Published online by Cambridge University Press: 20 April 2006
The gravity-wave scattering matrix for a barrier reef that separates two different depths of water is calculated by an extension of a variational analysis of diffraction by a discontinuity in depth (Miles 1967). The resulting transmission coefficient for equal depths differs from that obtained by Johnson, Fuchs & Morison (1951), which appears to be incorrect. The results are applied to the calculation of resonant amplification of incoming swell or tsunamis by a shallow lagoon that is bounded by the reef and a vertical inner boundary. The results for this last problem agree closely (exactly for equal depths) with those obtained by Tuck (1980) through a rather different approach.