Monitoring and Predictions of Rip Currents on So California Beaches
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Transcript of Monitoring and Predictions of Rip Currents on So California Beaches
Monitoring and Predictions of Rip Currents on So California Beaches
Drs. C-S Wu and 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
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Rip Current Deaths by State1999-2008
Rip Current Deaths by State 1995-2008
– a jet-like seaward flow across the surf zone Xb of a beach.
_ a transient eddy
_ local vortex motions
Rip currents on beaches:
“ “ Monitor rip currents so as to reduce the hazard Monitor rip currents so as to reduce the hazard to the public….” on 2004 Rip Current Workshopto 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
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Seasonal Wave Characteristics on Moonlight Beach
Mean Waves Surf Height (ft) Wave period (sec) Surf Zone Width (ft)
Season
Winter 3.50 13.3 330
Post-Spring 2.89 9.8 270
Summer 2.61 12.6 242
Fall 2.70 11.6 250
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 Weak Moderate Strong No rips
Season
Winter 30 36 16 32
Spring 33 26 7 12
Summer 53 49 3 16
Fall 37 25 3 14
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 Weak Moderate Strong No RipSurfs H (m)
H < 0.5 2 0 0 16
0.5< H < 1.0 9 7 2 10
1.0< H < 2.0 5 10 6 2
2.0< H <3.0 0 1 5 2
Winter: 12/7/09-1/20/2010
Rip Current cases versus Tide levels Moonlight Beach, California
Intensity Weak Moderate Strong No rips
Tide level
Low 16 13 5 7
Mid 12 <18> 4 6
High 14 10 2 9
Extreme case
0 0 1 1
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)
0.60 405 80
High tide
(3.0 m)
0.25 200 40
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
Moderate StrongAngle of incidence to shore normal (0)
0 -10 1 3 4
40-70 5 2 0
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 - 2.0 2.0 – 3.0 3.0 - 5.0
Preventive warnings
Tide level
Low 2 5 3 1270
Mid 3 9 0 456
High 0 4 1 49
Sub-total 5 18 4 1775
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 = Hb / 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 < ε= Abω² /gtan 2β => wave steepness/beach slope < 25.
II. Rip strength estimate: Mass Flux balance (Aaagard, Wu)• (sufficient condition)
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Omega
More sensitive to beach face slope, limited
Beach cusps, Ω < 1 or ε < 2.0, waves are reflected (Short, 1989)
Critical Skill PerformanceDuring 4/15/08 - 8/16/2008 at Moonlight Beach
SCORE PODProbability
of detection
FARFalse Alarm
rate
SRSuccess
ratio
CSICritical success index
Methods
Wave-Sand
M2
0.81 0.25 0.75 0.631
Surf - Beach
M3
0.63 0.43 0.57 0.495