What conditions cause rip currents to develop on the Great lakes?
Rip currents develop when there is an uneven distribution of water onshore. The uneven distribution could be caused by varying wave heights, varying wave periods (how often the waves come onshore), and varying wave directions. When waves come into the shore at an angle, they develop what is known as a longshore current. This current will span the entire width of the surf zone (the area where you swim), and is maximized at the center. If this current is generated and intersects something like a pier or break water, then the water will pile up along the structure and shoot out into the lake. This is known as a structural rip current. Structural rip currents are the reason people are not encouraged to jump off piers or swim near them…because the current can pull even the best swimmers out to deeper water in a matter of minutes.
Image depicts what happens when a longshore current intersects a pier or shoreline structure.
Image is from the COMET program.
Below are images of the beaches in lower Michigan that have trouble with rip currents. Note a common theme...structures and sandbars.
Left: Muskegon State Park (top) and Pere Marquette Park. To the Right: Mears (top) and Silver Lake State Park.
Images obtained from googlemaps.com.
Above is an image depicting the high and low pressure areas that develop as
water unevenly piles onshore. Image is from theCOMET Program.
Sandbars can cause water to unevenly pile up. Eventually when a
break occurs in the sandbar, a rip current develops.
Image is from the comet program.
Lastly, a third type of rip current is associated with the mouths of rivers on the lake. The water exiting the river mouth is enhanced and goes out into the lake (as a rip current). This is also enhanced by the longshore current if it is in place, serving as a "feeder" to the rip current.
Au Train, MI. This location has had several river-mouth and sandbar
rip current incidents. Image is from googlemaps.com
So if a person is caught in a rip current, how should they escape? Check out this document provided by the US Lifesaving Association:
What is a channel Current? A channel current is NOT the same as a rip current. Channel currents form when there is an island or structure just offshore from a beach. Typically, in the cases on the Great Lakes, this structure is joined to the beach by some sort of sandbar (also known as a Tombolo). When water is squeezed between the shore and the rocks (or structure), it speeds up and causes what is known as the channel current. This current is enhanced when the longshore current (see above) is strongest, which is typically when larger waves come in at an angle to shore. The channel current is seen in locations such as Picnic Rocks, Marquette MI and Hogs Island in Mackinac, MI.
Above is a picture used in the comet module: Rip Currents: Nearshore fundamentals by the COMET program.
This is a diagram showing how the longshore current develops as waves break at an angle to shore.
This picture is of Picnic Rocks in Marquette, MI. The Tombolo is seen stretching from shore to rocks. This is the
typical set up for channel current development. Image is from Googlemaps.com
How does one escape a channel current?
One could escape a channel current by swimming back towards the shore. Many people make the mistake of swimming against the current as they are trying desperately to get back to the sandbar. Think of it as an underwater treadmill. In order to get off the treadmill, one needs to step off to the site of it. The channel current will be moving parallel to shore, so in order to escape, swim perpendicular to the shore. Below is a diagram demonstrating escape.
Sea Grant diagram depicting the channel current. Escape is found by swimming back to shore.
What is the difference between a channel current and a rip current?
Not knowing the difference between a channel current and a rip current could cost a person their life. A rip current is a jet like stream of water extending perpendicular to shore…where a channel current runs parallel to shore. The directions to escape are OPPOSITE. See a comparison of escape routes below:
Escape from a rip current is found by swimming parallel to shore. Escape from a
channel current is found by swimming perpendicular to shore.
Image is from Sea Grant
For more information on rip currents or channel currents, see the National Rip Current Page or the National Weather Service Marquette’s rip current page. Tomorrow we will discuss the common rip and channel current problem areas on Lake Superior.