DEFINITION OF GROIN

People are modifying the shoreline environment on a

massive scale with the construction of new homes, resorts,

and structures that attempt to reduce or prevent erosion

along the beach. These modifications have been changing the

dynamics of the beach in drastic ways, and most often they

result in degradation of the beach. In many cases, obstacles

are constructed that disrupt the transportation of sand along

the beach in longshore drift. This causes sand to build up at

some locations, and to be removed from other locations further

along the beach. Some of the worst culprits are groins,

or walls of rock, concrete, or wood built at right angles to the

shoreline, designed to trap sand from longshore drift and

replenish a beach. Groins stop the longshore drift, causing

the sand to accumulate on the updrift side and to be removed

from the downdrift side. Groins also set up conditions favorable

for the formation of rip tides, which tend to take sand

(and unsuspecting swimmers) offshore, out of the longshore

drift system. The result of groin construction is typically a

few triangular areas of sand next to the rocky protrusions,

along what was once a continuous beach. Little or no sand

will remain in the areas on the downdrift sides of the groins.

Therefore, when groins are constructed it usually becomes

necessary to begin an expensive program of artificial replenishment

of beach sands to fill in the areas that were eroded

by the new pattern of longshore drift set up by the groins.

Construction or stabilization of inlets though barrier

islands or beaches often includes the construction of groinlike

jetties on either side of the channel, to prevent sand from

entering and closing the channel. Like groins, these jetties

prevent sand transportation by longshore drift, causing

beaches to grow on the updrift side of the jetty. Sand that

used to replenish the beach on the downdrift side gets

blocked, or washed around the jetty into the tidal channel,

where it moves into the lagoon to form tidal deltas. The

result is that the beaches on the downdrift side of the jetties

become sand-starved and thin, eventually disappearing.

See also BEACH.

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