STUDY OF TIDAL VORTICES AT THE NARUTO STRAIT i] 
THROUGH MULTI LEVEL REMOTE SENSING 3 
by 
T. MARUYASU, S. ONISHI and T. NISHIMURA 
Science University of Tokyo, 2641 Noda City, Chiba, JAPAN 
ABSTRACT 
Stirring whirlpools in the tidal stream at the Naruto Strait were | 
surveyed through multi level remote sensing from airplane and Landsat. | 
Small scale tidal vortices were surveyed from air at lower altitude, | 
and the large scale tidal vortices were surveyed from Landsat and from air at I 
higher altitude. 
It was revealed that the tidal vortices take the main role in the 
water mixing around the strait, after the hydrodynamical interpretation of the 
remotely sensed data aided by the hydraulic model tests. 
o o 
1. OUTLINE OF THE "VORTEX OF NARUTO" 
  
The Naruto Strait is located in the eastern part of the Seto Inland 
Sea, coastal region of which is one of the most industrialized area in Japan 
(Fig. 1). The strait is a very narrow water course between the Shikoku and the 
Awaji Island and connects the Harima Sea to the Kii Channel. The water course | 
is, as is shown in Fig. 2 (a), contracted by shoals stretching out sharply from li 
0ge Island in Shikoku and from Tozaki Cape in Awaji Island. The most contracted IBI 
section has a V-shaped configuration, as is shown in Fig. 2 (b), the width and | 
the maximum depth of which is about 1,100 meter and about 80 meter. UM 
  
  
The most distinguished characteristic of the Naruto Strait is the 
existence of the fast tidal current and the stirring whirlpools. Tidal level lili 
in each wide sea area around the strait ebbs and rises with a period of about 12 Il 
hours, accompanying with periodic tidal current. At a spring tide, the differe- il 
nce of the tidal levels reaches about 1.5 meter and causes the current velocity Ili 
of about 10 knots. The stirring whirlpools are generated in the narrow water I 
region between the fast tidal current and the dead water region. The diameter 
and the surface concavity of the whirlpools reach about 20 meter and 1.5 meter, 
respectively, under the spring tide condition. Such dynamic whirlpools are quite 
famous in Japan for a name of the "Vortex of Naruto". 
  
  
  
The tidal condition at the Naruto Strait is illustrated in Fig. 3. It 
shows an example of the relationship between the tidal current through the strait 
and the tidal levels at Anaka Port and Fukura Port, inside and outside of the 
strait. One can see that there exists a time lag of 6 hours between each of the 
tidal phases of the ports, that is a phase lag of 7L . This phase lag of 7L 
causes effectively the strong tidal current through the strait, direction of 
which turns reversely with a period of half a day. The reasons of the phase lag 
of JT is given as follows. A tidal wave propagates through the Tomogashima 
Strait and the Akashi Strait, and then arrives the Harima Sea side of the Naruto 
Strait about 6 hours later than its direct arrival at the Kii Channel side. 
Therefore, when the high tide reaches the Harima Sea, for example, the tide in 
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