Question

Followings are some of the statements related to drop spillway
(A). Drop spillway should be located on a reasonable straight section of channel with no upstream and downstream curves within 50 m reach of the structure.
(B). Transverse sill create a hydraulic jump at the toe of the structure.
(C). Longitudinal sill helps to straighten the flow.
(D). The cutoff wall depth should be sufficient to increase uplift pressure and reduce piping under the structure.
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (B) and (C) only.
• C. (B), (C) and (D) only.
• D. (A), (B), (C) and (D).

Hint:

To prevent ______piping failure______ in hydraulic structures:
We use a ______cutoff wall______ to force water to travel a longer path under the structure.
According to ______Bligh's Creep Theory______, this reduces the exit gradient and minimizes downstream ______uplift pressure______.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A drop spillway is a weir structure used to convey water safely down steep slopes or gully heads.
Its primary components (apron, sills, cutoff walls) are designed to control flow velocity, dissipate kinetic energy, and prevent structural failure due to scouring or piping.

Step 2: Detailed Explanation:
Let us analyze each design statement:
- ______Statement (A):______ To prevent turbulence and uneven scouring of the banks, the spillway must be located on a straight channel section. Curves within $50\,\text{m}$ upstream or downstream are avoided. (Correct)
- ______Statement (B):______ A transverse sill placed at the end of the apron helps form and stabilize a hydraulic jump on the apron floor, dissipating the kinetic energy of the falling water at the toe. (Correct)
- ______Statement (C):______ Longitudinal sills run parallel to the flow direction on the apron. They act as guides to keep the flow straight, preventing lateral spreading and eddy formation. (Correct)
- ______Statement (D):______ The cutoff wall (or toe wall) is constructed beneath the structure to lengthen the seepage path.
A longer seepage path ______decreases______ (not increases) the hydraulic gradient and the resulting uplift pressure on the floor, thereby reducing the risk of piping failure.
Therefore, Statement (D) is incorrect because the cutoff wall's function is to *decrease* uplift pressure, not increase it.

Step 3: Final Answer:
The correct statements are (A), (B), and (C) only, which corresponds to option (A).