Water wave diffraction by annular plates having a same center of curvature, rigidly fixed on shallow water, is analysed under the assumptions of linear long wave theory. The annular plates are anticipated to concentrate wave energy upon the center of curvature due to certain resonance, independently of wave incidence angles. In this paper, amplification of water elevation at the center of rings is confirmed, then effects of stationary waves by the smallest ring and interactions of diffraction waves between adjacent rings on the energy concentration at the center are investigated.