Analytical solutions for long waves over a circular island

Abstract

In this study, we derive an analytical solution for long waves over a circular island which is mounted on a flat bottom. The water depth on the island varies in proportion to an arbitrary power, gamma, of the radial distance. Separation of variables. Taylor series expansion, and Frobenius series are used to find the solutions, which are then validated by comparing them with previously developed analytical solutions. We also investigate how different wave periods, radii of the island toe, and gamma values affect the solutions. For a circular island with a small value of gamma (e.g. gamma = 2/3, as in the equilibrium beach (Bruun, 1954)), the wave rays approaching near the island center reach the coastline, whereas the rays approaching away from the center bend away from the coastline, leading to smaller wave amplitudes along the coast. However, for a circular island with a large value of gamma, e.g. gamma = 2, all the rays on the island reach the coast, giving large coastline wave amplitudes. If the island domain is small compared to the wavelength, the wave amplitudes on the coastline do not increase significantly: however, when the island domain is not small, the wave amplitudes increase significantly. If gamma is also large, the amplitudes can be so large as to cause a disaster on the island.