Feedback from the presentation…
L to R: Ted Gilfillan Estuary Care; Dr Eckart Schumann, Dr Tony Ribbink, CEO Sustainable Seas Trust
In introducing Dr Schumann Tony Ribbink said Eckart is a ‘true’ scientist whose specialty is the ‘physical side of the ocean’. His qualifications come from the University of KZN, UNISA and Cambridge University. He is currently a Consultant and Research Associate at Nelson Mandela Metropolitan University. He is also the Chairman of the South African Marine Rehabilitation & Education Center (SAMREC)
Dr Schumann’s presentation TIDES, WAVES and SEDIMENTS in the BUSHMANS ESTUARY gave a technical explanation of the effects of tides, winds and currents on the pattern of tidal ebbs and flows and the resultant accumulation of sediment in our estuaries.
He explained that South African tides were primarily semi diurnal, i.e. two high tides and two low tides per day, but were relatively small – maximum variation at spring tide is around 2 m. However, the south coast is unique in that substantial longer period waves (called shelf waves, with periods of days) caused additional water level variations in the sea of more than 0.5m. These fluctuations were amplified in the estuary, causing anomalous conditions where the low tide level on one day could be higher than the high tide level the next day.
Actual water level variations in the Bushmans Estuary (blue line), with the longer period fluctuations shown in black. In the extracted section the low tide on 7 September was higher than the high tide the following day.
The sill at the mouth of most South African estuaries also causes an asymmetry in the tide, with the flood tide shorter than the ebb tide. This means that flood currents are also stronger than ebb currents, important for the movement of sediment.
Waves approaching the shore at an angle can move substantial volumes of sediment, and the dominant waves from the Southern Ocean have built up the Alexandria dunefield (with the wind). It is not known how much sand is transported past the Bushmans, but the potential is probably substantial.
Waves and sediment transport in Algoa Bay and at the Bushmans (200,000 m3 pa = 10-ton truck every 10 minutes, 24/7)
Marine Sediments are mainly made up of quartz and shell fragments, however the distribution varies widely, for example Algoa Bay is about 60% quartz and clay minerals. On the other hand, fluvial sediments associated with rivers, streams and estuaries are primarily sand, mud and clay.
Cohesiveness in sediments comes mainly from the presence of clay particles which tend to form aggregates in which individual flakes are held together by electrostatic attraction and surface tension. They give clay their glutinous property and increase overall cohesion of the bed. Mud also settles out due to biological aggregation and flocculation when fresh water from the river meets salt water from the sea. Finally, the reworking of sediments by animals and plants in which biologically produced films bind the surfaces of sediment particles together form rich sediments in which other plants can grow.
Resulting sediments will vary throughout the estuary, with non-cohesive marine sediments the easiest to move. Consolidated cohesive sediments with extensive reworking by animals and plants will require strong currents to move, and may only come loose as clumps.
Consolidated sediment with an overlay of marine sand in the Keurbooms Estuary.
Because of stronger flood tidal currents and waves at the mouth, more sediment is transported into the estuary on the flood than is taken out again on the ebb. The narrow mouth also means that currents slow down when exiting the mouth area, dropping sediment and forming tidal deltas or sand banks.
There appears to be only one detailed analysis of sediment in the Bushmans Estuary, made by Baird, Uncles and Reddering (1981). They estimated the rate of sediment transport into the estuary at 20 m3 per day.
Various techniques can be used to remove unwanted sediment from the estuary. The most obvious is mechanical dredging, with other techniques using increased ebb flow: tidal flooding was proposed by DuraDams and Dr Dennis Loubscher around 2000.
However, a technique not used in South Africa is that of agitation dredging, where various means such as propwash, rakes and drag beams and water injection are used to raise sediment into the ebb flow stream. This is a more controlled and cheaper process than other forms of dredging, and can also be used to target consolidated sediment areas.
Dr Schumann indicated that management of the estuary should include regular monitoring and suggested that GPS could be used at spring high and low tides to map out estuary banks and other sand banks; he had used the technique successfully in the Keurbooms Estuary. Moreover, data from a tide gauge at the R72 bridge was available from DWA, while data on ocean waves was also freely available. This should be tied into weather, in particular wind and rain. Furthermore, it should be noted that wind and sand dunes were not addressed in the talk, and these played an important part in the overall sediment processes in the Bushmans Estuary.
