Optical Measurements of Low Frequency Cross-Shore Flows

Shear waves, a recently described surf zone motion, are manifested as low frequency (10^-2 s^-1) meandering in longshore currents. Due to their relatively high energy, shear waves may be an important process for cross-shore momentum flux, significantly altering the cross-shore structure of the mean longshore current. Despite this potential importance, field observations of shear waves have been limited, primarily due to the expense and logistical complications of maintaining arrays of in situ instruments. Thus it is difficult to address basic questions about their frequency of occurrence or their possible turbulent nature, particularly over a wide range of conditions and at different sites. In this abstract, we propose an optical technique to measure low frequency cross-shore motions in the surf zone, complimentary to our previously presented optical method to measure longshore currents. Time-exposure images of surf zones, taken from video monitoring stations, represent the cross-shore pattern of wave breaking over an offshore bar as a band of elevated image intensity with the intensity maximum corresponding roughly to the bar crest. We hypothesize that intensity pattern shifts in the cross-shore direction in response low frequency cross-shore flows will force a measurable time variation in image intensity, given by {{∂ I} / {∂ t}} = u(τ ) {{∂ I} / {∂ x}} where τ implies the slow variations typical of shear waves. Early investigation has shown evidence for this phenomenon by the similarity between frequency-wavenumber spectra of intensity time series from longshore pixel arrays and f-k spectra from an analogous set of current meters (Lippmann, 1992). We will investigate this technique, called gradient imaging, by comparing the low frequency variations in pixel intensity time series to low frequency variations in time series of cross-shore velocity from the 1997 SandyDuck field experiment conducted at Duck, NC. Additionally, the possible dependence of this technique on cross-shore location relative to the bar and wave height conditions will be explored.