Large Amplitude Internal Wave Transformation Into Shallow Water

Abstract Large amplitude internal solitary wave (ISW) shoaling, breaking and runup was tracked continuously by a dense and rapidly sampling array spanning depths 500 m to shore near Dongsha Atoll in the South China Sea. Incident ISW amplitudes ranged between 78 m and 146 m with propagation speeds between 1.40 ms−1 and 2.38 ms−1. Wave amplitude ratio to a critical amplitude A0, related to the wave speed cp and depth, controlled breaking type. Fissioning ISWs generated larger trailing elevation waves when the thermocline was deep, and evolved into onshore propagating bores in depths near 100 m. Collapsing ISWs contained significant mixing and little upslope bore propagation. Bores contained significant onshore near-bottom kinetic and potential energy flux, and significant offshore rundown and relaxation phases before and after the bore front passage, respectively. Bores on the shallow forereef drove bottom temperature variation in excess of 10 °C and near-bottom cross-shore currents in excess of 0.4 ms−1. Bores decelerated upslope, consistent with upslope two-layer gravity current theory, though runup extent Xr was offshore of the predicted gravity current location. Background stratification affected the bore runup, with Xr farther offshore when the Korteweg-de Vries nonlinearity coefficient α was negative. Fronts associated with the shoaling local internal tide, but equal in magnitude to the soliton-generated bores, were observed onshore of 20 m depth.