FIELD OBSERVATIONS OF BAR-TRAPPED EDGE WAVES

Edge waves which appear to be topographically trapped by surf-zone bar s are observed in data collected during the DELILAH experiment, at Duc k, North Carolina, in October 1990. Edge waves are theoretically trapp ed on bars when the phase speed is between (gh(bar))(1/2) and (gh(trou gh))(1/2), where the effective depth h is the actual depth modified by the longshore current [Howd et al., 1992]. Spectral analysis of the l ongshore component of orbital velocity data from two longshore instrum ent arrays, one in the trough and one seaward of the bar, showed conti nuous, distinct, diagonal lines of variance, extending into incident w ave frequencies. These lines of variance have the same frequency-waven umber coordinates as theoretical calculations of bar-trapped edge wave s. When the instrument array is on the top of the bar, the bar-trapped waves with the simplest shape are dominant; however, when the array i s farther from the bar, more complex bar-trapped waves dominate, In th e latter case, the simplest bar-trapped edge waves may exist on top of the bar and may have decayed to an insignificant size at the instrume nt location. Isolated cases exist where the simplest bar-trapped waves are still dominant even in the trough. These instances correspond to days with a strongly bimodal incident wave spectrum, which is broadban ded in both frequency and incident direction. These field observations show that bar-trapped edge waves can be the dominant edge wave modes in the longshore component of velocity data collected near or on the b ar over a wide range of frequencies, including incident frequencies; t heir potential contribution to nearshore hydrodynamics should be addre ssed in future studies of barred beaches.