Lecture 29Subsurface drainage – Investigations

Subsurface drainage refers to the removal of excess water present below the ground surface.

Agricultural lands affected by high water table generally need subsurface drainage.

While surface drainage removes the excess rainwater before it enters the root zone, subsurface drainage lowers the water table and provides a better environment in the root zone.

While subsurface drainage problems could be natural, often these problems are manmade.

BENEFITS OF SUBSURFACE DRAINAGE1.Aeration of root zone for maximum development

of the plant roots.2.Opportunity for desirable soil micro organisms to

develop through aeration and higher soil temperatures.

3.Availability of the soil for early cultivation and thus increased crop growth period.

4. Improvement of soil moisture conditions for operation of farm machinery.

5.Removal of undesirable salts from the root zone.6.Greater storage of rainwater in the root zone

because of a low initial water table before the rains.

INVESTIGATION FOR SUBSURFACE DRAINAGE1.Topographic map of the area.2.Data on soil salinity and alkalinity, drainable porosity

etc.3.Position and fluctuations of water table levels relative

to the ground surface and artesian pressures.4.Ground water quality.5.Logs of soil and subsoil materials.6.Hydraulic conductivity measurements.7.Crops proposed to be grown and their drainage

requirements.8. Irrigation practices and requirements.

The topographic map gives the details of land slope, possible outlets, existing drainage pattern, undulating land areas etc., and serves as the base map for preparing the water table contour maps.

Information on soil salinity and alkalinity is needed if surface drainage systems are to be planned along with reclamation of such soils.

Lecture 30Hooghoudt’s steady state equation

DRAINABLE POROSITYDrainage porosity is the volume of water released from a known volume of saturated soil under the force of gravity and inherent soil water tensions. It is expressed as a percentage of the total volume of saturated soil. It is also frequently referred to as the specific yield.

DRAINAGE COEFFICIENT or DRAINAGE DESIGN RATE

Drainage coefficient or drainage design rate with reference to subsurface drainage system is the design value at which water is to be removed per unit of time. It is also sometimes referred to as drainage modulus.

DRAINAGE EFFICIENCYDrainage efficiency is the ratio of the

volume of water discharged by the drains during a certain period to the precipitation generated in that period.

STEADY FLOWSteady is one in which the volume of water

passing a given point per unit of time remains constant.

The movement of water into subsurface drains is influenced by the following factors:

1.The hydraulic conductivity of the soil horizons;2.The configuration and location of the free water

surface, and the presence and magnitude of artesian pressure or of the back pressure in the drains;

3.Depth of drain below ground surface and location of drain with respect to various soil horizons;

4.The horizontal distance between individual drains;5.The diameter of drain;6.The tile joint spacing/ diameter and spacing of holes,

in case of PVC pipes and 7.The depth to impervious layer below the ground

surface.

Let,S = Spacing of drains, md= depth of impermeable layer below the

drain axis, mH = height of water table at mid point between

the drains above drain axis, mR = Constant rate of recharge due to rain/

irrigation, mm/hourK = Hydraulic conductivity of the soil,

mm/houri = Hydraulic gradient = dh/dx

Now assume a vertical section A-A at a distance x from the drain axis and let “h” be the height of water table/ phreatic line above drain axis, Using Darcy’s law, the flow across section A-A towards the drain per unit length can be expressed as, qx =Kh (dh/dx) ……(1) In which qx is the discharge per unit length of drain at a section x distance away from the drain,

The recharge per unit length of drain will be given by, qx = (S/2 - x) . R ………(2)

Equating equation (1) and (2), we get,

dx

dhhKRx

S..

2……(3)Re-

writing (3),

dxxS

K

Rdhh

2

.. ……….(4)

Re-writing (3),

Hd

d

s

dxxS

K

Rdhh

2/

0 2.

2/

0

22

22

.

2

sHd

d

xxS

K

Rh

On integrating (4) between limits, we get,

842

2222 SS

K

RdHd

5.0

24

HdR

KHS

…….(5)Re-writing (5) for S, we get,

In case if the drain is assumed to be placed at the junction of two layered soil with K1 and K2 as hydraulic conductivity for the top and bottom layer than, 5.02

12 48

R

hKHdKS

In case of pipe drains the term d is to be replaced by, d(e), called equivalent depth to moderate the effect of raising water table due to the presence of impermeable layer at shallow depths.

Lecture 32Random drainage - herringbone -

grid iron types

Lecture 33Pipe materials - tile, plastics cement -

Envelope materials. Load factors - blind inlet - filters - mole drains,

drainage wells

Pipe materials satisfy the following conditions: 1.The pipe materials should withstand various

pressure and stresses like tensile, Compression and hoop under water hammer condition.

2. It should be resistant to corrosion and abrasion caused by the water.

3. It should be durable having sufficient strength to bear the external loads coming over it.

4. It should be structurally safe.5. It should have minimum possible weight.6. It should be economical and uniform in size and

shape.7. It should be capable of easy hoisting and handling

at site.

CONSTRUCTIONS1. Pipe inlet laid below the ground

surface.2. Tile line section placed over the

inlet3. Coarse materials are filled on the

inlets.4. Size of the materials is become

less towards the surface5. Finally back filling with sand.

LOAD FACTORThe load factor is the ratio of the strength of a rigid conduit under given bedding conditions to its strength as determined by three edge bearing test. Generally it ranges from 1.2 to 1.5 for drainage pipe laying conditions.

DRAINAGE WELLThe use of wells for the purpose of

draining land is called drainage well. The soil permeability plays an important role in determining the feasibility of well drainage.