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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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Bowen, A.J., 1969: Rip currents, 1: Theoretical Investigations. 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. [http://www.meted.ucar.edu/marine/ripcurrents/NSF/index.htm] Cook, D.O., 1970: The occurrence and geological work of rip currents off southern California. Marine Geology, 9, 173-186. Dalrymple, R.A., 1975: A mechanism for rip current generation on an open coast. Journal of Geophysical Research, 80, 3485-3487. Dalrymple, R.A., 1978: Rip currents and their causes. Proc. of the 16 th International Conference of Coastal Engineering, Hamburg, American Society of Civil Engineers, 1414-1427. Engle, J., J. MacMahan, R.J. Thieke, D.M. Hanes, R.G. Dean., 2002: Formulation of a rip current predictive index using rescue data. Proc. of the National Conference on Beach Preservation Technology, Biloxi, MS. Florida Shore and Beach Preservation Association. Google, 2011: Google Maps. [Available online at http://www.googlemaps.com]http://www.googlemaps.com Guenther, D., 2003: Rip current case study 3, 4 July 2003. Marquette Michigan National Weather Service Office Report. Hite, M.P., 1925: The Undertow. Science, 62, 31-33. Hydrometeorological Prediction Center, 2011: Hydrometeorological Prediction Centers Surface Analysis Archive. [Available online at http://www.hpc.ncep.noaa.gov/html/sfc_archive.shtml]http://www.hpc.ncep.noaa.gov/html/sfc_archive.shtml Hydrometeorological Prediction Center, 2011: The Daily Weather Map. [Available online at http://www.hpc.ncep.noaa.gov/dailywxmap/] http://www.hpc.ncep.noaa.gov/dailywxmap/ Lascody, R.L., 1998: East Central Florida rip current program. National Weather Digest, 22(2), 25-30. Lushine, J.B., 1991: A study of rip current drownings and related weather factors. National Weather Digest, 16, 13-19. Meadows, G., H. Purcell, D. Guenther, L. Meadows, R.E. Kinnunen, and G. Clark, 2011: Rip Currents in the Great Lakes: An Unfortunate Truth. Rip Currents: Beach Safety, Physical Oceanography, and Wave Modeling, S. Leatherman and J. Fletemeyer, Eds., CRC Press, 199-214. McKenzie, R., 1958: Rip current systems. Journal of Geology, 66, 103-113. Munk, W.H., 1949: The solitary wave theory and its application to surf problems. Ann. N.Y. Acad. Sci., 51, 376-424. Nicholls, C.P.L., 1936: Rip Tides and How To Avoid Their Perils. Calif. Beaches Assoc., vol. 1 No. 9, 12. Shepard, F.P., 1936: Undertow, rip tide or rip current. Science, 84, 181-182. Shepard, F.P., K.O. Emery, and E.C Lafond., 1941: Rip Currents: A process of geological importance. Journal of Geology, 49, 338-369. Shepard, F.P., D.L. Inman., 1950: Nearshore circulation. Proc. of the 1 st Conference on Coastal Engineering, Berkeley, CA, Council on Wave Research, 50-59. Short, A.D., 1985: Rip current type, spacing and persistence, Narrabeen Beach, Australia. Marine Geology, 65, 47-71. Sonu, C.J., 1972: Field observations of nearshore circulation and meandering currents. Journal of Geophysical Research, 77, 3232-3247. Tang, E. and R.A. Dalrymple., 1989: Rip currents, nearshore circulation, and wave groups. In Nearshore Sediment Transport, R.J. Seymour, editor, New York, NY, Pelenum Press, 205-230. Wood, W.L., and G.A. Meadows., 1975: Unsteadiness in longshore currents. Geophysical Research Letters, Vol 2, No 11. Wright, L.D. and Short, A.D., 1984: Morphodynamic variability of the surf zones and beaches: A synthesis. Marine Geology, 56, 93-118.