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Why are tidal currents strongest at lagoon inlets?

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lagoons — lagoon Lagoons lie behind the coral reef and barrier islands surrounding Raiatea and Tahaa in the Society Islands, Pacific Ocean.

Tidal currents are strongest at lagoon inlets because these areas act as a constriction in the coast through which a large volume of water must pass during each flood or ebb-tide cycle.1

The speed of these currents is dictated by the "tidal prism," which is the total volume of water that must move through the inlet. This volume is the product of the tidal range and the surface area of the coastal bay or lagoon being served by the inlet. Consequently, even in regions with a low tidal range, currents can become very swift if the lagoon is large, as a significant amount of water is forced through a relatively narrow opening. Because of these high velocities, only coarse sediment can typically rest within these channels, while finer sediments settle in the quieter waters of the lagoon itself.123