The moon's pull causes the surface of the ocean to rise up, travelling from east to west as though following the moon itself. The waves created by the high tides flow to the north through the Kii and Bungo Channels. The waves that enter the Bungo Channel cause the water level to the west of the Seto Inland Sea to rise up and flow towards the Harimanada area of ocean. The waves that enter the Kii Channel caused by the high tides split into two, and flow towards the Naruto Strait and Osaka Bay. That wave then goes on to travel through the Akashi Strait and enter Harimanada, then mix with the high tide wave that has entered the Bungo Strait. It takes approximately 6 hours for the high tide to enter Harimanada after it has entered the Kii Channel. During this time the Kii Channel side changes to a low tide, and a difference in water level of up to 1.5m can be seen where Harimanada and the Kii Channel meet on either side of the Naruto Strait. The water begins to flow rapidly caused by the differences in the high and low tides, causing the whirlpools to form in the central area due to the slow flowing and fast flowing tides meeting.
Structure of whirlpools
The pull of the moon and the flow of the tides
The Naruto Strait is a V-Shape
The area directly below the Onaruto Bridge comes together in a v-shape, reaching a depth of up to 90m. The Naruto Strait has a depression extending from the south side (Pacific Ocean side) where it is 140m deep, and the Seto Inland Sea side where it is 200m deep. This uniquely shaped sea floor causes the flows of the low tides to come together, creating the Naruto whirlpools. The whirlpools can be up to 20m in width, and are said to be the largest in the world. The thundering noise they make is the origin of their name, with one of the meanings of "naru" being "to roar".
How are such large whirlpools formed?
The full moon and low moon cause high tides to occur twice every month. A high tide is caused when the pull of the moon and the sun are at their highest, causing the full tides and ebbing tides to have a large difference in water level, thus causing the water to flow fast. In the case of Naruto Strait, the high tides in spring and autumn can cause water to flow at speads of up to 2km/h, creating some of the largest whirlpools. In particular the period between the end of March and end of April is said to be the best time to see the whirlpools.
Going to see the whirlpools
Visitors can see the whirlpools from a glass-floored bridge suspended 45 metres above the ocean, known as the "Tokushima Prefectural Uzu-no-Michi", or if they don't mind getting a bit wet, from aboard a whirlpool sightseeing boat. They can also gain splendid views of the sunlit Uchinoumi sea, passing ships, and the mountain-backed shores from the Naruto Skyline, Eska Hill, or Ohnaruto Bridge Crossing Memorial Hall Eddy. Views of the whirlpools can also be enjoyed while eating traditional Japanese confectionary and souvenir shopping at Senjojiki or the Tea Garden Observation Deck.
Uzu-no-michi
Visitors can see the whirlpools from a glass-floored bridge suspended 45 metres above the ocean, known as the "Tokushima Prefectural Uzu-no-Michi", or if they don't mind getting a bit wet, from aboard a whirlpool sightseeing boat. They can also gain splendid views of the sunlit Uchinoumi sea, passing ships, and the mountain-backed shores from the Naruto Skyline, Eska Hill, or Ohnaruto Bridge Crossing Memorial Hall Eddy. Views of the whirlpools can also be enjoyed while eating traditional Japanese confectionary and souvenir shopping at Senjojiki or the Tea Garden Observation Deck.
Naruto Sightseeing Boats
The Wonder Naruto sightseeing boat has room for up to 400 passengers, and provides comfortable viewing of the whirlpools. Aqua Eddy is a smaller capacity boat, and provides passengers with up-close views of the whirlpools from 1m below the water.
Uzushio Sightseeing Boats
The Uzushio sightseeing boats are a new design of vessel. They provide passengers with views of the whirlpools at distances close enough to get sprayed with water by them. They're a great way to truly gain an understanding of the enormous size of the Naruto whirlpools.
Ohnaruto Bridge Crossing Memorial Hall Eddy
This memorial hall is based around the whirlpools and the bridge. It provides visitors with information about the Naruto whirlpools' workings, the structure of the bridge, and more, through imagery and simulations via a variety of angles.
Eska Hill Naruto
Eska Hill is located in front of the Naruto National Park #1 carpark, and acts as a viewing facility for both the whirlpools and the bridge. It features a sightseeing escalator with the biggest height difference in Asia - 34m - and boasts a length of 68m. The platform provides views of the entirety of Naruto Park.
Naruto Skyline
The 8.1km Naruto Skyline scenic drive provides views of the Naruto Straits, famous for their whirlpools, the still Seto Inland Sea, and Uchinoumi - famous for its beautiful scenery. The drive consists of 7 bridges strung over the Horikoshi and Konaruto straits, and also leads to a golf course.