Channel Currents & Longshore Currents at Picnic Rocks
September 17, 2012
Picnic Rocks is a popular beach area in Marquette. I thought that I’d come up with a good story for the name, but instead, I learned about the formation and dangers of what are known as channel & longshore currents. The Marquette National Weather Service explains:
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.
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.
…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.
Read on for more including a diagram demonstrating the best way to escape if you’re caught in these currents. A swimmer recently drowned here and this summer has been a deadly one – please use your knowledge to help others stay safe!
So we don’t end on a down note, let me say that in good weather, Picnic Rocks is a fantastic, natural playground for folks of all ages!!
More from Marquette on Michigan in Pictures.