tidal rise and fall is said to be at a maximum of 37 feet in at least one place on earth/ although a rise/ fall of 4 feet could be sufficient. point being the water weight can be used to generate electricity. IF you create a barrier/ dam, to separate high tide from low tide. water rises and then forces water through the turbine to generate electricity. ideally, the holding area is big enough to continue power generation through the cycle; in both directions. however when the water displacement meets on both sides turning shall stop for a time.