Mechanisms and Characteristics of High-Speed Reef Rip Current

Ryuichiro Nishi, Mario P. de Leon, Kouji Horinouchi,Akira Ohtani, Nicholas C. Kraus, and Julianti K. Manu Many people utilize beaches, but who is responsible for safe utilization? Coastal scientist and engineers care? The project on the safe utilization in nearshore zone with Japan Coast Guard;

Part 1. Rip current on a sandy beach ( 2002 to 2004)

Part 2. Reef rip current in a coral reef and carbonate beach system (2005 to 2006)

Part 3. Offshore current in a river mouth and inlet (2007 to 2008) Motivation; Coral reef and carbonate beach attract many tourists for instance, six million people in Okinawa. However, drowning accidents by a strong offshore current happen every year. SOMEBODY has to study and minimize the RISK. The project on Reef Rip Current (the strong offshore current in a coral reef);

Part 1. Drowning accident data analysis (risk analysis)

Part 2. Field study of current, waves. and topography (will be presented at this symposium). Risks in carbonate beach utilization 1

Record of drowning accidents in Okinawa Prefecture

Record of casualties (1990-2004) ) ) r (1998-2006) s Swimming 250 a n e o Coral reef areas y s / r

Okinawa s e

n 100 p o

200 f s r o Tokyo

e r

p 80

Kagoshima ( e b

150 s t m n u

e 60 n d ( i Snorkeling c s

t 100 c Boat sailing n a 40 e g d i n

i Diving c n c 50

w 20 a Surfing

o r g d n i

l n 0 a 0 w u 1 2 3 4 5 6 7 8 o 5 10 15 20 25 30 35 n r n D Prefecture reference A Marine related activities Fig. 1 Record on the number of Fig. 2 Record on the number of drowning accidents in Japan from drowning accidents in Okinawa 1990 to 2004 (Nishi et al., 2007). based on marine related activities Coral reef coasts are in Okinawa, from 1998 to 2006 (Nishi et al., Kagoshima (partially), and 2007) Tokyo(partially) . Two sisters, 12 and 5 years old, Risks in carbonate drowned on Tomori beach, beach utilization 2 Kagoshima 2:50 pm. A man tried to rescue them said that it was like a river. 8 years ago, medical doctor 28 years old drowned.

Three high school students (18 years old) drowned. One was injured, one was dead and one was missing at Nishihama beach, Okinawa during school excursion. Two years ago, a 20-year old university student drowned. Three years ago, a 30-year old man drowned. Technical questions by rescue agent and beach users on reef rip currents; Where and when, then how strong? Scientific questions by researchers; What is a mechanism and can be predictable?

Where is a reef rip current generated? Where is the area that beach user should avoid his/her use. Solution Inspection of aerial photographs; A few thousands aerial photographs supported by the Department of Civil Engineering, Okinawa Prefecture and Japan Geological Survey, were inspected to identify gaps in the reefs. Solution for where; reef gap Examples of reef gap Objectives of field study (mechanism and public awareness)

1. To characterize nearshore hydrodynamics in coral reefs through field measurement 2. Then establish the geomorphology-wave-tide – current relationship 3. Apply the result to educate rescue agents (how to find reef rip currents and how to act.). 4. Apply the results to public education on safe utilization of beaches (how to minimize risk). Field study in carbonate beach and coral reef system have been conducted since 2005 in Kagoshima and Okinawa, Japan to reveal the strong offshore current mechanisms and reduce drowning accidents. (Where, when, and how strong) 1. Tomori beach, Kagoshima Prefecture in 2004 (pre-project) an 2005. 2. Yoshiwara beach, Okinawa Prefecture in 2004 (pre-project), 2005 (summer season) and 2006 (winter season). Field study on Reef

Rip Current. Warm Kuroshio Japan Current -> coral reef

Kagoshima

Okinawa Tomori beach

Yoshiwara beach FIELD OBSRVATION: The entire project includes bathymetry survey by laser and aerial photographs, nearshore hydrodynamics (wave, current, mean water level, tide and temperature) study, flow visualization by drogue and dye experiment, wind observation, numerical simulation of downward current, public awareness program, and internet access.

