The notch angle for maximum discharge over the triangular notch is

Concept:

A V-notch weir is also called the triangular notch or weir. The discharge over a triangular weir or notch is given by,

\({\rm{Q\;}} = {\rm{\;}}\frac{8}{{15}}{{\rm{c}}_{\rm{d}}}\sqrt {2{\rm{g}}} \tan \frac{{\rm{θ }}}{2} \times {{\rm{H}}^{5/2}}\)

Where,

Q = Discharge, Cd = co-eff of discharge,

H = Head above the bottom of the notch, and θ = total included angle of the notch

The actual measured flow is about 40 percent less than this, due to contraction similar to a thin-plate orifice.

In terms of an experimental discharge coefficient, the recommended formula is

\({\rm{Q\;}} = {\rm{\;}}\frac{8}{{15}}\times{0.44\times}\sqrt {2{\rm{g}}} \tan \frac{{\rm{θ }}}{2} \times {{\rm{H}}^{5/2}}\)  & 20° < θ < 100° (for heads H > 50 mm.)

The angle of the triangular notch

∴ For maximum discharge, θ should be 90° < 100 °

so that, tan θ/2 = 1

θ = 90° 

Mistake PointsIn triangular notch, the angle varies between (20° < θ < 100°), so 120° mathematically cannot be the answer.