Rainfall
Plate 4-10: Example of photographs used in the determination of beach profile
The method used to obtain the beach profiles had several flaws:
• Although the tripod was levelled, working on sand resulted in several photographs containing a tilt. Where possible the horizon was used to compensate for this.
• The position of the camera varied from day to day and in some instances was varied for specific cross-sections.
• Lighting was occasionally a factor distorting post analysis of photographs.
The digital camera used to take the pictures has a resolution of 1600 by 1200 pixels. The error associated with a half degree tilt of the camera over thirty metres is approximately 260 mm in the vertical and approximately 1 mm in the horizontal.
5. DISCUSSION OF FIELDWORK
5.1. IntroductionThe main objective of the fieldwork was to determine the effects of flow on mouth state. Over the study period flow rates, water levels and mouth state were measured or observed, from which threshold flows for the various mouth states are discussed. The data collected also provides information on the losses associated with the system. As rainfallisan easier parameter to measure, and rainfall data is readily available this section also deals with the relationship between mouth state and rainfall. Mechanisms related to the mouth state such as failure and closure of the berm are discussed from observations made together with the data collected. The time scales associated with both the breaching and closure of the estuaries are also discussed.
5.2. Data collected
During the observation period from March 2002 to August 2003, field data was collected at both Mhlanga and Mdloti Estuaries. The data collectedis presented in sections 5.2.1 to 5.2.5, and comprises of data on rainfall, mouth state, flow rates, water levels and beach profiles. This data provides the basis for the analysis which follows in sections 5.3 to 5.6.
5.2.1. Rainfall Data
Figures 5-1 and 5-2 present the daily rainfall events over the observation period from March 2002 to August 2003. This data was obtained from the South African Weather Bureau, and was collected at the Mount Edgecombe station (reference number 0241072)
The Mean Annual Precipitation (MAP) for Mhlanga and Mdloti Estuaries (quaternary catchment area U30B) is approximately 982 mm. The expected average rainfall for the Mt. Edgecombe weather station, near Mdloti and Mhlanga Estuary, between the months of January and August, is approximately 60% of the MAP, however the region received less than 30% of the MAP between January and August in 2003. Over the entire observation period (March 2002 to August 2003) the area received approximately 65% of the expected rainfall. A plot of the monthly rainfall over the observation period against the average monthly rainfall is shown in figure 5-3.
Dally rainfall recorded at the Mt Edgecombe weather station
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Figure 5-1: Daily rainfall occurring between March 2002 and mid November 2002.
Dally rainfall recorded at the Mt Edgecombe weather station
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Figure 5-3: Comparison of monthly rainfall over the observation period against expected average monthly rainfall
5.2.2. Mouth State
Data gathered on mouth state for Mdloti and Mhlanga Estuaries is presented in figures 5-4 and 5-5 respectively. The mouth states at Mdloti and Mhlanga Estuaries over the observation period are summarized in table 5-1. Appendix F contains all the data recorded relevant to mouth state.
Table 5-1: Summary ofthe mouth state at Mhlanga and Mdloti Estuaries.
Parameters Quantity Comment
No. of breaches 9 6 within 3 days of neap tide, 2 on neap tide C No. of partial breaches 4 3 within 3 dayS of neap tide
B
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=00 No. of dayS closed 328 69%
~ Total no. days with data 478 100%
No. of breaches 18 9 within 3 dayS of neap tide, 3 on neap tide
C 6 within 3 days of neap tide, 1 on neap tide, 1
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~ No. of dayS fully open 146 27%
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No. of dayS partlY open 97 18%No. of dayS closed 296 55%
Total no. days with data 539 100%
Ramm et al. (1986) stated that Mhlanga Estuary is typically closed 96% of the time, while Mdloti Estuary is closed 64% of the time. CSIR, 2002 reports that Mhlanga and Mdloti Estuaries are closed/semi-closed 75% and 73% of the time respectively, under present conditions It was therefore expected that Mdloti Estuary would breach more frequently than
Mouth State of Mdloti Estuary over time
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Mouth State of Mhlanga Estuary over time
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Figure 5-5: The mouth state at Mhlanga Estuary over the observation period.
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.