This paper considers the effect of a hard-wall beach on the downstream side
of submerged parallel bars in a breakwater. In previous research, it was assumed that
the beach can absorb all of the transmitted wave energy, when an optimal dimension
for a submerged parallel bar is obtained and the wave amplitude is reduced as more
bars are installed. However, for a hard-wall beach there are waves reflected from the
beach that change the long-term wave interaction. We adopt the linear shallow water
equations in Riemann invariant form and use the method of characteristics, in a procedure
applicable to various formations of submerged rectangular bars. The distance from
the parallel bar (or bars) to the beach determines the phase differences between right
running waves in the beach basin and whether they superpose destructively or constructively
before hitting the beach, to define the safest and the most dangerous cases. Our
numerical calculations for one bar, two bars and for periodic rectangular bars confirm
the analytical formulae obtained.