Geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach

Abstract

[1] Extensive measurements of ripple characteristics and dynamics along with associated suspended sediment fluxes and hydrodynamic conditions were made in the shoaling and surf zones of a macrotidal coarse grained beach at Sennen Cove, Cornwall, England (median grain diameter 0.69 mm). Suborbital vortex ripples were observed during the majority of the study period with height $5 cm and length $35 cm. The scale and shape of the ripples did not vary significantly as the bed shear stress increased during wave shoaling and breaking. However, ripple migration rates (onshore directed) were strongly dependent on their location relative to the breakpoint, increasing from $0.1 cm min À1 under shoaling waves to 2 cm min À1 in the outer surf zone during low-energy conditions. Farther inside the surf zone, ripples persisted but migration rates slowed, probably owing to the presence of the offshore-directed mean flow which impedes landward migration of the ripples. Under low-wave conditions (during which measured sediment fluxes peaked around the outer surf zone and decreased through the saturated surf zone), bed form transport rates under shoaling waves were of the same magnitude as net suspended sediment fluxes but at least an order of magnitude smaller in the outer surf zone. Under high-energy conditions (during which suspended sediment fluxes in the surf zone were offshore directed owing to the presence of the seaward directed mean flow), bed form transport rates were several orders of magnitude smaller than suspended fluxes.Citation: Masselink, G., M. J. Austin, T. J. O'Hare, and P. E. Russell (2007), Geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach,

Interactions between Hydrodynamic Forcing, Suspended Sediment Transport, and Morphology in a Microtidal Intermediate-Dissipative Beach

“…In a similar way, during the dry season, cross-shore q G was significantly higher in the inner surf zone, decreasing near the swash zone. These observations are consistent with previous works [39,67], which indicate that SS-waves might be the most important component of sediment transport in the surf zone and near the swash zone, where the percentage of broken waves was higher. Cross-shore q IG was higher in the surf zone (with a lower break wave percentage) than near the swash zone (with a higher percentage).…”

Interactions between Hydrodynamic Forcing, Suspended Sediment Transport, and Morphology in a Microtidal Intermediate-Dissipative Beach

“…In a similar way, during the dry season, cross-shore q G was significantly higher in the inner surf zone, decreasing near the swash zone. These observations are consistent with previous works [39,67], which indicate that SS-waves might be the most important component of sediment transport in the surf zone and near the swash zone, where the percentage of broken waves was higher. Cross-shore q IG was higher in the surf zone (with a lower break wave percentage) than near the swash zone (with a higher percentage).…”

Sediment transport processes and morphodynamics on a reflective beach under storm and non-storm conditions

“…The wave reflection coefficient increased from the pre-storm ( R = 0.40) into the post-storm phase ( R = 0.60) which is qualitatively consistent with the trends in orbital velocity skewness; this was initially small and positive but became negative (and small) in the storm and post-storm phases (Table 1). Hence, the present sediment transport measurements contrast sharply with Masselink et al (2007) who observed dominant onshore-directed oscillatory transport under shoaling waves. Those measurements were obtained using electromagnetic current meters and stacks of OBS-sensors penetrating the seabed on an LTT beach at Sennen Cove, UK.…”

“…In fact, onshore-directed oscillatory transport due to incident wind waves has often been the most important component in measurements of suspended sediment transport on both the lower and the upper shorefaces (e.g. Ruessink et al, 1998;Masselink et al, 2007). There may be two reasons why the present observations differ from earlier reports.…”

Observations of bedforms on a dissipative macrotidal beach

“…Differences may also result because the eye is able to give an overall impression of change, by averaging over many different time lags. The results indicate migration rates that are of broadly similar magnitude to previous measurements by Masselink et al (2007) and are consistent with close examination of the time-separated scans. For the megaripples, filtering out the wave ripples gave a much clearer migration rate than carrying out the cross-correlation approach on an unfiltered bedform trace.…”

Interactions between Hydrodynamic Forcing, Suspended Sediment Transport, and Morphology in a Microtidal Intermediate-Dissipative Beach

“…In a similar way, during the dry season, cross-shore q G was significantly higher in the inner surf zone, decreasing near the swash zone. These observations are consistent with previous works [39,67], which indicate that SS-waves might be the most important component of sediment transport in the surf zone and near the swash zone, where the percentage of broken waves was higher. Cross-shore q IG was higher in the surf zone (with a lower break wave percentage) than near the swash zone (with a higher percentage).…”

Sediment transport processes and morphodynamics on a reflective beach under storm and non-storm conditions

“…The wave reflection coefficient increased from the pre-storm ( R = 0.40) into the post-storm phase ( R = 0.60) which is qualitatively consistent with the trends in orbital velocity skewness; this was initially small and positive but became negative (and small) in the storm and post-storm phases (Table 1). Hence, the present sediment transport measurements contrast sharply with Masselink et al (2007) who observed dominant onshore-directed oscillatory transport under shoaling waves. Those measurements were obtained using electromagnetic current meters and stacks of OBS-sensors penetrating the seabed on an LTT beach at Sennen Cove, UK.…”

“…In fact, onshore-directed oscillatory transport due to incident wind waves has often been the most important component in measurements of suspended sediment transport on both the lower and the upper shorefaces (e.g. Ruessink et al, 1998;Masselink et al, 2007). There may be two reasons why the present observations differ from earlier reports.…”

Observations of bedforms on a dissipative macrotidal beach

“…Differences may also result because the eye is able to give an overall impression of change, by averaging over many different time lags. The results indicate migration rates that are of broadly similar magnitude to previous measurements by Masselink et al (2007) and are consistent with close examination of the time-separated scans. For the megaripples, filtering out the wave ripples gave a much clearer migration rate than carrying out the cross-correlation approach on an unfiltered bedform trace.…”

Interactions between Hydrodynamic Forcing, Suspended Sediment Transport, and Morphology in a Microtidal Intermediate-Dissipative Beach

“…In a similar way, during the dry season, cross-shore q G was significantly higher in the inner surf zone, decreasing near the swash zone. These observations are consistent with previous works [39,67], which indicate that SS-waves might be the most important component of sediment transport in the surf zone and near the swash zone, where the percentage of broken waves was higher. Cross-shore q IG was higher in the surf zone (with a lower break wave percentage) than near the swash zone (with a higher percentage).…”

Sediment transport processes and morphodynamics on a reflective beach under storm and non-storm conditions

“…The wave reflection coefficient increased from the pre-storm ( R = 0.40) into the post-storm phase ( R = 0.60) which is qualitatively consistent with the trends in orbital velocity skewness; this was initially small and positive but became negative (and small) in the storm and post-storm phases (Table 1). Hence, the present sediment transport measurements contrast sharply with Masselink et al (2007) who observed dominant onshore-directed oscillatory transport under shoaling waves. Those measurements were obtained using electromagnetic current meters and stacks of OBS-sensors penetrating the seabed on an LTT beach at Sennen Cove, UK.…”

“…In fact, onshore-directed oscillatory transport due to incident wind waves has often been the most important component in measurements of suspended sediment transport on both the lower and the upper shorefaces (e.g. Ruessink et al, 1998;Masselink et al, 2007). There may be two reasons why the present observations differ from earlier reports.…”

Observations of bedforms on a dissipative macrotidal beach

“…Differences may also result because the eye is able to give an overall impression of change, by averaging over many different time lags. The results indicate migration rates that are of broadly similar magnitude to previous measurements by Masselink et al (2007) and are consistent with close examination of the time-separated scans. For the megaripples, filtering out the wave ripples gave a much clearer migration rate than carrying out the cross-correlation approach on an unfiltered bedform trace.…”