GROINS
Groins are constructed perpendicular to shore and extend, finger-like, out into the water. Used
singly or in groups known as groin fields, they trap sand or retard its longshore movement (Figure 10).
Sand tends to accumulate on the updrift side of a groin while erosion occurs downdrift. This will cause
the shoreline to rotate and align itself with the crests of the incoming waves, gradually decreasing the
angle between the waves and the shore. In turn, the longshore transport rate will decrease and the
shoreline will stabilize. The fillets of sand that collect on the updrift sides of the groins act as protective
buffers. Storm waves attack these accumulations first, before reaching the unprotected backshore.
Without the sand fillets, groins cannot protect the shoreline from wave action, nor are they
effective where the waves approach perpendicular to shore. Groin installations also require an adequate
sand supply and are not effective where the littoral materials are finer than sand. Silts and clays tend to
move in suspension and are not retained by groins on the beach.
When a groin is first built, the sand trapped on its updrift side is no longer available to downdrift
beaches and erosion may result. When the updrift fillet is completely formed, the sand will pass around
or over the groin to the downdrift shore, but at a slower rate than before it was built. If erosion of the
downdrift shore is unacceptable (it usually is), an alternative is to build more than one groin and fill the
area between with sand. This minimizes the downdrift damages and limits the erosion at the groin's
shoreward end.
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