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 = 2.5 mn = 0.03

Water flows uniformly 2.5 m deep in a rectangular grass-lined earth channel 1.5 m wide (Manning n = 0.03). The bed slope is S = 0.0015. Use Manning’s equation in SI units to find the mean velocity V.

b = 1.5 my = 2.5 mn = 0.03S = 0.0015
V = m/s
2.Easy
b = 4.5 my = 1.7 mn = 0.03

Water flows uniformly 1.7 m deep in a rectangular grass-lined earth channel 4.5 m wide (Manning n = 0.03). The bed slope is S = 0.0015. Use Manning’s equation in SI units to find the mean velocity V.

b = 4.5 my = 1.7 mn = 0.03S = 0.0015
V = m/s
3.Medium
b = 2.5 my = 1 m1.5 : 1n = 0.014, S = 0.0015

A trapezoidal unfinished concrete channel has a bottom width of 2.5 m and side slopes of 1.5:1 (horizontal:vertical). Water flows uniformly 1 m deep. Manning n = 0.014 and the bed slope is S = 0.0015. Find the flow rate Q in m³/s.

b = 2.5 my = 1 mz = 1.5n = 0.014S = 0.0015
Q = m³/s
4.Medium
b = 1.5 my = 1.1 mn = 0.025, S = 0.0005

A rectangular gravel channel is 1.5 m wide. Water flows uniformly 1.1 m deep. Manning n = 0.025 and the bed slope is S = 0.0005. Find the flow rate Q in m³/s.

b = 1.5 my = 1.1 mn = 0.025S = 0.0005
Q = m³/s
5.Hard
b = 4.5 my = 2.3 mn = 0.015, Q = 34 m³/s

A rectangular brickwork channel 4.5 m wide must carry 34 m³/s in uniform flow at a depth of 2.3 m. Manning n = 0.015. Find the required bed slope S (in m/m).

Q = 34 m³/sb = 4.5 my = 2.3 mn = 0.015
S = m/m
6.Hard
b = 2.5 my = 1.6 m3 : 1n = 0.015, Q = 29 m³/s

A trapezoidal brickwork channel with a bottom width of 2.5 m and side slopes of 3:1 (horizontal:vertical) must carry 29 m³/s in uniform flow at a depth of 1.6 m. Manning n = 0.015. Find the required bed slope S (in m/m).

Q = 29 m³/sb = 2.5 my = 1.6 mz = 3n = 0.015
S = m/m

Answer key

  1. 1. V = 0.8947 m/s
  2. 2. V = 1.264 m/s
  3. 3. Q = 8.347 m³/s
  4. 4. Q = 0.8614 m³/s
  5. 5. S = 0.002045 m/m
  6. 6. S = 0.001538 m/m

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