Water Channel Flow Calculator
Plan your civil engineering project with our free water channel flow calculator. Get precise measurements, material lists, and budgets.
Reviewed by Abdullah, Technical Content Specialist
Formula
Q = (1.49/n) x A x R^(2/3) x S^(1/2)
Manning equation for open channel flow in US customary units. Q is discharge in cfs, n is the roughness coefficient, A is the flow cross-sectional area in sq ft, R is the hydraulic radius (A/P) in ft, and S is the channel slope in ft/ft. The hydraulic radius R equals the flow area divided by the wetted perimeter.
Worked Examples
Example 1: Trapezoidal Earth Channel
Problem:Calculate flow in a trapezoidal channel with 4 ft bottom width, 2 ft depth, 2:1 side slopes, 0.5% slope, and n = 0.025.
Solution:A = (4 + 2x2) x 2 = 16 sq ft\nP = 4 + 2(2)sqrt(1+4) = 12.94 ft\nR = 16/12.94 = 1.236 ft\nV = (1.49/0.025)(1.236)^0.667 x (0.005)^0.5 = 4.87 fps\nQ = 4.87 x 16 = 77.9 cfs
Result:Q = 77.9 cfs, V = 4.87 fps
Example 2: Concrete Rectangular Channel
Problem:Find the discharge through a 3 ft wide by 1.5 ft deep concrete rectangular channel on a 1% slope with n = 0.013.
Solution:A = 3 x 1.5 = 4.5 sq ft\nP = 3 + 2(1.5) = 6 ft\nR = 4.5/6 = 0.75 ft\nV = (1.49/0.013)(0.75)^0.667 x (0.01)^0.5 = 7.52 fps\nQ = 7.52 x 4.5 = 33.8 cfs
Result:Q = 33.8 cfs, V = 7.52 fps
Frequently Asked Questions
What are common channel cross-section shapes?
The most common channel shapes are rectangular, trapezoidal, triangular, and circular. Rectangular channels are simple to construct and common in concrete-lined applications. Trapezoidal channels are the most hydraulically efficient open channel shape and are standard for earthen canals and ditches. Triangular channels (V-ditches) are used for shallow roadside drainage. Circular cross-sections are used for pipes and culverts. The most hydraulically efficient trapezoidal section has a bottom width equal to twice the depth times the tangent of 30 degrees, forming a half-hexagon shape.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy