Structural Currents and Longshore Currents

 

Longshore Currents

When waves approach the shoreline at an angle, they develop a longshore current. A longshore current is a lake current that moves parallel to shore. Shore-parallel sandbars, like those seen on the Great Lakes can greatly enhance the speed of this current. Longshore currents can be strong enough to prevent swimmers from being able to keep their feet on the lake bottom, making it difficult to return to shore. One of the most dangerous aspects of longshore currents is that they can feed directly into a rip current or structural current. These currents are very dangerous, as they pull swimmers out into deeper water.

 

Drifting Down the Beach

Swimmers often notice the longshore current when they place their belongings in one specific spot on the beach and get into the water. About 10 minutes later, they notice they are quite a distance down the beach from their belongings. This can be dangerous for small children, as parents can quickly lose sight of them when they drift down the beach. Be mindful of this current. Ensure that you are not drifting towards a hazardous swimming area, such as a breakwall or pier.

 

Image 1.  Diagram depicting various currents at the beach. The longshore current (yellow) is generated when waves approach the shoreline at an angle (white), and moves parallel to the shore. The longshore current can carry swimmers down the beach. Sometimes, the longshore current can draw swimmers into a rip current (red), or into a structural current near the breakwall (red). Image of Grand Haven State Park from: http://greatlakes.usace.army.mil/

 

Escaping a Longshore Current

To escape a longshore current, simply exit the water. This may be difficult because the current can be strong enough to prevent you from keeping your feet on the lake bottom. The best way to avoid getting caught in a strong longshore current is to stay out of the water when waves reach heights greater than 3 feet.

 

Image 2. Depiction of a longshore current. To escape, swim towards the shore. This diagram is from the Sea Grant Dangerous Currents Page.

 

Structural Currents

A structural current forms along shoreline structures, like piers or breakwalls, where the longshore current and wave action flow into the structure. Though these currents are actually a type of rip current, we set them apart as a separate current type because the traditional escape route for rip currents (swim parallel to shore) will not work for a structural current. Structural currents are responsible for a majority of the current-related drowning fatalities and rescues on the Great Lakes. NEVER swim near shoreline structures like piers and breakwalls.

 

Image 3. Diagram of a Structural current from the Sea Grant Dangerous Currents Page

 

Escaping a Structural Current

Escape from a structural current is made virtually impossible for several reasons (reference Image 1). First, a swimmer trying to swim parallel to the shore (moving away from the breakwall) will swim directly into the longshore current, which will keep forcing them toward the breakwall. Secondly, if the swimmer attempts to swim towards the shore, they will be fighting the structural current that forces them into the lake. Lastly, if the swimmer lets the current pull them lakeward (floating) they will be carried into much deeper water where they may not be able to swim. All currents aside, the swimmer will also be combating high waves crashing over the breakwall in rapid succession. Those rescued from structural currents often mention that they could not swim parallel to shore, and had trouble floating due to the waves. The only hope those caught in a structural current have is:

 

1. Getting to the ladder on the breakwall (if there is one).

2. Have someone on the breakwall throw a life ring.

3. Try to float (if you are floating, you are more relaxed and conserve energy). This may be difficult due to the high wave action near the breakwall.

 

The best way to escape a structural current is to AVOID getting caught in one. Never swim near shoreline structures such as breakwalls, piers, jetties, and groins.

 

The Most Problematic Dangerous Current on the Great Lakes

In an examination of Great Lakes current-related incidents from 2002-2013 (see the Great Lakes Current Incident Database), most of the fatalities and rescues on the Great Lakes were related to structural currents.

Image 4: Type of current related incidents in the Great Lakes, 2002-2013 (GLCID). Structural currents are responsible for a majority of the current related incidents on the Great Lakes. Struct/class or outlet/class are incidents where it was not known where the victim was swimming at the time, therefore it could have been a classic rip current (classic rip/class) or structural current/outlet current, depending on the beach. Current related means that incidents were likely a combination of waves and currents.

 

Structural Currents Still Present During Low Wave Heights

Although a majority of the Great Lakes current-related incidents occurred during high wave action, roughly one third of the incidents occurred with waves that were less than 3 feet. When these incidents were broken down by location, a majority of them were due to structural currents. Others were caused by channel currents and outlet currents. A percentage of the incidents occurred when wave heights were diminishing (meaning there were high waves earlier in the day), suggesting the currents remain long after the conditions that create them subside. Overall, it appears dangerous currents still exist near shoreline structures and river mouths regardless of the wave heights. Swimmers should avoid swimming near these features at all times.

 

 

Image 5. A pie chart depicting percentages of incidents with wave heights less than or equal to three feet. Sandbars indicates that the dangerous current was a classic rip current (no influence of shoreline structures or water outlets). Most of these low-wave height incidents were near a river mouth or shoreline structure. Data from the GLCID, 2013.


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