Question

The maximum permissible velocity of water in the clayey Channels is

• A. 0.45 $\text{m.s}^{-1}$
• B. 0.60 $\text{m.s}^{-1}$
• C. 0.70 $\text{m.s}^{-1}$
• D. 0.65 $\text{m.s}^{-1}$

Hint:

Cohesive soils (clays) can withstand higher water velocities than non-cohesive soils (sands).
- Sandy channels: Max velocity \( \approx 0.45 \text{ m/s} \).
- Clayey channels: Max velocity \( \approx 0.70 \text{ m/s} \).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The maximum permissible velocity (or non-eroding velocity) is the highest mean velocity of water that will not cause erosion or scouring of the channel bed and sides.
This velocity depends on the cohesive properties of the soil forming the channel boundary.

Step 2: Detailed Explanation:
When designing unlined earth channels, the water velocity must be kept below a critical limit to prevent soil erosion.
This limit is determined by the soil type:
1. Sandy and Light Sandy Soils: These soils are non-cohesive and easily eroded, so their maximum permissible velocity is low, typically around \( 0.30 \text{ to } 0.45 \text{ m/s} \).
2. Alluvial Silts and Loams: These have moderate cohesion, with permissible velocities ranging from \( 0.50 \text{ to } 0.60 \text{ m/s} \).
3. Clayey and Stiff Clay Soils: Clay particles are highly cohesive, which helps them resist the shear forces exerted by flowing water.
Because of this high cohesive strength, clayey channels can withstand higher water velocities without eroding.
According to standard irrigation engineering design guidelines (such as IS: 10430 and Fortier & Scobey's values), the maximum permissible velocity for unlined channels in average clayey soils is \( 0.70 \text{ m/s} \) (or up to \( 0.80 \text{ m/s} \) for very stiff clays).
Keeping the velocity near this limit helps prevent sediment deposition (silting) while avoiding channel erosion.
Therefore, \( 0.70 \text{ m.s}^{-1} \) is the correct design value.

Step 3: Final Answer:
The maximum permissible velocity of water in clayey channels is 0.70 $\text{m.s}^{-1}$.