Effect of local ground slope on the performance of tile drains in a clay soil

Abstract

The hydrograph response of tile drains laid in a heavy clay soil on a convex slope is described. Drainage efficiency is shown to be inversely related to local ground slope, with short-term storm outfall on a 5·0% slope only 0·55 that on a 3·6% incline. Peak drain discharge is also inversely related to ground slope, particularly during winter when water contents are high. However, the drain hydrograph shows a quicker response to rainfall on steeper slopes. The time of concentration is reduced by 1·5 h for every 1% increase in ground slope. This differential behaviour of tile laterals serves to indicate the pathways taken by rainwater once it enters the soil. Desiccation cracks are used to bypass the topsoil in autumn. But once the soil has been swollen by autumn rain, plough-layer interflow becomes a more significant route for drainage. Water passes to the drainage installation down the looser soil overlying the tile laterals or down persistent cracks left by the leg of the mole plough after lateral flow within the topsoil. The catchment of an individual tile drain is shown to be inversely related to local ground slope, at least in winter. This is due to the drainage divide shifting upslope in response to increasing slope gradient. In convex situations this results in a loss of contributing area on the downslope side of the lateral that is not compensated for by commensurate gains on its upslope side. Short-term drainage efficiency, that is the percentage of incident rainfall which is removed as drainflow within 40 h, falls from 82 to 45% as slope increases from 3·6 to 5·1%. Since the object of a drainage installation is rapid evacuation of rainwater, this unexpected effect of local ground slope in reducing drain efficiency has considerable significance for the design of underdrainage schemes in undulating topography.