A dam impounds many tons of water. If its mass and muscle are not adequate, the heaviness of the water may be enough to collapse the dam or slide it downstream. Also, water leakage under the dam can cause it to settle, break and eventually burst. To guard against this, a mixture of cores, barriers, and asphalt blankets are typically built-in into the dam formation. Another danger is the possibility of flood waters that may over flow a dam, eroding and disintegrating it as they pour downstream. To meet the threat, an overflow spillway should be built either in the dam itself or as a separate pone exit channel cut into the hillside to one side of the dam. On a site where the flood potential is great spillway may be the dam's dominant feature.
Rock fill dam requires rock-hard base of bedrock, compressed sand or gravel to put off settling and rupture of watertight facing. When constructing on bedrock, secure dam with bolts and seal joint with concrete.
Frame Dam is made-up from planks that have been covered with such stabilizers as creosote or pentachlorophenol to prevent decaying. To prevent leakage, face upstream side of dam with asphalt or a layer of fine silt or clay.
Concrete dam is chosen whenever overflowing is possible, since a spillway can easily be included. To prevent wearing down under the spillway, pile rocks at the base or shape the base to redirect the down warded rush of water.
Log dam can be built of treated 6 inch logs, such as oak with stone or gravel used as fill. Face the upstream side with seepage-proof boards. Wood dams do not last nearly as long as stone or earth dams and should not be more than 4 feet high.
Earth dam is the oldest and most ordinary type. For steadiness its slopes must be very preliminary. To inhibit leakage, a core of resistant clay or concrete may be used.