Acknowledgements
4. Methods
4.3 Data collection and processing
4.3.7 Stream level data
Stream flow data were initially only required for model calibration and dry weather baseflow estimation; long-term stream level data were obtained from CCT to fulfil this requirement.
However, since CCT only operates one stream gauge in the study area, water level sensors were installed as part of this study to obtain additional data. Later in the study, data from the level sensors were also used in the EMC estimation process (explained in Section 5.5.1).
Development of the water level sensors and data processing of all water level data are explained in the following sections.
4.3.7.1 Long term data from CCT
The only long-term stream data available in the study area is from a CCT stream level gauge, KEYS05DR, located in the central region of the study area. This station measures water depth in the channel using an ultrasonic sensor, installed under the culvert where the river crosses the M3 freeway (Figure 4-5). According to CCT, a rating curve was not available for this site.
Manning’s equation (Equation 4.1) was used to approximate volumetric flow from the water depth. The use of Manning’s equation requires a prismatic channel of relatively constant roughness. This channel has a natural lining and is not prismatic throughout its length. Although the conditions were not ideal for the use of Manning’s equation, it was used as there was no other long-term data to use for model flow calibration, and more detailed assessments that required additional site measurements were difficult to execute as these calculations were done during the initial Covid lockdown.
(4.1)
where Q = flow (m3/s); A = cross-sectional flow area; P = wetted perimeter; S = slope and n = Manning’s roughness coefficient.
Q = A5/3P3/2S1/2 n
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Figure 4-5: The Keysers River flowing underneath the M3 freeway through Dreyersdal (Google Maps, 2022)
Figure 4-6: The Keysers River just upstream of the M3 culvert. The vegetation growth obscures the view of the culvert (Source: Geordie Thewlis)
ZA Ghoor: Managing nutrient flows into the Zandvlei Estuary, Cape Town using Sustainable Drainage Systems (SuDS)
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4.3.7.2 Development of level sensors
Since stream level data was only available at one location throughout the study area, a part of this study was dedicated to the development and installation of ultrasonic flow level sensors to generate additional stream data. A programmer with experience in constructing sensors for river monitoring applications was consulted. The design and set-up of the required sensor was subject to the following considerations:
• Safety – the set-up had to be small or inconspicuous enough to be easily hidden or disguised to reduce the risk of it being stolen.
• Cost – the limited project budget necessitated a low-cost solution, thus precluding the selection of expensive components.
• Travel limits – a set-up that limited the amount of times required to travel to the monitoring sites was preferred; this would lower cost for the current and future projects that would utilise these sensors.
After a series of meetings with the programmer, the following set-up was selected: an ultrasonic sensor programmed with a low-cost MCU (microcontroller unit) using the GSM (global system for mobile communications) network to transfer data. The device was battery powered. This was chosen for the following reasons:
• Ultrasonic sensors are cost-effective, easy to install and can be set up to provide contactless readings (Abdelal & Al-Hmoud, 2021).
• Transferring data to a server using the GSM network, instead of storing it on the device itself, saves time and money by limiting trips to the monitoring sites and also ensures the data cannot be lost in the event that the device is stolen or tampered with.
• Batteries were the most convenient and cost-effective choice of power supply; although they needed to be recharged and replaced, they were a cheaper and safer option than solar panels, which would attract attention and be vulnerable to theft.
After consulting with various outlets, components were selected based on cost, availability and salesmen recommendations. Once all the components were gathered, the programmer assembled and coded the device.
The ultrasonic sensor was made up of a transmitter and receiver; the transmitter emitted ultrasonic sound waves at a known speed towards the water surface, which reflected them back to the receiver. The receiver used the speed and return time to calculate the distance to the water surface. Using a SIM card and the GSM module, this measurement was sent to a web server via a communications provider, using the GSM network (colloquially known as the cellular network). These processes were dictated by the microcontroller based on the original input
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coding. The depth of the water in the channel was calculated by subtracting the reading given by the receiver from the distance between the sensor and the bottom of the channel.
For this study, sensor units were powered by a set of eight rechargeable lithium-ion batteries. The device transmitted readings every 30 minutes, entering sleep mode in between transmissions to preserve battery life. During initial tests, the batteries lasted 7-8 days before needing to be recharged. The components were arranged in a plastic container, with a hole drilled through the centre to allow the protrusion of the ultrasonic sensor. The arrangement of the components in the container is displayed in Figure 4-7. The containers were closed with plastic bolts and attached to concrete culvert soffits with a two-part epoxy adhesive.
Figure 4-7: Flow sensor setup
Locations to place the sensors were chosen based on strength of network signal, safety and ease of access (to replace batteries) and availability of a mountable surface. Additional considerations included safety of the unit against theft and being washed away by flooding.
Water level sensors were placed at one location in the lower reaches of each river system.
The Keysers River sensor was placed underneath the Tokai Road bridge, just downstream of the river’s intersection with Main Road (Figure 4-8), while the Westlake sensor was placed underneath the bridge that enters the Camargue housing complex off Kerner Close, just upstream
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of Main Road. Both these locations were used as sample sites; their descriptors are Site K2 and Site W5 respectively. A full list and map of the sample locations are given in Section 5.2.1.
Figure 4-8: Replacing batteries in the Keysers River level sensor
Water levels were checked by measuring the water level with a custom-made metre stick (ruler) whenever the batteries were changed, and then comparing these values to the sensor readings recorded at the same time. Levels were within 20 mm for the Westlake sensor, but the Keysers sensor showed differences of up to 300 mm and sometimes the level did not change during a storm. The Keysers level data were deemed unreliable and therefore discarded. Unfortunately, the programmer was not available to help fix the sensor during the data collection phase. The level sensor locations are displayed in Figure 4-9.
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Chapter 4: Methods
Figure 4-9: Location of level sensors and CCT stream gauge