2. Mathematical model
The radiation and diffraction of linear water waves by a rectangular structure over a sill
in water of finite depth will be considered here. The geometry of the structure and the
arrangement of Cartesian coordinate system are shown in Fig. 1. The origin of the
coordinate system is at the undisturbed water surface with the positive z pointing upwardly
and the positive x directing to the right. Here the structure and the sill are assumed infinite
in the y direction, so the problem considered here is two-dimensional. The widths of the
structure and the sill are 2a, the water depths to the left and right of the sill are both h1,
the draft of the structure is d1, the height of the sill is h4, the distance from the still
water surface to the sill is e1 and the clearance between the structure and the sill is