Analysis of three-dimensional ponded drainage of a multi-layered soil underlain by an impervious barrier

A general analytical model is proposed for predicting three-dimensional seepage into ditch drains through a soil column comprising of three distinct vertical anisotropic soil layers and underlain by an impervious barrier, the drains being fed by a distributed ponding head introduced at the surface of the soil column. The problem is solved for three different situations resulting from three different locations of the water table in the ditches, namely, when the water level lies in the first layer, when it lies in the second layer and finally when it falls in the third layer. The derived solutions are validated by comparing with analytical solutions of others for a few drainage scenarios; in addition, a few numerical checks on them have also been carried out by making use of the Processing MODFLOW environment. From the study, it is seen that ponded drainage of a multi-layered soil is mostly three-dimensional in nature, particularly in locations close to the drains and that the directional conductivities of the layers play a pivotal role in deciding the hydraulics of flow associated with such a system. Further, it has also come out of the study that by suitably altering the ponding distribution at the surface of the soil, the uniformity of water movement in a multi-layered drainage system can be considerably improved mainly if the directional conductivities of the bottom layers are relatively lower than those of the top layer. As soils in nature are mostly stratified and as no analytical solution to the three-dimensional ponded ditch drainage problem currently exists for a layered soil, the proposed solutions are expected to be important additions to the already existing repertoire of drainage solutions on the subject, particularly when looked in the context of reclamation of water-logged and saline soils in layered field situations.