PURPOSE: To obtain an useful tide embarkment by a method in which the internal pressure of a flexible film rising and falling dam is specifically designated and the variation in the heights of the dam with the variation in the water depth of up and downstreams is lessened.

CONSTITUTION: In a flexible film rising and falling dam, under the condition of upstream water depth=height of dam and downstream water depth=0, the internal pressure P0 of the bottom of the dam is controlled to 1.6≤P0/H0≤2.0, where H0 is the basic height of dam. Since the variation in the dam height with the variation in the water depth of up and downstreams is small, the frequent falling of the dam by small-scale flooding can be avoided, and the overflow vibration of the dam in a high-overflow state can also be avoided. The dam is thus useful for tide embarkment in places where the water depth of downstream is varied.