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Plate 5-10: Build up of sand, deposited during high tide
6. CONCLUSION
6.4. Question 3: Tidal influence
What,
if
any, tidal influence exists in perched, temporary open estuaries such as Mhlanga and Mdloti Estuaries?While the estuaries are open tidal influence exists despite the perched nature of the estuary.
This phenomenon is as a result of wave runup superimposed on the natural tidal fluctuations.
At MWanga Estuary data captured on the water level monitor showed asymmetrical fluctuations, exhibiting signs of flood domination, corresponding with the tidal fluctuations.
Salinity readings were used to confIrm the existence of tidal influence at Mdloti Estuary, as no breaching events occurred after the installation of the continuous water level monitors.
6.5. Question 4: Mechanisms affecting the mouth
What are the mechanisms involved in breaching and closure of an estuary mouth, and what are the timescales implicated?
Breaching mechanisms were also explored in this dissertation. The mechanism by which an estuary breaches is dependant on the relationship between the inflows and outflows from the system. Where the inflows are of the same order of magnitude as the maximum outflows, the water level rises until the maximum breaching level is reached and seepage failure is likely to occur. However should the inflows be far greater than the outflows, failure through overtopping should occur. Inthe breaching events captured by the continuous water level monitors the water level tended to drop for 5 hours after commencement of breaching, until a low water level was reached.
Tidal influence is instrumental in the closure of Mhlanga Estuary, by progressively building up the mouth with each high tide. In the events captured on the water level monitors the closure took between 3 and 5 days. Length of time required for closure to occur could be prolonged if the freshwater inflow to the estuary is strong enough to scour the deposited sediment form the mouth. The estuary closed at high tide trapping a large volume of saline water in the estuary.
6.6. Summation
From the research conducted it was discovered that the mouth state of Mhlanga and Mdloti Estuaries is dependant on the water level within the estuary. There is a correlation between sustained high flows and an open estuary mouth and between sustained low flows which maintain the estuary in the closed state. However for the intermediate flow regimes, as well as high and low flows with short residence times, it is not as easy to determine the mouth state of the estuary. Anestuary is a complex system and determination of the mouth state requires an understanding of the water levels, losses, residence times and flows.
6.7. Suggestions for further research
In order to further this research the continuous water level recordings need to be coupled with continuous flow measurements. Combining these two data sets will provide more accurate information on the relationship between flow and water level as well as providing more precise details on the water balance and hence the losses from the system. The flow records during open phases can also be used to indicate the magnitudes of flow required to keep the estuary open. It would also be valuable to obtain further data for these case study estuaries under different climatic conditions, for instance the affects of a wet year and average year on flows and mouth state.
Surveys of the estuary are valuable in determining the storage of the estuary. Sediment build up in the estuary affects the morphology of the lagoon. Sediment within an estuary provides habitat to several organisms. The creation and destruction of this habitat could greatly influence the food chain. During the closed phase a sediment yield analysis (e.g. Modified Universal Soil Loss Equation) could be used to determine the sediment entering the estuary, which can be confmned with surveys. The effect of estuary breaching is to scour sediment from the estuary.
The water levels, flow rates and mouth size affects the amount of sediment scoured from the estuary and in turn the health of the estuary, therefore particularly for artificially breached estuaries an understanding of the water levels and mouth size is required for adequate scouring to occur.
The water level at which breaching occurs is dependant on the characteristics of the berm. A study focussed on the affect of berm characteristics on the breaching of an estuary could provide an explanation as to how and when an estuary is likely to breach. For instance, as Mdloti
Estuary has remained closed for a large portion of 2003 and the berm has gradually become wider over this period, does this affect the flows required for the estuary to breach.
When the estuary is in the open phase, tidal influence is evident. A long term study could be used to determine the relationship between the tide level, significant wave height and the volume exchange within the estuary. A secondary affect would be to see how these characteristics affect the transport of sediment in and out of the estuary.
There are several other abiotic characteristics influencing the estuarine environment, for instance turbidity and salinity, both of which are linked to mouth state. The relationship between flows, mouth state and abiotic characteristics could be further explored to obtain an understating of the estuary.