52 HEALTH AND SANITATION
Hence, the larger the catchment area, the greater the quantities of rainwater collected per millimetre of rainfall.
Impervious areas Rainwater that falls on impervious surfaces such as sidewalks, driveways, parking lots, and streets, can also be channelled to an underground tank.
a.
b.
The curve number method The method was developed by the United States Department of Agriculture Soil Conservation Service( now Natural Resources Conservation Service), from an empirical analysis of run-off from small catchments and hillslope plots( NRCS, 1986).
The major disadvantages of the method are sensitivity of the method to Curve Number( CN) values, fixing the initial abstraction ratio, and lack of clear guidance on how to vary Antecedent Moisture Conditions.
The method is used widely and is accepted in numerous hydrologic studies and models such as the SWAT model( Neitsch et al., 2002).
Run-off coefficient method The run-off coefficient( C) is a dimensionless coefficient relating to the amount of run-off to the amount of precipitation received. It is a larger value for areas with low infiltration and high run-off( pavement, steep gradient), and lower for permeable, well vegetated areas( forest, flat land).
CATCHMENT MATERIAL The roofing material is the outermost layer on the roof of a building, sometimes self-supporting, but generally supported by an underlying structure. A building ' s roofing material provides shelter from the natural elements.
The outer layer of a roof shows great variation dependent upon availability of material, and the nature of the supporting structure. Those types of roofing material which are commercially available range from natural products such as thatch and slate, to commercially produced products such as tiles and polycarbonate sheeting.
The most common roofing material can be categorised as:
• Thatch— roofing made of dry vegetation such as straw, water reed, sedge( Cladium mariscus), rushes, or heather. This type of roof is common in both urban and rural South Africa.
• Shingle or roofing slate— the generic term for a roofing material that is in many overlapping sections, regardless of the nature of the material, are made of various materials such as wood, slate, flagstone, fibre cement( in the past, the fibre in the cement material was asbestos which has been banned for health reason), metal, plastic, and composite material such as asphalt shingles. Ceramic roof tiles, which still dominate in Europe and some parts of Asia, are still usually called tiles.
• Membrane roofing— a type of roofing system for buildings and tanks. It is used on flat or nearly flat roofs to prevent leaks and move water off the roof. Membrane roofs are most commonly made from synthetic rubber, thermoplastic( PVC or similar material), or modified bitumen. Membrane roofs are most commonly used in commercial application, though they are becoming increasingly more common in residential applications.
• Metal roofing— a type of roofing system made from metal pieces or tiles. Metals used for roofing are: lead; tin and aluminium; copper; galvanised steel; blend zinc, aluminium and silicon-coated steel; stainless steel, and so on.
• Concrete or fibre cement, usually reinforced with fibres of some sort. Concrete tiles are made from sand of various grading and cement fibre cement is a composite building and construction material, used mainly in roofing and also façade products because of its strength and durability.
• Structural concrete roofing are usually used for flat roof constructions of large buildings. The three main categories of structure concrete roof are: precast / prestressed, cast-in-place, and shell.
The type of catchment material used by an RWH affects the proportion of rainfall collected during a rainfall event, defined as the‘ collection efficiency’ from the roof catchment, and the quality of harvested rainwater. In South Africa, most houses are mostly covered with ceramic tiles, concrete tiles, corrugated iron, and thatch. Structural concrete mostly cover administrative buildings.
a.
Run-off coefficient Theoretically, one litre of run-off can be collected from each millimetre of rainfall falling on a one m 2 surface area. In reality, some losses occur following contact with the catchment surface. These losses vary depending on the type of catchment material and the geometry of the roof and should be considered when estimating the amount of rainwater that can be collected and utilised by the RWH system.
These losses can be characterised by:
1. an initial loss factor( in mm of rainfall) due to the absorbency of the catchment material, and continuous losses( in percentage of rainfall) from wind and leaks;
2. the run-off coefficient— a dimensionless value that estimates the portion of rainfall that becomes run-off,
October 2018 Volume 24 I Number 8 www. plumbingafrica. co. za