Open-channel flow (Manning) worksheet with answers

Free printable Open-channel flow (Manning) worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy
b = 1.5 my = 1.4 mn = 0.03

Water flows uniformly 1.4 m deep in a rectangular grass-lined earth channel 1.5 m wide (Manning n = 0.03). The bed slope is S = 0.003. Use Manning’s equation in SI units to find the mean velocity V.

b = 1.5 my = 1.4 mn = 0.03S = 0.003
V = m/s
2.Easy
b = 3.5 my = 0.8 mn = 0.022

Water flows uniformly 0.8 m deep in a rectangular clean earth channel 3.5 m wide (Manning n = 0.022). The bed drops 0.8 m per kilometre. Use Manning’s equation in SI units to find the mean velocity V.

b = 3.5 my = 0.8 mn = 0.022drop = 0.8 m/km
V = m/s
3.Medium
b = 5 my = 1 m3 : 1n = 0.022, S = 0.0005

A trapezoidal clean earth channel has a bottom width of 5 m and side slopes of 3:1 (horizontal:vertical). Water flows uniformly 1 m deep. Manning n = 0.022 and the bed slope is S = 0.0005. Find the flow rate Q in m³/s.

b = 5 my = 1 mz = 3n = 0.022S = 0.0005
Q = m³/s
4.Medium
D = 900 mmn = 0.013, S = 0.001

A concrete storm sewer with an inside diameter of 900 mm flows half full. Manning n = 0.013 and the slope is S = 0.001. Find the flow rate Q in m³/s.

D = 900 mmn = 0.013S = 0.001
Q = m³/s
5.Hard
b = 3.5 my = 1.5 m1 : 1n = 0.022, Q = 21 m³/s

A trapezoidal clean earth channel with a bottom width of 3.5 m and side slopes of 1:1 (horizontal:vertical) must carry 21 m³/s in uniform flow at a depth of 1.5 m. Manning n = 0.022. Find the required bed slope S (in m/m).

Q = 21 m³/sb = 3.5 my = 1.5 mz = 1n = 0.022
S = m/m
6.Hard
D = 375 mmn = 0.013, V = 1.2 m/s

A concrete sewer with an inside diameter of 375 mm (Manning n = 0.013) must reach a self-cleansing velocity of 1.2 m/s when flowing just full. Find the minimum slope S (in m/m).

D = 375 mmn = 0.013V = 1.2 m/s
S = m/m

Answer key

  1. 1. V = 1.132 m/s
  2. 2. V = 0.862 m/s
  3. 3. Q = 6.45 m³/s
  4. 4. Q = 0.2862 m³/s
  5. 5. S = 0.003959 m/m
  6. 6. S = 0.005714 m/m

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