A flood barrier system for sealing an opening in a wall structure against the inflow of flood waters. The system comprises a plurality of elongated, horizontally disposed barrier elements of a length somewhat greater than the width of the opening. The barrier elements are stacked vertically, one upon the other against the outer face of the wall structure. End clamps engage opposite ends of the individual barrier elements to press them snugly against the outer surface of the wall, providing seals along the opposite vertical side edges of the opening. A simplified clamping mechanism is provided above the uppermost barrier element, with provisions to apply vertically downward clamping pressure on the uppermost barrier element, and thus upon the entire series of vertically stacked barrier elements. Each of the barrier elements has a resilient seal along its lower surface, engageable with an upper surface of the neighboring barrier element below such that, when the stack of barrier elements is under clamping pressure from above, effective water seals are provided between each of the barrier elements. An important benefit and advantage of the arrangement is the facility and thickness with which an effective flood barrier may be installed when flooding conditions are imminent.