Which of the following statements is/are true for an open channel flow? i. Energy grade line is obtained by adding pressure head and velocity head ii. Hydraulic grade line is the free surface itself
- AOnly (i)
- BOnly (ii)Correct
- CBoth (i) and (ii)
- DNeither (i) nor (ii)
Solution
Concept
Energy and hydraulic grade lines in open channel
Formula
NONE
Step-by-step solution
1. In open channel flow, the energy grade line represents the total head above a datum, incorporating pressure head and velocity head. 2. The hydraulic grade line corresponds to the free water surface elevation along the channel. Answer: B
Explanation
In open channel flow, the surface of the water is exposed to the atmosphere, so the pressure head at the surface is zero and the hydraulic grade line coincides with the free water surface. Statement i is false because the energy grade line is formed by adding the velocity head to the hydraulic grade line. Therefore, option B is correct.
Why the other options are wrong
C) Statement (ii) is false, so both statements cannot be true.
Shortcut
In open channel flow, the Hydraulic Grade Line (HGL) directly coincides with the free water surface.
Common mistake
Selecting option C by forgetting that datum/elevation head must be included along with pressure and velocity head to get the energy grade line.
Correct answer: B) Only (ii)