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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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