Outflows

Rainfall

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

5.3.1. Outflows

Inthe case of Mhlanga and Mdloti Estuaries the two main losses from the systems, when they are closed, are seepage and evaporation. This scenario is likely to be typical for perched estuaries with narrow sand bars. Both seepage and evaporation losses are storage dependant, with seepage increasing as the water level within the estuary rises and evaporation increasing as surface area increases.

Where estuaries are perched, the seepage losses can be substantial, and can dominate the evaporation losses. The seepage losses can be estimated by combining water level data and flow readings. When the water level remains constant in the estuary, there is an equilibrium as the inflows are equal to the outflows. Therefore flow readings taken at constant water levels are equal to the seepage losses at that water level. To determine the maximum seepage from the system, the relationship between flow rate and water level needs to be projected to the maximum water level.

During May 2003 the water level monitor at Mhlanga Estuary captured two different stages at which the water level remained constant for a period of time. Flow measurements were made during each of these two occasions. The water levels and flows recorded are tabulated in table 5-3.

Fortunately at Mhlanga Estuary a flow rate was captured the morning of a breach after the estuary had remained closed at the high water level for several days. This flow rate of 0.25m³/s

indicates the maximum seepage from the estuary. The average evaporation from the system was estimated by multiplying the MAE by the surface area. For MWanga Estuary the estimated evaporation from the system was calculated as 0.02m³/s, which is relatively small «10%) compared to the estimated seepage and can therefore be ignored.

Table 5-3: Constant water levels and the corresponding flow rates obtained at Mhlanga Estuary.

Water level Flow rate

Date Comment

(m+MSL) (m³/s)

7 May 2003 2.79 0.19 Constant approx. 20 days

23 May 2003 2.96 0.25 Morning of breach

Two data points, shown in table 5-4, were obtained for Mdloti Estuary, neither of which corresponded to the high water level of the estuary.

Table 5-4: Constant water levels and the corresponding flow rates obtained at Mdloti Estuary.

Water level Flow rate

Date Comment

(m+MSL) (m³/s)

19 Feb 2003 2.09 0.20 Constant> 10 days

25 Jun - 8 Jul '03 2.57 0.34 Averaged over 2 weeks

There was not enough data available to determine an accurate relationship between water level and seepage. The rate of seepage increases with an increase in water level. From the information available figure 5-19 was produced, from which the maximum seepage was estimated The maximum seepage was therefore estimated to be 0.53m³/s at a water level of 3.2m above MSL. The maximum evaporation losses from the system, which occur when the estuary is full and the maximum surface area exposed, is approximately 0.02m³/s or 5% of the maximum seepage. Table 5-5 contains a summary of the losses from MWanga and Mdloti Estuaries.

The residence time for any flow Q may be defmed as TR

=

S/Q where S is the storage when the estuaryisfull. The residence time represents the time required for a specific flow to replace the storage volume. The residence time associated with the maximum seepage outflows therefore

gives an indication of the time between successive breaching events when the flow is just large enough to overcome seepage losses.

Table 5-5: Summary of losses and related information.

Mhlanga Estuary Mdloti Estuary

Seepage (mJ/s) 0.25 0.53

Evaporation (m³/s) 0.02 0.02

Critical Tr (days)= S/QCR 34 20

% of MAR 68 21

Determining maximum seepage

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o 0.5 1.5 2 2.5 3 3.5

Water level (m+MSL)

• Seepage-Mhlanga - Seepage-Mdbti MIx. WL-Mdloti - MIx. WL-Mhlanga

Figure 5-19: Plot of seepage against water level used to determine maximum seepage.

The seepage is also dependant on the characteristics of the berm and can therefore change depending on the morphology of the berm.

5.3.2. Low flows

Low flows are previously defined, insection 2.3, as the state under which outflows exceed the inflows and therefore the water level within the estuary does not rise. Both Mhlanga and Mdloti Estuaries are perched with low water levels at 670 and 780 mm above MSL respectively. The maximum losses, due to seepage and evaporation, from the system occur when the water level

in the estuary is at a maximum. Itcan therefore be deduced that as long as the flows into the system are less than the maximum outflow from the system, the estuary will reach a equilibrium below the breaching water level, remaining closed until such time as the inflows increase above the maximum outflow. Therefore this state can be defmed as:

where I represents the inflows and Omax represents the maximum possible outflow (approximately equal to the seepage in this case).

Under this condition should the inflows into the system be reduced to less than the outflows (eg.

because of reduced rainfall or increased abstractions), the water level in the estuary will drop until an equilibrium is reached.

Mdloti Estuary has remained closed since February 2003 and therefore provides a good example of the low flow regime. The flow rates and water level between February and August 2003 are shown in figure 5-20. Superimposed on the figure are the maximum water level and the maximum outflow, estimated in section 5.3.1.

Water level and flow against time

4000 1.20

3500 1.05

_ 3000

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29-Jan-03 23-Feb-03 20-Mar-03 14-Apr-03 9-May-03 3-Jun-03 28-Jun-03 23-Ju~03

Time

• Measured WL - - Max. WL • Measured flow - - Max. seepage

Figure 5-20: Water levels and flow rates over time for Mdloti Estuary, with an indication of the maximum water level and maximum seepage.

From the water levels it can be seen that although there were variations in the water level the maximum water level was not reached and the estuary did not breach.

During a two week survey conducted from the 25 June to the 8 July 2003 daily flow rates and water levels were recorded. Over this period the water level was fairly constant, with the flows generally below the maximum seepage. Although there is one defined instance in which the flow rate was above the estimated maximum seepage, the flow was not sustained for a sufficient period of time to cause the water level to rise to or above the maximum water level and cause failure of the berm.