A rectangular channel of bed width 2 m is to be laid at a bed slope of 1 in 1000. Find the hydraulic radius of the canal cross-section for the maximum discharge condition? Take Chezy’s constant as 50

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  1. 0.5 m
  2. 2 m
  3. 1 m
  4. 0.25 m

Answer (Detailed Solution Below)

Option 1 : 0.5 m
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Detailed Solution

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Concept:

Most efficient channel: A channel is said to be efficient if it carries the maximum discharge for the given cross-section which is achieved when the wetted perimeter is kept a minimum.

Rectangular Section:

Full Test 2 (31-80) images Q.55

Area of the flow, A = b × d

Wetted Perimeter, P = b + 2 × d  

For the most efficient Rectangular channel, the two important conditions are

  1. b = 2 × d
  2.  \(R = \frac{A}{P} = \frac{{b\times d}}{{b + 2 \times d}} = \frac{{2{d^2}}}{{4d}} = \frac{d}{2}= \frac{b}{4}\)

Calculation

Given: b = 2 m

 \(R = \frac{2}{4}\)

R = 0.5

quesImage111

Rectangular channel section Trapezoidal channel section
  1. R = y / 2
  2. A = 2y
  3. T = 2y
  4. P = 4y
  5. D = y
  1. R = y / 2
  2. \(A = \sqrt 3 {y^2}\)
  3. \(T = \frac{{4y}}{{\sqrt 3 }}\)
  4. \(P = {{2y\sqrt 3}}{{ }}\)
  5.  D = 3y / 4

Where R = hydraulic radius, A = Area of flow, P = wetted perimeter, y = depth of flow, T = Top width

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