Analysis of mouth operation.

As can be seen from Figure 35, the behaviour of the mouth of the lake can have a major effect on the salinity of the lake.

  
Figure 35: Effect of various mouth operation scenarios on the salinity of North Lake.

In figure 35, the effect of the following mouth operation scenarios are illustrated. The scenarios are listed in order of decreasing effectiveness in decreasing the lake salinity.

virgin_mouth

The mouth is allowed to open and close naturally assisted by the natural movement of the Mfolozi mouth. This is the most effective scenario for lowering the salinity of the lake as the mouth naturally closes when there is a major influx of sea water into the lake. Note that there is a large degree of uncertainty about how exactly the mouth would behave, the point is that if the mouth behaved as modelled, the salinity would be as shown.

close

The mouth is kept closed and the lake St. Lucia ceases to be an estuary and becomes a land-locked lake like lake Sibaya. This would be an ecological disaster as the lake is a major marine nursery.

mouth_open and mouth_wide

In the mouth_open scenario the mouth is kept open and works according to Huizenga's lake level / outflow graph. In the mouth_wide scenario, water is allowed to flow through the mouth completely unimpeded. Note that there is very little difference between these two options. ie. Dredging the mouth more that it is currently will have very little or no effect.

mouth_only

The mouth is allowed to operate naturally, but the Mfolozi mouth and the lake mouth are kept separate. As shown in figure 14 the salinity heads rapidly towards a singularity if the mouth closure occurs when the lake is already hypersaline.

In view of the above results, judicious closure of the mouth could result in substantial improvements of lake salinity at relatively low cost.

However, the situation is complicated by the following factors...

• Closing the mouth stops skiboat fishermen from using the estuary as a harbour.
• Closing the mouth stops spawning runs and migrations of marine species.
• Closing the mouth when the mouth is already hypersaline will lead to rapid increases in salinity.
• As the future rainfall is unknown, all mouth operation rules will be suboptimal.

• Behaviour of the mouth under benchmark conditions.