Note on a Paper by Liu on the Scattering of Water Waves by a Pair of Semi-Infinite Barriers

Recently, a problem, whose solution was well known in exact form, has been analyzed by Liu (Scattering of Water Waves by a Pair of Semi-Infinite Barriers, ASME Journal of Applied Mechanics, Vol. 42, 1975, p. 777), by the method of matched asymptotic expansions. From the known exact solution a simple expression is obtained for the transmission coefficient. The exact expression for the transmission coefficient when expanded for low frequency incident waves differs from Liu’s result, and therefore casts doubt on Liu’s analysis and physical conclusions.