Field observation of nearshore circulation and meandering currents

Abstract

Field observations were made of wave‐induced nearshore circulations and meandering longshore currents on an undulatory surf‐zone bed, under the action of uniform incident waves. Circulations were associated with normal‐wave incidence; meandering currents were associated with oblique‐wave incidence. The transport in the observed circulations generally agreed with Bowen's (1967) linear theory based on the concept of radiation stress (Longuet‐Higgins and Stewart, 1962, 1964), provided that a friction coefficient C = 0.014 was assumed. The longshore current near the shore line moved from a shoal to a depression as predicted, but this movement was also directed from an area of high waves to one of low waves, which is different from the case of a circulation driven by nonuniform breaker heights on the bar. Spilling breakers over a shoal underwent greater energy dissipation than plunging breakers in the rip current. Observed streamlines were narrow in the outflow and broad in the inflow, a characteristic that was probably associated with a nonlinear mechanism arising from a steep depression in the rip channel, as previously explained by Arthur (1962). These circulations were pulsational, unlike the circulations of a steady‐state solution. Occasional strong outflows at beat frequencies caused water to escape from the circulation. For a given surf‐zone undulation, breaking over the inner bar was essential to the formation of a circulation, and the intensity of breaking, controlled by tide, corresponded with a proportionally stronger circulation. Thus, circulations were generally stronger during low tide than during high tide. Low rip‐current velocities at high tide fluctuated with incoming swells, whereas high velocities at low tide tended to fluctuate at surf beat frequencies. In proportion to increasing rip velocities, the rip pulsation tended toward lower intervals. Mean surface slopes caused by wave set‐up and set‐down agreed with trajectories of neutral‐density balls released in the circulation. Meandering currents associated with oblique‐wave incidence could be explained as a combined effect of circulation cells and parallel longshore flows.

Temporal observations of rip current circulation on a macro-tidal beach

“…This is consistent with the findings of Sonu (1972) and amongst others, who provide strong evidence that wave breaking on the adjacent bars is required to drive rip circulation. It also highlights the transient nature of rip activity in a macro-tidal environment when compared to the observations of Brander and Short (2001), who measured a continual variation in rip velocity over the tidal cycle in a micro-tidal climate, in which there was persistent wave breaking throughout the tide.…”

“…In common with several previous studies (e.g. McKenzie, 1958;Sonu, 1972;Brander, 1999) the strongest rip flows are observed around low tide; outside of this low tide window, flows are predominantly alongshore. The tidal modulation is observed for both Eulerian and Lagrangian flow data.…”

“…This modulation has been linked to temporal changes in the expression of the morphology causing: (1) spatial and temporal variation in the pattern of wave dissipation; and (2) morphological flow constriction. Sonu (1972), and MacMahan et al (2006) state that wave breaking over nearshore bars is essential for the formation of rip circulation and that the intensity of breaking corresponds with a proportionally stronger circulation. Considering the case of low tide bar/trough morphology, at high-and midtide the amount of wave dissipation over the bars will be small and hence the alongshore set-up gradient is small.…”

Morphological coupling in multiple sandbar systems – a review

“…The predominance of the Idt coupling type is related to the fairly large waves that persistently arrive with a large angle of incidence (30 • ). We hypothesised that such wave conditions drive a meandering alongshore current (Sonu, 1972;MacMahan et al, 2010) that prevents the outer-bar horns from welding to the inner bar and leads to downdrift-positioned landward perturbations in the inner terrace. When the meandering current is less strong (smaller wave height or more shore-normal incidence), the outer-bar horns can weld ashore and lead to the Odt coupling type.…”

“…Model studies first explained alongshore sandbar variability from a hydrodynamic template in the water motion (Bowen and Inman, 1971;Holman and Bowen, 1982); present-day models rely on the principle of self-organisation (Hino, 1975;Sonu, 1972;Falqués et al, 2000;Coco and Murray, 2007), in which a crescentic sandbar forms spontaneously through the positive feedback between the flow, sediment processes and the evolving morphology. The genesis of crescentic patterns in single sandbar systems is thus reasonably well understood.…”