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

Water flows uniformly 2.1 m deep in a rectangular gravel channel 3 m wide (Manning n = 0.025). The bed slope is S = 0.0005. Use Manning’s equation in SI units to find the mean velocity V.

b = 3 my = 2.1 mn = 0.025S = 0.0005

V = m/s

2.Easy

Water flows uniformly 1.9 m deep in a rectangular gravel channel 1.5 m wide (Manning n = 0.025). The bed drops 1.5 m per kilometre. Use Manning’s equation in SI units to find the mean velocity V.

b = 1.5 my = 1.9 mn = 0.025drop = 1.5 m/km

V = m/s

3.Medium

A concrete storm sewer with an inside diameter of 750 mm flows just full (as an open channel, not under pressure). Manning n = 0.013 and the slope is S = 0.003. Find the flow rate Q in m³/s.

D = 750 mmn = 0.013S = 0.003

Q = m³/s

4.Medium

A concrete storm sewer with an inside diameter of 900 mm flows half full. Manning n = 0.013 and the slope is S = 0.0015. Find the flow rate Q in m³/s.

D = 900 mmn = 0.013S = 0.0015

Q = m³/s

5.Hard

A concrete sewer with an inside diameter of 600 mm (Manning n = 0.013) must reach a self-cleansing velocity of 1 m/s when flowing just full. Find the minimum slope S (in m/m).

D = 600 mmn = 0.013V = 1 m/s

S = m/m

6.Hard

A concrete sewer with an inside diameter of 200 mm (Manning n = 0.013) must reach a self-cleansing velocity of 0.9 m/s when flowing just full. Find the minimum slope S (in m/m).

D = 200 mmn = 0.013V = 0.9 m/s

S = m/m

Answer key

1. V = 0.8182 m/s
2. V = 1.024 m/s
3. Q = 0.6098 m³/s
4. Q = 0.3506 m³/s
5. S = 0.00212 m/m
6. S = 0.007432 m/m

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