Sand bags are used to prevent flooding or to divert water or debris. They are made of a heavy-duty canvas or vinyl and filled with sand, dirt, or gravel. Sand bags are often used in emergency situations to divert water away from low-lying areas, to prevent flooding, and to prevent erosion. They are also used to secure tents, buildings, and other structures. They are often used in construction, landscaping, and gardening projects, as well as in military applications.