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 = 4 my = 2.1 mn = 0.012

Water flows uniformly 2.1 m deep in a rectangular finished concrete channel 4 m wide (Manning n = 0.012). The bed slope is S = 0.005. Use Manning’s equation in SI units to find the mean velocity V.

b = 4 my = 2.1 mn = 0.012S = 0.005
V = m/s
2.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 = 0.0005. Use Manning’s equation in SI units to find the mean velocity V.

b = 5 my = 0.8 mn = 0.014S = 0.0005
V = m/s
3.Medium
b = 5.5 my = 1.9 m2 : 1n = 0.014, S = 0.002

A trapezoidal unfinished concrete channel has a bottom width of 5.5 m and side slopes of 2:1 (horizontal:vertical). Water flows uniformly 1.9 m deep. Manning n = 0.014 and the bed slope is S = 0.002. Find the flow rate Q in m³/s.

b = 5.5 my = 1.9 mz = 2n = 0.014S = 0.002
Q = m³/s
4.Medium
b = 5.5 my = 0.9 m3 : 1n = 0.03, S = 0.003

A trapezoidal grass-lined earth channel has a bottom width of 5.5 m and side slopes of 3:1 (horizontal:vertical). Water flows uniformly 0.9 m deep. Manning n = 0.03 and the bed slope is S = 0.003. Find the flow rate Q in m³/s.

b = 5.5 my = 0.9 mz = 3n = 0.03S = 0.003
Q = m³/s
5.Hard
b = 5 my = 1.2 m3 : 1n = 0.03, Q = 21 m³/s

A trapezoidal grass-lined earth channel with a bottom width of 5 m and side slopes of 3:1 (horizontal:vertical) must carry 21 m³/s in uniform flow at a depth of 1.2 m. Manning n = 0.03. Find the required bed slope S (in m/m).

Q = 21 m³/sb = 5 my = 1.2 mz = 3n = 0.03
S = m/m
6.Hard
D = 450 mmn = 0.013, V = 1.2 m/s

A concrete sewer with an inside diameter of 450 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 = 450 mmn = 0.013V = 1.2 m/s
S = m/m

Answer key

  1. 1. V = 5.988 m/s
  2. 2. V = 1.144 m/s
  3. 3. Q = 65.93 m³/s
  4. 4. Q = 10.21 m³/s
  5. 5. S = 0.004858 m/m
  6. 6. S = 0.004481 m/m

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