Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Assumptions

The following assumptions that were made concerning the roof surface in Paarden Island included:

• It was assumed that the GIS Data that was given was as reliable as possible. The cadastral data that was received was dated August 2011 and since that time there have been a few developments that have occurred which have modified the layout of the ground surfaces with some of these surfaces now having been built on. Thus for this thesis the volume of potential rainwater that could be collected was thus estimated for Paarden Island August 2011;

• It was assumed that all ground surface runoff would be collected using the stormwater network that is already in place;

• It was assumed that all ground surfaces that were delineated where able to collect rainwater. This is not entirely true however since some of these surfaces are not connected to stormwater pipe systems and flow directly into the stormwater channels. The current stormwater system flows in two directions, one half of the system flows into the Salt River and the other into a stormwater channel that flows into the harbour.

• The difficultly in capturing the ground surface runoff was also not considered in the calculation of the rainwater collection. It would be very difficult to capture all the rainwater that landed on the ground surface since most of the stormwater pipes lead into the Salt River or stormwater channels at various point along their lengths. In other words there is no single drainage point from which the water could be captured;

• It was assumed that all the rainwater that comes from the ground surfaces could be used. The second difficulty with this method is that the storage of water that comes from the ground surfaces may prove problematic since there is very little land available in Paarden Island that can be used to build water reservoirs for storage;

• It was assumed that the amount of rainwater that could be collected within a month was collected over the whole month and not just on the few days that it rained. The number of days in which rained occurred during certain months varied from between 1 to 10 days as shown in tables 9-6 to 9-8; and

• It was assumed that the amount of rainwater that was collected for each month would be a reliable source of freshwater. This assumption would not be accurate since annual rainfall varied considerably from year to year and that some years received above average rainfall and other below average rainfall.

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Delineation of ground surfaces in Paarden Island

The delineation of the ground surfaces where done in ArcGIS using the cadastral GIS data that was sourced from the UCT GIS Department. Using the terrain map different types of ground surfaces were identified which include roof surface, paved or tarred surfaces, gravel, and grass and vegetated areas. The area of each ground surface was then calculated using the ArcGIS geometry calculator.  Figure 5-4 shows the delineation of various ground surfaces.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                         Figure 5‑4: Delineation of various ground surfaces in Paarden Island

 

Estimation of ground surface runoff coefficients

All roof surfaces larger than 50m2 were eliminated from the catchment area since they were already accounted for and would not contribute to the ground surface rainwater.

Paved and tarred areas

Paved and tarred ground surfaces were given a runoff coefficient of 0.8 based on the recommended values in table 3-2. Most of Paarden Island is classified as light industry and has a substantial amount of paved and tarred surfaces. The only infiltration areas from the paved surfaces come from the cracks that have developed over the years of use.

Gravel areas

The gravelled surfaces were given a runoff coefficient of 0.17 based on the recommended values in table 3-2. Gravelled areas located in Paarden Island are highly compact due to the area being constantly used by heavy vehicles that travel over the gravelled surfaces. The natural landscape of Paarden Island is relatively flat with very little change in altitude with in Paarden Island.

Grassed and vegetated areas

Grassed and vegetated areas were given a runoff coefficient of 0.15 based on the recommended values of table 3-2. Most of the grassed areas in Paarden Island are located near the Salt River, in the park (Mosselbaai Street and Lowestoft Street) and along Marine Drive. The soil is a dense with thick grass growing in most of these areas.

 

                      Table 5‑2: Summary of runoff coefficients used for various ground surfaces in Paarden Island

 

 

 

 

 

Calculation of potential rainwater that can be harvested from ground surfaces

The rainwater that can be harvested is calculated using equation 3.1. The value of the runoff coefficient for all different ground surfaces are shown in table 5-2. The result of these calculations can be seen in table C-2 and are summarised in table 5-3.

 

A total of 232 000 kL of rainwater can be collected from the ground surface runoff. 94% of the ground surface rainwater comes from the tarred and paved surfaces. Ground surface rainwater is able to supply almost 44% of the water demand for Paarden Island. Based on the standard deviation of annual rainfall in Cape Town in Appendix A the yearly collected amount of rainwater can vary by as much as 28% from the average. The estimated collectable rainwater per annum varies from 167 000 kL to 297 000 kL. Ground surface runoff rainwater does have a number problems associated with it. The collection of ground surface runoff is very difficult in Paarden Island since there is a lack of a single drainage point. Based on the layout of the stormwater network in Paarden Island as shown in figure 5-5 there seems to be a watershed that has formed. The stormwater in Paarden Island either gets taken to the Salt River or it is discharged in the stormwater channel that runs through the south western section of Paarden Island. The stormwater pipes in Paarden Island do not form a single larger network of pipes but rather many smaller stormwater pipes that separately discharge themselves into the river or channel.

 

Ground surface runoff will also be more contaminated than roof surface runoff. Most contaminants will come from the tarred and paved surfaces which represents a large portion of the rainwater supply. Most contaminants will come from motor vehicles that used the roads. Contaminants will also come from erven due to spillages that occur on the factory floor and are then washed away into the stormwater system during a rain event. The following section Chapter 5 Section 2 identifies more sources of contaminants for ground surface runoff.

Treatment plants and storage facilities that would be required to be implemented in Paarden Island to facilitate ground surface runoff would require large amounts of land area. Currently Paarden Island has very little free land available to implement these facilities and would instead have to purchase private land which could be a large additional cost.

 

The harvesting of ground surface rainwater would also have to be implemented by the municipality since the rainwater is collected from various erven owned by different parties. The rainwater that is harvested would also be too great for any single business entity to consume and thus would require access to the municipal water supply network so that the collected and treated rainwater can be redistributed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                              Figure 5‑5: Location of stormwater network and ground surface drainage flow

 

                         Table 5‑3: Summary of potential rainwater harvesting from ground surfaces in Paarden Island