SEDIMENT ADVECTION AND DIFFUSION BY OBLIQUELY DESCENDING EDDIES

Three-dimensional vortex structures involving obliquely descending eddies (ODE), produced by depth-induced breaking-waves, has been proved to be associated with local sediment suspension in the surf zone (Zhou et al., 2017); vertical velocity fluctuations around the ODEs induces sediment suspension near the bed. Otsuka et al. (2017) explained the mechanical contributions of the ODEs to enhance local sediment suspension under the breaking waves and modeled the vortex-induced suspension to predict the profile of the equilibrium sediment concentration in the surf zone. In order to predict local behaviors of sediment, however, sediment-turbulence interactions in the transitional turbulence under breaking waves need to be understood. The interaction may be described in terms of Schmidt number (Sc). Sc has been empirically determined for trivial steady flows such as open channel or pipe flows. In the surf zone where organized flows evolve into a turbulent bore, the interaction may vary with the transitional feature of turbulence during a wave-breaking process, and thus Sc may be variable in time and space. No appropriate Sc model has been proposed for the surf zone flow. A parametric study on the sediment motion with respect to the variation of Sc is required for better prediction of sediment transport in the surf zone. In this study, contributions of the sediment advection and diffusion in the vortex structure to the concentration are computationally investigated. Effects of Sc to the sediment suspension and diffusion process will be also discussed in this work.

Signal Analysis of Impact Test Due to Tidal Bore Using Empirical Mode Decomposition (EMD)

Tidal bore is a series of waves propagating upstream as the tidal flow turns to rising. The Qiantang Bore has aroused the curiosity of scientists and tourists all over the world in the past centuries as a strong turbulent bore. We consider here tidal bore impact measurements, recorded in the Qiantang River using pressure transducers. Based on the empirical mode decomposition (EMD) algorithm, a signal processing method for impact test due to the Qiantang Bore is proposed. As a self-adaptive approach, EMD method can be used to decompose the local characteristics of the no-stationary and nonlinear signals into several fluctuations and trend items with different time scales step by step. The separation of the quasi-static and dynamic pressures is conducted by using the empirical mode decomposition method.

Radar and Profiler Analysis of Colliding Boundaries: A Case Study

The kinematics of a head-on collision between two gust fronts, followed by a secondary collision between a third gust front and a bore generated by the initial collision, are described using analyses of Weather Surveillance Radar-1988 Doppler (WSR-88D) and Mobile Integrated Profiling System (MIPS) data. Each gust front involved in the initial collision exhibited a nearly north–south orientation and an east–west movement. The eastward-moving boundary was 2°C colder and moved 7 m s−1 faster than the westward-moving boundary. Two-dimensional wind retrievals reveal contrasting flows within each gravity current. One exhibited a typical gravity current flow structure, while the other assumed the form of a gravity wave/current hybrid with multiple vortices atop the outflow. One of the after-collision boundaries exhibited multiple radar finelines resembling a solitary wave shortly after the collision. About 1 h after the initial collision, a vigorous gust front intersected the eastward-moving bore several minutes before both circulations were sampled by the MIPS. The MIPS measurements indicate that the gust front displaced the bore upward into a neutral residual layer. The bore apparently propagated upward even farther to the next stable layer between 2 and 3 km AGL. MIPS measurements show that the elevated turbulent bore consisted of an initial vigorous wave, with updraft/downdraft magnitudes of 3 and −6 m s−1, respectively, followed by several (elevated) waves of decreasing amplitude.

Review Lecture - Water waves and their development in space and time

Most practical predictions of water-wave propagation use linear approximations based on the concepts of ‘geometric’ rays and group velocity. Although this is successful, or adequate, in many instances, there are phenomena that can only be fully understood in terms of nonlinear effects. The recent boom in soliton-related studies has shed much light on the nonlinear aspects of wave propagation in shallow water. However, for waves on deeper water some of the nonlinear effects are only now being appreciated. A few, such as the focusing pattern of steady wave fields have direct parallels in shallow water; while others, such as deep-water soliton solutions, have their own rich structure. In deep or shallow water, wavebreaking is the most eye-catching development of a wave field. With the exception of the classical turbulent bore or hydraulic jump, our present models are still some way from giving a quantitative appreciation of important effects such as energy dissipation and momentum transfer, but causes of breaking for deep-water waves are now a little better understood.

A turbulent bore on a beach

A theoretical model is developed giving a moderately detailed description of the flow in a turbulent bore, the velocity profiles, the shear stresses, the energy dissipation, etc. An analysis of the flow conditions at the toe of the turbulent front indicates significant differences from the usual description based on the finite-amplitude shallow-water equations, and it is shown that the present model gives a closer description of the actual physical conditions. Finally, numerical results are presented that illustrate how the model works, and test its validity on an example with known properties.

MODELING TURBULENT BORE PROPAGATION IN THE SURF ZONE

Initial efforts to numerically simulate surf zone waves by using a modified form of the nonlinear shallow water equations are described. Turbulence generated at the front of the moving bore-like wave spreads vertically downward to significantly alter the velocity profile and hence the horizontal momentum flux. This influence of turbulence is incorporated into the momentum balance equation through a momentum correction coefficient, a which is prescribed based in part upon the theoretical a(x) distribution beneath stationary hydraulic jumps. The numerical results show that with a suitably chosen a(x) distribution, the equations not only dissipate energy as the waves propagate, but also that the wave shape stabilizes as a realistic profile rather than progressively steepening as when the nonlinear shallow water equations are employed. Further research is needed to theoretically determine the appropriate a(x,t) distribution.