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An Examination of Rip Current incidents on the Great Lakes

An Examination of Rip Current incidents on the Great Lakes. Megan Babich NWS Marquette, MI. Overview. What is a rip current ? Conditions necessary for rip current development The Great Lakes Current Incident Database

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An Examination of Rip Current incidents on the Great Lakes

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  1. An Examination of Rip Current incidents on the Great Lakes Megan Babich NWS Marquette, MI

  2. 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

  3. 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. Grand Marais, MI: Lake Superior Photo Courtesy of Don Rolfson, NWS Marquette Shepard et al. 1941

  4. 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

  5. What causes these variations in “stress”? • Differences in wave characteristics: • Height • Period • Tidal influences/Seiches • Shoreline Structures

  6. 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

  7. Limitations • Media coverage of incidents • Media assumptions and error • Limited observations of nearshore environment • Observations may not be representative of true environment

  8. 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) Ludington, MI: Lake Michigan Photo Courtesy of Univ. of Michigan Coastal Engineering Department

  9. Rip current fatalities and rescues on the Great Lakes 2002-2011

  10. 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.

  11. Where Do Dangerous Rip Currents On The Great Lakes Form? Near Shoreline Structures!

  12. Locations of ocean rip currents Courtesy of NCbeaches.com: Fishing Piers Courtesy 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 Courtesy of Dennis Decker, WCM, NWS Melbourne, FL Shepard et al. 1941, Wright and Short 1984

  13. Beach Types Wright and Short, 1984: Brander, 2012

  14. Shoreline structures + LTB state Grand Haven, MI Google Maps

  15. River mouths or outlets + RBT… Au Train, MI Google Maps

  16. At beaches with only complex morphology-TBR Grand Marais, MI Google Maps

  17. Most rip currents on the Great Lakes occur at beaches with shoreline structures!!!!!

  18. What Conditions Were Observed During Rip Current Incidents On The Great Lakes? • Wave Heights • Wave Periods • Wind Speed and Direction

  19. Wave Height Observations Waves between 2 and 4 feet Sandbar incidents rare if waves less than 2 feet

  20. 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

  21. Total incidents used: 344/346

  22. Why so many low-height incidents?

  23. Why so many low wave height incidents?

  24. 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 Google maps Park Point, MN frequently sees rip current incidents when waves are in the 1 to 3 ft range

  25. Wave Period Observations Short: 3 to 5 Seconds

  26. 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

  27. Wave periods typically 3-5 seconds on the Great Lakes

  28. Wind observations Onshore or parallel to shore

  29. 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)

  30. Wind speeds during G.L. incidents

  31. Wind orientation to shore during G.L. incidents Only 45% of onshore cases within 30 degrees of normal

  32. Offshore winds: Possible explanations

  33. Forecasting application • Onshore /Parallel winds: Rip Current Development • Know your beach! • Remember: Indirectly Related Off Highway 2: Mackinac County, MI Image Courtesy of Steve Hernek

  34. Synoptic Pattern Cold Frontal Passage

  35. Synoptic pattern

  36. Cold frontal passage August 16, 2010: 3 Fatalities 4 Rescues

  37. Cold frontal passage August 1, 2009: 1 Fatality 8 Rescues

  38. Cold frontal passage August 5-8, 2010: 7 Fatalities 5 Rescues

  39. Popularity + favorable conditions PURE MICHIGAN

  40. 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

  41. Questions/Comments: Megan.Babich@noaa.gov Website: http://www.crh.noaa.gov/mqt/?n=rip_toc Ludington, MI. Photo by Megan Babich

  42. References 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 16th 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] 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 Center’s Surface Analysis Archive. [Available online at 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/] 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 1st 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.

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