Abstract: A valve is installed into an underground conduit to control drainage of water through that conduit. The drainage control valve includes a perforated plate across the conduit. The latter plate is formed with a peripheral groove rectangular in cross section and a hollow tube made of flexible material is inserted into this groove to seal the joint between the plate and the inner surface of the conduit. A flap also made of flexible material is applied against the upstream surface of the plate and is formed with a peripheral edge taper whereby pressure applied on the flap by water upstream of the drainage valve forces the edge taper against the upstream plate surface to form a sealed joint between these flap and plate. The upper portion of the flap is attached to the upper portion of the plate through screw and nut assemblies. These assemblies traverse rigid washers inserted in respective holes made in the flap.