Dangerous currents along breakwalls pose serious risk to swimmers and recreational fishers

New technology used to monitor dangerous currents near breakwalls in Lake Michigan

A structural current is one that forms along a shoreline structure. It develops as a longshore current until it intersects a breakwall or peninsula. Then the water moves swiftly parallel to the structure. These currents are extremely dangerous because they are difficult to escape. Swimmers can be caught in these structural currents when they swim near a breakwall and are then carried lakeward along the breakwall. If a recreational fisher accidently falls off a breakwall while fishing, he/she will be caught in this structural current. In fact, structural currents account for most of the Great Lakes dangerous currents related incidents that are reported.

To avoid being caught in a structural current, swimmers should only swim in designated areas and avoid swimming near the base of breakwalls. If caught in a structural current, it is best to shout for help and have someone throw you a life ring or something that floats. If help is not available, you should try to make your way to one of the ladders on the breakwall.

In the spring 2014, a team of researchers from Michigan Technological University, University of Michigan, Michigan Sea Grant and Michigan State University Extension (led by Guy Meadows  from Michigan Technological University), conducted a research project at the Grand Haven and Holland State Parks breakwalls in southern Lake Michigan. The goal of this field experiment was to document structure-related rip currents at Grand Haven and Holland State Parks. Weather conditions during the experiment were under the influence of the passage of a series of cold fronts and the associated wind shift from southerly to northerly flow. Calm conditions allowed for acquisition of the bathymetric data. Rip current flow measurements along the breakwall were acquired as the wind increased in velocity directed toward the structure.

At Grand Haven, the bathymetry immediately south of the harbor breakwall showed a deep channel reaching 9 feet of water depth approximately 400 feet from shore. The transition to this channel is abrupt in the alongshore direction, shifting from 5 feet of water depth to 9 feet in less than 50 feet. The field crew also characterized the nearshore bathymetry on the north side of the Grand Haven breakwall and found the bathymetry at this site comparable to the south side of the structure, revealing a channel of similar depth and configuration immediately adjacent to the breakwall. At Holland, north of the harbor breakwall, the bathymetry was dominated by a deep channel originating as a longshore trough that turns offshore at the interface of the bathymetry with the harbor structure. The channel is approximately 20 meters in width.

In the presence of a large harbor breakwall, satellite/GPS drifters were swept lakeward near the structure reaching over 1000 ft offshore. Average drifter velocities were 0.7 to 1.6 ft/s with maximum velocities of 4.6 to 11.8 ft/s under relatively high incident wind and wave conditions. In less stormy conditions, flow velocities reached 1.5 ft/s at the structure, with an average of 0.6 ft/s, with higher velocities concentrated in a narrow channel adjacent to the structure and closer to shore.

Funding for this project came from the Michigan Coastal Zone Management Program, Michigan Department of Environmental Quality, through a grant from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.

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