This dissertation, "The Propagation of Nonlinear Waves in Layered and Stratified Fluids" by Wing-chiu, Derek, Lai, 黎永釗, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of the thesis entitled THE PROPAGATION OF NONLINEAR WAVES IN LAYERED AND STRATIFIED FLUIDS submitted by Derek Wing-Chiu Lai for the degree of Doctor of Philosophy at the University of Hong Kong in April 2001 In this thesis the propagation of nonlinear waves in layered and stratified fluids is investigated. In the first part of this research, "unconventional" solitary waves are obtained and their interactions are investigated by the Hirota bilinear transformation. Such solitary waves are "unconventional" because they can be expressed analytically as some mixed exponential - algebraic expressions. Furthermore, the separation of the crests goes like a logarithm, rather than a linear function, in the time scale. In a proper frame of reference these unconventional solitary waves are usually counterpropagating waves. These counterpropagating waves and their interactions are investigated for several nonlinear evolution equations which are of fluid dynamical interests. Firstly, 2- and 3-soliton expansions are obtained for the Manakov system, a coupled set of nonlinear Schrodinger equations arising from the propagation of multiphase modes when the group velocity projections overlap. A pair of counterpropagating waves is observed if the technique of "merger" of the wavenumbers is performed for a 2-soliton expansion, and the separation of the crests goes like a i logarithm in time. Furthermore, temporal modulation of the amplitude is observed if the same technique is applied to a 3-soliton expansion. A similar procedure is then applied to the (2+1)-dimensional (2 spatial and 1 temporal dimensions) long wave-short wave resonance interaction equations in a two-layer fluid. Such long-short resonance interactions can be considered as a degenerate case of triad resonance. The required condition is that the phase velocity of the long wave matches the group velocity of the short wave. The "merger" technique can also be extended to the dromion solutions. Dromions are exact, localized solutions of (2 + 1) (2 spatial and 1 temporal) dimensions that decay exponentially in all directions. In a two-layer fluid the modified Korteweg-de Vries (mKdV) systems will be the governing equation if the quadratic nonlinearity vanishes. The required condition for the case of irrotational flow is that the density ratio is approximately equal to the square of the depth ratio. Under the irrotational flow assumption only the mKdV systems with the cubic nonlinear and the dispersive terms of opposite signs (mKdV-) exist. Our contribution here is to investigate the wave propagation in a two- layer fluid with shear flows in order to demonstrate the existence of mKdV systems with the cubic nonlinear and the dispersive terms of the same sign (mKdV+). A class of counterpropagating waves and their interactions are studied for the mKdV+. From the perspective of fluid dynamics the propagation of nonlinear waves in the first part of this research is considered in the ii weakly nonlinear regime. In the second part of this research fully nonlinear internal solitary waves in stratified fluids are calculated. Such internal waves for the exponential and linear density profiles are obtained by computing the higher order terms in an asymptotic expansion where the Boussinesq and long wave parameters are comparably small. With increasing amplitude the wavelength of the solitary waves generally decreases and