Rainfall
Plate 5-1: Water hyacinth preventing flow measurements upstream ofMdloti estuary
MWanga Estuary. Even though Mhlanga and Mdloti Estuaries were open 27% and 21%
respectively, Mhlanga Estuary breached double the number of times Mdloti Estuary breached.
Since February 2003 Mdloti Estuary has remained closed, reflecting the lowered rainfall, however Mhlanga Estuary has continued to breach regularly.
5.2.3. Flow Data
Figures 5-6 and 5-7 show a plot of the flow rates measured at Mdloti and Mhlanga Estuaries respectively. Where flow measurements were made upstream of a WWTW discharge point, the estimated discharges, obtained from the EMWS, were added to the measurement. Appendix C contains the locations of the flow readings along with the cross-sectional data and velocity readings.
No flow measurements were made for Mdloti Estuary between February and June 2003, due to accessibility problems encountered upstream of the estuary, as seen in plate 5-1.
Flow rates measured at Mdloti Estuary
17-Aug-03 28-Jun-03
9-Way-03 20-War-03
,,,
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,29-Jan-03 10-Dec-02
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21-Oct-02 ,+--'
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1-Sep-02
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24-Way-02 13-Ju1-02
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4-Apr-02 • - -- - - -- - - -- - - - - - . . . .
5
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Date
Figure 5-6: Spot flow data measured at Mdloti Estuary over the observation period. (Points joined by dotted line for clarity).
Flow rates measured at Mhlanga Estuary
17-Aug-03 28-Jun-03
9-Way-03 20-War-03
29-Jan-03 10-Dec-02
Dale 21-Oct-02
1-Sep-02 9
8 ~
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I I
6 ,
5 ' \
4 "
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4-Apr-02 24-Way-02 13-Ju1-02
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Figure 5-7: Spot flow data measured at Mhlanga Estuary over the observation period. (points joined by dotted line for clarity).
Some of the other problems encountered in measuring flows at the estuaries included wind and reeds adversely influencing the drogues. The Swoffer instrument was only effective where wading was possible and required a minimum velocity of 0.03 m/s.
The average measured flow for each estuary, over the observation period, was determined using a simple arithmetic average of all the samples yields a value of 1.09m3/s for Mdloti Estuary and 0.75m3/s for Mhlanga Estuary. However these figures are biased as samples were taken more frequently in June and July of 2003 than in the other months. Therefore for each month an average of the flow readings was taken and one reading form each month averaged to obtain 2.08m3/s and 1.40m3/s for Mdloti and Mhlanga Estuaries respectively. A flow of 8.35m3/s was measured at Mhlanga Estuary in January 2003 is included in the latter average even though it was a short lived flooding event that was not representative of the flows for that month. If the flooding event is excluded the average flow is 0.70m3/s.
The expected flow rate or MAS (inclusive of the effects of Hazelmere Dam, WWTW discharges and abstractions) was estimated., in chapter 3.1.5, to be 76.5 million m3/yr and 20.1 million m3/yr for Mdloti and Mhlanga Estuaries respectively. This is equivalent to 2.42m3/s and 0.64m3/s. The average flow rate of 2.08m3/s obtained for Mdloti Estuary is approximately 79%
of the expected flow, which is reasonable as the estuary experienced only 65% of the expected precipitation. At Mhlanga Estuary the measured flow (excluding the flood event) was approximately 110% of the expected stream-flow. Mhlanga Estuary should have an average flow less than the expected stream-flow due to the low rainfall. This suggests that either the flow measurements were not representative of the actual average flows or that there were higher discharges into the estuary from the WWTW. Alternatively the MAR for Mhlanga Estuary could have been underestimated.
5.2.4. Water levels
The water level data was as expected lower when the estuary was open and shortly after closing than when it had been closed for a period of time. From the water level data for Mdloti Estuary figure 5-8 was produced. Figures 5-9, 5-10 and 5-11 represent the water level data for Mhlanga Estuary. Appendix G contains the water level data obtained from photographs of the bridges as well as an example of the data captured by the water level monitors.
Water levels recorded at Mdlotl Estuary over time
- - - - - - - ~ -
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.§.. 2000
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i
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o
4-Apr-02 24-May-02 13-Jul-02 1-Sep-02 21-0ct-02 10-Dec-02 Dilte
29-Jan-03 20-Mar-03 9-May-03 28-Jun-03 17-Aug-03
Figure 5-8: Spot water level data measured at Mdloti Estuary over the observation period. (Points joined by dotted line for clarity).
Water levels recorded at Mhlanga Estuary over time
- - -
-
--
- - - -- - - - -
- - - - --
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i
> 1000~ 500
o
10-Mar-02 18-Jun-02 26-Sep-02
Dilte
4-Jan-03 14-Apr-03 23-Jul-03
Figure 5-9: Spot water level measurements at Mhlanga Estuary over the observation period. (Points joined by dotted line for clarity).
Water levels recorded at Mhlanga Estuary over time
/"""".. ~
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--
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" .3500 :::; 3000
!
1/1 2500!
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..
1500~ 1000
~ 500
o
20·Mar-03 3O·Mlr-03 9-Apr-03 19-Apr-03 29-Apr-03
Date
9-May-03 19·May-03 29-May-03
Figure 5-10: Continuous water level data measured, using water level monitors, between March 2003 and August 2003.
Water levels recorded at Mhlanga Estuary over time
- - - -
----
.-.-r'"
~" .
/" . /
,..Ur
•
'n.,.
3500 :::; 3000 1/1::E 2500 + E 2000
.§.
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1500 :: 1000 S~ 500o
29-Mly-03 8-Jun-03 18-Jun·03 28-Jun-03 8-Ju~03
Date
18-Ju~03 28-Jul-03 7-Aug·03
Figure 5-11: Continuous water level data measured, using water level monitors, between March 2003 and August 2003.
Infigures 5-10 and 5-11 the continuous water level monitoring data over the period from the 23 May 2003 to the 30 May 2003 was erroneous due to instrument failure. From the water level data the maximum and minimum water levels for the estuaries was estimated. The maximum water level recorded at Mhlanga Estuary was 2960 mm above MSL, while the minimum water level recorded was 670 mm above MSL. At Mdloti Estuary the maximum water level recorded was approximately 3200 mm above MSL and the minimum water level was 780 mm above MSL.
From plate 5-2 and 5-3 of the M4 bridges over the two estuaries, distinct high water marks are visible. The various water marks indicate the different breaching levels attained by the estuaries.