Section 4: Soil and Water Conservation Engineering
Soil and Water Erosion
Subtopic: Gully control structures – drop, drop inlet and chute spillways
Question Statement:
Which of the following criteria dictate the selection and hydraulic application of gully control spillways?
Options:
(A) Straight drop spillways are ideal for low total drops (typically $Y \le 3.0\text{ m}$) where a rectangular weir discharges into a horizontal stilling basin
(B) Drop inlet (pipe) spillways are preferred where high drops (gt; 3.0\text{ m}$) accompany an earthen embankment bund with substantial flood detention storage
(C) Chute spillways are well suited to steep gully headcuts where runoff is conveyed down an open paved trough at supercritical velocities
(D) Drop spillways require neither stilling basins nor energy dissipators because free-falling water loses all energy in air friction
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
- Statements A, B, and C are correct engineering guidelines for gully control:
1. Straight drop spillways: for drops up to $3\text{ m}$ with hydraulic jump stilling basins.
2. Drop inlet spillways: conduit under pressure flow through an earthen dam, providing temporary flood storage.
3. Chute spillways: reinforced concrete steep chutes terminating in SAF stilling basins.
- Statement D is false: The high kinetic energy of the free-falling nappe requires a designed stilling basin with floor blocks, baffle sills, and end sills to prevent devastating downstream scour.