Hydrodynamic Performance of Pipe Underdrains

Theoretical analysis and laboratory investigations of flexible-flap drain valve in high-velocity open channel flow are presented. Main hydrodynamic characteristics of the valve are: (1)Radial diffuser action, which prevents the valve from opening at low pressures; and (2)uplift forces, caused by high-velocity flow in the channel, helping the valve to open. The most severe condition for the valve operation is when there is little or no flow in the channel, but still high ground-water table around the channel. Flap-valves are found suitable for use in high-velocity flows, provided that the ratio of radii, R₂/R₁, between the circular flap and the valve opening is kept as low as compatible with engineering considerations.