GUTTERING SYSTEM DESIGN FOR RAINWATER HARVESTING APPLICABLE FOR IBADAN METROPOLIS, NIGERIA

Abstract

The study is on Ibadan (Nigeria) water supply which lags dangerously behind the demands of the fast-increasing population now put at about smillion. Since one source is proving inadequate, an integrated water management scheme (IWMS) is proposed, to accommodate surface and bore-holes as well as rainwater. Guttering, a vital aspect of rainwater harvesting gulps up to 50% of the cost of rainwater harvesting. This work, on guttering system design for Ibadan, has the objective of producing a more efficient and cost-effective design. Rainfall data for 24 years (1980-2003) were collected. The probabilities of accidence and the return periods of the data were calculated using we bull plotting position formula. The coefficient of variation was found to be as low as 17%. The return period of 1.92 years ensures the reliability of the rainfall pattern of Ibadan. A rainfall intensity of 2mm/minute entreasured for any given rain, was used in the design of the gutter and downpipe, using the long established manning’s formula. Ibadan has a bimodal rainfall pattern with annual rainfall intestly of 0.0025mm/minute and with erratic pattern between November and February. Rainfall amount not less than 1315.7mm has a return period of 1.92 years justifying using 1310mm mean anuall rainfall in the design. A semi-circular PVC pipe was used in the design. Gutier diameter as a function of roof area, roof type and slope was found to conform to the equation:

                                                d_g=2,445x10² (A₁C₂ Cosø)^1/2

Where: d= gutter diameter (m), A, = roof plan area (m²), C, =run-off coefficient, o= roof slope (degrees). The downpipe diameter was found to be ds=  3.08

The design gives an efficient shape and configuration, about 20% above what is generally in place as gutter for rainwater harvesting in some buildings in Ibadan.