The nondimensional unapproximated form of the equation describing the motion of the water level in a simple nonthrottled open surge tank operating under the condition of constant hydraulic power to the turbine is shown to have two singular points. One of these is at the steady-flow hydraulic grade-line level and accounts for small displacement phenomena. The other is at a lower elevation and accounts for the occurrence of drainage due to insufficient power being available at the turbine.
An approximate condition for the occurrence of drainage of this type is given for the case of sudden acceptance of full load from a zero-flow condition. This result and the condition for the perpetuation of oscillations of constant amplitude are compared with experimental data obtained by the author from a hydraulic model.