Effect of Drain Placement in a Low Hydraulic Conductivity Zone in a Two Layered Soil Profile on Transmissivity of the Shallow Aquifer

An inverse procedure for assessing the transmissivity of shallow aquifers has been used to evaluate the performance of a relatively shallow (1.20 m deep drains) and a relatively deep (1.75 m deep drains) subsurface drainage system in a two layered profile. In the case of deep drains where drains are placed at the interface of two layers, the transmissivity or the hydraulic conductivity is within 20 per cent of the average values obtained with conventional procedures. Such differences are as per expectations. On the contrary in the shallow drainage system where the drains have been placed in less permeable layer, the differences in the values increase to 56 per cent. Such differences cannot be explained due to the uncertainty in the parameters alone. The changes in hydraulics of flow in such a case seems to be responsible for larger decrease in the values. Based on these results and on the basis of spacings calculated for different situations, it is concluded that as far as practical, the drains may be placed at the interface of the layers. If practical situations do not permit, the likely reduction in transmissivity may be considered while deciding the lateral drain spacing.