A weir is a notch of regular form through which water flows. The term is also applied to the structure containing such a notch. Thus a weir may be a depression in the side of a tank, reservoir, or channel, or it may be an ovrflow dam or other similar structure. Weirs are classified in accordance with the shape of the notch; there are rectangular weirs; triangular, or V-notch, weirs; trapezoidal weirs; and parabolic weirs. The edge or surface over which the water flows is called the crest of the weir. The overflowing sheet of water is termed the nappe. A broad-crested weir may be defined as one having a horizontal or nearly horizontal crest sufficiently long in the direction of flow so that the nappe will be supported.
