A water barrier is a physical obstruction or membrane designed to block, divert, or control the flow of water, used for flood defense (like levees, floodgates), preventing water ingress in buildings (like waterproofing membranes, water stoppers), or managing moisture in construction (moisture barriers). They range from massive civil engineering structures to temporary, deployable systems like water-activated tubes or reusable modular barriers, functioning to protect property, infrastructure, and maintain structural integrity.