Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

The distribution of rainfall needs to be considered. If the design requires that water be provided throughout the year then the catchment area and storage size must be designed to accommodate the longest dry interval between rain events (Krishna, 2005).

 

In Cape Town the wet period is in the winter and the dry period is during the summer. The highest water usage is also during summer, thus the longest dry period also corresponds to the highest usage period. This makes it very difficult for rainwater harvesting systems to be implemented and achieve complete autonomy. These systems if they were possible will most likely be more expensive due to the high cost of storage.

 

The main aim of rainwater harvesting during the winter would be to help offset the water demand during this period and allow the dams in the surrounding areas to fill up and store water. The water would then be used during the summer periods.

 

Monthly Rainfall

The monthly rainfall that is expected during a month is usually taken as the average rainfall over a 10 year period (Khoury-Nolde, 2008). This is the value that is used to calculate the potential rainfall that will be captured during a particular month. The average rainfall allows the designer to estimate what would be the likely amount of rain that can be collected during a given month but does not take in to account the effects of different rainfall events having different rainfall intensities. I.e. the effects of major and minor storms are not considered, thus the average rainfall for a given month has the potential to be collected within a single rain storm event.

If the rainfall event does not produce more than 1mm (100L per 100m²) during a rainfall event then there exists the probability that very little rain will be harvested from roof surfaces since the first flush diverter will first need to be filled before any water is collected and some of the water will still be left of the collection surface of the roof. This issue is even further compounded for ground surfaces which require an even higher volume of rainfall before conveyance of water can begin. Figure 3-1 shows an example of a monthly rainfall distribution for a particular area.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3‑1: Graph showing the estimated average monthly rainfall distribution for Pinelands from 1985 to 2004

Source: The City of Cape Town Department of Catchment Stormwater and River Management