Boussinesq’s theory are used in this study on watr waves entering shallow water showing that the spacial variation of the wave amplitude is nonlinear, and is governed by the Duffing equation usually applied to describs nonlinear oscillation in nature. Boussinesq’s theory are used in this study on watr waves entering shallow water showing that the spacial variation of the wave amplitude is nonlinear, and is governed by the Duffing equation usually applied to describs nonlinear oscillation in nature.