19.
REEF CREST LONGSHORE DRIFT STUDY
(Based on an original project by Mary Lou Carle)
Purpose
The aims of this project are to:-
(a)
discover if longshore drift is consistent in direction and speed along a straight stretch of reef crest
when measured at 15m intervals at the same time.
(b)
find out if longshore drift is consistent at differ ent distances from the crest
(c)
manipulate data in graphical form
(d)
estimate the positions of rip currents and describe some of their effects
(t)
Optional if activity can be completed over tide:- to estimate if the longshore drift changes before and
after high and low tides.
Defintions
Longshore drift is the movement of water parallel to the shore caused by waves striking the beach at an
angle, forcing water to move parallel to the shore. On the reef crest it is the current set up beside the coral as
the tide changes. This could
be
compared with grabbing a piece of wet soap and propelling it foreward. The
thumb (representing the beach's inertia and the fingers, (representing the oblique wave pressure)
I mportance
Longshore drift moves sand along the coastline, making problems for mariners wishing to enter and exit from
rivers and bays.
A rip is an underwater channel that returns incoming water out to sea resulting in a current moving away from
shore which dissipates as the channel widens.
Coral zone
As the tide goes out, currents are set up along the crest. These currents can be quite strong and pose a
danger to snorkelers or divers who are unaware of their strenghts. They are at their extremes at half tides
when the rule of twelfths is at its maximum fall.
