Longshore currents:
Waves usually approach the coast at some
acute angle rather than exactly
parallel to it.
Because of this, the waves are refracted as they enter
shallow water, which in turn
generates a current along the
shore and parallel to it. Such a current is called a longshore
current, and it extends from the
shoreline out through the zone of
breaking waves. The speed
of the current is related to the size of the waves and to their
angle of approach. Under rather
quiescent conditions, longshore currents move only about 10-30
centimetres per second;
however, under stormy conditions they may exceed one metre per second. The
combination of
waves and longshore current acts to transport large quantities of
sediment along the shallow
zone
adjacent to the shoreline.
These
currents also cause a person to drift down the beach while
swimming in the ocean. They are also the cause of
rip currents (known incorrectly as
riptides). The longshore currents flow behind
sandbars but if the sand bar is broken at a point, causing water in front of the
bar (on the beach side) to flow out towards sea. This current is very strong and is the cause of many
swimming accidents.
Important: Swimmers caught in a rip current should not attempt to swim
shoreward directly against the current. Instead, it is best to swim a short distance parallel to
the beach to emerge from the rip current before returning to shore.