Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Assumptions on collected data and assessment

The following assumptions were made concerning rainfall data that was collected for this research project:

• The data that was given by the City of Cape Town department of Department of Catchment Stormwater and River Management was as accurate and reliable as possible. A warning of caution to be taken with the recorded data was given by one of the city officials;

• Days that were not present in some of the data was assumed to not have had any rainfall on that day;

• The average rainfall that was calculated for each rain gauge station was a good estimate of the actual average in that area. The effect of droughts and wetter periods were considered but due to the complexity of accounting for each of these different climate conditions the use of the average rainfall was assumed to be acceptable;

• All erven in Paarden Island have access to a municipal freshwater supply. This would allow the use of the average rainfall to be used during the design since it would not be critical to have rainwater, supply all the freshwater for the erf; and

• Removal of rainfall events in which the rainfall was 1mm or less. This assumption was justified in that the first 1mm of rainfall that fell onto roof surfaces would be collected in the first flush devices and not contribute to the supply of rainwater.

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Assessment of the annual rain gauge rainfall data 

The results showed what the average rainfall of the various rain gauge stations are and by how much the annual average can vary for different years. Rainfall around Cape Town can be highly variable. Certain rain gauge stations recorded higher rainfall in certain years than other stations and would then record lower rainfall in other years. A period of higher rainfall was visible in 1989 to 1993 which was followed by a drought period from 1997 to 2000.

 

The Athlone rain gauge rainfall data was removed from the data set due to the low number of data points. The modified data for Athlone resulted in a 33% higher average rainfall than Pinelands which is located within 2 km of Athlone. The Athlone data set was also recorded during periods of high rainfall which could have resulted in the higher average.

 

The rainfall data showed that the highest rainfall event average of 43 mm accounted for 8% of the total annual modified average rainfall of 522 mm. The median rainfall event of 5.7 mm was offset to the mean rainfall of 8.5 mm that was recorded. The data also showed that on average only 60 days of the year have rainfall events that are able to contribute to the collection of rainwater.

 

It could then be concluded that most of the rainfall that occur are from a few large rainfall events. Majority of rainfall events that occur have very little rainfall which could result in problems with the collection of rainwater since for over 50% of the rainfall events would have an efficiency of about 80% or less due to the first flush device alone. The higher rainfall events that occur could result is loss of rainwater due to storage tanks being already partially full during these rainfall events and then being filled to capacity before the rainfall event is over resulting in loss of rainwater.  

 

Analysis of modified monthly rain gauge rainfall data

The highest periods of rainfall were mostly recorded between the months of May and August. The results also showed that during certain years, a rainfall event occurred in which the maximum rainfall was much higher than the average monthly rainfall.

 

Calculation of modified monthly rainfall for Paarden Island

The results of the average rainfall figures are used for the calculation of roof and ground surface runoff. The overall modification factor of 0.66 showed that about 1/3 of the rain that falls in Cape Town would not be able to contribute to fresh water supplies from roof surface runoff and even less if ground surfaces were to be included which would require more than 1mm before the ground would actually produce runoff.  Areas that receive higher annual rainfall also tend to have a much higher modification factor which means that most of the actual rain that does fall in these areas will be able to contribute to fresh water supplies. The table below shows the results of the estimated modified monthly rainfall (mm) in Paarden Island using data from worldclim.org.