Weirs broad-crested

Fig. 10. Stream flow over (a) a broad-crested, rectangular weir (b) a cross-current view of the rectangular and CipoUetti weirs (c) a trape2oidal-notch or CipoUetti weir (d) a sharp-crested, triangular, or V-notch weir (e) a cross-current view of the V-notch and hyperboHc-notch weirs and ( a...

Sharp-edged weirs have edges like those of square or sharp-edged orifices (see subsection Orifice Meters ). Notched weirs are or(iuarily sharp-edged. Weirs not in the sharp-edged class are, for the most part, those described as broad-crested weirs. 

The upstream face of the weir plate should be smooth, and the plate should be strictly vertical. The crest should have a sharp, square upstream edge, a top width of He to % in, and a bevel on the downstream side, so that the nappe springs clear, making a line contact for all but the very lowest heads. If it does not spring clear, the flow cannot be considered as true weir flow, and the coefficients do not apply. If the crest extends downstream so that it supports the nappe, the weir is broad-crested. The velocity at any point in the nappe can be determined. The approach channel should be long enough so that normal velocity distribution exists, and the surface should be free of waves. It may sometimes be necessary to install baffles to ensure a quiet flow of approach. All deviations from proper weir construction affect the flow in the same way. That is, they increase it over the computed value. 

Woodbum, J. G., Texts of Broad-Crested Weirs, cited in R. L. Daugherty and A. C. Ingersoll, Fluid Mechanics, McGraw-Hill, New York, 1954. 

Woodbum, J.G. (1932). Tests of broad-crested weirs. Trans. ASCE 96 387-453. 

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