5 seconds), wind-generated waves dissipate while passing a floating structure. Such waves have short
wavelengths that may be less than the structure width.
APPENDIX C contains information about specific breakwater configurations.
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 (Figure 10). 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 the
downdrift beach, resulting in erosion. Prior to construction, it should be realized that any erosion of
neighboring downdrift property is unacceptable. 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. An alternative is to
build more than one groin and fill the area between (as shown on the figure) with sand. This will
minimize downdrift damages and limit scour at the groin's shoreward end.
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