Rainfall
Plate 3-4: The affect ofthe 1987 flood on the sand berm ofMdloti Estuary. (perry, 1989)
4. FIELDWORK METHODOLOGY
4.1. Introduction
During the project a range of data was collected in the field. Water levels and flow rates were a large portion ofthe fieldwork for this project. Surveying was also required and this ranged from the use of a GPS to link the water levels within the estuary to mean sea level, to a photographic survey of the beach to monitor the change in the beach profJle. KZN Wildlife provided daily monitoring of the mouth state, and data on salinity concentrations were obtained by the UND biology department. The methodology and theory are discussed in this section to establish the nature and relevance of the fieldwork.
4.2. Flow measurements
Direct measurements of the inflows into the estuary were made at several stations upstream of the estuary. Initially flow measurements were made using a drogue comprising of a float and a drag vane shown in plate 4-1 a). The method involved timing the drogue, as it travelled a known distance at discrete intervals across the width of the river. Where possible the vane was set to approximately 0.6 times the river depth in order to obtain the average velocity of the vertical velocity profile. The method was limited as it required a uniform straight channel and wind and reeds sometimes interfered with the drogues. Depth was also a limiting factor, as only the surface velocity of shallow water could be measured. Cross-sections were obtained manually using a sounding rod while wading across the river as shown in plate 4-1 b). It is estimated that the error associated with the use of drogues varies from 10 to 40%.
Since February 2003 the Model 3000 Swoffer instrument, shown in plate 4-2, has been used to determine the velocities and the flow rates upstream of the estuaries. The instrument is a propeller type velocimeter which uses a 2 inch propeller and a photo-fibreoptic sensor to determine the velocity of the water from the rotation rate of the propeller. The velocity range of the instrument is from 0.03 to 7.5 meters per second, with an accuracy of approximately 1% (Swoffer Instruments, 2002). The velocimeter is fitted to a wading rod which is used to determine the river depth at discrete intervals across the river width. Both the depth and distance across the width of the estuary are manually entered into the portable micro-computer.
The average velocity is measured at each position, with the propeller set at 0.6 of the river
depth, and recorded in the portable micro-computer. The flow rate through the cross-section is computed by numerical integration.
a) b)
Plate 4-1: The photographs show: a) the drogues and b) the determination ofthe cross-sectional area.
a) b)
Plate 4-2: The plates show: a) the Swoffer instrument and b) the Swoffer instrument in use upstream ofMdloti estuary.
4.3. Water level monitoring
A key aspect of the study of estuarine dynamics is the monitoring of changes in water level.
Initially the water level was monitored using photographs of the M4 bridges at Mhlanga and Mdloti Estuaries. The measured height of the top beam and the height of the column were used
to scale the water height from digital photographs, relative to the pile cap. Figure 4-1 shows the dimensions of the bridges used to scale the water level from the photographs.
880
3640
Mhlanga Mdloti
Figure 4-1: The dimensionsof the M4 bridges over Mhlanga and Mdloti Estuaries.
The digital camera used to take the pictures to determine the water level has a resolution of 1600 by 1200 pixels. Depending on the position from which the photograph was taken relative to the bridge the data extracted from the photographs has an accuracy ranging from 10 to 300 mm. Lower accuracies occurred on days when there was poor visibility when photographs were taken from the beach. The quality of photographic data improved with the discovery of access to the bridges alongside the M4.
The need for continuous water level monitoring became apparent in 2002 as weekly water levels did not provide an adequate level of accuracy in terms of temporal resolution. High resolution records of water level fluctuations can be used to provide a record of when the estuary changes state, (i.e. whether it is open or closed) as well as giving an indication of the volume of water stored in the estuarine system. The data recorded could also show short term changes due to rain events, or when the water level remains constant it isindicative of the system losses, such as seepage, being equal to the inflows into the estuary. Any tidal influx into the estuary when in the open state, would also be visible from the data logged. It was therefore decided that an instrument would be developed to continuously monitor the water level in the estuaries.
The instrument was required to be small, compact and waterproof so that it could be placed below the low water level, out of sight. Itwas important to limit visibility and accessibility to the instrument in order to diminish the risk of vandalism and theft, particularly when the water level is low. Alternatively a big structure could have been used to protect the device, however this was less desirable as it would interfere with the estuary aesthetically, attract unwanted attention and it would be cumbersome to install.
Based on the above considerations it was decided to use a pressure sensor linked to a small data logger, housed in a sealed capsule. The capsule was placed in a perforated container which formed a permanent fixture during the monitoring period.
Figure 4-2 is a schematic diagram of the preliminary design of the water level monitor (WLM), incorporating the use of the Tinytalk TK0702 miniature data logger.
GIOJrd
Power
GroJod
Tinytalk TK0702
Figure 4-2: A schematic diagram which was the basis ofthe WLM design. Where PT is the pressure transducer and VR the voltage regulator.
Inthis application it was decided that logging was to be done at hourly intervals. This time span is more than adequate during the closed phase. However a higher resolution is preferable during breaching and the open phase which can display changes in water level of up to 1240 mm and 300mm in one hour respectively. A compromise between the requirements for the open and closed phases was required as the logger can only store in the region of 1800 data points, therefore the data logger can operate for 75 days set on the hourly interval. The tinytalk logger logs the voltages it receives from the pressure transducer after it outputs a sensing signal pulse to indicate a reading is imminent. The sensing signal was used to trigger the powered warm-up period for the pressure transducer prior to taking the reading, allowing it to stabilise.
The Motorola MPX 5050 pressure transducer was chosen based on availability, function and affordability. A differential pressure transducer vented to the atmosphere can provide a direct water level reading accounting automatically for barometric pressure variations. However a problem arises with visibility of the instrument, as a vent to the atmosphere would be visible above the water line and was therefore undesirable. It was therefore decided to simply determine the water pressure, relative to the pressure within the sealed capsule and to subsequently compensate for changes in barometric pressure during post processing. The pressure transducer has an error estimatedtobe 2.5% of the total range under all conditions.
A problem arose with the initial design of the water level monitor as the sensmg signal generated by the logger only allows for a 150 millisecond warm up period, whilst the pressure transducer requires 15 seconds to warm up. To overcome this problem a PlC microcontroller was incorporated with the technical assistance ofMrRVan Zyl, from the University of Natal Pietermaritzburg (UNP). This microcontroller was programmed to continuously monitor the time between sensing signals and used this time to turn on the voltage regulator for the required 15 second warm up period prior to sampling. Plate 4-3 shows a Tinytalk data logger and the pressure transducer mounted on a printed circuit board designed and built by MrRvan Zyl of UNP and incorporating the PlC microcontroller. The circuitry for the board, the component list and costs are given in appendixD.
The Tinytalk TK0702 has an eight bit resolution; therefore it has 256 levels (from 0 to 255) to measure a 5 meter column of water giving an accuracy of approximately 20 millimetres.
Compensation for changes in atmospheric pressure is required as variations of 200 mm can occur over extended time periods. The manufacturer's specifications are included in appendix E.
a) b)
Plate 4-3: Different views ofthe Tinytalk data logger and the pressure transducer mounted on a circuit board along with the microprocessor, are shown in a) and b).
The pressure transducer also required an external power source in the form of batteries. In determining the battery specifications a compromise between size and durability was required.
The pressure transducer requires a 5 volt supply, and in order for the voltage regulator to supply a regulated 5 volts, an minimum input voltage of 5.5 volts was required, therefore 4 AAA alkaline batteries were used, supplying a 6 volt voltage.
Plate 4-4: The materials for the capsule comprising ofperspex