Abstract: For the purpose of providing a more economical alternative to conventional control gates for the quasi-permanent closure of all or part of the length of the overflow spillway of a dam except when discharging extraordinary floods, the invention consists of installing on the sill of the spillway a water level raising means comprising at least one heavy element, held by gravity on the sill, the water level raising means or its element being designed to overturn at a predetermined head corresponding to a headwater level not higher than the predetermined maximum water level in order to discharge extraordinary floods, smaller floods being discharged by a separate flood discharge structure.

Abstract: For the purpose of effecting a quasi-permanent raising of the normal water level of an impounded reservoir and thereby augmenting its storage capacity except during the passage of major floods, the invention consists of rigidly fixing to the sill of the spillway a water level raising means comprising at least one vertical plate capable of resisting the water loads when spilling moderate heads (for discharging the floods of shorter recurrence intervals) but bending at its lower portion where it is fixed to the sill at a predetermined head not higher than the maximum water level in order to discharge larger floods.

Abstract: For the purpose of effecting a quasi-permanent raising of the normal water level of an impounded reservoir and thereby augmenting its storage capacity except during the passage of major floods, the invention consists of installing on the sill of the spillway a water level raising means comprising at least one heavy element, the said means or water level raising elements being capable of resisting the water loads when spilling moderate heads (for discharging the floods of shorter recurrence intervals) by virtue of their own weight but breaching by overturning at a predetermined head corresponding to a level not higher than a predetermined maximum water level in order to discharge larger floods.