The history and technical capabilities of Argus

The subject of ocean turbulence is in a state of discovery and development with many intellectual challenges. This book describes the principal dynamic processes that control the distribution of turbulence, its dissipation of kinetic energy and its effects on the dispersion of properties such as heat, salinity, and dissolved or suspended matter in the deep ocean, the shallow coastal and the continental shelf seas. It focuses on the measurement of turbulence, and the consequences of turbulent motion in the oceanic boundary layers at the sea surface and near the seabed. Processes are illustrated by examples of laboratory experiments and field observations. The Turbulent Ocean provides an excellent resource for senior undergraduate and graduate courses, as well as an introduction and general overview for researchers. It will be of interest to all those involved in the study of fluid motion, in particular geophysical fluid mechanics, meteorology and the dynamics of lakes.

https://assets.cambridge.org/97805218/35435/frontmatter/9780521835435_frontmatter.pdf

The Turbulent Ocean

[1] The morphodynamic response of the nearshore zone of an embayed beach induced by wave groups is examined with a numerical model. The model utilizes the nonlinear shallow water equations to phase resolve the mean and infragravity motions in combination with an advection-diffusion equation for the sediment transport. The sediment transport associated with the short-wave asymmetry is accounted for by means of a time-integrated contribution of the wave nonlinearity using stream function theory. The two-dimensional (2-D) computations consider wave group energy made up of directionally spread, short waves with a zero mean approach angle with respect to the shore normal, incident on an initially alongshore uniform barred beach. Prior to the 2-D computations, the model is calibrated with prototype flume measurements of waves, currents, and bed level changes during erosive and accretive conditions. The most prominent feature of the 2-D model computations is the development of an alongshore quasi-periodic bathymetry of shoals cut by rip channels. Without directional spreading, the smallest alongshore separation of the rip channels is obtained, and the beach response is self-organizing in nature. Introducing a small amount of directional spreading (less than 2°) results in a strong increase in the alongshore length scales as the beach response changes from self-organizing to being quasi-forced. A further increase in directional spreading leads again to smaller length scales. The hypothesized correlation between the observed rip spacing and wave group forced edge waves over the initially alongshore uniform bathymetry is not found. However, there is a correlation between the alongshore length scales of the wave group-induced quasi-steady flow circulations and the eventual alongshore spacing of the rip channels. This suggests that the scouring associated with the quasi-steady flow induced by the initial wave groups triggers the development of rip channels via a positive feedback mechanism in which the small scour holes start attracting more and more discharge.

Morphodynamic modeling of an embayed beach under wave group forcing

Hydrodynamics and sediment transport in the swash zone: a review and perspectives

Mechanisms for bar/trough generation are examined using velocities measured in the field applied to the Bowen [1980]/Bailard [1981] energetics-based sediment transport model. Measurements consist of a cross-shore array of nine electromagnetic current meters spanning the surf zone and daily bathymetric surveys during a 10-day period during which two storms occurred, when the bathymetry evolved from a three-dimensional terrace to a well-developed linear bar. The model predicts bed and suspended load transport separately based on various velocity moments. The velocities are partitioned into mean currents, low-frequency infragravity and shear instabilities ( 0.05 Hz) to determine the relative importance of various mechanisms to the total transport. Velocity moments are computed over 90-min intervals to resolve tidal fluctuations. Tidal signatures were apparent in all modes of transport. Predicted transport rates are integrated and compared with daily cross-shore bathymetric profiles (averaged over a 400-m length of beach). The suspended load terms were an order of magnitude greater than bed load terms owing to the low fall velocity of the fine-grain sand within the surf zone. Model results for this experiment indicate the dominant mechanism for bar development was sediments mobilized by the strong longshore current and incident short waves within the surf zone and transported offshore by the mean undertow and shoreward transport onshore due to short wave velocity skewness. Using standard coefficients, the model correctly predicted the first-order movement of the bar during storms, but underpredicted trough development, and did not always perform well during mild wave conditions.

Bar/trough generation on a natural beach

A new type of alongshore progressive wave with periods and alongshore wavelengths of the order of 102 seconds and meters, respectively, has been observed in the surf zone. These periods fall into the lower end of the much studied infragravity frequency band previously shown to contain surface gravity edge and leaky waves. However, their short wavelengths (more than an order of magnitude smaller than the mode 0 edge wave) distinguish these new waves from surface gravity waves. In addition, they are only observed in the presence of mean longshore current and they change celerity (O(1 m/s)) and direction with the mean current. Alongshore wavenumber-frequency spectra clearly identify these waves, distinct from edge and leaky waves, by their approximately linear dispersion line at wavenumbers greater than the mode 0 edge wave dispersion curve. Their rms horizontal velocities can exceed 30 cm/s. These waves are shown to be consistent with a model [Bowen and Holman, this issue] of vorticity waves generated by the shear instability of the mean longshore current.

W. A. Birkemeier

Papers

Shear instabilities of the mean longshore current: 2. Field observations