Some factors affecting flow into drainpipes

Factors influencing flow into subsurface drainpipes were investigated in an experimental tank. Where available and experimental results were compared to theory. It was found that doubling drain diameter from 2 to 4 inches increased the flow 35 to 60 per cent. An increase of drain diameter from 2 to 7.48 inches caused a 90 to 130 per cent increase in flow depending on the water-table height. Decreasing pipe-segment length from 3 to 1 foot increased the rate of flow into the pipe by 2½ times. Wrapping the pipe with glass fiber reduced the effect to about a 50 per cent increase.

Discharge rates are greater when the holes are at the bottom of the pipe because of the greater hydraulic head. When outflow is adjusted for the hydraulic head there is no difference due to location of holes. In all the measurements, except for the largest drain and for the lowest reservoir level, the water table did not intersect the drain but was above it even though the drains were not running full. The only observable effect of various water levels in the drains (open, one-quarter or one-half full) was due to a slight change in the total hydraulic head. The effect was negligible for the cases investigated. Flow into the drains was directly proportional to water-table height at the midpoint.