DESIGN OF SUB SURFACE DRAINS

Abstract

The basic purpose of sub surface drainage system in irrigated areas is to provide a soil moisture regime conducive to better plant growth. Where the :deep percolation from irrigation is common, the water table may rise very rapidly into the root zone and even to the soil surface. In such case, the function of a sub surface drainage system would be to lower the water table within the root zone fast enough after irrigation to avoid damage to the crop. The design criteria of sub surface drains accounting falling water table are based on the unsteady state formula. One of the, unsteady state formulae for determining .• depth and spacing of parallel drainage system has. been derived by Glover (1954), which popularly known as U.S.- Bureau of Reclamation formula. The formula has been derived on the basis of the Dupuit-Forchheimer assumption. The cost of drains increases with 'depth of placement of drains because of increase in excavation cost. On the - other hand the shallow drain will require closer drain spacing and the drainage system would require more number of drain pipes leading to increase cost of material. In this study, the economical depth and spacing of parallel drains have been. determined. The .economical design is based on the.Glover's formula. Two cases have been dealt. In the first case, the recharge originates only from local irrigation application and in the second case the recharge originates from local as well as external source.