Megan Babich NWS Marquette, MI

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

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 Photo Courtesy of Don Rolfson, NWS Marquette Grand Marais, MI: Lake Superior

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

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

The Great Lakes Current Incident Database 346 current related incidents collected from media articles/eyewitness reports Weather and lake conditions during incidents documented swim seasons

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

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

Rip current fatalities and rescues on the Great Lakes

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.

Near Shoreline Structures!

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

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

Shoreline structures + LTB state Google Maps Grand Haven, MI

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

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

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

Wave Heights Wave Periods Wind Speed and Direction

Waves between 2 and 4 feet Sandbar incidents rare if waves less than 2 feet

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

Total incidents used: 344/346

Why so many low-height incidents?

Why so many low wave height incidents?

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

Short: 3 to 5 Seconds

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

Wave periods typically 3-5 seconds on the Great Lakes

Onshore or parallel to shore

Winds: Indirectly related to rip current development Lushine: Similar study on ocean in % cases onshore 90% cases within 30 degrees of normal to shore Wind speeds: 15 mph (6.7 m/s)

Wind speeds during G.L. incidents

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

Offshore winds: Possible explanations

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

Cold Frontal Passage

Synoptic pattern

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

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

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

Popularity + favorable conditions PURE MICHIGAN

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

Website: Questions/Comments: Ludington, MI. Photo by Megan Babich

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