Narrow Results

An approach by which the stochastic properties of the net bedload transport rate can be derived from the irregular nonlinear wave motion outside the bottom boundary layer is presented. It is demonstrated how bedload transport formulas valid for regular nonlinear waves can be used to find the probability distribution function of the net bedload transport rate for nonlinear (asymmetric) irregular waves. Here the bedload transport formula for second order Stokes waves proposed by Damgaard et al. (1996) combined with the Soulsby (1997) wave friction factor have been chosen to illustrate the method. The Madsen and Grant (1976) bedload transport formula combined with the same friction factor has also been included for comparison. Generally, it is recommended that a stochastic approach should be used rather than using the rms values in an otherwise deterministic approach.

An approach by which the stochastic properties of the bed shear stresses can be derived from the irregular nonlinear wave motion outside the laminar bottom boundary layer is presented. It is demonstrated how bed friction formulas valid for regular second order Stokes waves can be used to find the cumulative distribution function of individual shear stress maxima for nonlinear irregular waves. The friction factor for nonlinear random waves is also determined. An example is given, and an extension to nonlinear random waves plus current flow is also suggested.

The suspended sediment concentration under large random waves alone as well as under large random waves plus current has been examined by a classical sediment diffusion model in conjunction with a dynamic turbulent boundary layer model based on the linearized boundary layer equations with horizontally uniform forcing. The turbulence closure is provided by a high Reynolds number k - ∊ model. Under such conditions wave or current induced sea bed ripples are washed out, such that a sheet flow and a suspension layer of high sediment concentration exist. This sediment diffusion model has been verified against a range of existing measurements, for regular waves alone, as well as regular waves plus current. Overall, these comparisons show that the present sediment diffusion model does a fair job except in the outer suspension layer, where the existence of a 'convective' peak near flow reversal becomes important. These peaks, which are possibly generated by instability mechanisms within the water-sediment boundary layer, can not be captured by standard sediment diffusion models. Correlations between suspended sediment concentration and boundary layer quantities due to the grouping of the largest waves in a realistic sea state have been examined. The resulting mean sediment concentration and mean sediment flux for random waves plus current has been shown to agree reasonably well with those obtained by an equivalent sinusoidal wave plus current. Overall, the results obtained in this paper support the application of an equivalent wave to predict mean sediment concentrations, mean sediment fluxes, as well as the total sediment transport under sheet flow conditions.

The paper considers the effects of wave age and air stability on whitecaps at sea. This is made by using the logarithmic mean wind velocity profile including a stability function as well as adopting a recent wave age dependent sea surface roughness formula. The results are valid for wind waves in local equilibrium with the steady wind. An example of results demonstrates clear effects of wave age and air stability on whitecaps.

An approach by which the stochastic properties of the bottom friction under nonlinear random waves plus a relatively weak current can be derived from the irregular nonlinear wave motion outside the rough bottom boundary layer is presented. It is demonstrated how bed friction formulas valid for regular second order Stokes waves can be used to find the cumulative distribution function of individual shear stress maxima for irregular second order Stokes waves. The friction factor for nonlinear random waves is also determined, and an example of the calculation procedure is given.

The flows, at Reynolds number (Re, based on the free stream velocity and cylinder diameter) ranging from 1×10⁴ to 4.8×10⁴ in the subcritical flow regime, around a two-dimensional (2D) circular cylinder close to a non-movable flat bed have been investigated numerically using 2D Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with a standard high Reynolds number k-ε model. The results are compared with published experimental data and numerical results by considering effects of gap ratios, incident boundary layer thicknesses, Reynolds numbers and roughness of the flat bed. Studies on the suppression and the development of the vortex shedding are performed and reasonable flow mechanisms are observed. Under-predictions of the essential hydrodynamic quantities of the cylinder are seen due to the limitation of the turbulence model. The mean pressure distribution along the flat bed is predicted reasonably well as compared with the published experimental and numerical results.

A practical method for estimating the wave run-up height on a slender circular cylindrical foundation for wind turbines in nonlinear random waves is provided. The method is based on the velocity stagnation head theory and Stokes second order wave theory by assuming the basic harmonic wave motion to be a stationary Gaussian narrow-band random process. The predicted wave run-up heights agree fairly well with the measurements by De Vos et al. (2007).

This paper investigates the effect of residual currents on the combined tidal and wind driven flow and the resulting bedload sediment transport at intermediate water depths. Numerical simulations are done by using a simple one dimensional two-equation turbulence closure model. Predictions of the combined tidal and wind driven flow with given residual currents are presented, showing that the residual current has a substantial effect on both the depth averaged mass transport and the mean bedload transport directions. Finally, the bottom roughness is observed to have a small effect on the surface drift and the depth averaged mass transport.

Velocity and suspended sediment concentration data from field measurements at Pearl Beach in New South Wales, Australia, have been analysed. The analysis reveals that ripple-induced streaming induces an onshore current in the close vicinity of the bed in several of the time series. A simple boundary layer sediment model has been applied, yielding a qualitatively fair agreement between the predicted and measured mean velocity and suspended sediment concentration profiles although the predicted suspended sediment concentration is one order of magnitude smaller.