Analysis of Infragravity Frequency Sediment Transport on Macrotidal Beaches
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Many cross-shore sediment transport models use simple treatments of infragravity frequency (0.005- 0.05Hz) processes. For example, infragravity waves have been assumed to provide solely a 'drift velocity' for transport of sediment mobilised by incident frequency waves (0.05-0.5Hz) and be 100% reflected at the shoreline. Furthermore, numerous models calculate broken incident wave heights on the basis of water depth only. This work investigates both the processes underlying infragravity frequency variations in the crossshore velocity field, and the resulting effect of such variations on sediment suspension and transport. Data were selected from three beach experiments in order to compare observations from a range of energetic conditions and positions in the nearshore. Experiments conducted on a dissipative beach at Llangennith, and an intermediate beach at Spurn Head, form part of the pre-existing British Beach And Nearshore Dynamics dataset. The third deployment, at a dissipative site at Perranporth (Cornwall), provided new data for analysis. At Llangennith, high swell waves (significant wave height 3m) were observed, and the measurements come from an infragravity wave dominated saturated surf zone. At Perranponh, locally generated wind wave heights were 2m and measurements came from an incident wave dominated saturated surf zone. Conditions at Spurn Head saw swell wave heights of 1.5m, and observations were made in both an incident wave dominated non-saturated surf zone and the incident wave shoaling zone. Analysis of the data revealed that, in the surf zone, the nature of the infragravity wave field was dependent upon the distribution of energy between higher (>0.02Hz) and lower (<0.02Hz) infragraviiy frequencies. Lower frequency infragravity waves were found to shoal as free waves, while higher frequency infragravity waves were dissipated near to shore on low gradient beaches. Inftagravity wave reflection coefficients showed a dependence on frequency and beach slope (parameterised by the Iribarren number), varied between 50-90% for lower infragravity frequencies, and could be less than 50% for higher infragraviiy frequencies. Incident wave heights were modulated in the shoaling zone with a 'groupy' form. Modulation was also observed in the surf zone, but in the form of individual large waves occurring at low frequency. In the shoaling zone and very close to shore, non-linear interactions occurred between the incident and infragravity components, and calculated phase values between modulated incident waves and infragravity waves indicated a phase shift from a value of less than 180° in the shoaling zone toward 0° close to shore. However, the two signals were not significantly correlated for much of the surf zone. High velocities resulting from a combination of the mean, infragravity and incident wave components drove sediment suspension. Large suspension events occurring at infragravity frequencies were correlated with incident wave groupiness in the shoaling zone, and in high energy conditions with infragravity waves near to the swash zone. Such variations in suspension were related not only to velocity magnitude, but the duration for which a threshold for suspension was exceeded. The bed response to forcing also varied during a tide, possibly as a result of changing bed conditions (e.g. due to bedforms). The infragravity contribution to suspension was independent of the magnitude of suspended sediment concentration, and increased from approximately 30% at the breaker line to 90% in an infragravity wave dominated inner surf zone. The contribution of the infragravity component to transport did not show a similar behaviour, due to phase effects, which produced a reversal in the transport direction between higher and lower infragravity frequencies. Comparison of the observations of sediment transport with energetics predictors identified several cases where the observed transport was qualitatively different from the model prediction as a result of sediment transport thresholds being exceeded at, or for, infragravity timescales.
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