Sources of error. The unavoidable lack in drifter coverage may cause quantities
such as mean velocity, at a particular location to be biased. An expression for the true
mean velocity was obtain by taking the time average of the product of velocity and drifter
concentration. The velocity and drifter concentration were both separated into mean and
fluctuating components.
uc = (u +')(c + c')
uc = (u)(c) + (u')(c')
u = -f (3-2)
In the third line of Equation 3-2, u is the true mean velocity and the first term on
the right hand side is the apparent mean velocity, which is measured by the method of
VDT presented in this study. This apparent mean velocity may differ from the true mean
velocity, u, due to the effects of the second term on the right hand side of Equation 3-2.
If u' and c' are correlated than two separate scenarios could alter the apparent mean
velocity, causing it to differ from the true mean velocity, which include: 1) if there is a
greater concentration of drifters during high velocities then VDT will tend to over predict
the true mean velocity within a particular computational bin and 2) if there is a greater
concentration of drifters during low velocities then VDT will tend to under predict the
true mean velocity within a particular computational bin. It is possible that u' and c' are
not correlated in which case there would be no bias or the two scenarios may nearly
cancel each other out. The consequence of this bias on the true fluid velocities of the rip
current system can not be determined. At least 20 drifter velocity measurements, within