Mechanisms and Characteristics of High-Speed Reef Rip Current

Transcription

1 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

2 Many people utilize beaches, but who is responsible for safe utilization? Coastal scientist and engineers care?

3 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)

4 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.

5 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).

6 Risks in carbonate beach utilization 1 Drowning accidents (number of persons) Record of casualties ( ) Tokyo Coral reef areas Okinawa Kagoshima Prefecture reference Fig. 1 Record on the number of drowning accidents in Japan from 1990 to 2004 (Nishi et al., 2007). Coral reef coasts are in Okinawa, Kagoshima (partially), and Tokyo(partially). Annual drowning accidents (persons/year) Record of drowning accidents in Okinawa Prefecture ( ) Swimming Snorkeling Boat sailing Surfing Diving Marine related activities Fig. 2 Record on the number of drowning accidents in Okinawa based on marine related activities from 1998 to 2006 (Nishi et al., 2007)

7 Two sisters, 12 and 5 years old, drowned on Tomori beach, 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. Risks in carbonate beach utilization 2 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.

8 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.

9 Solution for where; reef gap Examples of reef gap

10 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).

11 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)

12 1. Tomori beach, Kagoshima Prefecture in 2004 (pre-project) an Yoshiwara beach, Okinawa Prefecture in 2004 (pre-project), 2005 (summer season) and 2006 (winter season). Field study on Reef Rip Current. Japan Warm Kuroshio Current -> coral reef Kagoshima Okinawa Tomori beach Yoshiwara beach

13 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

14 Study area:yoshiwara coast, Ishigakijima,Okinawa, Japan

15 Laser survey was conducted by JCG Offshore Reef flat Reef flat Carbonate beach Onshore River Bathymetry of the study area (Yoshiwara coast, Okinawa, Japan)

16 0 Transect line (No.1) Transect line (No.2) Height ( m ) Transect line (No.1) Transect line (No.2) Offshore distance ( m ) reef gap well developed reef Cross-shore profiles in a reef gap and well-developed reef

17 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)

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

19 1200 Offshore distance ( m ) Staff with GPS Float 1 Float 2 Float 3 Float 4 Float 5 Shoreline (L.W.L) Sand beach Staff Reef flat edge (schematic) N 0 Shoreline(H.W.L.) Longshore distance ( m ) Flow visualization (GPS floats were transported into a reef gap)

20 7 Offshore velocity (m/s) O rig in a l d a ta (1 s e c. in te rv a l) 1 0 s e c. m o v in g a v e ra g e ( m / s ) E lla p s e d tim e (s e c.) Example of offshore velocity of GPS drogue around a reef gap.

21 平均水位 (m) Current velocity (m/s) 計測時間 ( 時間 ) 平均水位 W H B Zoom-up Offshore High speed reef rip current develops nearly once in a day at low-low tide in calm waves and weak wind. Offshore Offshore Onshore Average current velocity (WHB) Offshore Offshore Onshore Onshore Elapsed time (hours) Onshore Offshore Onshore 20-min mean current velocity during normal condition(small wave and wind) (June 25 July 1, 2006)

22 High speed reef rip current develops nearly once in a day at low-low tide in calm waves and weak wind. 1.2 Current velocity(m/s) Low tide WHB High tide Water level(m)

23 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)

24 Spatial gradient of mean water level caused by tide v 2gh m/ 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)

25 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 m/ s?

26 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 area). 20 minutes average velocity(m/s) (-); Onshore (-); Onshore (+); Offshore (+); Offshore N-S component(m/s) E-W component(m/s) Ellapsed time (day) 20-min mean current velocity during winter season 2007(high waves and strong wind).

27 High speed reef rip current develops not only at low tide, but also under high waves and strong onshore wind. 20 minutes average vel.(m/s) & Tidal level(m) m inute average velocity(m/s) Tidal level(m) E lapsed tim e(day) Tide and velocity record in winter season 2007

28 High speed reef rip current; strong enough on the sea bed (hard coral bottom) in winter condition second 20 秒平均流速 average velocity (cm/s) (cm/s) Velociyty 10cm above sea bed Ellapsed time (day)

29 Q; When does a high-speed reef rip current develop? Current velocity (m/s) E l a p s e d t i m e ( h o u r s ) 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 ) 20 minutes average vel.(m/s) & Tidal level(m) m i n u t e a v e r a g e v e l o c i t y ( m / s ) T i d a l l e v e l ( m ) E l a p s e d t i m e ( d a y ) A; (1)Low tide under calm waves; (2) high waves; and (3) strong onshore wind.

30 How strong is a reef rip current? 20 minutes average vel.(m/s) & Tidal level(m) Not for official use! 20 minute average velocity(m/s) Tidal level(m) Extreme danger Strong danger Danger Elapsed time(day) Swimming speed Safe student Current No one can swim against a high-speed reef rip current!

31 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.

32 CONCLUSION: The study has revealed that (ii) The mass of the water contained in the lagoon is dependent on tide, waves, and wind.

33 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.

34 Sorry, I am going home! Q & Answer ; (tough question; send an to Nishi Sediment_24@hotmail.com)

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36 0 Transect line (No.1) Transect line (No.2) Height ( m ) Transect line (No.1) Transect line (No.2) Offshore distance ( m )

37 Grid system for a down current simulation Wall (Slip condition) Flow 0 10 Outflow Boundary Z ( m ) 20 Wall (Non-slip condition) X ( m )

38 High tide condition u=1, 2, 3m/s 0 Z ( m ) 10 3m/s 2m/s 1m/s (m/s) X ( m ) 0 Z ( m ) 10 3m/s 2m/s 1m/s (m/s) X ( m )

39 Low tide condition U=1, 2, 3m/s 0 Z ( m ) 10 3m/s 2m/s 1m/s (m/s) X ( m ) 0 Z ( m ) 10 3m/s 2m/s 1m/s (m/s) X ( m )