High flows

Rainfall

Plate 5-2: The high water mark on the M4 bride over the Mhlanga Estuary

5.3.4. High flows

When the inflows into the estuarine system exceed the maximum seepage, the water level will rise continuously. Initially the water level rises rapidly, slowing as the water level increases and seepage increases. The water level will continue to rise until the water level in the estuary exceeds the lowest point of the sand berm resulting in overtopping and scour. This flow regime is defined by:

Jezewski (1984) states the one in two year flood at Mdloti Estuary has a peak discharge of approximately 76m³/s. Under these flow conditions the outflows from the system are negligible in comparison to the inflows. The peak flow rate translates to a residence time of approximately 4 hours. Hazelmere Dam probably has little effect in attenuating floods of this magnitude.

During the 1987 floods Mdloti Estuary experienced an extreme flood flow of approximately 2000m³/s (perry, 1989). This flow was much larger than the maximum seepage which is of the order 0.53m³/s. The flood flow translates to a residence time of approximately 10 minutes.

Hazelmere Dam will not have been able to attenuate the flood as the volume of the dam was far exceeded by the volume of the flood.

5.3.5. Summary

Mhlanga and Mdloti Estuaries appears to breach at a consistent high water level (approximately 3 and 3.2 m above MSL respectively). In determining how flows affect the mouth state a simple relationship was not found. The mouth state is easily determined for sustained high and low flow rates, with high flows leading to open mouth conditions and low flows causing the estuary to remain closed. The intermediate flows (flows of similar magnitude to the maximum seepage) are not as easily defined. This flow regime is likely to result in seepage failure, however the estuary may not breach immediately after reaching the maximum water level, but may remain in this unstable state. These scenarios only apply to sustained flows, for example should a high flow be attained for a short enough period of time the estuary probably will not breach unless it was almost full when the high flow occurred.

5.4. Effects of rainfall on mouth state

5.4.1. Mdloti Estuary

Figure 5-22 shows the plots of rainfall and mouth state over time. The rainfall experienced by the Mdloti Estuary during the various closed periods ranges from 0 to 70 mm over the catchment area. From the data there is no clear correlation evident between amounts of rainfall and breaching, with rainfall events of varying intensity coinciding with breaching events. A general trend exists as in 2003 the estuary breached less than in 2002, coinciding with a reduction in rainfall from 2002 to 2003.

Mouth State relative to Rainfall for Mdloti

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a)

Mouth State relative to Rainfall for Mdloti

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b)

Figure 5-22: Plot of mouth state and rainfall over the observation period.

Figure 5-23 was produced in order to further explore the effects of rainfall on the mouth state through plotting the rainfall accumulated in the month as well as the percentage of time the estuary was closed each month against time. There is a weak general trend between the amount of rainfall in a month and the percentage of time the estuary was closed within that month in that the rainfall decreased as the percentage closed increased. The computed Spearman's linear correlation coefficient of 0.37, was found insignificant at the 95% confidence level, but significant at the 85% confidence level.

Plot of ranked rainfall vs ranked %closed

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Figure 5-23: Ranks of monthly rainfall and percentage oftime in the corresponding month that the estuary mouth was closed.

*(Where a rank of I is assigned to the largest number)

5.4.2. Mhlanga Estuary

Comparing the data collected in 2002 to the results in 2003 for the months March through to July Mhlanga Estuary was, as expected, open less frequently in 2003 than 2002 as the rainfall during the 5 months of 2002 amounted to 336 mm, while the same 5 months in 2003 only received 145 mm. Between March and December 2002 the estuary was closed approximately 57% of the time, whilst between March and December 2003 the estuary was closed 87% of the time. Between 1 August 2002 and 28 February 2003, Mhlanga Estuary was closed approximately 36% of the time, with a rainfall during this period of approximately 356 mm. In order to further explore the effects of rainfall on the mouth state figure 5-24 was produced.

Again breaching coincided with varying intensities of rainfall and in some cases no rainfall was recorded over the closed period yet breaching still occurred.

Mouth State relative to Rainfall for Mhlanga

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Mouth State relative to Rainfall for Mhlanga

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b)

Figure 5-24: Plot of rainfall and mouth state over the observation period.

As with Mdloti Estuary figure 5-25 was produced for Mhlanga Estuary in order to further explore the effects of rainfall on the mouth state by plotting the rainfall accumulated in the month as well as the percentage of time the estuary was closed each month. At MWanga Estuary thereisa stronger general trend than at Mdloti Estuary between the amount of rainfall in a month and the percentage of time the estuary was closed within that month in that as the rainfall decreased the percentage closed increased. The Spearman's linear correlation coefficient of 0.61, is significant at the 95% confidence level.

Plot of ranked rainfall vs ranked%closed

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Figure 5-25: Ranks* of monthly rainfall and percentage oftime in the corresponding month that the estuary mouth was closed against time.

*(Where arankof I is assigned to the largest number)

5.4.3. Summary

The effect of rainfall on mouth state was sought as a long term rainfall data is more readily available than flow data. It could be expected that the conclusions obtained for determining mouth state from stream flow would be applicable when determining the effect of rainfall on mouth state. Therefore estuaries should open after receiving large amounts of rainfall and remain closed under drought conditions. However this cannot be looked at in isolation. For instance both Mhlanga and Mdloti Estuaries experienced drought conditions in 2003, and while Mdloti Estuary remain closed as expected Mhlanga Estuary continued to breach regularly, in large, due to the addition of treated effluent.

Although Mhlanga Estuary shows a correlation between rainfall and the percentage of time the estuaryisclosed, this method of determining mouth state may not work on all estuaries or under all conditions. The weak correlation between rainfall and mouth state found for Mdloti Estuary confirms that rainfallisnot a reliable method of determining mouth state.