Calculate the effective unit tractive force that causes bed load transportation if the unit tractive force produced by the flowing water is 3.20 N/m^2. The Manning’s coefficient on an unrippled bed is 0.0108 and the actual observed value of Manning’s coefficient on ripped channels is 0.0222. Take the value of critical shear stress that is required to move the grain particle as 0.5 N/m^2.

Question

Calculate the effective unit tractive force that causes bed load transportation if the unit tractive force produced by the flowing water is 3.20 N/m^2. The Manning’s coefficient on an unrippled bed is 0.0108 and the actual observed value of Manning’s coefficient on ripped channels is 0.0222. Take the value of critical shear stress that is required to move the grain particle as 0.5 N/m^2.

asked Sep 16, 2023 in Irrigation Engineering by Carter (75.2k points)

Calculate the effective unit tractive force that causes bed load transportation if the unit tractive force produced by the flowing water is 3.20 N/m^2. The Manning’s coefficient on an unrippled bed is 0.0108 and the actual observed value of Manning’s coefficient on ripped channels is 0.0222. Take the value of critical shear stress that is required to move the grain particle as 0.5 N/m^2.

(a) 1.58 N/m^2

(b) 0.222 N/m^2

(c) 1.08 N/m^2

(d) 0.58 N/m^2

The question was asked during an online exam.

My doubt stems from Sediment Transport topic in division Sediment Transport and Irrigation Channels Design of Irrigation Engineering

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Greg · Answer 1 · 2023-09-25T17:06:52+0000

Correct answer is (d) 0.58 N/m^2

The explanation: The unit tractive force causing bed load to move is reduced by ripples in the ratio of

S0‘= S0 (n’/n) ^3/2 = 3.2 (0.0108/0.0222)^3/2 = 1.08 N/m^2.

The effective unit tractive force = S0’ – Sc = 1.08 – 0.5 = 0.58 N/m^2.

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