ESTUARIES IN SOUTH AFRICA

Main Page	General Information	Catchment Characteristics	Land and Estuary Uses	Ecology
Threats	References	Water Quality	Hydrodynamics and mouth dynamics	Climate
GROOT BRAK ESTUARY: Physical Features

	Topography
	The Great Brak river rises on the slopes of the Engelsberg in the Outeniqua Mountain range 25km in a straight line from the river mouth. The catchment is relatively long and narrow being about 25km long and reaching a maximum of about 8 km in width. Much of the river system drains an elevated coastal platform 150 - 300m high. (Ref 1)
	Bathymetry
	The area between The Island and the bridges is relatively shallow i.e. 200mm during low water and about 800mm during high spring tides. (Ref 1)
	Obstructions
	There are five bridges across the estuary of the Great Brak River. That nearest to the mouth is a narrow single lane road bridge giving access to The Island. This bridge obstructs water flow in the northern channel: "Unfortunately when the bridge was built an inadequate number of spans was designed and a large amount of fill was dumped into the river which directly influenced the flow of water and resulted in the main channel changing its course" (WMS Franklin, 1979, in litt.). In times of flood the river opens the southern channel and discharges directly into the sea. When the river flow returns to normal, sand transported by longshore currents and waves closes the southern channel, and the river flow reverts to the northern channel. However, the road bridge to The Island has attenuated the flow in the northern channel with the result that sand, blown in from the dunes on the bluff to the east, is no longer adequately scoured from the channel. (Ref 1).
	The effect of this obstruction by the bridge and consequent shallowing of the northern channel is that the southern channel remains open for much longer than it did in the past (WMS Franklin, 1979, in litt.). The natural tendency is for the southern channel to migrate northwards towards The Island. The channel remains open to the sea close to the south side of The Island, and thus exposes The Island to wave attack. (Ref 1).
	The railway bridge, the old National Road bridge and the new paired freeway bridge all cross the estuary relatively close to one another. In addition, the pylons of the old railway bridge still stand between the new railway bridge and the old National Road Bridge. The embankments of the new freeway have drecreased the capacity of the floodplain to absorb floodwater and have blocked a flood dissipation route over the old National Road. All floodwater is now forced to pass under three bridges before reaching the lower estuary. The railway bridge, has very little clearance at high spring tides when the mouth is open. The flood situation could be further aggravated if floodwaters were to reach the pole yard, as there is a possibility that poles could be carried downstream and form a dam at the three bridges which could cause backflooding of the whole floodplain. (Hill Kaplan Scott, 1974; Stauth, 1982). (Ref 1)
	The fifth bridge is situated 4,8 km upstream from the mouth and links the eastern and western parts of Great Brak River village. The bridge is near the limit of tidal effect in the estuary. This bridge, the Charles Searle bridge, was built in 1964 to replace the causeway constructed in 1850 (Frankilin, 1979). The original 1850 causeway was designed with floating decks so that it would be passable under flood conditions. However, the mechanism never functioned as designed (Frankilin, 1979). The Charles Searle Bridge was constructed on the original 13 piers of the causeway and there has therefore been no reduction in the degree of obstruction to the river flow although Sales (1977) states that the bridge has little effect in reducing river flow. (Ref 1)