Why can tidal currents be especially fast in passes through coral reefs?

In coral atoll lagoons, tidal currents reach their maximum velocities within the passes because these openings serve as the primary conduits for seawater to move in and out of the lagoon.1

Unlike other types of lagoons that may have significant freshwater runoff, coral lagoons typically lack such input, meaning the water circulation is almost entirely dictated by the tidal exchange through these restricted channels. As the tide rises and falls, the entire volume of water moving into or out of the lagoon must be funneled through these relatively narrow passes, resulting in significantly increased current speeds. For example, velocities of 19 to 22 km (12 to 14 miles) per hour have been recorded in the Hao Channel of the Tuamotu Archipelago.1