Rainwater Harvesting
A joint effort of:
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Purpose-build 40 sqm corrugated-iron ‘roof’ catchment from Botswana
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Streched-woven hessian sacks used for a 6 sqm ‘Sarisa’; purpose-build catchment in Kenya
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All drawings taken from: Gould/Nissen-Petersen «Rainwater Catchment Systems for Domestic Use» |
Gutters that are horizontal or sloping away from the tanks (1)
Overflow pipes placed well below the top of the tank (2)
Outlet taps high above the base of the tank (3)
Down pipes wasting water (4)
Only part of the roof area being used (5)
Less storage capacity (6)
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A better design and the explanation of the weak points see below!
The poorly designed system above is probably only about 10 per cent efficient! Less than a quarter of the roof area is being effectively utilised, and only half of the storage capacity.
Unfortunately, this is based on a real example!
Important points:
1- Gutter slope
2- Height of overflow
3- Height of tap
4- Efficient catchment area
5- Storage efficiency
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Sheet-metal (estimate):
0.8 - 0.85
Cement tile:
0.62 - 0.69
Clay tile (machine made:
0.30 - 0.39
Clay tile (hand made):
0.24 - 0.31
see also: sizing2, supply side calculation with runoff coefficient
Source: Gould/Nissen-Petersen