Dam

Dam, in Civil Engineering, is a structure erected to prevent the flow of water from one level to another, or to regulate this flow. Weirs and coffer-dams are treated separately, and this article confines itself to the important class of structures known as masonry-dams and earth embankments. In the formation of a reservoir for the storage of water a spot is frequently chosen where the disposition of the ground is such that by building a masonry or earthen embankment of comparatively short length a large basin may be enclosed for the reception of water. Earthen embankments are much employed in India, where they are made of clay soil well watered and trampled down. The water side is built at a much steeper angle than the outer face, and is generally protected by a bed of puddle or small stone. The danger is lest the water should leak through the embankment, undermine its foundations, and carry it away. Masonry dams can only be constructed on rock foundations, but are much to be preferred. They may be built to great heights, the Furens dam in America being 200 feet high, the proposed Quaker-bridge dam 250 feet high, and the Vyrnwy dam 100 feet above ground, with 60 feet of foundation. The slopes on each side are steeper than in the case of earthworks, steeper on the water side than on the outer face. The shape of section is carefully designed, so that every layer of stone is of sufficient dimensions to withstand the weight of masonry above it and the possible pressure of water on that masonry; the resultant pressure on any such layer should pass through its middle third both when the reservoir is empty and when full. The overflow from the reservoir is carried off over a waste-weir. It is of much importance that the waste-weir should be adequate at all times.