The world’s largest “Naruto whirlpools” that look through the glass 45m above from the sea is so thrilling! My legs were unconsciously frozen with fear of falling the glass!

Naruto Strait is a 1.3-km strait between Awaji Island in Hyogo Prefecture and Oge Island in Tokushima Prefecture. Naruto Strait is very famous for its whirlpools and is said to be one of the world’s three major tidal currents, along with the “Strait of Messina” in Italy and the “Seymour Narrows” in the United States.

Mechanism of whirlpools in the Naruto Strait

The whirlpools in the Naruto Strait are related to the tidal currents caused by the ebb and flow of the Harima Sea and the Kii Channel, which occur approximately every six hours. The Naruto Strait connects to both the Harima Sea in the Seto Inland Sea and Kii Channel. When Harima Sea is at high tide, the Kii Channel is at low tide, so the tide flows into the Kii Channel from Harima Sea (this is called the “Nan Ryu – south current”). When the Kii Channel has a high tide, the Harima Sea has a low tide, so the tide flows Into the Harima Sea from the Kii Channel (this is called the “Hoku Ryu – north current”). During spring tide, the difference in sea level between Harima Sea and Kii Channel is 1.5m.

The topography of the Naruto Strait seafloor and the tidal currents generate whirlpools. The bottom of the Naruto Strait is deeply indented in a V-shape, reaching approximately 100 meters at its deepest point. When the tide passes through the Naruto Strait, it flows fast in the center of the strait because there is little resistance, and it flows slowly on both banks because there are shoals creating resistance. The difference in the speed of the tidal currents creates a rotational force that generates many large and small whirlpools on the ocean surface. The largest whirlpools are 15 meters in diameter but can be as large as 30 meters in diameter during spring tides. In the Naruto Strait, whirlpools occur alternately on both sides of Harima Sea and Kii Channel every six hours.

Based on the mechanism that whirlpools are generated near the boundary between fast and slow tidal currents, it is supposed that “right-handed whirlpools” and “left-handed whirlpools” are generated near both banks of the Naruto Strait. However, actually, when the tidal current flows from Harima Sea to Kii Channel (Nan-Ryu), a right-handed whirlpool is generated on the Naruto side, and conversely, when the tidal current flows from Kii Channel to Harima Sea (Hoku-Ryu), a right-handed whirlpool is generated on the Awaji Island side.

Thus, most of the whirlpools in Naruto are right-handed whirlpools, but left-handed whirlpools rarely occur on the Awaji Island side during the south current. These left-handed whirlpools are not as powerful as the right-handed whirlpools. Left-handed whirlpools do not occur on the Naruto side during the north current due to the topography of the seafloor.

Onaruto Bridge

The bridge over the Naruto Strait is “Onaruto Bridge”, completed in 1985. This is the suspension bridge with 1629m length and 144m high main tower. This bridge has a two-layer structure, upper and lower. The upper part is a motorway and the lower part is for railways and Shinkansen can run this part.

It was once planned to construct the bullet train route connecting Kobe in Honshu to Tokushima in Shikoku via the Akashi Kaikyo Bridge (completed in 1998), Awaji Island, and the Onaruto Bridge. However, since the Akashi Kaikyo Bridge connecting Awaji Island and Honshu was constructed as a stand-alone bridge for motorway use only, Tokushima Prefecture took the initiative in 2000 to renovate the Onaruto Bridge’s railroad section as a 450-meter promenade called “Uzu-no-Michi”. The observation room of the promenade located 45m above from the sea surface partly has a glass floor and you can see whirlpools from there. And also, there are four rest areas along the promenade, so you can take a seat and enjoy the view from Onaruto Bridge.

Uzu no Oka Onaruto Bridge Memorial Hall

The Uzu no Oka Onaruto Bridge Memorial Hall, located on Awaji Island, Hyogo Prefecture, is a complex facility where visitors can enjoy entertainment and dining. The Uzushio Science Museum, where visitors can learn about the “Naruto Strait whirlpools,” is located on the first floor. There is a large objet d’art of onion, a specialty of Awaji Island, with the Onaruto Bridge and the Naruto Strait in the background for a commemorative photo.

Onaruto Bridge Crossing Memorial

The Onaruto Bridge Crossing Memorial Hall (nicknamed “Eddie”), located in Tokushima Prefecture, is a facility that introduces the mechanism of whirlpools in Naruto and the structure of the Onaruto Bridge under the themes of “whirlpools” and “bridges”. At the Ohnaruto Bridge Crossing Memorial Hall, you can have the simulated experience of whirlpool using projection mapping. When you touch it, the image will change and you can create a whirlpool in the whole room.

If you take a sightseeing boat, you can enjoy extremely powerful whirlpool up close.