Structural Currents and Longshore Currents on the Great Lakes

 

Longshore Currents

When waves approach the shoreline at an angle, they develop 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. This current is the strongest typically when waves approach at a near 45 degree angle from the shoreline. If a swimmer is caught in this current, they will be pulled along the shore of the beach. An example is when a swimmer places their belongings in one specific spot on the beach and gets into the water, only to realize 10 minutes later they are quite a distance down the beach from their belongings. To escape a longshore current, simply get out of the water (swim towards the shore). Never try to swim against the current! Longshore currents can be dangerous, as they can knock swimmers off their feet, or worse, carry them towards a shoreline structure, into a structural current (see below).  A best practice for beachgoers: Keep an eye on your belongings. If you notice you are drifting farther from your belongings, get out of the water, walk back to your belongings, and get back in the water. This is especially true if the longshore current is taking you towards a pier or breakwall!

 


 

Figure 1: From the COMET program (2011). The longshore current is seen moving parallel to the shore,

strongest at the center of the surf zone, or the place where you swim. Longer arrows indicate faster currents.

 

 Structural Currents

A structural current is simply a rip current that occurs near a shoreline structure, such as a pier, breakwall, groin, peninsula, or jetty. This current develops as the longshore current intersects a shoreline structure. Though these currents are a type of rip current, we set them apart as a separate current type because the escape route for classic rip currents will not work for a structural current. In fact, there is no recommended escape route for structural currents. The National Weather Service suggests that you STAY AWAY FROM SHORELINE STRUCTURES WHEN SWIMMING! One must also take precautions to not let the longshore current carry them into the structural current, as often happens.

Figure 3. Diagram of a Structural current [NWS/Sea Grant 2013]

 

 

FIGURE 3.  Example of  a structural current. This Googlemaps (2011) image, annotated by  NWS Marquette depicts the approaching wave angle (shown in blue) and the resulting longshore current (red arrows). The longshore current moves parallel to the beach and intersects the shoreline structure, moving out into the lake as a structural current.

Escape from the structural current is made virtually impossible for several reasons. First, anyone trying to swim parallel to the shore (moving away from the breakwall) will swim directly into the longshore current, which will fight to keep them against the breakwall. Secondly, if the swimmer attempts to swim towards the shore, they will be pushed further out into the lake against the structural current. Lastly, if the swimmer lets the current pull them lakeward (relax) 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 (every 3 to 5 seconds, normally). Some victims that were rescued from structural currents report not being able to get out of the currents, and being smashed into the breakwall, leading to cuts and broken bones (in extreme cases). Luckily, some of these survivors were rescued by Emergency Crews, life rings thrown by those near the breakwall, or just happening to get to safety by chance (on top of the breakwall). This is why it is important to keep an eye on your belongings-to ensure the longshore current does not carry you into the structural current!

 The Most Common Dangerous Current on the Great Lakes

In an examination of Great Lakes current-related incidents from 2002-2012 (see the Great Lakes Current Incident Database page), it became clear that most of the drowning fatalities and rescues on the Great Lakes were related to structural currents.

Figure 3: Type of current related incidents in the Great Lakes, 2002-2012 (GLCID). Structural currents are the most common cause of these drowning fatalities and rescues.

Although a majority (68%) of the Great Lakes current-related incidents occurred during high wave action, roughly 32% 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 structural currents, some were channel currents and others were river outlet-type rip currents. This would suggest that these dangerous currents can occur near shoreline structures and river outlets even when the lake conditions seem benign!! Again, a best practice is to avoid swimming in these locaitons. Some 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.

 

 

Figure 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 river/powerplant drainanges and outlets). Most of these low-wave height incidents were near a river mouth or shoreline structure.

 

 If structural currents and longshore currents have led to numerous drowning incidents at your local beach, and the nearest NWS office issues Recreational Beach Forecasts/Beach Hazard Statements, you can find out if these dangerous currents are possible on any given day in your area (summer only). To read about what offices offer the forecast, and to read about the forecast and statements in general, click HERE.


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