Question

Which of the following permanent gully control structure is used at the head to convey the water safely to the gully bed?

• A. Brushwood check dam
• B. Drop inlet or pipe spillway
• C. Drop spillway
• D. Chute spillway

Hint:

Gully head ($H \le 3\text{ m}$) $\rightarrow$ Drop spillway.
Gully drop ($H > 3\text{ m}$) $\rightarrow$ Chute spillway.
Detention dam $\rightarrow$ Drop inlet spillway.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Permanent gully control structures stabilize active overfall gully heads and dissipate kinetic energy of concentrated storm runoff.
Key Formula or Approach:
\[ Q = C L H^{3/2} \quad \text{(Weir Flow over Drop Spillway Crest)} \]

Step 2: Detailed Explanation:
Permanent soil conservation structures for gully stabilization:
1. Drop Spillway: Installed directly at the gully head for drops generally up to \(3.0\text{ m}\). Runoff passes over a straight or box weir crest and falls vertically onto an apron/stilling basin, safely lowering water into the gully bed and halting headward gully retreat.
2. Chute Spillway: Used for steep drops and larger drop heights (\(> 3\text{ m}\)) conveying water along a paved open channel.
3. Drop Inlet Spillway: Used where high embankments create temporary detention ponds.
4. Brushwood dam: A temporary, non-permanent structure.

Step 3: Final Answer:
Therefore, the permanent structure used at the gully head is a Drop spillway, corresponding to option (C).