Internet access was available Study area:Yoshiwara coast, Ishigakijima,Okinawa, Japan Laser survey was conducted by JCG

Offshore

Reef flat Reef flat

Carbonate beach Onshore River

Bathymetry of the study area (Yoshiwara coast, Okinawa, Japan)

Transect line (No.1) 0 Transect line (No.2)

-10

) Transect line Transect line

m (No.1) (No.2)

(

t h g

e -20 H

-30

-40 300 400 500 600 700 800 900 1000 Offshore distance ( m )

reef gap well developed reef

Cross-shore profiles in a reef gap and well-developed reef

Wave Hunter (water level, wave height, current and direction, water temp.） ECM (current direction, water temp.） Water level gage (level, water temp.） Monitor camera

6m depth

Location of wave gage, electronic current meters, tidal gages, monitoring camera (i.e. Winter, 2007) Speed up

Flow visualization (dye pattern shows an offshore current toward a reef gap)

1200 ④ 1000 Staff with GPS ③ Float ① ) Reef flat edge Float ② m ② (schematic)

( 800

Float ③ e

c Float ④ n

a Float ⑤

t 600 s i N d

e r 400 o

h Shoreline (L.W.L) s f f Sand beach Staff O 200 ⑤ ①

0 Shoreline（H.W.L.） -200 -600 -400 -200 0 200 400 Longshore distance ( m ) Flow visualization (GPS floats were transported into a reef gap)

6 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） 5 ) s / m (

4 y t i c o l 3 e v e r o 2 h s f f

O 1

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 E lla p s e d t im e (s e c .)

Example of offshore velocity of GPS drogue around a reef gap.

2 . 6 平均水位 W H B 2 . 4 2 . 2 High speed reef rip current develops nearly 2 . 0 ）

ｍ 1 . 8 （

位 1 . 6 水

均 1 . 4 平 1 . 2 once in a day at low-low tide in calm waves 1 . 0

0 . 8

0 . 6 0 1 2 2 4 3 6 4 8 6 0 7 2 8 4 9 6 1 0 8 1 2 0 1 3 2 1 4 4 1 5 6 1 6 8 1 8 0 計測時間（時間） and weak wind. 1.4 Offshore Average current velocity (WHB) Offshore 1.2 ) Offshore Offshore s /

m 1.0 (

y t i 0.8 Onshore Onshore Onshore Offshore c Zoom-up o

l Onshore e

v 0.6

n Offshore e 0.4 r Onshore r u

C 0.2

0.0 0 12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 Elapsed time (hours)

20-min mean current velocity during normal condition(small wave and wind) (June 25 – July 1, 2006) High speed reef rip current develops nearly once in a day at low-low tide in calm waves and weak wind.

1.2 WHB 1.0 Low tide ）ｓ

/ 0.8 ｍ（ y t i

c 0.6

o l e v

t 0.4 n e r r u

C 0.2 High tide

0.0

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 Water level（ｍ） Why does a high speed reef rip current develop at low tide?

Orientation of water level gages (Gage 16 at the reef gap, Gage 26 on the carbonate beach, Gage 25 in the reef lagoon, and Gage 24 on the reef edge) Spatial gradient of mean water level caused by tide

v 2gh 2 9.8 0.3 2.4m/s?

Mean water level outside of coral reef (No. 16), on reef edge (No. 24), in the middle of the lagoon (No. 25), in front of the carbonate beach (No. 26) Reef flat (reef edge) and lagoon system contains mass of water similar to a hydraulic dam on the other hand, a reef gap where the elevation is lower has a function to discharge water in the lagoon during low tide.

v 2gh 2 9.8 0.3 2.4m/s? High speed reef rip current develops not only at low tide, but also under high waves and onshore strong wind (typically winter in the study are a). 1.8 N-S component(ｍ/ｓ）

） 1.6 E-W component（ｍ/ｓ）ｓ

/ 1.4 (+); Offshore ｍ

（ 1.2 (+); Offshore y t i

c 1.0 o l e

v 0.8

g 0.6

a r

e 0.4 v a

s 0.2 e t

u 0.0 n i m

-0.2 (-); Onshore 0

2 -0.4 (-); Onshore -0.6 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Ellapsed time (day) 20-min mean current velocity during winter season 2007(high waves and strong wind). High speed reef rip current develops not only at low tide, but also under high waves and strong onshore wind. ) m ( l e 2.4 v

e 20 m inute average velocity(m /s) l 2.2 l Tidal level(m ) a 2.0 d i T

1.8 &

) 1.6 s

/ 1.4 m (

l 1.2 e v 1.0 e g 0.8 a r

e 0.6 v a 0.4 s e 0.2 t u

n 0.0 i 0 2 4 6 8 10 12 14 16 18 20 22 24 26 m

0 Elapsed time(day) 2

Tide and velocity record in winter season 2007 High speed reef rip current; strong enough on the sea bed (hard coral bottom) in winter condition. 100

