Notch weirs are classified by the shape of their notch; rectangular weirs; triangular, or Vnotch, weirs; trapezoidal weirs; and parabolic weirs.
The picture above shows a V-notch weir. The edge the water cascades over is called the crest and the overflowing water sheet is called the nappe. Today weirs are still used to determine flows from open water sources such as streams. A typical 90° V-notch will be beveled at 45° so the edge is less than 0.08” thick, and the angle of the notch will be precisely 90°.
Water flow over the weir is calculated by the equation : Q=2.49h1 2.48 , where h1 = head on the weir in ft and Q = discharge over weir in ft3 /s. It is easy to see that this is a simple measurement technique can be used on nearly any open flowing body of water. Its simply a matter of building a large enough weir plate. It is just as obvious that this technique wont work in an enclosed pipe, and it certainly wont work for gasses.
The measurement of head is the height of the water above the lowest portion of the weir, and should be made at least four times that height, back from the weir.
Credits - Mitchell Cottrell