50 HEALTH AND SANITATION
Rainwater catchment and conveyance
A key component of any rainwater harvesting system is collecting rainwater from a catchment surface and conveying it to a tank for storage and future use.
By Water Research Commission
Table 4.1
APPLICABLE CODES, STANDARDS, AND GUIDELINES SANS 10400-L: 2011 The application of the National Building Regulations Part L: Roofs SANS 10400-R: 1990 The application of the National Building Regulations Part R: Stormwater disposal SANS 10252-1:2012 Water supply and drainage for buildings Part 1: Water supply installations for buildings
Rainwater harvesting( RWH) systems most often utilise the roof of a house or building for collecting rainwater. It is possible to collect rainwater from other surfaces such as lawns and parking lots, but these catchments are not addressed in this manual due to concerns surrounding the quality of rainwater collected from these surfaces.
This article focuses exclusively on the collection of run-off from roof surfaces, or roof catchments. Once rainwater has been collected from the catchment surface, it must be conveyed to the storage tank by means of a‘ conveyance network.’
The most common method of conveying rainwater is through the use of gravity flow, whereby rainwater is transported to the storage tank without the use of pumps or other means of assistance.
APPLICABLE STANDARDS AND GUIDELINES The applicable standards found in the South African National Standards are listed in Table 4.1.
SELECTED PROVISIONS AND DESIGN AND INSTALLATION IMPLICATIONS 4.3.2 Gutters and downpipes
RR3 Valleys and gutters
5 Materials, pipes, fittings, components and fixtures 5.1 General 5.2 Pipes and pipe fittings 6.7 Pipes 7.6 Pipe sizing 8.5 Joints 8.6 Laying of pipes
For most RWH systems collecting rainwater from a roof catchment, the size of the catchment area is usually predetermined by the size of the existing house or building. In such cases, one means of collecting additional rainwater is to utilise multiple roof catchments and convey rainwater to one central or‘ communal’ storage tank.
Alternatively, it may sometimes not be feasible or beneficial to collect rainwater from the entire catchment area due to rainwater quality concerns, location / placement of rainwater storage tank, or for other reasons. These and other issues are discussed further in the design and installation guidelines.
CATCHMENT AREA Rooftops The size of the catchment area or roof determines how much rainwater can be harvested. The area is based on the‘ footprint’ of the roof( Figure 4.1), which is basically the sum of the surface area of the building and the surface area of the roof’ s overhang.
Differences in roof shapes do not change a building’ s catchment area. Nevertheless, the slope of the roof determines how quickly water will run-off during and after a rainfall event. A steep roof will shed run-off quickly and more easily clean the roof of contamination.
On a roof with gentle slopes, run-off moves more slowly, raising the potential for contamination to remain on the catchment surface
Figure 4.1: Roof footprint of a building.
October 2018 Volume 24 I Number 8 www. plumbingafrica. co. za