Diffraction of water waves by a small cylindrical elevation of the bottom of a laterally unbounded ocean covered by an ice sheet is investigated by the perturbation analysis. The ice sheet is modelled as a thin elastic plate. The reflection and transmission coefficients are evaluated up to the first order in terms of integrals involving the shape function representing the bottom elevation. Three particular forms of the shape function are considered for which explicit expressions for these coefficients are obtained. For the particular case of a patch of sinusoidal undulations at the bottom, the reflection coefficient up to first order is found to be an oscillatory function of the ratio of the wavelength of the bottom undulations and that of the incident wave train. When this ratio approaches 0.5, the reflection coefficient becomes a multiple of the number of undulations and high reflection of the incident wave energy occurs if this number is large. Reflection coefficient is depicted graphically to visualize the effect of the presence of ice-cover and the number of undulations.
IntroductionProblems of water wave diffraction by obstacles situated at the bottom of water basins of finite depth having a free surface at the top, have been investigated assuming linear theory in the literature of the last few decades. The earliest work in this area involves propagation of long waves over an abrupt change in the depth of the basin, which was briefly discussed in the treatise of Lamb [1, Art 176]. Conformal transformations were used in [2] to investigate the reflection of water waves by obstacles of arbitrary shape at the bottom. For rectangular obstacles situated at the bottom of basins of uniform finite depth, a variational formulation was used in [3], and more recently [4] and [5] presented multi-term Galerkin approximations in the papers' analysis, to obtain accurate numerical estimates for the reflection coefficient. In [6], water-wave reflection by a patch of sinusoidal undulations at the bottom was considered. The problem was solved by using the Fourier transform, after introducing a linear friction term in the free surface condition for the purpose of ensuring