 1980). (Ref 1)


	Mouth dynamics
	The mouth of the Gamtoos River is situated in a dynamic environment of bare transverse coastal sand dunes. The mouth is therefore highly susceptible to the combined effects of the predominant wind, wave and longshore current regimes. According to Heydorn and Tinley (1980) the predominant wind direction at Port Elizabeth, 60km to the east of Gamtoos, is SW and WSW and to a lesser degree E and ESE in January, while in July the wind blows mainly from the west. (Ref 1)
	Furthermore, they state that the dominant wave directions on the south coast are SSW to SSE. Under the influence of all these forces, the mouth of the Gamtoos River tends to move eastwards but at times of flood it breaks through in the west. The rate that the mouth moves eastwards will vary according to the intensity and duration of the combined effects of the wind, waves and longshore currents. (Ref 1)
	Reference to a series of aerial photographs taken from 1971 to 1980, indicate that initially the mouth moved eastwards and subsequently slightly westwards to its position at the time of the ECRU survey. Throughout the above-mentioned period, the main channel at the mouth has been offset to the west with a flat shallow sandbar on the eastern side of the mouth. A possible reason for the offset tidal inlet is that a delta on the eastern side formed by sediment brought down by the river, retracts the waves to cause a local transport direction from east to west (Dr H Swart pers. comm.). (Ref 1)