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Mechanisms and Characteristics of High-Speed Reef Rip Current

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Mechanisms and Characteristics of High-Speed Reef Rip Current 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 i 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 N 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) 7 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 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 t 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 e 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 ( .
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