ESTUARIES IN SOUTH AFRICA

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SWARTVLEI ESTUARY: Hydrodynamics and mouth dynamics

	Hydrodynamics
	If open the tidal inlet causes a considerable damping of the sea tide (extreme range 1,98m). Only 0,5km upstream of the mouth the tidal range is reduced by one third, while the tidal lag (with outgoing tide) is two hours. At the upper end of the estuary the tidal range is reduced by 90 percent and upstream of the SAR Bridge by as much as 99 percent, while the tidal lag between in and outgoing tides amounts to six and a half hours. (Ref 1)
	There are only two places at which resistance to water flow is high, that is, the narrow estuary mouth and at the rail bridge. (Ref 1)
	During neap tides there is virtually no inflow from the sea. During spring high tides the upper estuary is filled to a high water level which causes strong outflow during spring low water. Because of the pan-like bathymetry of the upper estuary and the relatively narrow estuary channel this high water level remains while the sea tide is already receding. Even a backflow to the lake was observed under this condition. In the lake the water level varies by 0,2m between spring tide and neap tide, and in the lake and upper estuary the low water level during neap tides is lower than at the low water springs (Howard-Williams and Allanson). The strong outflow conditions during falling spring tide were confirmed by current measurements in the mouth by Moes in May 1976 (Hidrouliese studie van Swartvlei estuarim, 1978) and during the ECRU – survey on 7 August 1983. During the latter survey outflow velocities of up to 1,67m/s versus inflow velocities of 0,8m/s were measured after heavy rainfalls. During the former survey outflow velocities of 1,4m/s and inflow velocities of 1,2m/s were recorded. The total evaporation from Swartvlei is 1 127mm per year which at the normal lake level of 0,65 m above GSL, is equivalent to a total volume lost from the lake and estuary combined of 12,1x10⁶m³. Inflow from the catchment is five times this volume, so at present it is unlikely that Swartvlei will ever become hypersaline, this is however dependent on future water abstraction from the catchment area. Swartvlei is normally a meromictic lake ie a body of water in which vertical mixing is restricted by differences in density. In Swartvlei this is caused by differences in salinity and this has a marked effect on the fate of inflowing river water. River inflows have a low density and during periods when inflow is strong, flood water moves over the top of the lake in a layer 0,2-0,5m thick. If there are not strong winds during this period, surface fresh water will extend across the lake to flow out under the railway bridge and very little mixing with the lake water will occur. However, there is likely to be appreciable mixing caused by wind, during the low inflow periods, especially when the mouth is closed. (Ref 1)
	Mouth dynamics
	According to an eleven your old record of mouth conditions the estuary inlet is open about 50 percent of the time. The mouth is almost always closed during winter ie 97 percent of the time, whereas it is closed only about 30 to 40 percent of the time during the other seasons. (Ref 1)
	Mouth closures are, however, not associated with seasonal floods because heavy rainfalls can occur in this area at any time of the year. The predominance of mouth closures in winter appears to be caused mainly by the rough south-westerly wave conditions which occur during this season. During the last few decades artificial breechings of the mouth have been carried out at water levels of MSL + 1,5 to 2,25m to prevent flooding of low lying developments. (Ref 1)