Horizontal, sharp edged weirs (Bazins formula)

Q = 0.66 x cB x (2g)^0.66 x H^1.5

where;

Q = water flow rate, m³/sec

B = width of the weir, metres*

c = discharge coefficient, average 0.62

g = gravitational constant, 9.81

H = Height of the water over the weir, measured behind the weir edge, m 

* Note’; if the weir is narrower than the channel feeding it, shorten B by 0.2xH

Broad crested (flat topped) weirs

Q = 0.35 x B x (2g)^0.5 x H^1.5

where;

Q = water flow rate, m³/sec

B = width of the weir, metres

g = gravitational constant, 9.81

H = h x (V2)/2g where, h = height of the water over the weir at the discharge edge of the weir, m

V = velocity of the stream across the weir crest, m/sec

V notch, sharp edged weirs

Q = 0.266 x cB x (2g)^0.5 x H^1.5

where;

Q = water flow rate, m³/sec

B = width of the weir at the flowing rate

c = discharge coefficient, average 0.62

g = gravitational constant, 9.81

H = Height of the water over the weir, measured behind the weir edge, m