Weir flow calculations

Weir Type CalculationHorizontal, sharp edged weirs(Bazins formula)

Q = 0.66 x cB x (2g)0.66 x H1.5

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 theweir 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 H1.5

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 weirat the discharge edge of the weir, m

V = velocity of the stream across the weir crest, m/secV notch, sharp edged weirs Q = 0.266 x cB x (2g)0.5 x H1.5

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 theweir edge, m