Nonlinear waves on collinear currents with horizontal velocity gradient

Analytical and experimental research of wave dynamics on adverse and following currents with horizontal-velocity gradient, was conducted. Laboratory tests were carried out at the Tainan Hydraulics Laboratory of the National Cheng Kung University of Taiwan, where a special setup aimed at accelerating/decelerating currents was designed, constructed and employed. In the case of accelerating adverse currents, the wave behaviour is strongly nonlinear and leads to downshifting of the wave energy which allows the waves to penetrate the blocking current. For the case of decelerating currents, linear behaviour should lead to amplification of wave amplitude and increase in steepness, which is indeed observed, but downshifting also happens if the initial waves are steep enough. Such results point out to the physics which is presently not accounted for in wave forecast models.