A construction for protecting the shore side of a shore line water barrier wall from flooding from a storm surge overtopping the barrier wall, comprising: a water trap arranged above shore side grade on the shore side of the barrier wall and comprising a front wall and lateral walls (5, 5', 6) connecting to the front wall, both the front wall and the lateral walls extending above shore side grade, the front wall being contiguous with the barrier wall, for entrapping storm water overtopping said front wall to form a pool, a recess on the shore side of said barrier wall with a bottom below said shore side grade and a top ledge at shore side grade, said recess being spaced from said water trap front wall and said barrier wall, at least one normally horizontal buoyant gate comprising at least one gate unit housed in horizontal disposition in said recess below shore side grade inside the water trap and spaced from said front wall sufficiently to allow formation of a pool between the gate and the front wall, the gate comprising lateral sides (35, 35'), an underside face and a length spanning from an end distal to said barrier wall to an end proximal to said barrier wall and greater than the height of the front wall, the gate being rotatingly pivotable from horizontal to a full upright position about an axis at said distal end generally parallel to the barrier wall, at least two of said lateral walls being proximately adjacent said sides of said gate and having a height at least as tall as the height of the front wall, said front wall in concert with the lateral walls serving to impound storm water overtopping the front wall to a pool depth sufficient to produce a hydrostatic pressure, impressible on the underside face of the gate or each gate unit after the gate is rotatingly buoyed partially upward from horizontal by water accumulating in the water trap, effective to rotate the gate or gate unit to a fully upright position. a water trap arranged above shore side grade on the shore side of the barrier wall and comprising a front wall and lateral walls (5, 5', 6) connecting to the front wall, both the front wall and the lateral walls extending above shore side grade, the front wall being contiguous with the barrier wall, for entrapping storm water overtopping said front wall to form a pool, a recess on the shore side of said barrier wall with a bottom below said shore side grade and a top ledge at shore side grade, said recess being spaced from said water trap front wall and said barrier wall, at least one normally horizontal buoyant gate comprising at least one gate unit housed in horizontal disposition in said recess below shore side grade inside the water trap and spaced from said front wall sufficiently to allow formation of a pool between the gate and the front wall, the gate comprising lateral sides (35, 35'), an underside face and a length spanning from an end distal to said barrier wall to an end proximal to said barrier wall and greater than the height of the front wall, the gate being rotatingly pivotable from horizontal to a full upright position about an axis at said distal end generally parallel to the barrier wall, at least two of said lateral walls being proximately adjacent said sides of said gate and having a height at least as tall as the height of the front wall, said front wall in concert with the lateral walls serving to impound storm water overtopping the front wall to a pool depth sufficient to produce a hydrostatic pressure, impressible on the underside face of the gate or each gate unit after the gate is rotatingly buoyed partially upward from horizontal by water accumulating in the water trap, effective to rotate the gate or gate unit to a fully upright position.