Currents and Current Loadings

Currents induce additional forces (drag and lift) on ocean structures that on occasion may be of far greater magnitude than is initially suspected. This occurs when the currents and surface gravity water waves have components which coincide directionally. Thus, since the drag force is usually taken to be proportional to the square (rather than the first power) of the fluid particle velocity, the resulting force, due in part to the coupling (cross-multiplication products) terms, may be considerably enlarged. Currents also have another effect, namely that of refraction of surface waves which occurs at the boundary of an ocean current and still water. The end result is a change in the ‘apparent’ wave lengths and direction of the incident waves. The change in wave lengths also appears as a change in wave steepness. For details, the reader is referred to the paper by Johnson (1947).