Choice of Pipe Material Influences Drain Spacing and System Cost in Subsurface Drainage Design

Abstract

HighlightsFor 3- or 4-row regular-perforated pipes, the effective radius ranged from 0.3 to 0.9 cm (average 0.6 cm).For 8-row regular-perforated pipes, the effective radius was 1.9 cm.The effective radius of sock-wrapped pipes ranged from 5.7 to 6.0 cm (average 5.9 cm).The 8-row sand-slot pipes had a lower initial system cost than the sock-wrapped pipes.The 8-row regular-perforated pipes had a lower initial system cost than the 4-row regular-perforated pipes.Abstract. Knitted-sock envelopes are applied in agricultural subsurface drainage to prevent sediment clogging of the drain pipes. In the United States and Canada, sand-slot pipes are sometimes used as a cheaper alternative to sock-wrapped pipes. However, their initial system cost has not been compared. The main objective of this study was to evaluate the effect of pipe material on drain spacing and initial system cost. First, the theoretical effective radius of each pipe material was estimated. Then, the drain spacing was calculated for each pipe material in a drainage design, such that each pipe would provide the same design drainage rate (i.e., same water removal rate). The results showed that the effective radius of sock-wrapped pipes (average 5.9 cm) was much greater than that of 4-row (average 0.4 cm) and 8-row perforated sand-slot pipes (average 1.6 cm). Rows refer to number of longitudinal rows of perforations. The sock-wrapped pipes considerably increased the effective radius of the pipe by reducing the entrance head loss. Furthermore, the sock-wrapped pipes allowed for a wider drain spacing (ranging from 0.8 to 5.4 m wider) in soil with risk of drain sedimentation, thereby reducing the total length of the lateral drain pipe needed for drainage design compared to both 4- and 8-row sand-slot pipes. The 8-row regular-perforated pipes allowed for a wider optimum drain spacing, thereby reducing the initial system cost in soil without a drain sedimentation issue compared to 4-row regular-perforated pipes. In conclusion, even though the sock-wrapped pipe reduced the total length of the lateral drain pipe, the 8-row sand-slot pipe had a lower initial system cost compared to the sock-wrapped pipe, when designed at the same design drainage rate and drain depth. Keywords: Drain spacing, Effective radius, Entrance resistance, Geotextile, Knitted sock, Perforation.