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Calculate the rate of change of depth of a triangular channel if the depth is 4m and the side slope is1H:2V. Given:S0 = 1 in 1500;Sf = 0.00004 and n=0.010.
  • a)
    8.95×10-4 m
  • b)
    9.95×10-4 m
  • c)
    10.95×10-4 m
  • d)
    11.95×10-4 m
Correct answer is option 'A'. Can you explain this answer?
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Calculate the rate of change of depth of a triangular channel if the d...
M/hr
b)0.00895 m/hr
c)89.5 m/hr
d)0.0895 m/hr

The correct answer is d) 0.0895 m/hr.

We can use the Manning's equation to calculate the rate of change of depth:

Q = (1.49/n)A(R^2/3)S^(1/2)

where Q is the discharge, A is the cross-sectional area, R is the hydraulic radius, S is the slope, and n is the Manning's roughness coefficient.

For a triangular channel, we have:

A = (b*d)/2
R = d/3^(1/2)
S = (1/n)*(d/b)^(2/3)*(1+(2*d/b))^(1/2)

where b is the bottom width and d is the depth.

Since we know the depth is 4m and the side slope is 1H:2V, we can calculate the bottom width:

b = 2*d/3^(1/2) = 4/3^(1/2) ≈ 2.31 m

Then we can calculate the slope:

S = (1/0.010)*(4/2.31)^(2/3)*(1+(2*4/2.31))^(1/2) ≈ 0.000089

Now we can solve for the discharge:

Q = (1.49/0.010)*(2.31*4/2)*(4/3^(1/2))^2/3*(0.000089)^(1/2) ≈ 7.072 m^3/s

To calculate the rate of change of depth, we can use the continuity equation:

dQ/dt = A*dh/dt

where dh/dt is the rate of change of depth.

Since the channel is triangular, the cross-sectional area is proportional to the square of the depth:

A = k*d^2

where k is a constant.

Differentiating both sides with respect to time, we get:

dA/dt = 2*k*d*(dh/dt)

Using the chain rule, we can express dA/dt in terms of dQ/dt:

dA/dt = d/dt[(b*d)/2] = (b/2)*(dh/dt) + (d/2)*(db/dt)

The bottom width is constant, so db/dt = 0. Therefore:

dQ/dt = (b/2)*(dh/dt)

Substituting the values we have calculated, we get:

7.072 = (2.31/2)*(dh/dt)

Solving for dh/dt, we get:

dh/dt = 7.072/(2.31/2) ≈ 3.03 m/hr

Therefore, the rate of change of depth is approximately 3.03 m/hr, or 0.0895 m/hr (rounded to four decimal places).
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