A photographic survey of the berm at the mouth of MWanga was used to observe the effect of beach processes on the mouth area. -1: Summary of mouth conditions at MWanga and Mdloti estuaries 70 Table 5-2: Mouth conditions, rainfall and flow during the observation period 80 Table 5-3: Constant water levels and corresponding flow rates obtained at MWanga estuary.
INTRODUCTION
- Introduction
- Motivation for the study
- Objectives of the study
- Outline of the dissertation
What are the mechanisms involved in opening and closing the mouth and what are the time frames. The condition of the estuary is an important feature of this research, so the mechanisms of estuary breakthrough and closure are analyzed.
BACKGROUND & LITERATURE REVIEW
Introduction
Biotic component
- Biotic background
- Biological importance of hydrodynamics
A determining influence on the biomass, productivity and diversity of microalgae is the condition of the estuary mouth. The condition of the estuary mouth has a major influence on zooplankton biomass, productivity and diversity, with a reduction in macrobenthos typically occurring during open, high-flow conditions.
Water Balance
Rainfall
Climate
Schulze (1997) states that runoff in South Africa is on average 9% of precipitation, which is well below the world average of 35%. The province in South Africa with the highest runoff is KwaZulu Natal with 16.5% of the average annual precipitation (MAP), but this is still less than half the world average.
Mechanisms of breaching
As the hydraulic gradient increases, the rate of seepage through the berm increases until the point is reached where soil stability is lost. This scour causes the berm to subside, allowing water from the estuary to overflow the berm and scour the rest of the mouth, as shown in step 3.
Seepage at Mhlanga Estuary
Mhlanga Estuary after breaching
Once this erosive seepage reached the crest of the dam, plate 2-4, the breach occurred within minutes and continued to expand, as seen in plate 2-5, for about an hour thereafter. These similarities are seen in the initial photos of seepage and the later photos of the breached estuary and failed dam.
Seepage reaches crest of Teton Dam (Olson, 1976)
- Overtopping
- Wave Action
It is interesting to note the similarities between the estuary breach event and the failure of an earth dam. In extreme flood conditions, the flood wave will rapidly overtop the berm and generate strong scours, breaching the estuary.
Depletion and accretion as observed on Mhlanga beach
- Studies relating flow and mouth state
- Mdloti Estuary
- The Great Brak
- Summary
- CASE STUDIES
- Catchment area
- Lagoon area
- Weather data
- Mdloti Estuary
To protect homes and facilities built too close to the edge of the estuary and. The storage capacity of the estuaries was determined by multiplying the area of the estuary by the depth.
The effect ofthe construction ofthe M4 bridge
The artificial breakthrough of the Mdloti estuary, in the middle of the berm, results in the estuary draining completely. The north embankment of the bridge lies directly in the path of floodwaters, and both embankments limit the flood flow.
An aerial photograph taken in 1985, showing pre-flood conditions
The affect ofthe 1987 flood on the sand berm ofMdloti Estuary. (perry, 1989)
- Mhlanga Estuary
- Summary
- FIELDWORK METHODOLOGY
- Introduction
- Flow measurements
The dikes that form part of the bridge over the estuary encroach on the natural habitat of the estuary, reducing the reed beds in the area. Direct measurements of the inflow into the estuary were carried out at various stations upstream of the estuary. Both the depth and the distance across the width of the estuary are manually entered into the portable microcomputer.
An important aspect of the study of estuarine dynamics is monitoring changes in water levels. Initially, water levels were monitored using photographs of the M4 bridges at the Mhlanga and Mdloti estuaries.
The materials for the capsule comprising ofperspex tubing and HDPE end plugs
The pressure transducer requires a 5 volt supply, and in order for the voltage regulator to supply the regulated 5 volts, a minimum input voltage of 5.5 volts was required, so 4 AAA alkaline batteries were used to supply 6 volts. Because the bladders proved unreliable in the field, they were discarded and the recorders were simply placed in the estuary with a porous geotextile cover over the end cap, as shown in Plate 4-6(b). The geo-fabric allowed water to reach the pressure transducer, but trapped dirt from the water and prevented it from interfering with pressure readings or blocking the pressure port.
To ensure that the water level monitor is always placed in the same place and cannot drift away, a perforated container is fixed in the estuary. Scuba divers, shown in plate 4-9, were used to place the containers under the stack caps.
The canister was placed under the pile cap ofthe first column ofthe northern end ofthe M4 bridge at Mdloti Estuary
The initial design of the canister was a section of steel pipe closed at one end with a cap at the other, attached to a steel stake approximately 1 meter long. The design idea was to drive a stake into the bottom of the estuary near the M4 bridge, but this did not work in practice as it was not easy to drive stakes into the ground underwater. Another design consisted of a PVC pipe with an end cap on one side and a cap on the other.
This container is attached to the piles under the bridge pile caps with stainless steel brackets, rope and stainless steel wire. The position of the containers at Mdloti and Mhlanga estuaries are shown in plates 4-7 and 4-8 respectively.
At Mhlanga Estuary the canister was placed under the pile cap ofthe first column ofthe M4 bridge on the Northern
Water level gauges were acquired regularly, not exceeding 75 days, as this was the limit set by available logger memory when using hourly logging intervals. Water level gauges were placed in the test chamber at various pressures both before and after placement in the estuary to calibrate the instrument.
