Siphons

"Many older dams were not constructed with a means of draining the reservoir. Lowering the reservoir may be needed for temporary construction, for emergency response, or when regular releases aren't needed. Siphons can be a low-cost means of providing a reservoir outlet if one does not exist. The key operational parameters are: (1) the required hydraulic lift cannot exceed the effective local atmospheric pressure adjusted for vapor pressure and frictional losses; (2) the discharge point of the siphon must be lower in elevation than the body of water to be siphoned; and (3) the pipe or hose used for the siphon must be designed to operate at less than atmospheric pressures. Note that siphon outlets can be difficult to prime."^[1]