Improving subsurface drainage design: minimising the environmental impacts

This paper addresses how the design of subsurface drainage systems impacts on the drainage volume and salinity of drainage water generated. The effects of drain depth and spacing are reviewed and discussed. The conceptualisation of a subsurface drainage system which incorporates drainage water quality into the design is presented. This system, known as a Multi-Level Drainage System, aims to minimise offsite impacts associated with subsurface drainage while still providing adequate protection from waterlogging and salinity of the plant root zone. This is achieved through the use of a shallow closely spaced drainage system (0.7 m deep at 3.3 m) underlain by a deeper, widely spaced drainage system (1.8 m deep at 20 m). Field investigations show that the shallow drains had approximately five times lower salinity than deep drains, with median values being 5.5 dS/m and 28 dS/m respectively. The results indicate that, by re-thinking subsurface drainage design to incorporate water quality aspects, alternative designs can be formulated which go some way to meet the present day environmental constraints placed on subsurface drainage systems.