Shaft Spillway

in hydrotechnics, an automatic tubular spillway used chiefly to discharge excess water from reservoirs with dams, from canals, and from the head basins of hydroelectric power plants. The inlet of a shaft spillway is usually flared and is submerged below the level of the head in order to prevent the entrapment of air, litter (such as driftwood), and ice. The advantages of a shaft spillway include the large carrying capacity and the feasibility of constructing the spillway after the main construction has been completed, for instance, in the event it becomes necessary to expand the spillway facilities.