Rainfall
Plate 2-8: Depletion and accretion as observed on Mhlanga beach
3. CASE STUDIES
3.1.2. Lagoon area
The area of the lagoon portion of each estuary was measured off a set of orthophotos dated 1992. Unfortunately the detail provided by the 5m contour interval on the orthophotos was not sufficient to determine the true head of the estuary. Therefore the head of the estuary was found by projecting the maximum water level, determined from fieldwork, upstream using the 2m contour data and mapping, dated 2003, supplied by the Durban City Engineers. Figures 3-3 and 3-4 show the Mhlanga and Mdloti lagoons as determined from the contours depicted. Not all of the contours are displayed in the figure as this would clutter the plot, therefore 2m contours are displayed up to 10m above MSL, and thereafter random contours are shown. Note that the values obtained for the surface areas of the estuaries are much larger than those defmed by literature (e.g. Begg, 1984), this is probably linked to the definition of the estuary head.
Figure 3-3: Map showing the extent of Mhlanga Estuary.
Figure 3-4: Map showing the extent ofMdloti Estuary.
3.1.3. Survey
All the water level data collected at Mhlanga and Mdloti Estuaries was referenced to the pile cap of the respective bridges. In order to report these measurements relative to MSL a GPS survey was conducted at both Mhlanga and Mdloti Estuaries. From this survey the heights of the pile caps at Mhlanga and Mdloti Estuaries were measured as 1274 and 962 mm above MSL
respectively. The maximum and minimum water levels were then determined from the data collected relative to MSL (see section 5.2.4 for data collected on water levels).
The GPS survey was also used to measure the length, width and height of the sand berm.
3.1.4. Storage capacity
It is important to note that the estuarine environment is dynamic and therefore the volume can change depending on the water level of the estuary and the morphology of the lagoon.
The dynamic storage within the estuary is an important hydrological feature of temporary open estuaries. No hydro-graphic survey was conducted as part of this study, therefore a model has been used to estimate the storage capacity of the estuary. A possible model for storage of an estuary is a triangular prism, allowing for sedimentation and dead storage in the lower reaches, as seen in figure 3-5. The resulting cross-section representing the dynamic storage is trapezoidal.
vLWL
.~"\.•..
Dynamic storage
Sediment& dead storage
Figure 3-5: Storage model used to represent the storage within an estuary. HWL and LWL refer to the high water level and low water level.
Unfortunately there was insufficient data for this model therefore a rough estimate of storage was determined by:
Inthis equation V represents the volume of the estuary while d is the maximum change in water level and A is the surface area of the estuary when full. Table 3-2 contains the approximate storage for both Mhlanga and Mdloti Estuaries.
Table 3-2: Determination of dynamic storage.
Mhlanga Mdloti
Max. change in water level (m) 2.29 2.42
Estuary area(mL) 700000 800000
Storage (m3) 800000 970000
Observations during this study indicate that the actual morphology of the estuaries is far more complex. Several cross-sections of both Mhlanga and Mdloti Estuaries were measured for flow determination. Examples of cross-sections measured at Mdloti and Mhlanga Estuaries are presented in figures 3-6 and 3-7 respectively. A complete record of all the cross-sections and the location at which each ofthe cross-sections were measured are included in appendix C.
Cross-Sectional Profile
10 8 9
7 Width (m)
4 5 6
3
o 2
0.00~--~--'----~-~--~--~--~--~-_~_ _--.
0.10 0.20 0.30
I
0.40 .t:i
0.50o 0.60 0.70 0.80
0.90 - ' - - - - ---J
Figure 3-6: The cross-sectional pronte at Mhlanga Estuary measured on the 13 November 2002. The surface water level relative to MSL is 0.25m.
Cross-Sectional Profile Width (m)
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
o
0.00t-~-~----'--~----'--~~...-~~-~~--,---~-~~-...--.----,--~~
0.20 0.40 0.60
I
0.80 :; 1.00~0. 1.20 1.40 1.60 1.80
2.00 - ' - - - - _
Figure 3-7: The cross-sectional profIle at Mdloti Estuary measured on the 22 January 2003. The surface water level relative to MSL is 2.25m.
3.1.5. Abstractions and discharges
The Department of Water Mfairs and Forestry (DWAF) provided information on the discharges from Hazelmere dam into the Mdloti River. The average discharge stated in table 3-1 was calculated from 20 years of complete data between 1977 and 2002, excluding the 1987 data.
The 1987 data was excluded as the flood event occurring during this year was reported by Perry (1989) as probably the maximum experienced by Mdloti Estuary and therefore would bias the data. eThekweni Municipality Water and Sanitation (EMWS) provided information on the volumes of treated effluent discharged from the Mhlanga and the Phoenix Waste Water Treatment Works (WWTW) into the Ohlanga River upstream of Mhlanga Estuary (shown in figure 3-2). The current annual discharge or capping flow is approximately 7.2 X 106m3/yr (0.23m3/s). The effluent discharges from the Verulam and the Mdloti WWTW (see figure 3-2) have a current annual discharge of approximately 2.7 x 106m3/yr (0.086m3/s). These WWTWs are situated downstream of Hazelmere. Table 3-3 presents the discharges from the various WWTWs.
Table3-3: Summary of discharges.
Discharge Discharge WWTW (millionm3/yr) (m3/s)
Mblanga 2.62 0.083
Phoenix 4.75 0.15
Mdloti 0.37 0.012
Verulam 2.33 0.074
Water is abstracted from Mdloti River by Tongaat-Hullet for irrigation and domestic use. The estimated abstraction is 5.9million cubic meters per year (0.19m3/s). Water meters were recently installed; however this data is not yet available (Gurney, 2003).
The natural MAR reported in Midgely and Pitmann (1994) for the quaternary catchments was scaled by the catchment areas of Mhlanga and Mdloti Estuaries to obtain the natural MAR figures in table 3-1. For Mhlanga Estuary the present day Mean Annual Stream-flow (MAS) was determined by adding the discharges from WWTWs to the natural runoff. The present day MAR for Mdloti Estuary was calculated by adding the discharge from Hazelmere Dam and the WWTW to the natural runoff for Mdloti Estuary for the catchment area in U30B, and subtracting the abstractions. The discharge from Hazelmere Dam effectively replaces the runoff from quaternary catchment U30A, as it falls upstream of Hazelmere Dam, which would naturally enter Mdloti's estuarine system.