The relaxation of pore pressure in the core of a rockfill dam, that is caused by a rapid discharge of the water from the reservoir, is controlled by two different time scales. The time scale due to de-consolidation is irrelevant, whereas the time scale due to the displacement rate of the phreatic surface is equal to the ratio between the conductivity and the porosity. If the core is vertical and slender, the motion of the phreatic surface is governed by an ordinary first order differential equation. If the discharge is instantaneous, the equation is separable and has been solved. Experiments with a Hele Shaw cell show that the agreement between theory and experiment is not too good. The explanation is that the discharge of liquid from the reservoir in the Hele Shaw cell is not effectively instantaneous. The practical result implies that the relaxation time is approximately 1 year for a core of a real dam composed of soil whose height and width are 50 and 4 meters, respectively.