Drainage

UNIT III
 Water Logging and Salinity
An agricultural land is said to be water-logged, when its
productivity gets, affected by the high water table.
Apart from ill-aeration of the plants, many other problems are
created by water-logging, as discussed below :
• The normal cultivation operations, such as tilling, ploughing, etc.
cannot be easily carried out in wet soils.
• Certain water loving plants like grasses, weeds, etc. grow
profusely and luxuriantly in water-logged lands, thus affecting
and interfering with the growth of the crops.
• Water-logging also leads to salinity, as explained below : If the
water table has risen up, or if the plant roots happen to come
within the capillary fringe, water is continuously evaporated by
capillarity. Thus, a continuous upward flow of water from the
water table to the land-surface, gets established. With this
upward flow, the salts which are present in the water, also rise
towards the surface, resulting, in the deposition of salts in the
root zone of the crops.
Causes of water logging
(i) Over and Intensive Irrigation.
(ii)Seepage of Water from the Adjoining High
Lands.
(iii)Seepage of Water through the Canals.
(iv)Impervious Obstruction.
(v)Inadequate Natural Drainage.
(vi)Inadequate Surface Drainage.
(vii)Excessive Rains.
(viii)Submergence due to Floods.
(ix)Irregular or Flat Topography.
Water Logging control
• Lining of canals and water courses
• Reducing the intensity of irrigation
• Crop rotation
• By optimum use of water
• By providing intercepting drains
• By provision of an efficient drainage system
• By improving the natural drainage of the
area
• By adopting the consumptive use of surface
and sub-surface water
Reclamation of saline and
alkaline land
• Land reclamation is a process by which an unculturable
land is made fit for cultivation.
• Every agricultural soil contains certain mineral salts in it.
Some of these salts are beneficial for plants as they
provide the plant foods, while certain others prove
injurious (alkali salts) to plant growth.
• The soluble alkali salts also move up with water and get
deposited in the soil within the plant roots as well as on
the surface of the land. This phenomenon of salts coining
up in solution and forming a thin (5 to 7.5 cm) crust on
the surface, after the evaporation of water, is called
effeorescence. Land affected by effeorescence is called
saline soil.
Leaching
• In this process, the land is flooded with adequate
depth of water.
• The alkali salts present in the soil, get dissolved in
this water, which percolate down to join the water
table or drained away by surface and sub-surface
drains.
• The process is repeated till the salts in the top
layer of the land are reduced to such as extent
that some salt resistant crop can be grown. This
process is known as-leaching.
Land Drainage
• While designing a canal irrigation system, it
is sometimes desirable to provide a suitable
drainage system for removing the excess
irrigation water.
• This may be necessary in areas of high
water table and in river delta.
• The types of drainage system that can be
provided are
• Surface Drainage
• Sub-surface drainage/Tile drainage/Under-
ground drainage
Surface Drainage or Open Drainage
• Surface drainage is the removal of excess rain water falling on the fields or the
excess irrigation water applied to the fields, by constructing open ditches, field
drains, and other related structures.
• The land is sloped towards these ditches or drains, as to make the excess water
flow in to these drains.
• When irrigation is extended to arid regions, drainage ditches become necessary
to remove water required for leaching undesirable salts from the soil, and to
dispose off the excess rainfall.
• The open drains, which are constructed to remove the excess irrigation water
collected in the depressions on the fields, as well as the storm (rain) water, are
broad and shallow, and are called shallow surface drains.
• These drains carry the runoff to the outlet drains, which are large enough to
carry the flood water of the catchment area from the shallow surface drains, and
are of sufficient depths to provide outlets even for the underground tile drains, if
provided.
• These outlet drains may be called deep surface drains.
• Surface drains constructed for removing excess irrigation water
applied to the farms and the storm water, cannot and should not be
deep enough, as to interfere ,with the agricultural operations.
• They are, therefore, designed as shallow surface drains.
• and grading, which results in a continuous land slope towards the
field drains, is an important part of a surface drainage system.
• Land grading or land levelling is also necessary for surface irrigation.
• The shallow surface drains are trapezoidal, in cross-section.
• Strictly speaking, they should be designed to carry the normal storm
water from the fields, plus the excess irrigation water.
• Many a times, the excess irrigation water is neglected and these
drains are designed only for the runoff resulting from the average
storms.
• It is neither economical nor desirable to design these drains for
exceptional storms.
• Kutter's or Manning's equations may be used to design these drains,
keeping the velocity within the limits of the critical velocity, and
thereby avoiding silting or scouring.
• Manning's equation is, however, generally used for the design of
shallow as well as deep surface drains.
• Deep surface drains or outlet drains carry the storm
water discharge from the shallow surface drains, and the
seepage water coming from the underground tile drains.
They are, therefore, designed for the combined discharge
of the shallow surface drains as well as that of the tile
drains.
