ABSTRACT
The rectangular weir and the triangular weir (V-notch) are the com monest and most efficient types of sharp-crested structure. Other forms include the trapezoidal weir and the compound weir.
9.3.2 Rectangular Weir Fig. 9.4 shows the pattern of stream lines in two-dimensional flow over a rectangular weir. The water is accelerated as it approaches the weir, the increase in velocity being derived from a reduction in potential head which manifests itself in a downward slope of the surface. A continued acceleration under gravity takes place downstream of the crest, the overflowing sheet of water being called the nappe. When the weir is operating correctly the nappe springs clear of the crest and this condi tion is called free. The upward component of velocity adjacent to the upstream face of the weir produces an initial sharp curvature and slight rise in level of the underside of the nappe. The falling nappe has an approximately parabolic trajectory in accordance with the relation ships set forth in Ch. 4, Sect. 4.6.5. Both the upper and lower surfaces
of the nappe are exposed to the atmosphere so that the pressure dis tribution throughout is near to atmospheric.
