The nearshore current field, defined here as the residual horizontal flow after averaging over the incident wave period, exhibits variability at a range of time and space scales. Some of the variable currents are low frequency gravity wave motions. However, variable, rotational (in the sense of possessing vertical vorticity) flow can also exist as part of the overall nearshore current field. A field and numerical modelling investigation of these variable rotational currents has been carried out. Drifters, which were developed for surfzone use, enabled measurement of the nearshore current structure; the design and testing of these new instruments is described. Two sets of field measurements, using the new drifters and Eulerian instruments were carried out for conditions with swell perpendicular to a plane beach and in strong longshore currents. In the perpendicular swell conditions, an interesting and well-defined feature of the measured trajectories was the development of transient rip currents. Discrete vortices were also observed. In the longshore current case, trajectories with the longshore current displacement removed had complex meandering paths. Lagrangian data were used to make estimates of length scales and dispersion, both of which provide strong evidence that the current field cannot be due to low frequency gravity waves alone. Under the assumption of equipartition of kinetic and potential energy for low frequency gravity waves, Eulerian measurements of velocities and pressure show significant energy due to non-divergent, rotational flow in both the perpendicular swell and longshore current case. A numerical model that can simulate horizontal flow with a directionally spread, random wave field incident on a plane beach was implemented. The model developed transient rip currents that are qualitatively very similar to those seen in the drifter trajectories from the field. The number and intensity of rip currents in the model depended on the beach slope and incident wave spectra. The energy content and cross-shore flux (and hence transport of material) of the rotational current flow component in the simulated flow fields is comparable to that due to low frequency gravity waves. The modelling also provided some evidence that there may be universal characteristics of the rotational currents. The field results and modelling show that variable rotational currents are ubiquitous in the field even when longshore currents and hence shear waves are not present. The term “infragravity turbulence” is suggested to describe the general class of nearshore hydrodynamics not directly associated with shear waves, which is largely disorganised, but contains well defined features such as transient rips currents and large scale horizontal vortices. The results have important implications in the understanding of the transport of material, including sediment, biological material, pollution, and sometimes bathers, in the nearshore zone.
| Identifer | oai:union.ndltd.org:ADTP/221035 |
| Date | January 2004 |
| Creators | Johnson, David |
| Publisher | University of Western Australia. Centre for Water Research |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright David Johnson, http://www.itpo.uwa.edu.au/UWA-Computer-And-Software-Use-Regulations.html |
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