Download PDF - Anchor Environmental

3. ECOLOGICAL CHARACTERISTICS AND FUNCTIONING OF
THE ESTUARY
3.1 Mouth dynamics, hydrology and channel shape
The Uilkraals Estuary has been classified as a temporarily open-closed estuary (Whitfield 2000),
and is a mixed (in terms of salinity), blackwater system (van Niekerk unpubl. data). When
functioning naturally, the estuary has tidal exchange and a high frequency of connection to the
sea, similar to the Palmiet and Kleinmond estuaries, but is in a more advanced stage of
progressive infilling and reduction of the tidal prism (Harrison et al. 1995a). The road bridge,
which was constructed in 1973, is approximately 220 m long and crosses the river
approximately 800 m from the mouth (Figure 9 and Figure 10). It is supported on the eastern
side by a high embankment of rubble spanning almost two-thirds of the original high tide river
width (Gaigher 1984). The remaining 100 m are supported by concrete pylons, effectively
halving the width of the estuary there and concentrating the river flow against the western
bank (Heydorn & Bickerton 1982). In 1978 a 150 m long rubble and rock embankment was
built on the beach in front of the beach facing bungalows, forcing the estuary mouth
eastwards, away from the caravan park (Heydorn & Bickerton 1982). However, the
embankment was quickly eroded by wave and tidal action and a shallow, stagnant pool of
water and a series of sand dunes formed on the beach in front of the bungalows (Figure 9).
In the past the estuary mouth opened over a beach with a relatively flat profile and the open
mouth status was probably maintained by strong tidal currents (Harrison 2004). Tidal currents
play a major role in maintaining a connection with the sea in cool and warm-temperate
estuaries (unlike subtropical estuaries, where river flow is the major factor; Cooper et al. 1999,
Cooper 2001). Seasonal closure and migration of the estuary occurs due to strong seasonal
variations of river flow and wave climate where limited river flow allows the formation of a
sand bar across the estuary mouth.
Uilkraals Estuary Situation Assessment
18
Anchor Environmental