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BOT RIVER ESTUARY: Physical Features

Topography

Bathymetry

Using standard surveying techniques (Taljaard et al., 1999), the CSIR surveyed the bathymetry of the Bot Estuary and mouth in 1999. Depth soundings by echo sounder and a differential global positioning system (D-GPS), calibrated to official survey beacons, and depth data. Survey accuracy to MSL was 0.01 m only. The survey was conducted during a period of mouth closure and in windless conditions (Vonk, 2000). (Ref 1)

There is a narrow outlet channel towards the mouth near Meerensee, where breaching is effected, as well as a shallow link or overflow channel towards the Kleinmond estuary in the west. (Ref 1)
A bathymetry survey of the Botriviervlei was completed at the end of 1980 (Willis, 1981). The bathymetric chart shows shallow water over most areas of the vlei and confined deeper areas with bed level of 2 -2,5m below MSL. (Ref 2)
The water level usually fluctuates between about MSL + 1,4m and MSL + 2,7m, although when the vlei is opened to the sea it drops to approximately MSL. (Ref 2)
It can also be seen that the vlei is generally deeper along the eastern banks and that the deep water areas are congruent with the ancient  Bot River valley.  The slope of the bed across the centre of the vlei from the western bank to the Afdaks River bay is very flat, approximately 1 : 60, or less than one degree. (Ref 2)
There are some marginal low-lying areas which are flooded during periods of high water levels.   These are the arm South East of Sonesta, and the Rooisand area which serves as a link between the main vlei and the Kleinmond estuary. (Ref 2)

Obstructions 
There are no State-constructed dams in the catchment although many farmers have their own earth dams which collect rainwater for irrigation purposes.  Occasionally water is pumped fromt he rivers int he drainage basin into the farm dams in order to stock up for the summer months.
The only dam built across a river course is at the northeastern end of the Botriviervlei where the Hopies River has been dammed up by a large earthen dam shortly before it enters the vlei. (Ref 2)
The major obstruction to the Bot River over the last seven years has occurred during the construction of the road bridge which cuts across the extensive Prgmites floodplains at the head of the estuary where the river enters the vlei. (Ref 2)

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