Evaluation of seepage losses in unlined canals of Bura irrigation scheme

Abstract

The research project addresses itself to the evaluation of seepage losses in unlined canals in the scheme. The objectives of the study, were; to evaluate seepage losses in the conveyance and distribution system, investigate and analyse the factors influencing seepage losses; and develop management and operational strategies for improvement of conveyance efficiency in the supply and distribution systems of the scheme. Seepage evaluation was done in the major conveyance and distribution systems of the scheme, using the inflow-outflow, seepage meter and ponding methods. Seepage losses in the supply canal obtained by inflowoutflow and seepage meter methods are 22.6 l/s/km (1.1 %/km) and 9.49 l/s/km (0.38%/km), represent 39.8% and 14.0% respectively, in the supply respectively. These results loss of the inflow discharge, can aL, which is about 36.8 km long. Results reported from studies on seepage, elsewhere, indicate that seepage losses in the supply canal range from 3 to 86% of the inflow discharge. Evaluation in the night storage reservoirs indicate that seepage losses are 0.39 and 0.28 cm/day on average. Results obtained from other studies on general soils, report seepage to range from 20 to 58 cm/day. Assessment of effects of seepage on ground water rise indicate that the rise range from 0.03 to 0.01 m/year. Thus, it would take the ground water table (estimated at less or equal to 30 ro from ground surface) about 1,000 to 3,000 yars to reach the ground surface. The only adverse effect to be caused by rising ground water table, in the short run, would be expected to occur only in those sections overlying perched water table. Lining cqnsiderations on the conveyance and distribution systems and the consequent unjt cost of lining were assessed. Seepage losses were evaluated to be low. Lining option could be undertaken if cost of lining is less than the un i t cost of water lost thr ough seepage, report ed, as Kshs. 2.05 per metre square. Improvement on water use efficiency could be acheived in Bura Irrigation Settlement Scheme through better operation and maintenance programme in the supply and distribution system.