Chezy application to a full rectangular channel (b = 6 m, y = 3 m) A rectangular channel is 6 m wide and 3 m deep, running full on a bed slope S = 1/2000. If Chezy’s constant C = 54.8, identify the correct specifications (hydraulic mean depth, velocity, and discharge).

Difficulty: Medium

Correct Answer: All of the above

Explanation:


Introduction / Context:
Chezy’s formula is widely used for uniform open-channel flow to relate mean velocity to hydraulic radius and slope. This problem checks competence in computing section properties and applying the velocity formula correctly.


Given Data / Assumptions:

  • Rectangular section: width b = 6 m; depth y = 3 m (running full).
  • Chezy constant C = 54.8; bed slope S = 1/2000.
  • Uniform, steady flow; prismatic channel; water at ordinary conditions.


Concept / Approach:

Hydraulic mean depth (hydraulic radius) R = A / P for open channels. Chezy’s velocity: V = C * √(R * S). Discharge: Q = A * V.


Step-by-Step Solution:

Area A = b * y = 6 * 3 = 18 m^2.Wetted perimeter P = b + 2 y = 6 + 6 = 12 m.Hydraulic radius R = A / P = 18 / 12 = 1.5 m.Mean velocity V = C * √(R * S) = 54.8 * √(1.5 * 1/2000).Compute inside root: 1.5/2000 = 0.00075; √(0.00075) ≈ 0.027386; V ≈ 54.8 * 0.027386 ≈ 1.5 m/s.Discharge Q = A * V = 18 * 1.5 = 27 m^3/s.


Verification / Alternative check:

Dimensions: R in meters, S dimensionless; V in m/s; Q in m^3/s. Values are internally consistent and typical for a channel of this size and slope.


Why Other Options Are Wrong:

Since (a), (b), and (c) are all correct, any option omitting one of them is incomplete.


Common Pitfalls:

Using hydraulic depth A/b instead of hydraulic radius A/P; arithmetic slips with slope; forgetting the square root in Chezy’s relation.


Final Answer:

All of the above

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