A portable flood water barrier system is disclosed having one or more portable, attachable, flexible modules. Each module comprises an elongated tubular member sealed at either end by rigid, flanged end caps. Each module is provided with one or more openings to permit the introduction of water, sand or similar material into the tubular member interior cavity. Each module is also provided with one or more sealable openings to permit any water, sand or similar material to be removed from the tubular member interior cavity. The material provides weight the module to hold it in position against the force of the moving floodwaters. With the tubular member interior cavity partially filled with the material the portion of the tubular member in contact with the ground substantially conform in shape to the ground contours to minimize any water seepage under the tubular member. Each end cap has a plate with a center section providing shoulders about which a retaining ring can be used to fix the tubular member between the shoulders and the inside surface of the retaining ring. The perimeter section of the end plate forms the flange that is provided with a series of bolt openings to permit end caps from adjacent modules to be bolted together. The outer face of each flange member can be structured to receive a gasket or other sealing member to form a watertight seal between to adjacent attached end caps.