| A laboratory study was performed to identify the energy flux paths associated with the degeneration of basin-scale internal waves in lakes with sloping topography. The ambient fluid was two-layer stratified with the interfacial waves being generated at the basin-scale via a single forcing event. Preliminary results suggest that the interfacial waves may be decomposed into three discrete wave groups: basin-scale standing waves, nonlinear progressive waves and dispersive solitary waves. For moderately to strongly forced systems, up to 25% of the initial wave energy is transferred to the nonlinear wave group during the first basin-scale wave period and up to 40% if the nonlinear waves steepen thereafter. This steepening process is known to decrease the horizontal wavelength until it is balanced by dispersion, thus inducing the gradual development of a solitary wave packet. If fully developed, the packet may contain half of the nonlinear wave energy. The ratio of the steepening timescale, required for the evolution of the solitary waves, to the traveltime of these waves controls their development within the domain. When the characteristic lengthscale of a particular solitary wave is less than the imposed slope length, wave breaking occurs acting to low-pass filter the incoming wave group signal. Furthermore, the position of the density interface as determined by the temporal phase of the basin-scale wave, in turn influences whether the breaking of the solitary waves is characterized by shear or convective instability. These experiments qualitatively reproduce ubiquitous observations of internal waves in lakes with sloping topography at one end (e.g. Loch Ness, Babine Lake, Lake of Zurich). In particular, unidirectional propagating nonlinear fronts and associated solitary waves have been observed in both systems. |
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