Mechanisms and Characteristics of High Speed Reef Rip Current
Ryuichiro Nishi, Mario P. de Leon, Kouji Horinouchi,Yasuro Ohtani, Nickolas C. Kraus, and Julianti K. Manu
BACKGROUND: Tropical and subtropical regions characterized by coral reef and carbonate sandy beach have
attracted many tourists, however it gives rise to safety risk of coastal users who just visit a beach but does not know a
nearshore current system. For instance, five drowning accidents in Ishigakijima, Okinawa were documented by
Japan Coast Guard and were found to be caused by a strong offshore current commonly known as “reef current” in
Okinawa in 2004 and 2005. Hence, field study and analysis of drowning accidents in carbonate beaches and coral
reef system have been conducted since 2005 in Japan’s subtropical regions in Kagoshima and Okinawa Prefectures
particularly to establish and describe the strong offshore current mechanisms that is important for utilizing a beach
with minimum risk, understanding a circulation of water and sediment transport in a coral reef system.
FIELD OBSRVATION: The entire project includes bathymetry survey by laser and aerial photographs, nearshore
hydrodynamics (wave, current, mean water level, tide and temperature) study, drogue and dye experiment, wind
observation, numerical simulation of downward current and public awareness program. High speed offshore reef
rip current which exceeded 1.2m/s and caused by tide was measured in the summer of 2006, then other type of high
speed reef rip current which probably caused by a superimposition of tide, wave and wind effects was measured in
the winter of 2007 in Yoshiwara beach, Ishigakijima. In this paper, the later high speed reef rip current event is
described. Yoshiwara beach as shown in Photo. 1 is selected as one of the major study sites. Wave gages, ECMs,
tide gages and thermometers were installed in lagoons and narrow channel around reef gap, over a reef flat and
offshore slope of coral reef as well as a monitoring camera on the beach.
CONCLUSION: The study has revealed that (i) the maximum 20 minute average offshore velocity could be order
of 2.0m/s especially during winter season, because the northern wind that is perpendicular to the coast has been
almost persistent and generated reasonably high wave, (ii) reef flat (reef edge) and lagoon system has a function to
contain mass of water similar to a hydraulic dam or a reservoir, on the other hand a reef gap where an elevation is
lower and reef width is narrower has a function to discharge water in the lagoon, (ii) mass of the water contained in
the lagoon is dependent on tide, wave and wind. The authors would like to express their special appreciation to
whom assist the project nevertheless a risky sea condition and hope the project improve the safe utilization of coral
reef and carbonate beach.
7
O r ig in a l d a t a ( 1 s e c . in t e r v a l)
1 0 s e c . m o v in g a v e r a g e （ ｍ / ｓ ）
Offshore velocity (m/s)
6
5
4
3
2
1
0
0
20
40
60
80
100
120
140
160
E lla p s e d t im e ( s e c .)
20 minutes average velocity（ｍ/ｓ）
1.8
Fig. 1 Offshore velocity of GPS drogue around a reef gap.
N-S component(ｍ/ｓ）
E-W component（ｍ/ｓ）
1.6
1.4
(+); Offshore
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
(-); Onshore
-0.4
-0.6
0
2
4
6
8
10
12
14
16
18
20
22
24
26
Ellapsed time (day)
Fig. 2 20-minute average velocity in a reef gap.
20 minutes average vel.(m/s) & Tidal level(m)
Photo. 1 Overview of Yoshiwara beach.
2 .4
20 m inutes averag e velocity(m /s)
T idal level(m )
2 .2
2 .0
1 .8
1 .6
1 .4
1 .2
1 .0
0 .8
0 .6
0 .4
0 .2
0 .0
0
2
4
6
8
10
12
14
16
18
20
22
24
26
E llapsed tim e(day)
Fig. 3 Correlation between tide and current in a reef gap.