Rectangular Weir Flow
Rectangular Weir Flow
Weirs measure open-channel flow from the head over the crest, a simple and robust field method.
The discharge is Q = C_d·(2/3)·√(2g)·b·H^1.5. where C_d is the discharge coefficient, b the crest width and H the head over the crest.
Inputs
C_d := 0.62=0.62
g := 9.81 m/s^2=9.81 m/s^2
b := 2 m=2 m
H := 0.3 m=300 mm
Apply the rectangular sharp-crested weir formula.
Q := C_d·(2/3)·sqrt(2·g)·b·H^1.5=0.6 m^3/s
Results
Flow rises with the 3/2 power of head, so the weir is sensitive at low heads — good for measurement. Approach velocity and end contractions add corrections.
Assumptions & limitations
- Sharp-crested, fully ventilated weir.
- Free discharge.
- Negligible approach velocity.