• Generally, a cunnette of about 0.6 m depth is provided in
the centre of the drain-bed, so as to carry the seepage
water of the underground tile drains.
• A steeper slope is given to the cunnette and it is lined, so
as to withstand higher flow velocities, and thus, to inhibit
weed growth.
• The full section would be operative only during the rainy
season, as otherwise, the flow will be confined only within
Surface Inlet
• The surface water from the pot holes,
depressions, road ditches, farm steads,
etc. may be removed either by
connecting them with the shallow surface
drains, sometimes called random field
drains or by constructing an in-take
structure called an open inlet or surface
inlet
• A surface inlet is a structure constructed
to carry the pit water into the sub-sur-
face or tile drain.
• A cast-iron pipe or a manhole
constructed of brick or monolithic
concrete, is sufficient and satisfactory.
Manholes with sediment basins are
sometimes used as surface inlets.
• At the surface of the ground, a concrete
collar extending around the intake is
constructed on the riser to prevent
growth of vegetation and to hold its
place.
French Drain
• When the quantity of water to be
removed from the pits or depressions is
small, a blind inlet may be installed over
the tile drain.
• The blind inlet is also called French drain.
• These are constructed by back filling the
trench of the tile drain with graded
materials such as gravel and sands.
Bedding
• Bedding is a method of surface drainage which makes use
of dead furrows.
• The area between the two adjacent furrows is known as
bed.
• The depth of the bed depends on the soil characteristics
and tillage practices.
• In the bedded area the direction of farming may be
parallel or normal to dead furrows:
• Tillage practices, parallel to the beds, retard water
movement to the dead furrows.
• Ploughing is always parallel to the dead furrows.
• Bedding is most practicable on flat slopes of less than
15%, where the soils are slowly permeable and the
drainage is not economical.
Sub-surface Drainage or Tile Drainage
• Plants need air as well as moisture in their root zones for their
survival.
• Excess irrigation farm water is free to move into the
underground tile drains, if provided.
• This water, if not removed, retards the plant growth, because it
fills the soil voids and restricts proper aeration.
• Surface drains are; therefore, needed for removing the excess
farm water, for most of the cultivated crops on flat or undulating
topography.
• Sub-surface drains, on the other hand, are required for soils with
poor internal drainage and a high water table.
• If no impervious layer occurs below the farm land and the water
table is low (lower than about. 3m from the ground), internal
soil drainage may be sufficient and no tile drains needed.
Advantages of Tile Drains.
Tile drainage helps in increasing crop yields by drain-the water or by lowering
the water table in the following manner
(i) Removes the free gravity water that is not directly available to the plants.
(ii) Increases the volume of soil from which roots can obtain food.
(iii) Increases air circulation.
(iv) Increases bacterial activity in the soil, thus improving soil structure and
making the plant food more readily available.
(v) Reduces soil erosion. A well drained soil has more capacity to hold rainfall,
resulting in less runoff and hence, reduced erosion.
(vi) Reduces and removes toxic substances such as sodium and other soluble
salts, which when present in large concentrations may retard plant growth.
(vii) Lesser time and labour is required for tilling and harvesting the soils, as
these drains do not obstruct farming operations. With a crop such as corn,
a delay in planting may decrease the yields. Planting in wet soils is also
likely to decrease yields. All such troubles are removed in tile drained soils.
(viii) Tile drains permit deep roots development by lowering the water table,
especially during spring months.
 Outlet
Gravity Outlet
Pump Outlet
Drawdown
Depth and spacing of Tile
Drain
 Drainage Coefficient
• The rate at which water is removed by a drain is
called the drainage coefficient.
• It is expressed as the depth of water In cm or
meters, to be removed in 24 hours from the
drainage area.
Drainage Area
The area actually drained by the tile drain system is
called its drainage area.
Size of the tile drain
• The tile drains are designed according the
Manning's formula to carry a certain discharge
decided by D.C. and drainage area.
• The drains are laid on a certain longitudinal slope
varying from 0.05 to 3%.
Drainage Layout
Natural System
• The natural system is generally adopted in rolling
topography, where drainage of isolated areas is
required.
• This system is suitable when the land is not to be
completely drained.
Grid-Iron
• This system is adopted when the land is practically
level; or where the land surface slopes away from
the sub main on one side, and when the entire area
has to be drained.
Herring Bone
• This layout is adopted, when the main (or sub-
main) is laid in depression.
• The land along the main is double drained, but
since it exists in depression, it probably requires
more drainage than the land on the adjacent
slopes.
Double main
• This layout is adopted when the bottom of
depression is wide.
• This arrangement reduces the length of the laterals
and eliminates the break in slope of the lateral at
the edge of the depression
Intercepting tile drains
This arrangement is preferred when the main source
of drainage is from a hilly land.