Megan Babich NWS Marquette, MI. Overview What is a rip current? Conditions necessary for rip current...
43
An Examination of Rip Current incidents on the Great Lakes Megan Babich NWS Marquette, MI
Megan Babich NWS Marquette, MI. Overview What is a rip current? Conditions necessary for rip current development The Great Lakes Current Incident Database
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
Slide 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. Shepard
et al. 1941 Photo Courtesy of Don Rolfson, NWS Marquette Grand
Marais, MI: Lake Superior
Slide 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
Slide 5
What causes these variations in stress? Differences in wave
characteristics: Height Period Tidal influences/Seiches Shoreline
Structures
Slide 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
Slide 7
Limitations Media coverage of incidents Media assumptions and
error Limited observations of nearshore environment Observations
may not be representative of true environment
Slide 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) Photo Courtesy of Univ.
of Michigan Coastal Engineering Department Ludington, MI: Lake
Michigan
Slide 9
Rip current fatalities and rescues on the Great Lakes
2002-2011
Slide 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.
Slide 11
Near Shoreline Structures!
Slide 12
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
Slide 13
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
Slide 14
Shoreline structures + LTB state Google Maps Grand Haven,
MI
Slide 15
River mouths or outlets + RBT Google Maps Au Train, MI
Slide 16
At beaches with only complex morphology-TBR Google Maps Grand
Marais, MI
Slide 17
Most rip currents on the Great Lakes occur at beaches with
shoreline structures!!!!!
Slide 18
Wave Heights Wave Periods Wind Speed and Direction
Slide 19
Waves between 2 and 4 feet Sandbar incidents rare if waves less
than 2 feet
Slide 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
Slide 21
Total incidents used: 344/346
Slide 22
Why so many low-height incidents?
Slide 23
Why so many low wave height incidents?
Slide 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 Park Point, MN frequently sees
rip current incidents when waves are in the 1 to 3 ft range Google
maps
Slide 25
Short: 3 to 5 Seconds
Slide 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
Slide 27
Wave periods typically 3-5 seconds on the Great Lakes
Slide 28
Onshore or parallel to shore
Slide 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)
Slide 30
Wind speeds during G.L. incidents
Slide 31
Wind orientation to shore during G.L. incidents Only 45% of
onshore cases within 30 degrees of normal
Slide 32
Offshore winds: Possible explanations
Slide 33
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
Slide 34
Cold Frontal Passage
Slide 35
Synoptic pattern
Slide 36
Cold frontal passage August 16, 2010: 3 Fatalities 4
Rescues
Slide 37
Cold frontal passage August 1, 2009: 1 Fatality 8 Rescues
Slide 38
Cold frontal passage August 5-8, 2010: 7 Fatalities 5
Rescues
Slide 39
Slide 40
Popularity + favorable conditions PURE MICHIGAN
Slide 41
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
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.