Subsurface Drainage Design Questions and Answers

Q: One of the key assumptions made by Dupuit-Forchheimer about the flow behavior in the porous medium towards subsurface drains is

The assumption is horizontal streamlines.The Dupuit-Forchheimer assumptions for unconfined flow say that for a gently sloping water table the streamlines are horizontal and the velocity is uniform over the depth, proportional to the slope of the water table. This reduces the problem to one dimension and underlies the Hooghoudt and related drain-spacing equations.A homogeneous or isotropic medium and uniform recharge are assumptions of particular drainage formulas, but the defining Dupuit-Forchheimer assumption concerns the flow lines.

Q: The depths from the soil surface to subsurface tile drains, impermeable soil layer and the highest water table are measured as 2.8 m, 5.0 m and 0.8 m, respectively. The effective hydraulic head for drainage in meter is

The effective hydraulic head is 2.0 m.The head that drives flow into the drains is the height of the water table above the drain level, measured midway between drains at its highest. With depths measured from the surface:\[ h = 2.8 - 0.8 = 2.0\ \text{m} \]0.8 m is the depth of the water table itself, 2.2 m is the distance from the drains down to the impermeable layer, and 4.2 m is the water table's height above the impermeable layer; none is the head above the drains.

One of the key assumptions made by Dupuit-Forchheimer about the flow behavior in the porous medium towards subsurface drains is

A.homogeneous medium
B.isotropic medium
C.horizontal streamlines
D.uniform recharge

Exam Year(s)

GATE AG - 2018,

Answer: C

The assumption is horizontal streamlines.

The Dupuit-Forchheimer assumptions for unconfined flow say that for a gently sloping water table the streamlines are horizontal and the velocity is uniform over the depth, proportional to the slope of the water table. This reduces the problem to one dimension and underlies the Hooghoudt and related drain-spacing equations.

A homogeneous or isotropic medium and uniform recharge are assumptions of particular drainage formulas, but the defining Dupuit-Forchheimer assumption concerns the flow lines.

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The depths from the soil surface to subsurface tile drains, impermeable soil layer and the highest water table are measured as 2.8 m, 5.0 m and 0.8 m, respectively. The effective hydraulic head for drainage in meter is

A.0.8
B.2.0
C.2.2
D.4.2

Exam Year(s)

GATE AG - 2018,

Answer: B

The effective hydraulic head is 2.0 m.

The head that drives flow into the drains is the height of the water table above the drain level, measured midway between drains at its highest. With depths measured from the surface:

\[ h = 2.8 - 0.8 = 2.0\ \text{m} \]

0.8 m is the depth of the water table itself, 2.2 m is the distance from the drains down to the impermeable layer, and 4.2 m is the water table's height above the impermeable layer; none is the head above the drains.

Discuss in Forum

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Subsurface Drainage Design Questions and Answers

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