GROINS
Groins are constructed perpendicular to shore and extend out into the water. Used singly or in
groups known as groin fields, they trap sand or retard its longshore movement along beaches. Sand
accumulates in fillets on the updrift side of the groin, and the shoreline rotates to align itself with the
crests of incoming waves. As the adjustment proceeds, the angle between the shoreline and the waves
decreases and with it, the longshore transport rate. Sand fillets act as protective barriers, which waves
can attack and erode without damaging the previously unprotected upland areas. A groin, without a sand
fillet, cannot protect a shoreline from direct wave attack. A prime consideration with groin system
design is evaluation of the net direction and amount of longshore sediment transport. Successful
performance requires an adequate net longshore transport rate to form an updrift fillet. If the gross
transport rate is high but nearly equally divided in both directions (small net transport), groins do not
generally function well or successfully form large updrift fillets.
When first built, the sand trapped on a groin's updrift side is no longer available to replenish
downdrift beaches, resulting in erosion. When a groin fills to capacity, material passes around or over it
to the downdrift shore, but at a slower rate than before the groin was built. If downdrift erosion is
unacceptable (it usually is), an alternative is to build more than one groin and fill the area between with
sand (Figure 45). This will minimize downdrift damages and limit scour at the groin's shoreward end.
Groins are generally effective only when littoral materials are coarser than fine sand. Silts and
clays tend to move in suspension and are not retained.
Important design considerations for groins include their height, length, and spacing (for a groin
field). Height determines how much sand can pass over the structure; groins can be built either high or
low with respect to the existing beach profile. Low groins, which essentially follow a foot or two above
the natural beach profile, are widely used because they stabilize the beach but do not trap excessive
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