Rip Currents and Channel Currents on Lake Superior
Rip currents and channel currents occur all over the Great Lakes. Since 2002, there have been 263 current related incidents on the Great Lakes. 29 of those incidents have occurred on Lake Superior, with 11 drowning deaths and 18 rescues recorded in the National Weather Service (NWS) database. Where are all of these drowning deaths and rescues occurring? Why are these incidents occurring where they are? This page will give a brief overview of drowning deaths and rescues due to channel and rip currents on the Great Lakes.
Rip currents at the Grand Sable Dunes near Grand Marais, MI.
What causes rip currents and channel currents? A brief review...
Rip Currents are currents of water that flow outward from shore. Rip currents will develop under conditions that cause the uneven distribution of water onshore. The uneven distribution could be caused by sandbars that are parallel to shore, varying wave heights and periods (the time between each wave), and seiches (water sloshing back and forth in the lake). This uneven distribution will lead to areas of high and low pressure in the water. Water will flow towards the region of low pressure, and then jet out into the lake. Conditions where rip currents are likely are when winds are blowing towards the shore, and waves are two feet or larger. Rip currents can persist long after the conditions that generated them subside. Rip currents are typically found in locations with sandbars running parallel to shore, breakwalls, or river mouths. Escape is found by swimming parallel to shore until out of the current, then swimming back to shore.
Channel Currents are currents of water that flow parallel to shore. They develop when water is squeezed between the shore and an offshore structure (like an island). Typically there is a sandbar that connects the island to the mainland, and this is where people get into trouble. This current is enhanced by higher waves and stronger winds, however like the rip currents, they can persist long after the conditions that generated them disappear. Escape is found by swimming perpendicular to shore instead of trying to swim back to the sandbar.
Please note that RIP CURRENTS ARE NOT CHANNEL CURRENTS. The escape routes are OPPOSITE!
Beach Breakdown: Where are the incidents occuring and Why?
Twelvemile Beach: Grand Marais, MI
Since 2002, there has been one rescue at Twelvemile Beach in Grand Marais, MI. Note that there have been other incidents, but they were prior to 2002 and therefore not included in the database. One thing to immediately notice about the beach is the sandbars that run parallel to the shore. This can cause the uneven distribution of water in the surf zone (where you swim), and thus rip current development. There is also a breakwall at the end of the beach, which is also another favored rip current location, however the incident occurred far from the breakwall.
Photo from Googlemaps of Twelvemile Beach area near Grand Marais, MI. Note the sandbars and Breakwall.
At the time of the incident (2008), the winds were north to northeast at 22 mph...gusting to near 30 mph! Waves were four to six feet. One thing to stress about swimming in the lake: Avoid swimming during times of rough conditions! Not only is it difficult to swim in the choppy waves, but rip currents are more likely to develop! These types of conditions are typical after cold fronts move across the lakes. Note that north to northeast winds are onshore winds. With the aid of the sandbars, it is no suprise that rip currents developed.
Weather map obtained from the HPC Surface analysis archive. Incident was August 18, 2008. Note the cold
front just south of Lake Superior.
Au Train Bay in Alger County, MI
There have been at least 3 rescued from a rip current associated with either the sandbars or the river mouths in 2010. Conditions at the time of the events were northwest winds at 15 to 20 mph with waves one to three feet. Prior to the incidents though, waves were two to four feet. The cold front had passed 9 hours before the incidents, but the rip currents persisted even after conditions were no longer supportive of their development.
Weather map 9 hours prior (top) and during the time of the incident. Winds
were onshore. Cold front had passed earlier in the day.
Image from HPC surface analysis archive
Middle Bay Beach/Presque Isle: Marquette, MI
So far, two people have been rescued here (2010) and two people have drowned (2007). This location is unique in that Presque Isle acts similar to a breakwall or pier jetting out into the lake. As water piles up in the bay, a rip current develops alongside the Isle and heads into the lake. One thing to note is that the rip current typically only develops under conditions where waves are high and winds are fairly strong. Both of the incidents had winds in a northerly-oriented direction with gusts around 20 mph. Waves in both incidents were five to seven feet.
Presque Isle/Middle Bay in Marquette, MI. A rip current develops along the
side of presque Isle during times of high winds and waves.
Image is from Googlemaps.
Note the cold fronts passing during each incident, bringing a northerly-oriented flow and causing
development of a structural rip current. Images from HPC surface analysis archive.
Park Point, MN
The problem area near Park Point, MN is in the Twin Ports. This is near Duluth, MN. So far there has been one drowning, and ten rescues that we know of. There were three different days over which these incidents occurred, and all of them had northeasterly (onshore) winds. Waves during the incidents varied from one to four feet in some to four to eight feet in others. Note the sandbars along the shore. Similarly to the other cases, there was a cold front passing from north to south in each case.
Park Point, MN. Note the presence of sandbars along the coast.
Image is from the Duluth NWS
Note that all cases had a cold frontal passage with high pressure moving in behind it. Images from Dailyweathermap.com
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 two rescues. Below is a photo of the area and the escape route from the channel current.
Depiction of the Picnic Rocks Channel current. Escape is towards shore. Image is from googlemaps.
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 south and the north direction. 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 shore, maximizing the strength of the longshore current. If the waves are larger than two feet, 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 shore, they tried to swim back to the sandbar. Unfortunately, this is against the current, and as a result, they became exhausted and drowned. At the Marquette NWS, we use this image of the Marquette High School Football Field (scaled to the appropriate size) to show how far the rocks really are from the shore. As is seen, at the shortest distance, the sandbar goes for at least ONE of the football fields. Because the rocks are so large, they give off the impression 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, which of course, is the fatal mistake.
Image of Picnic Rocks with Football field scaled in. At its CLOSEST point, it is as far as one football field
from the shore to the rocks. Image from googlemaps.
More research still needs to be done on the specifics of the Picnic Rocks location. During summer of 2011, the city of Marquette will install a current meter between the shore and the rocks. The NWS will compare this data with surrounding locations, observations from lifeguards, nad wave model output in order to better understand the conditions that cause the current to strengthen and also to see how fast it actually is.
Lake superior may have some drowning deaths and rescues due to rip currents on the Canadian side, but that data has not yet been collected. Finally, note the common thread between rip current drowning deaths and rescues and the winds and waves. The higher the waves and the stronger the onshore winds, the more likely rip current development is. Typically, higher winds and waves result from a cold frontal passage. Lastly, Picnic rocks is a location on Lake Superior with a Channel Current problem-recall there is a difference between the channel currents and rip currents. The channel current at Picnic Rocks is enhanced by stronger southerly/northerly winds and larger waves.
For more information on rip currents and how they develop, visit the National Rip Current Page.
Tomorrow we will talk about how to stay safe on the beach. This includes a discussion on the flag system and how it works.