When an Unsurfaced Track Becomes Impassable: Gully Growth per Season and the Maintenance Interval That Prevents It

An unsurfaced track on sloping ground does not wear out slowly and evenly. It collects water, water cuts wheel ruts into channels, and the channels deepen each wet season until a vehicle that used the track last year cannot cross it this year. This paper computes that timetable and the maintenance that prevents it. For a stated hillside track (gradient 5%, width 3.0 m, contributing upslope width 20 m, wet-season rainfall 600 mm, runoff coefficient 0.35, silty clay loam soil), the runoff reaching each metre of track is 4.2 cubic metres per season. At a stated sediment transport concentration of 80 kg per cubic metre and a delivery factor of 0.55, the effective incision concentrated in the wheel ruts is 0.237 m, or 23.7 cm, per season. Against stated vehicle limits, a pickup with 0.21 m ground clearance fails at a rut depth of 0.18 m, reached in 0.8 seasons, and a hand cart fails at 0.25 m, reached in 1.1 seasons: on this track, with no drainage and no maintenance, both vehicles are lost within about one wet season, the pickup first. Cross-drains that limit flow accumulation hold the track passable: spacing for at most 10 mm of incision per season is 11 m, and the recommended spacing of 10 m gives 0.95 cm per season, with 100.0 drains per km. The holding program costs 6.0 labour-days per km per year, against 12.0 labour-days per km per year for repeated grading without drains, after a one-off drain installation of 30.0 labour-days per km.