Rip Currents on the Great Lakes
Image 1. Rip currents (sandy water moving outward from shore) along the Sleeping Bear Dunes near Traverse City, MI. Courtesy of the University of Michigan Coastal Engineering Department.
What is a rip current?
A rip current is a relatively small scale current of water flowing away from the beach. Rip currents develop when there is an uneven distribution of water onshore. The uneven distribution can be caused by a variety of conditions, including moderate to high wave heights, longer wave periods (how often the waves come onshore), or very direct wave angles of approach. As winds blow over the lake and generate waves, the waves move towards the beach and crash onto shore. Uneven distributions of water will cause areas of high pressure and areas of low pressure in the water. Fluids flow towards low pressure (in the atmosphere and water). As an example, think of what happens in the bathtub when you open up the drain. The water will flow into the drain (area of low pressure). The same thing happens on the beach, except the water converges and heads OUT towards the lake (Figures 1 and 2).
Figure 1. Areas of high and low pressure develop on the water surface, leading to convergence and eventually an outward flowing current of water, known as a rip current. Image from the COMET Program.
Figure 2. As the water unevenly piles up between the shoreline and the sandbars, rips develop in the sandbars
from the increasing water pressure. Image is from the COMET Program.
Common Rip Current Misnomers
There are many 'misnomers' for rip currents. The most common misnomers are "Undertow" and "Rip Tide." The most common misnomers and their true definitions are listed below (taken from the NWS National Rip Current Page):
Undertow: Scientists actually have no agreed on definition of undertow. Many people assume it is an underwater current that sucks people downward,however this is not the case. In fact, there is no force in the lake or ocean that is found in the nearshore environment that will pull unsuspecting swimmers underwater! Undertow is thought to be the backwash after a wave crashes on the beach. This can trip waders, move them seaward, and make them susceptible to immersion from the next incoming wave. This would make the swimmer feel as if they are being "pulled under."
Rip Tide: The word tide inplies the pull of the sun or the moon. Rip Tides are NOT THE SAME AS RIP CURRENTS. They occur during high or low tides and are found in areas where water moves through inlets, mouths of estuaries, harbors, and embayments as well as around points of land and islands.
Rip Current Examples and Identification
Sandbars help to 'pile up' water along the shoreline (Image 2). Once the water 'piles up' unevenly along the shore, rip currents can develop in weak spots within the sandbar, or in previously formed rip current channels (Image 3).