Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Roof surface runoff

This type of rainwater harvesting technique captures rainwater that falls on the roof of a structure. It is usually considered to be of a much higher quality than most other techniques and the quality of the rainwater is usually dependant on how clean the roof surfaces are (Khoury-Nolde, 2008, p. 3) and what material the roof is comprised of (UN-HABITAT, 2005, p. 11). Most of the contamination from roof runoff comes from dust that is blown onto the structure as well as biological contaminants such as leaves and bird excrement.

Ground surface runoff

This type of rainwater harvesting technique is used to capture large amounts of rainwater but also has the highest potential for contamination due to the types of surfaces that the water will be collected off such as urban roads which have high levels of oils, heavy metals and other pollution sources (Bannerman, et al., 1993). The efficiency of the rainwater yield will depend on what types of surfaces are used (Porter, Persyn & Silvy, 2008), from concrete paving which has a high runoff coefficient to loose vegetated soil which will have a low runoff coefficient. 

According to the South African Road Drainage Manual (2006, pp. 3.4-3.9) the surface runoff is dependent on a number of factors which include:

• Topographical factors;

  • Size of catchment;

  • Catchment shape;

  • Catchment slope;

  • Stream patterns;

  • Infiltration;

  • Soil type and geology;

  • Seasonal effects of vegetation.

• Antecedent soil moisture conditions;

• Developmental influences; and

  • Land Use;

  • Storage; and 

  • Reservoirs.

• Climatological variables.

  • Climate;

  • Rainfall as a flood parameter; and

  • Time and area distribution of rainstorms.

Groundwater recharge

This type of rainwater harvesting technique is used to recharge groundwater by infiltrating rainwater runoff into the soil at the source (Mechell & Lesikar, 2008) and storing it in the ground to be used when needed. The quality of the groundwater will depend on where the surface runoff is collected off and what kind of pre-treatment was done. Great care must be taken to ensure the groundwater is not contaminated.