What is a Groin?
A groin is rigid hydraulic structure built from an ocean shore or from a bank (in rivers) that interrupts water flow and limits the movement of sediment. It is usually made out of wood, concrete or stone. In the ocean, groins prevent the beach sand from being washed away by longshore drift. Submerged groins are a type of groin that is completely submerged underwater.
What is Longshore Drift?
Sand moves to and from the beach in an onshore-offshore direction, but it also moves along the shoreline (“longshore”). This longshore sand transport is referred to as longshore drift (fig. 2), and because of it, beaches have been called “rivers of sand.” Longshore currents on the east coast typically move north to south rather than south to north like the Gulf Stream Current. There have been special instances where the longshore currents have changed direction during storms, hurricanes, or an abrupt change of wind for a brief period of time.
The longshore current, the movement of water along the shoreline, is caused by
How do Groins work?
A groin creates and maintains a wide area of beach or sediment on its updrift side. It reduces erosion on the other downdrift side. It is a physical barrier to stop sediment transport in the direction of longshore drift (also called longshore transport). This causes a build-up, which is often accompanied by accelerated erosion of the downdrift beach, which receives little or no sand from longshore drift. Groins add sediment to the beach by capturing downward drift. However, this can cause severe erosion on shorelines downstream from the groin. If a groin is correctly designed, then the amount of material it can hold will be limited, and excess sediment will be free to move on through the system. However, if a groin is too large it may trap too much sediment, which can cause severe beach erosion on the down-drift side.
Do Groins Cause Rougher Currents?
Groins occasionally improve the shape of surfing waves by creating a rip current next to the rocks. The rip can be a hazard to swimmers. The rip can also divert beach sand onto offshore sand bars, thereby accelerating erosion. Groins can also ruin the surf. If the waves are reflected off the rocks, the waves may lose their shape and "close-out."
As soon as one groin is built, property owners downdrift of it may start clamoring for the government to build groins to save "their" beach. Eventually, the beach may become lined with groins. Since no new sand is added to the system, groins simply "steal" sand from one part of the beach so that it will build upon another part.
Logging Requirements:
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By using your information about groins and beach erosion, what is the direction of the prominent longshore current? Hint: observe the side of the groin where a sand build-up is most evident.
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Is this groin classified as a submerged groin?
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Is this groin successful in slowing/stopping the process of erosion?
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Sources:
https://en.wikipedia.org/wiki/Groyne
https://www.britannica.com/technology/groin
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