Looking for a new method to measure hidrodynamics

[Marcos Soares Barbeitos]

        Dear coral-listers,

        I'm looking for a simple way to quantify water movements at 5 m
depth. No method avaliable in literature is suitable for my needs, since I
intend to measure 90 points at the same time. My idea was to quantify the
amount of movement measuring the loss of material in small bodies placed
near my collecting points after 12 hours of exposure to the action of the
sea  I've already tried to make these bodies with chalk, salt and clay, but
this substances are eroded either too fast or too slowly. Is anybody aware
of any substance that can be employed succesfuly to this kind of experiment?
Does anybody know any other suitable method (it's gotta be cheap and easy
for my number of points is too large)? Looking forward to hearing from you,
kind regards.