Presentation on theme: "Monitoring and Predictions of Rip Currents on So California Beaches and Drs. C-S Wu and Stephan B. Smith NOAA National Weather Service Office of Science."— Presentation transcript:
Monitoring and Predictions of Rip Currents on So California Beaches and 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)
Beach hazards: Rip Currents Trends in the States Courtesy Miami Herald James B Lushine
230 1 13 44 0 18 4 3 13 0 0 0 2 0 9 19 0 1 6 56 3 1 0 38 91 0 34 0 0 Guam GuamGuGuamGu 34 Samoa 3 PR/VI 10 Rip Current Deaths by State 1999-2008 Rip Current Deaths by State 1995-2008
– a jet-like seaward flow across the surf zone X b of a beach. _ a transient eddy _ local vortex motions Rip currents on beaches:
“ 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. Background
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.
What causes Rip Currents ? Major physical factors : Surfs or wind wave breaking Water Level ( tides/set-up ) Beach State (bar, hole)
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.
Southern California Rip Current Monitoring Locations
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: _____________________
Rip Current Data Entry Form (MLB) https://bestpractices.nws.noaa.gov/contents/mdl/testrip/rip_form.php?wfo=KMLB
Buoy - Station 46225, 100 Torrey Pines Outer, CA CDIP- UCSD
Surf Height at Moonlight Beach
Seasonal Wave Characteristics on Moonlight Beach Mean Waves Surf Height (ft)Wave period (sec)Surf Zone Width (ft) Season Winter 3.50 13.3330 Post-Spring2.899.8270 Summer2.6112.6242 Fall2.7011.6250 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
Seasonal Rip Current frequency on Moonlight Beach, CA Intensity WeakModerateStrongNo rips Season Winter 3036 16 32 Spring 3326712 Summer 5349316 Fall 3725314 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
Rip Current observed versus Surf heights San Clemente Beach, California Intensity WeakModerateStrongNo Rip Surfs H (m) H < 0.520016 0.5< H < 1.097210 1.0< H < 2.051062 2.0< H <3.00152 Winter: 12/7/09-1/20/2010
Rip Current cases versus Tide levels Moonlight Beach, California Intensity WeakModerateStrongNo rips Tide level Low161357 Mid12 46 High 14 1029 Extreme case 0011 Selected cases during Summer (6/4-9/27/2008) and Winter and Spring(1/7-4/12/2009)
Two application cases at San Clemente Beach, California 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
Tide effects on Rip hazard Rip current results Max rip speed (m/sec) Rip pull Distance (m) Surf Zone Width (yards) Tide levels Low tide (0.5 m) 0.6040580 High tide (3.0 m) 0.2520040 Nile Mile Beach: H=0.65 m, T= 10 sec and slope = 1/25 (Short and Hogan, 1994)
Effects of wave direction on Rip hazard Rip current strength Weak Or no rips ModerateStrong Angle of incidence to shore normal (0) 0 -10 134 40-70 520 San Clemente Beach: H= 1.0-1.5 m, T= 12-13 sec
Number of Rescues under Waves and Tides at Moonlight Beach, California Surf (ft) 1.5 - 2.02.0 – 3.03.0 - 5.0 Preventive warnings Tide level Low2531270 Mid390456 High04149 Sub-total5 18 41775 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.
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.
1.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. Conclusion and Remarks
Parametric Analysis I. Rips on open beaches : A.Wave -Sediment ( by Short and Wright, Dean ) or M 2 Ω = H b / T w = wave particle speed/sediment falling speed Prevailing condition: 1 < Ω < 6 rips to appear. B.Surf scale parameter (Wave-Beach face slope) or M 3 2.0 wave steepness/beach slope < 25. II. Rip strength estimate: Mass Flux balance (Aaagard, Wu) (sufficient condition)
More sensitive to beach face slope, limited
Beach cusps, Ω < 1 or ε < 2.0, waves are reflected (Short, 1989)
Critical Skill Performance During 4/15/08 - 8/16/2008 at Moonlight Beach SCORE POD Probability of detection FAR False Alarm rate SR Success ratio CSI Critical success index Methods Wave- Sand M 2 0.810.25 0.75 0.631 Surf - Beach M 3 0.630.430.570.495