Transcript of Megan Babich NWS Marquette, MI. Overview What is a rip current? Conditions necessary for rip current...
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- Megan Babich NWS Marquette, MI
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- Overview What is a rip current? Conditions necessary for rip
current development The Great Lakes Current Incident Database
Comparisons between ocean rip currents and Great Lakes rip currents
based on collected data
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- What is a rip current? A narrow jet of water moving swiftly
away from shore, roughly perpendicular to the shoreline. A way for
water piled up on shore to escape back into the lake/ocean. Shepard
et al. 1941 Photo Courtesy of Don Rolfson, NWS Marquette Grand
Marais, MI: Lake Superior
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- How do rip currents develop? Variations in the stress on the
surface of the water lead to areas of high and low pressure.
Converging longshore (shore parallel) currents cause an outward
flow of water: a rip current Shepard et al, 1941; Shepard and Inman
1950; McKenzie 1958; Bowen 1969; COMET
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- What causes these variations in stress? Differences in wave
characteristics: Height Period Tidal influences/Seiches Shoreline
Structures
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- The Great Lakes Current Incident Database 346 current related
incidents collected from media articles/eyewitness reports Weather
and lake conditions during incidents documented 2002-2011 swim
seasons
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- Limitations Media coverage of incidents Media assumptions and
error Limited observations of nearshore environment Observations
may not be representative of true environment
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- Why is this important? Rip currents responsible for at least
150 drowning fatalities per year nationally (USLA, Lushine 1991)
Rip currents responsible for at least 10 drowning fatalities per
year on the Great Lakes (NWS MQT database) Photo Courtesy of Univ.
of Michigan Coastal Engineering Department Ludington, MI: Lake
Michigan
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- Rip current fatalities and rescues on the Great Lakes
2002-2011
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- Why is this important? NWS MISSION: Protection of Life and
Property Knowing the conditions and locations necessary for rip
current development on the Great Lakes will help forecasters to
better advise the public about these dangerous hazards.
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- Near Shoreline Structures!
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- Locations of ocean rip currents Shepard et al. 1941, Wright and
Short 1984 Courtesy of Dennis Decker, WCM, NWS Melbourne, FL
Courtesy of NCbeaches.com: Fishing PiersCourtesy of U.S Army Corps
of Engineers Digital Visual Library Piers, groins, and Jetties
River mouths or similar outlets Near complex sandbar structures :
intermediate beach types
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- Low energy, large sand grains No Rip Currents Steep Beach
Little Change over time Reflective Medium energy, Medium sand
grains Rip currents common Constantly changing-4 different states
Intermediate High energy, small sand grains No rip currents Flat
Beach Little change over time Dissipative Beach Types Wright and
Short, 1984: Brander, 2012
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- Shoreline structures + LTB state Google Maps Grand Haven,
MI
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- River mouths or outlets + RBT Google Maps Au Train, MI
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- At beaches with only complex morphology-TBR Google Maps Grand
Marais, MI
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- Most rip currents on the Great Lakes occur at beaches with
shoreline structures!!!!!
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- Wave Heights Wave Periods Wind Speed and Direction
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- Waves between 2 and 4 feet Sandbar incidents rare if waves less
than 2 feet
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- Wave height: Great Lakes Vs. Ocean Higher waves on ocean made
rip currents: Less numerous Stronger Lower waves on ocean made rip
currents: More numerous Weaker Shepard et al. 1941; Shepard and
Inman, 1951; Bowen, 1968
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- Total incidents used: 344/346
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- Why so many low-height incidents?
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- Why so many low wave height incidents?
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- Wave Heights: Conclusions Life Threatening rip currents not
likely if wave heights < 2 ft Shoreline structures River mouths
Threat increases once waves get to 2 ft Forecasting Application:
Know your beach! Always exceptions Park Point, MN frequently sees
rip current incidents when waves are in the 1 to 3 ft range Google
maps
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- Short: 3 to 5 Seconds
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- Wave period: Ocean Long wave periods: Greater than 9 seconds
Larger volume of water onshore Regularly spaced rip currents Wide
rip currents Short wave periods: Less than 9 seconds Numerous rip
currents Irregularly spaced rip currents Smaller rip currents Could
contribute to duress of swimmer Shepard et al. 1941, Shepard and
Inman, 1951; Bowen, 1968
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- Wave periods typically 3-5 seconds on the Great Lakes
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- Onshore or parallel to shore
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- Winds: Indirectly related to rip current development Lushine:
Similar study on ocean in 1991 100% cases onshore 90% cases within
30 degrees of normal to shore Wind speeds: 15 mph (6.7 m/s)
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- Wind speeds during G.L. incidents
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- Wind orientation to shore during G.L. incidents Only 45% of
onshore cases within 30 degrees of normal
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- Offshore winds: Possible explanations
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- Forecasting application Onshore /Parallel winds: Rip Current
Development Know your beach! Remember: Indirectly Related Image
Courtesy of Steve Hernek Off Highway 2: Mackinac County, MI
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- Cold Frontal Passage
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- Synoptic pattern
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- Cold frontal passage August 16, 2010: 3 Fatalities 4
Rescues
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- Cold frontal passage August 1, 2009: 1 Fatality 8 Rescues
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- Cold frontal passage August 5-8, 2010: 7 Fatalities 5
Rescues
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- Popularity + favorable conditions PURE MICHIGAN
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- Conclusions Forecasters: Know your beaches! Suggestion: Collect
your own data In my study of the Great Lakes: Most incidents were
near shoreline structures Wave heights 2 to 4 ft Shore
Parallel/Onshore Winds Wave period 3 to 5 seconds Cold front
passage was common problem-pattern
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- Megan.Babich@noaa.gov Website:
http://www.crh.noaa.gov/mqt/?n=rip_toc Questions/Comments:
Ludington, MI. Photo by Megan Babich
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Journal of Geophysical Research, 74, 5468-5478. Bowen, A.J., and
D.L. Inman., 1969: Rip currents, 2: Laboratory and field
observations. Journal of Geophysical Research, 74, 5479-5490
Brander, 2012. Science of the surf, 2012: SOS Fact Sheet: Beaches.
[Available online at
http://www.scienceofthesurf.com/downloads/SOSFact_Sheet_Beaches.pdf]
COMET, 2011: Rip Currents: Nearshore Fundamentals.
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