1. Home
  2. Computational Methods
  3. Weir Flow Calculations

Weir Flow Calculations

weir calculations

Studio Express uses a standard weir equation for Rectangular Sharp-crested, Compound, Cipolletti, Trapezoidal and Rectangular Broad-crested weirs.

Q = discharge over weir, cfs (cms)
L = length of the weir crest, ft (m)
H = distance between water surface and the crest, ft (m)
Cw = weir coefficient, typically 3.33 (1.84) for sharp-crested; 3.367 (1.83) for Cipolletti; 3.09 (1.7) for Broad-crested

Calculating Weir Flow
Sharp-crested weir. Maximum crest thickness is <= 1/2 depth.
Weir Flow Calculation
Broad-crested weir. Hydrostatic pressure is fully developed. Crest width is >= 2 x depth.

Trapezoidal with Sloped Sides

Total flow over trapezoidal weirs with side slopes is computed using the standard weir equation as shown above, plus two times the flow given from the following equation.

Q = discharge over side-sloped portion of weir, cfs (cms)
Z = side slope (Z horizontal to 1 vertical) of the weir crest
H = distance between water surface and the crest, ft (m)
Cw = weir coefficient, typically 3.367 (1.83)

Cipolletti weir calculations
Cipolletti weirs have .25 H to 1 V side slopes, Z = 0.25.


V-notch weirs are computed using this equation:

Where: Q = discharge over weir, cfs (cms)
Θ = angle of v-notch in degrees
H = head on apex of v-notch, ft (m)

Adjustment for Submerged Weirs

Weirs can be affected by submergence, i.e., when the user-inputted tailwater rises above the weir’s crest.

The equation for the reduction in flow is:

Qs = submerged flow, cfs (cms)
Qr = unsubmerged flow from standard weir equations, cfs (cms)
H1 = upstream head above crest, ft (m)
H2 = downstream head above crest, ft (m)

Submerged weir flow calculation
Submerged weir

New to our site? Learn more about Hydrology Studio’s complete suite of stormwater design software.

  • Hydrology Studio
  • Stormwater Studio
  • Culvert Studio
  • Channel Studio
  • Studio Express

Visit Hydrology Studio today.