Calculation of Flow Rate Over a Rectangular Weir

(Sharp-crested, Contracted)

Inputs Calculations

Height of wier crest above 1.11

1 ft

yes

4 ft

1.11

2 ft

yes

Measured head over

0.45 ft H/L = 0.23

yes

If all three answers are yes, flow is fully contracted.

Flow Rate, Q = 1.92 cfs

If the answer to any of the three questions above is "no," then the Kindsvater-Carter Equation,

0.5 0.0612 3.152

0.010 1.981 cfs

(B - L)/4H =

channel invert, P =

(B - L)/4H > 1 ?

Width of channel, B =

P/2H =

Length of weir, L =

P/2H > 1 ?

the weir, H =

H/L < 0.33 ?

Q = 3.33(L - 0.2 H)(H3/2)

given below, should be used: (Use the table below and the L/B value to get values for C1 and C2. Use

the graph below and the L/B value to get a value for kb. Enter C1, C2, & kb, and Q will be calculated.)

Q = (C1(H/P) + C2)(L - 0.003)(H + kb)3/2

L/B = C1 = C2 =

kb = Q =

Source: U.S. Dept. of the Interior, Bureau of Reclamation, 2001 revised, 1997, 3rd ed,

Water Measurement Manual

L/B

0.2 -0.0087 3.152

0.4 0.0317 3.164

0.5 0.0612 3.173

0.6 0.0995 3.178

0.7 0.1602 3.182

0.8 0.2376 3.189

0.9 0.3447 3.205

1.0 0.4000 3.220

Source: U.S. Dept. of the Interior, Bureau of Reclamation, 2001 revised, 1997, 3rd ed,

Water Measurement Manual

kb as a function of L/B

http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/index.htm

C1 C2

C1 and C2 as functions of L/B

http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/index.htm