A shallow self-actuating flood barrier assembly responsive to rising water for preventing entrance of flood waters though a horizontal opening in an upright wall 14) of a structure, comprising: a) a frame comprising a first upright sidewall and a second upright sidewall for mounting adjacent opposite sides of said opening to project transversely outwardly from said wall, b) a plurality of flood prevention units (27, 28, 29, 30) vertically stacked one over another inside said sidewalls, each unit comprising: i) a deck horizontally connecting said sidewalls, one deck for each unit, ii) a buoyant gate horizontally disposed between said sidewalls and below said deck of the unit in a plane oriented from substantially horizontal to less than vertical in the absence of a self-actuating force, such gate having an upper surface, lateral sides (36, 37), and front and rear ends (38, 39), the rear end of each gate being pivotally mounted about a horizontal axis transverse to said sidewalls by one or more pivotation members, each said pivotation member comprising a stationary member on a support connected to said frame and a moveable member moveably joined to said stationary member for pivotation of the gate rotationally upwardly about its horizontal axis between said sidewalls for engagement with said deck on rise of water buoyantly lifting the gate, iii) a limiter for limiting the extent of self actuated rotation of said gate, the gate of each of said plurality of units except the vertically lowermost unit being vertically spaced at its rear end from the rear end of the gate of a next lower unit by at least the sum of vertical thickness dimension of said deck and a vertical height of the maximum rise of the gate of such next lower unit about the horizontal axis on which said next lower unit can rotate upwardly, characterized in that a sealing gasket (51, 53) is arranged on the front end of each gate to sealingly wipe said deck of the unit on rise of said gate. a) a frame comprising a first upright sidewall and a second upright sidewall for mounting adjacent opposite sides of said opening to project transversely outwardly from said wall, b) a plurality of flood prevention units (27, 28, 29, 30) vertically stacked one over another inside said sidewalls, each unit comprising: i) a deck horizontally connecting said sidewalls, one deck for each unit, ii) a buoyant gate horizontally disposed between said sidewalls and below said deck of the unit in a plane oriented from substantially horizontal to less than vertical in the absence of a self-actuating force, such gate having an upper surface, lateral sides (36, 37), and front and rear ends (38, 39), the rear end of each gate being pivotally mounted about a horizontal axis transverse to said sidewalls by one or more pivotation members, each said pivotation member comprising a stationary member on a support connected to said frame and a moveable member moveably joined to said stationary member for pivotation of the gate rotationally upwardly about its horizontal axis between said sidewalls for engagement with said deck on rise of water buoyantly lifting the gate, iii) a limiter for limiting the extent of self actuated rotation of said gate, i) a deck horizontally connecting said sidewalls, one deck for each unit, ii) a buoyant gate horizontally disposed between said sidewalls and below said deck of the unit in a plane oriented from substantially horizontal to less than vertical in the absence of a self-actuating force, such gate having an upper surface, lateral sides (36, 37), and front and rear ends (38, 39), the rear end of each gate being pivotally mounted about a horizontal axis transverse to said sidewalls by one or more pivotation members, each said pivotation member comprising a stationary member on a support connected to said frame and a moveable member moveably joined to said stationary member for pivotation of the gate rotationally upwardly about its horizontal axis between said sidewalls for engagement with said deck on rise of water buoyantly lifting the gate, iii) a limiter for limiting the extent of self actuated rotation of said gate,