ESTUARIES IN SOUTH AFRICA

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EERSTE RIVER ESTUARY: Hydrodynamics and mouth dynamics

	Hydrodynamics
	In 1982, when the estuary was open, it was only slightly tidal, because of its high elevation of the sill at the mouth (2m above LWOST). Originally the lagoon extended to both sides of the river onto the beach area. The eastern arm was cut off by the construction of a causeway in 1977, which left a confined stagnant water body on its eastern side. This stagnant water body developed into a marshy area. At present only the western arm of the lagoon is functioning (Ref. 1).
	During a survey on 11 May 1982, during LWST, the water flow through the meander-shaped mouth channel, was measured as 3 mł/s (Ref. 1).
	The tidal range between Mean Low Water Spring and Mean High Water Spring in False Bay is approximately 1.48 m (Ref.1.)
	Mouth dynamics
	It 1978 (Ref.3.) it was reported that the estuary may take many forms, from a more widened outlet channel to a lagoon several hectares in extent, held by a wind-and-wave-built sandbar. The mouth may be greatly out of alignment with the river or may be closed entirely except at high spring tide. The dynamics of the Eerste River mouth are governed by seasonal variations in river flow and wave energy. The incidence of relatively high wave energy causes the build-up of a high beach bar. The lagoon breaches the bar during winter rains at a site where the bar is low. Strong meandering of this overflow channel indicates a rather attenuated pattern of discharge.
	In 1980 (Ref.2.) it was reported that the Eerste River Estuary mouth is closed during summer and breaks open after the first winter rains in about May. However in 1982 (Ref.1.) the mouth appeared to remain open for most of the year. Due to inflows from numerous waste water works this open mouth condition remaons until present.