Installing the water level monitors using the boat and divers. The plate on the left shows the divers holding the canister before installation
- Salinity
- Surveying of the berm
If one of the sampling points does not represent the conditions, large errors can be obtained in the data analysis. The seaward slope of the berm is continuously shaped by the sea, so a daily survey covering a two-week tidal cycle was carried out to cover tidal conditions from spring to near and back to spring in order to determine how they affect these changes break the estuary. The study was conducted in the Mhlanga estuary as it was the more active of the two estuaries at the time.
The measurements were taken at low tide, where there was maximum exposure of the beach slope. Poles, marked at 25 centimeter intervals, were placed at the edge of the estuary and the edge of the sea, with a rope between the two.
Example of photographs used in the determination of beach profile
- DISCUSSION OF FIELDWORK 1. Introduction
- Data collected
The main objective of the fieldwork was to determine the effects of leakage on the condition of the mouth. Mechanisms related to mouth conditions such as berm failure and closure are discussed from observations made along with collected data. During the entire observation period (March 2002 to August 2003) the area received approximately 65% of expected rainfall.
A plot of the monthly precipitation over the observation period against the average monthly precipitation is shown in Figure 5-3. Dally rainfall recorded at Mt Edgecombe weather station. Figures 5-6 and 5-7 show a plot of the flow rates measured at Mdloti and Mhlanga Estuaries respectively.
Water hyacinth preventing flow measurements upstream ofMdloti estuary
- Water levels
As expected, water level data were lower when the estuary was open and shortly after closure than when it had been closed for a period. Appendix G contains water level data obtained from photographs of the bridges as well as an example of data captured by the water level monitors. From the water level data, the maximum and minimum water levels for the estuaries were 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 approx. 3200 mm above MSL and the minimum water level was 780 mm above MSL.
The high water mark on the M4 bride over the Mhlanga Estuary
- Outflows
- Intermediate flows
- High flows
- Tidal exchange
The maximum losses, due to seepage and evaporation, from the system occur when the water level reaches. The intermediate currents lead to a rise in the water level initially when the water level in the estuary is low. However, the water level monitor recordings at Mhlanga Estuary indicated cyclical fluctuations in water level when the mouth of the estuary was open.
The narrowing of the mouth also results in the water level in the estuary lagging behind the tidal fluctuation (e.g. Schumann, Largier and Slinger, 1999). The rise in tidal level is greater than the rise in water level within the estuary.
III 0
Summary
However, the breach only occurred during neap tides, nine days after the estuary reached high water levels. At eleven o'clock on the morning of the breakthrough, the water level meter was replaced. The water level in the estuary was approximately 2915 mm above NAP, with the tidal situation shifting from dead to spring.
During the subsequent low tide, the water level in the estuary reached a low of 920 mm above MSL. On July 27, 2003, the estuary erupted for the third time since water level monitors were installed.
Seepage at the southern end ofMhlanga Estuary on the 26 June 2003
Seepage observed at Mhlanga Estuary on the 15 September 2003
In June and July 2003, it took 3.5 and 2.5 days, respectively, for the estuary to close, which is equivalent to 7 and 5 tidal cycles, respectively. The approximate amount of sediment lost from the sand bar when the estuary opened on 26 June 2003 was estimated at 1400 m3. It took 3.5 days for the estuary to close the sandbar within a channel built with a smaller channel that released water through the middle.
It took 7 high tides to close the estuary, so about 200 m3 of sediment was deposited in the estuary at each high tide, which amounts to 400 m3 per day. The Durban area has an average longshore sediment transport rate of 1400 m3 per day (Schoonees, 2000), so the estuary required approximately 30% of the foreshore sediment supply.
The characteristics of the mouth
Build up of sand, deposited during high tide
- Summary
- Modelling the relationship between flow and mouth state
- Flow duration curves
- CONCLUSION
- Introduction
- Question 1: Flow and mouth state
- Question 2: Rainfall and mouth state
- Question 3: Tidal influence
- Question 4: Mechanisms affecting the mouth
- Summation
- Suggestions for further research
An interesting aspect of the gag is that it occurs at high tide trapping a large volume of salt water in the estuary. The closure of the estuary is due to wave action depositing sediment at the mouth of the estuary. While estuaries are open, tidal influence exists despite the established nature of the estuary.
From the conducted research, it was found that the condition of the mouth of the Mhlanga and Mdloti estuaries depends on the water level in the estuary. The relationship between discharges, estuarine condition and abiotic characteristics could be further investigated to obtain an underestimation of the estuary.
Natal Estuaries Status Report No. 21 - The Sipingo Estuary, National Institute for Water Research of the CSIR. Natal Estuaries Status Report No. 20 - The Mzinto Estuary, National Institute for Water Research of the CSIR. Natal Estuaries Status Report No. 2 - The Sandlundlu Estuary, National Institute for Water Research of the CSIR.
Natal Estuaries Status Report No. 9 - The Kaba Estuary, National Institute for Water Research of the CSIR. Natal Estuaries Status Report No. 4 - The Mhlanga Estuary, National Institute for Water Research of the CSIR. Natal Estuaries Status Report No. 65 - The Mkomazi Estuary, National Institute for Water Research of the CSIR.
Report on the state of natal estuaries no. 62 - Amanzimtoti Estuary, CSIR National Institute for Water Research.
