Pressure drop and consecutive air demand behind high head gates during emergency closure is studied by physical and mathematical models. Measurements are done on hydraulic model of a leaf gate installed in the intake structure of a penstock. Local loss coefficients are determined as functions of Reynolds number and gate openings from measurements of discharge and piezometric levels at static positions of the gate. A mathematical model for the unsteady flow due to closing gate is formed by applying the integral continuity and energy equations on control volumes upstream and downstream of the gate. Dimensionless numbers relevant to the problem are obtained by dimensional analysis of the governing equations. Timewise variations of air discharge in the ventilation shaft and pressure behind the gate are obtained from numerical solution of the model equations. The relative air demand is computed over substantial ranges of dimensionless parameters and some design considerations are discussed.