Abstract
A laboratory soil tank was used to determine the effect of the drain tube perforations on transient drainage and subirrigation processes. Seven corrugated plastic drain tubes with varying total perforation areas and locations were tested. Experiments were also conducted with a standard drain tube surrounded with a 5-cm thick gravel envelope. Drainage and subirrigation experiments were conducted for three combinations of initial and final water table positions. The pressure head distribution, drainage and subirrigation flow rates were measured continuously during each test and the results are reported. There was an increase in drain flow, drawdown and subirrigation rates with increase in total perforation area up to an area of 38.5 sq cm/m. Doubling the perforation area 79 sq cm/m further increased the drawdown rate and had about the same effect as the use of a 5 cm gravel envelope for the cases tested. Tubes with smaller perforation areas had a greater proportion of the total hydraulic head loss occurring near the drain for both drainage and subirrigation. The water table was backed up over the drain for tubes with smaller perforation areas (1.8 sq cm/m - 8.8 sq cm/m) for more than 40 h after drainage began for all conditions tested. Location of the drain tube perforations had only a small effect on drawdown, drainage outflow and subirrigation rates. More work is needed to determine the effect of hole locations on transient state drainage. (Author 's abstract)