ESTUARIES IN SOUTH AFRICA

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BOT RIVER ESTUARY: Hydrodynamics and mouth dynamics

	Hydrodynamics
	The more important indicator is the calculation of the intertidal area, which is of great interest to ecologists to determine the extent of the available habitat to various (especially avian) life forms (Branch et al., 1985). The intertidal area of the estuary in an open state varies between 40 ha and 50 ha depending on the stage in the neap spring cycle and the duration since the last breaching. Continuous waterlevel recordings by DWAF show the estuary on average has a tidal amplitude of between 15 and 25 cm - typical for an estuarine lake with a large surface area and relatively small sea inlet. (Ref 1)
	The hydraulic behaviour of the Botriviervlei is governed byt he seasonal character of the feeder-rivers and by the overflow from the main vlei to the Kleinmond estuary via Die Keel, Rooisand and the Lamloch swamps or the dune hinterland channel.(Ref 2)
	During the winter floods the vlei is filled up to a very high water level which causes a freshening of the water, reduction in the flocculating effect of saline water on mud-laden fresh water, a high silt load, and a change from a saline to a brackish or even freshwater lake. Flooding of the low lying properties and erosion of banks are other effects of high water levels. (Ref 2)
	The overflow into the Kleinmond estuary provides only a very restricted contact with the sea with the advantage that the vlei cannot discharge its suspended mud load or obtain seawater by means of tidal exchange. The recruitment of marine fishes is also impeded by this limited connection. (Ref 2)
	Hydraulically, the overflow system acts as a one-way safety valve which allows excess floodwaters to escape from the Botriviervlei via the Kleinmond system, but prevents the vlei from breaking open to the sea by natural means. This situation is aggravated by the extraction of water in the catchment for agriculture and forestry and by dune consolidation close to the potential breaching site at Sonesta. (Ref 2)
	Mouth dynamics
	The Bot Estuary mouth has in recent years been mostly closed (or “blind”), since it is cut off from the sea by a berm and an adjacent belt of coastal dunes. The valleys between the dunes are sufficiently low in some places to permit occasional wave overtopping from the sea during exceptionally high tides. The coastal dune belt is shifting landward due to the deliberate stabilisation projects through the introduction of mainly Acacia cyclops since the 1940s (Bally, 1985). (Ref 1)
	At present the Bot Estuary is breached artificially approximately every two years. It was thought that it breached naturally only three times since 1940 (Bally and Branch, 1986). The Bot Estuary is normally breached at Meerensee (previously Sonesta) to create a tidal mouth between 80 and 110 m wide and -2.0 m to -2.5 m MSL deep. The estimates of the rate of outflow during breaching vary between 254 mł/s and 409 mł/s (Fromme, 1985a). The water level in the estuary ranges from around +2.7 m MSL before breaching to just above or near MSL after the event (Koop, 1982). (Ref 1)
	The characteristics and dynamics of a river mouth are closely related to the topography and geomorphology of the coast and to the wave and current regime of the nearshore and offshore zone. (Ref 2)
	Historically, the estuary was open to the sea but since the mid-1800s a large dune barrier dammed the mouth, near Sonesta, and the estuary changed to a semi-permanent closed system. Natural breachings take place sporadically when heavy rainfall in the catchment raises water levels in the estuary breaking the dune barrier and opening the mouth to the sea. Artficial breachings also occur and have resulted mainly from local fisherman wanting to replenish marine fish stocks. These artificial breachings, which also occur at the Kleinmond estuary, occur sporadically and result in dramatic drop in water levels. Studies (Heyl & Currie 1985; Harebottle & Delport 2000) have shown that opening the mouth too frequently, and at the wrong time, can have significant impacts on waterbird richness and abundance and may have detrimental effects on the reproductive success of those waterbirds breeding at the estuary. (Ref 3)