Influence of mean water depth and a subsurface sandbar on the onset and strength of wave breaking

Wave breaking in the open ocean and coastal zones remains an intriguing yet incompletely understood process, with a strong observed association with wave groups. Recent numerical study of the evolution of fully nonlinear, two-dimensional deep water wave groups identified a robust threshold of a diagnostic growth-rate parameter that separated nonlinear wave groups that evolved to breaking from those that evolved with recurrence. This paper investigates whether these deep water wave-breaking results apply more generally, particularly in finite-water-depth conditions. For unforced nonlinear wave groups in intermediate water depths over a flat bottom, it was found that the upper bound of the diagnostic growth-rate threshold parameter established for deep water wave groups is also applicable in intermediate water depths, given by k(0) h greater than or equal to 2, where k(0) is the mean carrier wavenumber and h is the mean depth. For breaking onset over an idealized circular arc sandbar located on an otherwise flat, intermediate-depth (k(0) h greater than or equal to 2) environment, the deep water breaking diagnostic growth rate was found to be applicable provided that the height of the sandbar is less than one-quarter of the ambient mean water depth. Thus, for this range of intermediate-depth conditions, these two classes of bottom topography modify only marginally the diagnostic growth rate found for deep water waves. However, when intermediate-depth wave groups ( k(0) h greater than or equal to 2) shoal over a sandbar whose height exceeds one-half of the ambient water depth, the waves can steepen significantly without breaking. In such cases, the breaking threshold level and the maximum of the diagnostic growth rate increase systematically with the height of the sandbar. Also, the dimensions and position of the sandbar influenced the evolution and breaking threshold of wave groups. For sufficiently high sandbars, the effects of bottom topography can induce additional nonlinearity into the wave field geometry and associated dynamics that modifies the otherwise robust deep water breaking-threshold results.

Hydrogen permeation and corrosion behaviour of high strength steel 35CrMo under cyclic wet-dry conditions

Hydrogen permeation behaviours of high strength steel 35CrMo under different cyclic wet-dry conditions have been investigated by using Devanathan-Stachurski's technique. Four electrolytes were used: distilled water, seawater, seawater containing 1500 ppm H2S and seawater containing 0.03 mol L-1 SO2. The corrosion weight loss of 35CrMo in the wet-dry cycles was measured simultaneously. The experimental results show that hydrogen can be detected at the surface opposite to the corroding side of the specimen during wet-dry cycles and the permeation current density during a wet-dry cycle showed a maximum during the drying process. The hydrogen permeation was obviously promoted by Cl- ions, H2S and SO2. The hydrogen permeation in the real marine atmosphere has also been investigated. There is a clear correlation between the amount of hydrogen permeated and the corrosion weight losses. Results show the importance of hydrogen permeation that merits further investigation.