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 = 5 my = 0.8 mn = 0.014

Water flows uniformly 0.8 m deep in a rectangular unfinished concrete channel 5 m wide (Manning n = 0.014). The bed slope is S = 0.005. Use Manning’s equation in SI units to find the mean velocity V.

b = 5 my = 0.8 mn = 0.014S = 0.005
V = m/s
2.Easy
b = 3 my = 2.3 mn = 0.015

Water flows uniformly 2.3 m deep in a rectangular brickwork channel 3 m wide (Manning n = 0.015). The bed slope is S = 0.005. Use Manning’s equation in SI units to find the mean velocity V.

b = 3 my = 2.3 mn = 0.015S = 0.005
V = m/s
3.Medium
b = 4 my = 2.2 m1 : 1n = 0.012, S = 0.0008

A trapezoidal finished concrete channel has a bottom width of 4 m and side slopes of 1:1 (horizontal:vertical). Water flows uniformly 2.2 m deep. Manning n = 0.012 and the bed slope is S = 0.0008. Find the flow rate Q in m³/s.

b = 4 my = 2.2 mz = 1n = 0.012S = 0.0008
Q = m³/s
4.Medium
D = 1200 mmn = 0.013, S = 0.005

A concrete storm sewer with an inside diameter of 1200 mm flows just full (as an open channel, not under pressure). Manning n = 0.013 and the slope is S = 0.005. Find the flow rate Q in m³/s.

D = 1200 mmn = 0.013S = 0.005
Q = m³/s
5.Hard
D = 300 mmn = 0.013, V = 0.6 m/s

A concrete sewer with an inside diameter of 300 mm (Manning n = 0.013) must reach a self-cleansing velocity of 0.6 m/s when flowing just full. Find the minimum slope S (in m/m).

D = 300 mmn = 0.013V = 0.6 m/s
S = m/m
6.Hard
b = 5 my = 0.7 m1.5 : 1n = 0.03, Q = 2.7 m³/s

A trapezoidal grass-lined earth channel with a bottom width of 5 m and side slopes of 1.5:1 (horizontal:vertical) must carry 2.7 m³/s in uniform flow at a depth of 0.7 m. Manning n = 0.03. Find the required bed slope S (in m/m).

Q = 2.7 m³/sb = 5 my = 0.7 mz = 1.5n = 0.03
S = m/m

Answer key

  1. 1. V = 3.617 m/s
  2. 2. V = 4.42 m/s
  3. 3. Q = 38.97 m³/s
  4. 4. Q = 2.757 m³/s
  5. 5. S = 0.001924 m/m
  6. 6. S = 0.0007871 m/m

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