The statement which is not correct for the porous medium is
Show Hint
- Seepage velocity is always greater than the Darcy's velocity
- Darcy's velocity is not exclusively controlled by soil porosity
- Seepage velocity increases with increasing surface ponding of water
- Darcy's velocity in unsaturated soil is always greater than that in saturated soil
The Correct Option is D
Solution and Explanation
Step 1: Darcy's velocity vs seepage velocity.
Darcy's velocity (\(v_d\)) is the discharge per unit total area.
Seepage velocity (\(v_s\)) is the actual velocity through pores:
\[
v_s=\frac{v_d}{n}, n=\text{porosity}.
\]
Since \(n<1\), it follows that \(v_s>v_d\). Therefore statement (A) is correct.
Step 2: Dependence of Darcy's velocity.
Darcy's velocity depends on hydraulic gradient and hydraulic conductivity (\(K\)), not only on porosity. Hence (B) is also correct.
Step 3: Effect of surface ponding.
If water ponds on the soil surface, the hydraulic head increases, increasing both Darcy's and seepage velocity. Hence (C) is correct.
Step 4: Unsaturated vs saturated soil.
In unsaturated soil, effective hydraulic conductivity decreases because part of the pore space is filled with air. Therefore, Darcy's velocity in unsaturated soil is actually lower than in saturated soil, not higher. Hence (D) is incorrect.
Final Answer:
\[
\boxed{\text{D}}
\]
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