Monitoring and Predictions of Rip Currents on So California Beaches

1. Monitoring and Predictions of Rip Currents on So California Beaches Drs. C-S Wu and Stephan B. Smith NOAA National Weather Service Office of Science and Technology Meteorological Development Laboratory Thanks to: Jason Taylor, Mike Churma, and John Schattel (MDL) David Soroka (OCWWS), Noel Isla, Michael Khuat, Tina Stall and Ivory Small (SGX), David Danielson (LOX)

2. Beach hazards: Rip Currents Trends in the States Courtesy Miami Herald James B Lushine

3. Rip Current Deaths by State 1999-2008 0 1 0 0 13 2 3 34 0 0 9 13 0 18 0 1 9 3 0 4 44 56 13 1 0 19 1 6 PR/VI Guam 34 10 230 Samoa 3 0 Rip Current Deaths by State 1995-2008 GuamGu Guam 38

5. Rip currents on beaches: – a jet-like seaward flow across the surf zone Xb of a beach. _ a transient eddy _ local vortex motions

6. Background “ Monitor rip currents so as to reduce the hazard to the public….” on 2004 Rip Current Workshop Observations of rip currents on beaches, HOW ? Un-manned radars? In-sit measurements ? Trained Spotters, Lifeguards.

7. Rip Currents on west coasts From U S Lifesaving Beach Safety Manual (Brewster,1995): Fixed: appear over a rip channel. Permanent: Behind or near a jetty Flash: wave sets, bi-modal waves, transient. Traveling or migrating: High waves strike the beach at large angle for a long duration.

12. What causes Rip Currents ? Major physical factors : Surfs or wind wave breaking Water Level (tides/set-up) Beach State (bar,hole)

14. Analyze Rip Current factors • 1. Surf heights (H, T) in bin-averaged set • 2. Tide levels • 3. Breaking wave direction • 4. Beach rescues • 5. No rip conditions. • In So Cal, most waves are swells with little coast winds, so we exclude winds.

15. Southern California Rip Current Monitoring Locations

16. Surf Zone Rip Monitoring Report FAX TO: (858)-675-8712 Beach ID: Moonlight Beach Date: ____/_____/_____ Observations range from B-street to D-street mm dd yyyy 10:00AM: ____ 4:00 PM: ____ Waves : Maximum Surf height (ft) =_______; Wave Period (sec.) = ______ Average Surf Zone Width (ft) =________; Tides: Low Rising High Falling Incoming wave direction: _____Directly on- shore ____ Oblique to the shore Rip Currents: Observed rips: ____YES ______NO. Estimate size of Rip pull: ________ Rip Strength: ____Weak, _____Moderate, _____ Strong Number of Rips: _____Single, _____Multiple (2-4), ______Wide stack (5+) Rip location relative to Street names: E---+---D---+---C---+---B---+---A Street Tower Comments: ( # of rescues, bars, rip life, beach face, cusps, wave sequences): _____________________________________________________________ Prepared by Lifeguard: _____________________

17. Rip Current Data Entry Form (MLB) https://bestpractices.nws.noaa.gov/contents/mdl/testrip/rip_form.php?wfo=KMLB

18. Buoy - Station 46225, 100 Torrey Pines Outer, CA CDIP- UCSD

19. Surf Height at Moonlight Beach

20. Seasonal Wave Characteristics on Moonlight Beach Winter:12/15/08-4/16/09 Spring: 4/16/08-06/10/08 Summer: 6/12/08-10/02/08 Fall: 10/03/08-12/14/08

21. Seasonal Rip Current frequency on Moonlight Beach, CA Winter: 12/15/08-4/16/09 Spring: 4/16-06/10/08 Summer: 6/12-10/02/08 Fall: 10/03-12/14/08

22. Rip Current observed versus Surf heights San Clemente Beach, California Winter:12/7/09-1/20/2010

23. Rip Current cases versus Tide levels Moonlight Beach, California Selected cases during Summer (6/4-9/27/2008) and Winter and Spring(1/7-4/12/2009)

24. 06/10/2010 2000UTC Surf: Hs= 0.87 m 1.6 ft S swell (13 sec), 3.5 ft wind swell (8.3 sec) from 270. Tide levels 1.5 ft Surf zone width: 75 yds, more cobble stones, inshore hole at knee-waist high, L-currents Strong rips 06/10/2010 1700UTC Surf: Hs=0.85 m 1.6 ft S swell (13 s), 2.8 ft wind swell (8.3 sec) from 267 Tide levels 3.5 ft Surf zone width; 35 yds, inshore holes at chest high NO rips, mini surf on bar Two application cases at San Clemente Beach, California

25. Tide effects on Rip hazard Nile Mile Beach: H=0.65 m, T= 10 sec and slope = 1/25 (Short and Hogan, 1994)

26. Effects of wave direction on Rip hazard San Clemente Beach: H= 1.0-1.5 m, T= 12-13 sec

27. Number of Rescues under Waves and Tides at Moonlight Beach, California Encinitas Lifeguards safety service record (7/1-8/30/2008) Many beach RESCUES are done in 2-3 ft waves at a variety of tide levels.

28. Diagnostic Approach to Rip Current Prediction Hypothesis: Surf heights, period, tide level, Beach sand, Surf zone width, winds etc. Wave-Sediment parameter Ω(H, T, tides, sand) Short (1982): 1 < Ω < 6 rip occurrence range.

29. Conclusion and Remarks • With beach observations, we can obtain guidance to issue rip currents risk and improve forecasting results. • 2. Data supports that low wave/swell (0.5 -1.0 m) at low tide or mid-tide level can result in dangerous hazards. • 3. Due to uncertainties at sea bottom, we can use 3-tier risk level (Caution/Moderate/High) or a 2-tier (Caution/Hazardous) level. • 4. Parametric model is limited, digital models are desired for better forecasting . • 5. Reducing Rip Current Risk requires a Safety – Education – Science (3 in 1) approach, particularly assistance from certified lifeguards.

30. Parametric Analysis I. Rips on open beaches: • Wave -Sediment ( by Short and Wright, Dean ) or M 2 Ω = Hb / T w = wave particle speed/sediment falling speed Prevailing condition: 1 < Ω < 6 rips to appear. • Surf scale parameter (Wave-Beach face slope) or M 3 2.0 < ε= Abω² /gtan 2β => wave steepness/beach slope < 25. II. Rip strength estimate: Mass Flux balance (Aaagard, Wu) • (sufficient condition)

32. More sensitive to beach face slope, limited

33. Beach cusps,Ω< 1 or ε < 2.0, waves are reflected (Short, 1989)

34. Critical Skill PerformanceDuring 4/15/08 - 8/16/2008 at Moonlight Beach