The shore side of a shore line water barrier is protected from flooding caused by storm surge over-topping the barrier. Water overtopping the barrier, such as from wind waves crashing against the barrier, is trapped, impounded and accumulated between a parapet atop the barrier and walls on the sides of a buoyant gate unit taller than the parapet and walls and normally resident below grade on the shore side of the barrier, hinged about an axis generally parallel to the barrier and spaced from the parapet by a distance not less than the height of the gate unit. The accumulated overtopping water is applied to rotationally float the gate unit from the horizontal position initially principally by buoyancy and then principally by hydrostatic pressure to drive the gate unit to an upright position to oppose storm surges having a water level higher than the parapet.
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