We present an optical method (optical current meter) to measure the longshore
component of nearshore surface currents by measuring the alongshore drift of persistent
sea foam in the surf zone. The method uses short time series of video data collected from
an alongshore array of pixels. These space-time data are first Fourier transformed to a
frequency-wave number spectrum and, finally, to a velocity spectrum. A model of the
velocity spectrum is fit to the observed spectrum to estimate the foam drift velocity.
Confidence intervals and other measures of the input and output data quality are
calculated. Field test comparisons were made against an in situ bidirectional
electromagnetic current meter on the basis of 1 month of video data from the 1997 Sandy
Duck field experiment. The root mean square error between the two approaches was
0.10 m/s. Linear regression analysis showed the gain between the two instruments to not be
statistically different from one. Differences between the surface and interior measurements
were compared to forcing mechanisms that may cause surface velocity shear. Velocity
offsets and alongshore wind stress were well correlated for cases when waves and wind
were not aligned to within ±45°, when wind- and wave-forced currents are reasonably
separable. Calculated wind-dependent surface current shear, modeled as a surface boundary
layer, correlated well with the observed velocity offsets for observations of nonalignment
between wind and waves. This technique can be applied to study large-scale coastal
behavior
