Dam A structure of earth, rock, or concrete designed to form a basin and hold water back to make a pond, lake, or reservoir. A barrier built, usually across a watercourse, for impounding or diverting the flow of water. General types of dams include: 1 Arch Dam – Curved masonry or concrete dam, convex in shape upstream, that depends on arch action for its stability, the load or water pressure is transferred by the arch to the Abutments. 2 Buttress Dam – A dam consisting of a watertight upstream face supported at intervals on the downstream side by a series of buttresses. 3 Cofferdam – A temporary watertight enclosure that is pumped dry to expose the bottom of a body of water so that construction, as of piers, a dam, and bridge footings, may be undertaken. A “diversion cofferdam” prevents all downstream flow by diverting the flow of a river into a pipe, channel, or tunnel. 4 Crib Dam – A barrier or form of Gravity Dam constructed of timber forming bays, boxes, cribs, crossed timbers, gabions or cells that are filled with earth, stone or heavy material. 5 Embankment Dam – A dam structure constructed of fill material, usually earth or rock, placed with sloping sides and usually with a length greater than its height. Types of embankment dams include: Earthfill or Earth Dam – A dam in which more than 50 percent of the total volume is formed of compacted fine-grained material obtained from a borrow area (i.e., excavation pit), Fill Dam – Any dam constructed of excavated natural materials or of industrial waste materials, Homogeneous Earthfill Dam – A dam constructed of similar earth material throughout, except for the possible inclusion of internal drains or drainage blankets, distinguished from a Zoned Earthfill Dam, Hydraulic Fill Dam – A dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water, Rockfill Dam – A dam in which more than 50 percent of the total volume is comprised of compacted or dumped pervious natural or crushed rock, Rolled Fill Dam – A dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipment, and Zoned Embankment Dam – A dam which is composed of zones of selected materials having different degrees of porosity, permeability, and density. 6 Gravity Dam – A dam constructed of concrete and/or masonry that relies on its weight for stability. 7 Inflatable Dam – A dam constructed of heavy-duty rubber or similar material and inflated with air or water and used for small-scale impoundment of flood flows or as flashboards for regulating the overflow of larger dams.. 8 Masonry Dam – A dam constructed mainly of stone, brick, or concrete blocks that may or may not be joined with mortar. A dam having only a masonry facing should not be referred to as a masonry dam. 9 Weir – A dam in a river to stop and raise the water, for the purpose of conducting it to a mill, forming a fishpond, or the like. When uncontrolled, the weir is termed a fixed-crest weir. Other types of weirs include broad-crested, sharp-crested, drowned, and submerged.