What you need to know
Channel Currents: What are they?
A channel current is caused when water is squeezed between the shore and an offshore structure or feature (such as an island). When water is squeezed it speeds up, thus causing the current. This is like putting a smaller nozzle on a garden hose. When the smaller nozzle is on, the water comes out faster.
Above is a picture of Picnic Rocks, located in Marquette, MI. When water gets squeezed between the rocks and the
shore, it speeds up and causes the current to form. Image was taken from Google Maps (2011). East is at the top of the image.
This current can be enhanced by what is known as a longshore current, a current that is generated by waves breaking onshore. As waves move onshore, they break in the direction they are moving in order to dissipate their energy. This causes the longshore current. Overtime, the current spans the entire width of the surf zone (the place where you swim). In the case of a channel current, the longshore current can speed up the channeling effect between the shore and the rocks, causing dangerous conditions to develop for those who are walking along the sandbar. The longshore current is maximized during times of higher waves that come in at a 45 degree angle to the shore.
Above is a diagram depicting the longshore current. This current develops as waves break at an angle to shore.
Diagram taken from the COMET module: Nearshore Fundamentals.
In summary, the channel current itself is from the convergence of water between the rocks and the shore. It is enhanced by the longshore current, a current that flows parallel to the shore. The longshore current (and thus the channel current) is enhanced when large waves come in at an 45 degree angle to the shore. Keep in mind that if conditions were choppy the night before a day at the beach (or even as much as a day), the current may still be strong even if the waves are small.
How does one escape a channel current?
One could escape a channel current by swimming back to towards the shore. Many people make the mistake of swimming against the current as they are trying to get back to the sandbar. Think of the current as an underwater treadmill. In order to get off the treadmill, one needs to step off to the side 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.
Above is a diagram from Michigan SeaGrant. Escape from a channel current is found by swimming towards 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, whereas 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 the shore.
Escape from a channel current is found by swimming perpendicular to the shore.
Images are from Michigan SeaGrant.
Escape a rip current: Since the current is flowing from the shore into the lake, one would swim parallel to the shoreline in order to escape. Once out of the current, swim back to shore.
Escape a channel current: Since the current is flowing along the shoreline, one would swim towards the shore in order to escape.
Where are channel currents a problem in the Great Lakes?
Picnic Rocks: Marquette, MI
The familiar set up for channel currents is found at Picnic Rocks in Marquette, MI. At this location, 15 people have drowned since 1963 as a result of the channel current here. In addition, there have been at least five rescues. Below is a photo of the area and escape route from the channel current.
The image above is from Google Maps (2011). Arrows were added in by NWS Marquette.
East is at the top of the image.
By studying the past events at Picnic Rocks, one thing has become clear. The winds at the time of the drowning deaths or rescues have been primarily out of the southerly and the northerly directions. In the above photograph, north is to the left, and east is at the top of the image. As more water is pushed between the rocks and the shore, it speeds up. When winds are out of the southeast and northeast, the waves come in at a 45 degree angle to the shore, maximizing the strength of the longshore current. If the waves are moderately high, the current can be even stronger. Many of the victims of this current were walking along the sandbar to get to the rocks and were suddenly pushed off. Instead of swimming back to the shore, they tried to swim back to the sandbar. Unfortunately, this is against the current, and as a result, they became exhausted and drowned.
One of the problems is that people don't realize how far the rocks are from the shore. At the Marquette NWS, we use this image of the Marquette High School football field (scaled to the correct size) to show how far the rocks really are from the shore. As is seen, at the shortest distance, the sandbar extends as far as at least ONE of the football fields. Because the rocks are so large, they give off the impression that they are much closer than they truly are. Once people realize how far out they are, they panic and reason that the sandbar is much closer. This is the fatal mistake.
The image above is from Google Maps (2011). Football field added in by Marquette NWS.
More research needs to be done on the specifics of the Picnic Rocks location. During the summer of 2011, the city of Marquette will install a current meter between the shore and the rocks. The National Weather Service will compare this data with observations from surrounding locations, lifeguards, and wave model output in order to better understand how fast the current is and what conditions cause the current to strengthen.
A quick look at other locations with channel current problems on the Great Lakes....
Saddlebag Island on Lake Huron. Located in Chippewa County, MI.
1 drowning occurred here. Note the similarities to Picnic Rocks in Marquette.
Image from Google Maps (2011).
Washington Island to Rock Island, WI on Lake Michigan/Green Bay.
One drowning occurred at this location. Note the sandbar connecting the islands.
Image from Google Maps (2011).
Round Island, MI. Located in Delta County.
1 drowning and 1 rescue occurred at this location.
Image from Google Maps (2011).
Hogs Island, MI in Mackinac County. One drowning has occurred at this location.
Image is from Google Maps (2011).
A similar pattern to the channel current diagram is seen in all of these locations. Picnic Rocks likely has more deaths and rescues because of the popularity of the location, and the higher population of the area. Marquette also sees many tourists. In fact, many of the victims at that location were not local residents.
In summary, remember that rip currents ARE NOT THE SAME as channel currents! Not knowing the difference could cost a person their life.