CHAPTER 3: METHODOLOGY
3.7 Stormwater Run-off Analysis
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Figure 3.6.3: Illustration of green roof temperature simulation model
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sufficient per trial. This assumption was first tested on the green roof model taking into account the fact that the growing medium will retain a volume of the water until complete saturation of the medium is attained. Once the benchmark had been tested and proved sufficient, it was applied throughout. The various roof models were placed on a large tray that would serve as a simulated catchment area and allow for the volume of run-off to be quantified (see Figures 3.7.1-3.7.3-graphical representations of the individual model setup). Utilising a measuring jug, the volume of water was passed through the modified polystyrene container and onto the roof models. The volume of water not absorbed by the materials and that which had passed onto the tray was then measured as the run-off volume. The experiment was then repeated three times per roof model in order to determine an average run-off volume for the various roof models. The trials were not conducted consecutively as this would influence the amount of run-off, particularly in the green roof due to saturation of the medium. As a result, the trials were conducted days apart from the preceding trials to ensure that the models would become dry and the green roof model become unsaturated.
Figure 3.7.1: Illustration of green roof model stormwater runoff simulation
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Figure 3.7.2: Illustration of concrete roof model stormwater runoff simulation
Figure 3.7.3: Illustration of tiled roof model stormwater runoff simulation
To test the run-off potential from a concrete roof, the model was cast with gaps between the container and the concrete to simulate drainage conduits i.e. gutters and downpipes that would typically transfer water from the rooftop to the drainage conduit at the base of the building. Theoretically, concrete will not allow water to pass through and if the model had been cast without any gaps it would result in pooling of water until the water had overflowed the container. This explains why water pools in some places on a roof after a rainfall event. However, not all water that falls upon the roof remains behind due to the camber on roof finishes for drainage. Subsequently, to determine the volume of run-off from a concrete roof, similar properties had to be incorporated into the concrete model.
To test the run-off potential from the tiled roof, the roof tile was angled to mimic that of a typical low-cost house. As such, based on gravity and the angle of the roof structure theoretically most of the water should run-off the structure. However, with build-up of
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particulate matter and the absorption from the tile material some water is expected to be retained by the tiled roof. It is based on these factors that each simulation had been tested on a dry tile.
The potential retention of storm water will be quantified in terms of the amount of carbon emissions that can be saved through a reduction in the utilisation of potable water that would have been used as hydration for the green roof. The difference between the average run-off volume of the green roof and that of the other roof models was utilised to determine the average volume of water retained that could be achieved if the green roof was utilised instead. The carbon emission savings are based on the emissions associated with the water treatment process from entry into the dam until entry into the distribution network. These highlight the emissions produced at each phase of the water treatment process which then allows for the total process to be added to determine the carbon emissions per kL for potable water. As such, the carbon emission factors for each process (see Figure 3.7.4) were added together and multiplied by the difference in average run- off from the roof models to determine the quantity of carbon emissions that are emitted and hence, the quantity that can be saved if a green roof is utilised upon the other roof type. This is further based on saturated volumes of the green roof that would be attained during a rainfall event and thereafter not require the use of potable water as a means to hydrate plant life and in addition, a reduced water volume going through the water treatment process.
Figure 3.7.4: Carbon emission factors associated with the water treatment process (after Buckley, et al., 2009)
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In order to model the maximum potential carbon emission savings from a roof area in the City of Durban, rainfall data was required. The data was obtained from readings of a rain gauge set up at the City Engineers Complex (see Figure 3.7.5) which is located in the Central Business District of Durban. The readings of the rain gauge are uploaded to a website from which the data can be sorted into hourly, daily or monthly data. This experiment utilised average monthly rainfall data from the beginning of the year 2013 to the end of year 2015, in order to determine an average monthly rainfall data set.
Figure 3.7.5: Location of rain gauge station (Source: Google Earth, 2016)
In summary, the Chapter above represents detailed methodologies and experimental procedures utilised to obtain and quantify the data presented in Chapters 5 through 9. It aims to give the reader an understanding of the processes involved to promote understanding of the information presented further on in the thesis.
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