A dam may be defined as an impervious wall of masonry, concrete, earth or lose rock which upholds a mass of water at its rear, while its face or lower side is free from pressure of water to any appreciable extent. The waste water of the reservoir impounded by the dam is disposed of by means of a waste weir or by a spillway clear of the work. Or in rare cases by sluice gates (openings) in the body of the dam
Weirs or over-fall dam, although often confounded with bulk-head dams, differ from the latter in the following points.
First, that the water overflows the crest.
Second, the tail water is formed below the dam.
These two differences often modify the conditions of stress which are applicable in the design of dams and consequently the subject of weirs demands separate treatment.