Groins, much like jetties, are long structures that are built perpendicular to the coast in an effort to interrupt the longshore water flow as well as the transportation of sediment. Also like jetties, groins are generally made of stone or concrete, and sometimes include less permanent wooden structures. Groins differ from jetties in that they are significantly smaller than jetties, and they are not a pair of structures used to maintain an open channel, rather, they are primarily implemented to slow coastal erosion processes that are caused by longshore drift. More specifically, the longshore current slowly wears away at the beach, but the groins can disrupt this phenomenon by breaking the current and consequently stopping the flow of sediment.
By disrupting the longshore drift, the groins can prevent coastal erosion while also causing the buildup, or accretion, of sediment on one side of the structure. This trapping of sediment is also referred to as beach evolution, as it can help extend the beach into the water as a result of the accretion. While this can be an effective way to maintain and expand the beach on one side of the groin, the opposite side suffers from beach starvation. As seen in the diagram below, this can reduce the size of the beach on the downdrift portion of the groin, rather than facilitating stability. It is fairly common for property owners to construct groins downdrift from their portion of the beach, which protects their home, but further jeopardizes their neighbor’s beach.
As a result of the joint effects of groins, they are often constructed in large numbers, with one following the previous groin to combat the beach starvation on the downdrift sides; this series of groins is called a “groin field.” Additionally, they are often erected in combination with seawalls to further stabilize the beach, however, this implementation also comes with its associated downfalls.
Like most other hard stabilization strategies, groins are often created due to their relatively low initial cost, but their short-term and long-term effectiveness is limited by the increased downdrift beach starvation that they cause.
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