Abstract: A device for effecting local reduction of sea wave amplitude includes a grid plate adapted to be supported in the sea at a predetermined depth ensuring that the grid plate remains submerged below the trough of the highest wave expected to be encountered in use of the wave reduction device. The grid plate member includes a plurality of interconnected elongate supporting members hingedly supporting flapper valve plates which are operable between substantially opened and closed positions, each flapper walve in its opened position permitting sea water to flow substantially unimpeded through the flapper valve and in its closed position substantially blocking the flow of sea water through the flapper valve.