Experimental study of nearshore dynamics on a barred beach with rip channels

TLDR

Laboratory experiments on a barred beach with rip channels measured wave and current fields, from which depth‑averaged currents and momentum‑balance terms were derived using conservation of mass and quantified relative magnitudes. The experiments show that rip channels dominate nearshore circulation, producing widespread cross‑shore flow and longshore pressure gradients, with breaker type and bathymetry driving the flow; cross‑shore momentum is controlled by pressure and radiation stress gradients, while longshore feeder currents are set by a balance of pressure gradient and radiation stress, with rip currents creating a positive feedback that amplifies feeder currents.

Abstract

Wave and current measurements are presented from a set of laboratory experiments performed on a fixed barred beach with periodically spaced rip channels using a range of incident wave conditions. The data demonstrate that the presence of gaps in otherwise longshore uniform bars dominates the nearshore circulation system for the incident wave conditions considered. For example, nonzero cross‐shore flow and the presence of longshore pressure gradients, both resulting from the presence of rip channels, are not restricted to the immediate vicinity of the channels but instead are found to span almost the entire length of the longshore bars. In addition, the combination of breaker type and location is the dominant driving mechanism of the nearshore flow, and both are found to be strongly influenced by the variable bathymetry and the presence of a strong rip current. The depth‐averaged currents are calculated from the measured velocities assuming conservation of mass across the measurement grid. The terms in both the cross‐shore and longshore momentum balances are calculated, and their relative magnitudes are quantified. The cross‐shore balance is shown to be dominated by the cross‐shore pressure and radiation stress gradients in general agreement with previous results, however, the rip current is shown to influence the wave breaking and the wave‐induced setup in the rip channel. Analysis of the longshore balance shows that the overall strength of the longshore feeder currents is governed by a balance between longshore pressure gradient forcing and an opposing radiation stress gradient. In addition, the longshore feeder currents show maxima in the bar trough region, providing experimental evidence that longshore pressure gradients can shift longshore current maxima shoreward from the bar crest. Finally, since the longshore radiation stress gradient in the surf zone is governed by the amount of wave dissipation on the rip current, there exists a positive feedback mechanism whereby a strong rip current can weaken the radiation stress gradient opposing the feeder currents and lead to even stronger feeder currents and rips.