Time Dependent Fluctuations in Longshore Currents

During constant sea state conditions, longshore current velocities were monitored continuously for fifteen minute periods separated by fifteen minute periods separated by fifteen minute intervals. Three ducted impellor flowmeters were placed at equally spaced vertical positions through the water column. Sequential measurements were made with similar vertical current meter arrays at different locations across the surf zone. Simultaneous measurements of wave height, period and celerity were made at stations placed at equal intervals from the outer surf zone to the beach. The fifteen minute continuous records were subjected to spectral analysis. This analysis showed that the major power associated with fluctuations in the longshore current velocity field occurs in two major frequency bands. A significant spectral peak was coincident with the breaker period of the incident wave field, 4.2 seconds and, another dominant signature occurred at 78.8 seconds. Attenuation with depth of both the steady and fluctuating components of the longshore current flow field was relatively small. The maximum observed velocities for each station and each vertical current meter position varied from 90 to 150 percent above the observed mean longshore current velocity. However, at each station, variation of the means with depth was not appreciable and thus supports the results from time and space averaged theories of vertical uniformity in longshore currents, away from the boundary layer. Results from the field investigation of Wood and Meadows (1975) indicated that the steady state components are dominated by the fluctuating portions of the flow field. Therefore, time averaging of conservation equations in longshore current theory is a physically inappropriate procedure. In order to evaluate the magnitude of the unsteady components, a close examination of surf zone dynamics was made. The most obvious contribution to unsteadiness in longshore currents arises from the longshore component of the maximum horizontal particle velocity. However, the magnitude of observed current fluctuations is too large to be completely accounted for by this component. Spectral peaks at longer periods appear to be related to modes of edge wave phenomena. The long period spectral signature of 78.8 seconds is in direct agreement with the calculated period for a zero mode edge wave in shallow water. This agreement supports the contention that oscillatory components, other than the longshore component of the wave particle velocity, are contributing to unsteadiness in longshore currents and are manifested throughout a wide range of frequencies.