Tidal whirlpools occur in the Naruto Strait, the channel between Magosaki, Naruto City and Awaji Island. The whirlpools become the largest during spring tides in spring and fall, and the fastest current achieves 20 kilometers per hour and its maximum diameter may reach 20 meters. The sight of the currents crossing and flowing with thunderous sound are spectacular. High tide and low tide occur twice each per day, and whirlpools can be seen during these high and low tides in the Naruto Strait. The best time to see the whirlpools is between one and a half hours before and after each tide.
The “Naruto Strait”, the “Strait of Messina” situated between Italy Peninsula and Sicily Island and the “Seymour Narrow”, between the western coast of North America and the eastern coast of Vancouver Island are called the “world’s three greatest currents”.
The bulges of the sea water surface, which are caused by the gravitation of the moon, move from east to west, following the movement of the moon. The high tide waves, caused by the bulges of the water, flow out towards the Kii Channel and Bungo Channel, and move northward. The high tide waves flowing into the Bungo Channel move from the western Seto Inland Sea to the Sea of Harima, raising the water level. The high tide waves flowed into the Kii Channel split into two ways and go towards the Naruto Strait and Osaka Bay. The waves, then, flow through the Akashi Strait, enter the Sea of Harima and join the high tide waves coming through the Bungo Channel. It takes approximately six hours for the tidal waves coming from the Kii Channel to enter the Sea of Harima. In the meantime, low tides occur in the Kii Channel, therefore, up to 1.5-meter difference is generated in the water level at the Naruto Strait between the Sea of Harima and Kii Channel. Then, the sea water flows rapidly from the high tide side—the higher water level—to the low tide. The difference in the speed between the current at the center (fast current) and the current outside (slow current) generates whirlpools.
Under the Onaruto Bridge, it drops deeply in a V-shape and the deepest point reaches approximately 90 meters in depth. There are natural deep depressions (Kaifu): the water depth of 140 meters in the south side—the Pacific Ocean side—and the water depth of 200 meters in the north side—the Seto Inland Sea side. The unique topography associated with the currents occurring due to the tides generate the Naruto Whirlpools. The size of the largest whirlpools reach up to 20-meter diameters and is said to be one of the largest whirlpools in the world. Its name, Naruto, is said to be derived from its thunderous sounds generated by the rapid currents in the Seto Inland Sea. (Naru plus To for Seto).
Spring tides happen twice a month—at full moon and new moon. At spring tides, the difference between the water level of high tides and low tides due to the gravities of the sun and the moon having an effect at the same time, then the current of the sea water becomes rapid. In the case of the Naruto Strait, during spring tides in spring and fall, the current speed reaches 20 kilometers per hour and large whirlpools can be viewed. The best period to view the whirlpools is between late March and late April.
“Tokushima Prefectural Uzu-no-Michi Walkway” with a glass floor 45 meters high up from the sea and the “whirlpool-seeing boats” which take you so close that you will get splashed by the whirlpools.
The Naruto Strait, Naruto City
The maximum diameter: 20 meters