An extendible flexible flood barrier comprising: a flexible wall comprising membrane material and textile material, said flexible wall being attached and sealed to a vertical anchor post with a mounting plate and mounting seal; a storage container that is integral with the vertical anchor post and holds the flexible wall when stowed; a receiver post that the flexible wall is attached to with a mounting plate when the flexible wall is in an extended configuration; at least one cable, the at least one cable attached to the vertical anchor post and spans the distance between the vertical anchor post and the receiver post; and tensioned; a plurality of cable clips, the cable clips being attached to an uppermost portion of the flexible wall and the at least one cable; the plurality of cable clips being configured to be slidably moveable along the at least one cable after the at least one cable is tensioned; and wherein the flexible wall further comprises a ground skirt extension with an integral seal compressed by ballast or bolted clamping bars, and at least one second cable, the at least one second cable being attached to the ground skirt extension with a plurality of second cable clips; the plurality of second cable clips being configured to be slidably moveable along the at least one second cable; the ground skirt extension being positionable to lay horizontally on the ground after it is extended along the at least one second cable during extension of the flexible wall, and released from the plurality of second cable clips when extended; and bolted clamping bars, whereby the ground skirt extension may be sealed to the ground by said ballast or the bolted clamping bars. a flexible wall comprising membrane material and textile material, said flexible wall being attached and sealed to a vertical anchor post with a mounting plate and mounting seal; a storage container that is integral with the vertical anchor post and holds the flexible wall when stowed; a receiver post that the flexible wall is attached to with a mounting plate when the flexible wall is in an extended configuration; at least one cable, the at least one cable attached to the vertical anchor post and spans the distance between the vertical anchor post and the receiver post; and tensioned; a plurality of cable clips, the cable clips being attached to an uppermost portion of the flexible wall and the at least one cable; the plurality of cable clips being configured to be slidably moveable along the at least one cable after the at least one cable is tensioned; and wherein the flexible wall further comprises a ground skirt extension with an integral seal compressed by ballast or bolted clamping bars, and at least one second cable, the at least one second cable being attached to the ground skirt extension with a plurality of second cable clips; the plurality of second cable clips being configured to be slidably moveable along the at least one second cable; the ground skirt extension being positionable to lay horizontally on the ground after it is extended along the at least one second cable during extension of the flexible wall, and released from the plurality of second cable clips when extended; and bolted clamping bars, whereby the ground skirt extension may be sealed to the ground by said ballast or the bolted clamping bars.