Velociyty 10cm above sea bed ) s / 80 m ） c ( ｓ

/ y t i ｍ c ｃ o l

（ 60 e v 速

e 流 g

a r 均 e 40 v 平 a

秒 d 0 n 2 o c

e 20 s

0 2

0 0 5 10 15 20 Ellapsed time (day) Q; When does a high-speed reef rip current develop?

1 . 4 A v e r a g e c u r r e n t v e l o c i t y ( W H B ) 1 . 2 ) s /

m 1 . 0 (

y t i 0 . 8 c o

l e

v 0 . 6

t n

e 0 . 4 r r u

C 0 . 2

0 . 0 0 1 2 2 4 3 6 4 8 6 0 7 2 8 4 9 6 1 0 8 1 2 0 1 3 2 1 4 4 1 5 6 1 6 8 1 8 0 E l a p s e d t i m e ( h o u r s ) )

m ( l

e 2 . 4 v

e 2 0 m i n u t e a v e r a g e v e l o c i t y ( m / s ) l 2 . 2

l T i d a l l e v e l ( m ) a 2 . 0 d i T

1 . 8 &

) 1 . 6 s

/ 1 . 4 m (

l 1 . 2 e

v 1 . 0

g 0 . 8 a r

e 0 . 6 v

a 0 . 4

e 0 . 2 t u

n 0 . 0 i 0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6 m

0 E l a p s e d t i m e ( d a y ) 2 A; (1)Low tide under calm waves; (2) high waves; and (3) strong onshore wind. Swimming speed How strong is a reef rip current? ) student m ( l

e 2.4 v Not for official Current

e 20 minute average velocity(m/s) l 2.2

l Tidal level(m)

a use! 2.0 d i T

1.8 &

) 1.6 s Extreme danger / 1.4 m (

l 1.2 e

v 1.0

g 0.8 Strong a r

e 0.6 danger v

a 0.4

s Danger

e 0.2 t u

n 0.0 Safe i 0 2 4 6 8 10 12 14 16 18 20 22 24 26 m

0 Elapsed time(day) 2 No one can swim against a high-speed reef rip current! CONCLUSION: The study has revealed that; (i) The maximum 20-min average offshore velocity could be order of 2 m/s especially during winter season, because the northern wind that is perpendicular to the coast is persistent and generates reasonably high waves;

(ii) A reef flat 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 the elevation is lower and reef width is narrower has a function to discharge water in the lagoon. CONCLUSION: The study has revealed that

(ii) The mass of the water contained in the lagoon is dependent on tide, waves, 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. Q & Answer ; (tough question; send an email to Sorry, I am Nishi going home! [email protected])

Transect line (No.1) 0 Transect line (No.2)

-10

) Transect line Transect line

m (No.1) (No.2)

( t h g i

e -20 H

-30

-40 300 400 500 600 700 800 900 1000 Offshore distance ( m ) Grid system for a down current simulation

Wall (Slip condition) 0 Flow Outflow Boundary 10 )

20 (

Z Wall 30 (Non-slip condition)

0 50 100 150 200 300 400 500 X ( m ) High tide condition u=1, 2, 3m/s

) 3m/s

m 2m/s

( 10 1m/s

0 1 2 3 (m/s) 20

0 50 100 150 X ( m )

) 3m/s

m 2m/s

( 10 1m/s

0 1 2 3 (m/s) 20

0 50 100 150 X ( m ) Low tide condition U=1, 2, 3m/s

0 ) 3m/s m

2m/s ( 10 1m/s Z

0 1 2 3 (m/s) 20

0 50 100 150 X ( m )

0 ) 3m/s m

2m/s ( 10 1m/s Z

0 1 2 3 (m/s) 20

0 50 100 150 X ( m )