Calculation of drain spacings for optimal rainstorm flood control

In many parts of the world the frequency of river floods (flash floods) seems to have increased during the past half century. Intensified agriculture is considered as a possible cause for the changed peak flow behavior. It is believed that a large-scale, narrowly designed subsurface drainage reduces the soil water retention in periods with excessive precipitation or snow melt. To increase the soil water retention, it may be necessary to reconsider conventional drain spacing design. The present study deals with the calculation of drain spacings for optimal rainstorm runoff control. A semi-analytical procedure is developed with which for a given extreme rainfall event the drain spacing is calculated that provides the highest possible soil water retention, but no surface runoff. The model considers two-dimensional unsteady water flow between parallel tile drains, with a rising water table. It combines an analytical rising water table model with an empirical spreading water table model. A comparison of the new and a conventional drain design system (Hooghoudt–Ernst) shows that with the newly designed system a considerable temporary soil water retention during heavy rainfall can be achieved. For example, for a soil with a hydraulic conductivity of 0.5 m d-1 that is underlain by an impervious barrier at the 2.0 m depth, and that is drained by tiles with a radius of 0.1 m at the 1.0 m soil depth, an additional soil water retention of 38 mm is obtained when the drain spacing is 46.0 m instead of 13.5 m for a rainfall event of 80 mm in a 4-day period. The newly proposed design system may help to reduce the flood threat in areas with large-scale agricultural drainage in periods with excessive rainfall or snow melt.