Complex sentence: When a tide higher than 110 cm is forecast, compressed air expels the water from the gates and they rise, rotating around the axis of the hinges until they emerge from the water, temporarily isolating the lagoon from the sea and stopping the tidal flow.
Split into simpler sentences: When a tide higher than 110 cm is forecast, the gates are emptied of water by introducing compressed air. They therefore rise, rotating around the axis of the hinges until they emerge from the water, temporarily isolating the lagoon from the sea and stopping the tidal flow.