Lesson Note On Land Drainage Systems

2.1    Sources of Excess Water

Direct rainfall constitutes the major and
most common source of excess water in an area. However, another major source of
excess water in many cold and moderate climates is snowmelt water during spring
seasons. Other sources of excess water are irrigation, seepage, runoff and
flood water, which are mostly of local importance.

The occurrence of excess rainfall applies
especially to humid climates. However, it may also occur in semi-arid climates
following the common type of intense, heavy storm or in general during the
rainy season. The drainage load from rainfall not only depends on the amount of
rainfall but also on the storage capacity of the soil and on the rate of
evapotranspiration. Part of the rainfall may be stored beneficially in the soil
profile or be readily evaporated so that only the remaining excess water needs
to be removed from the land.

2.2 Design
Considerations for Land Drainage

In the ICID definition of drainage given
in Lesson 1, the phrase ‘the removal of excess water’ indicates that land drainage
(or drainage) is an action by man (i.e., artificial action) who must know how
much excess water should be removed. Therefore, when designing a system for a
given area, drainage engineers should use certain criteria to determine whether
or not water is in excess. Water balance of the area to be drained is the most
accurate tool to calculate the volume of water required to be drained (Bos and
Boers, 1994).

Before carrying out the water balance of
an area, a number of field investigations should be undertaken which would
result in adequate hydrogeological, hydropedological and topographic maps (Bos
and Boers, 1994), among other information. Also, all subsurface water inflows
and outflows must be measured or estimated. The precipitation and relevant evapotranspiration
data from the area under investigation should be analyzed. In addition, all
relevant data on the hydraulic properties of the soil should be collected. The
above processes in drainage surveys call for a sound theoretical knowledge of
various subjects related to the field of Soil and Water Engineering. Detailed
information on field investigations required for the design and implementation
of drainage systems is given in Smedema and Rycroft (1983), Ritzema (1994), and
Michael and Ojha (2006). 

In some cases, a proper identification of
the source of ‘excess water’ can avoid the construction of a costly drainage
system. Some examples are as follows (Bos and Boers, 1994):

·        
If
irrigation water causes waterlogging, the efficiency of water use in the water-supply
system and at the field level should be studied in detail and improved.

·        
If
the surface water inflow from surrounding hills is a major cause of excess
water in an area, this water could be intercepted by a hillside drain which
diverts the water around the agricultural area.

·        
If
the problem of surplus water is caused by an inflow of saline groundwater, this
groundwater inflow could be intercepted by a series of tubewells, which can
dispose of effluent into a drain that bypasses the agricultural land.

·        
If an
area is partially inundated due to the insufficient discharge capacity of a
natural stream, a renovation of the stream may solve the drainage problem.

However, if the origin of excess water
lies in the agricultural area itself (e.g., excess rainfall or extra irrigation
water to meet the leaching requirement for salinity control), then the
installation of drainage facilities within the agricultural area should be
considered. Usually, drainage facilities consist of: (i) a drainage outlet,
(ii) a main drainage canal, (iii) some collector drains, and (iv) field drains
(also called ‘lateral drains’) as illustrated in Fig. 2.1.

Fig. 2.1. Schematic diagram of a drainage
system. (Source: Bos and Boers, 1994)

The main drainage canal is often a
canalized stream which runs through the lowest parts of the agricultural area.
It discharges its water into a river, lake, or sea by means of a pumping
station or tidal gate located at a suitable outlet point (Fig. 2.1). Main
drainage canals collect water from two or more collector drains. Although
collector drains preferably also run through local low spots, their spacing is
often influenced by the optimum size and shape of the area to be drained by a
field drainage system. However, the layout of collector drains is still
somewhat flexible because the length of field drains can be varied and
sub-collector drains can be designed. Furthermore, the length and spacing of
field or lateral drains are kept as uniform as applicable. Note that both the
collector drains and the field drains can be either open drains or pipe drains,
which are decided based on a number of factors such as topography, soil type,
farm size, and the method of field drainage.

2.3 Types
of Drainage Systems

Three most commonly used techniques for
removing (draining) excess water are: (a) surface drainage, (b) subsurface
drainage, and (c) vertical drainage (also known as ‘tubewell drainage’).
Besides these conventional drainage techniques, there is an emerging
non-conventional drainage technique known as biodrainage which is described in
Lesson 12. An introduction to the conventional drainage techniques is presented
below, and their details are provided in later lessons.

2.3.1 Surface
Drainage

Surface drainage can be defined as (ASAE,
1979): “Surface drainage is the removal of excess water from the soil surface
in time to prevent damage to crops and to keep water from ponding on the soil
surface, or, in surface drains that are crossed by farm equipment, without causing
soil erosion”. Surface drainage is a suitable technique where excess water from
rainfall or surface irrigation cannot infiltrate into the soil and move through
the soil to a drain, or cannot move freely over the soil surface to a
natural/artificial drainage channel. Surface drainage problems occur in flat or
nearly flat areas, in the areas having uneven land surfaces with depressions or
ridges preventing natural runoff, and in the areas where there is no outlet. A
detailed discussion of surface drainage technique is provided in Lesson 3.

2.3.2 Subsurface
Drainage

Subsurface drainage is defined as ‘the
removal of excess soil water in time to prevent damage to crops because of a
high water table’. Subsurface drainage problems occur in the areas having shallow
water table (e.g., canal commands), which occurs due to substantial groundwater
recharge and sluggish subsurface outflow. Subsurface field drains can be either
open ditches or pipe drains, but nowadays they are mostly pipe drains. Pipe
drains are installed underground at depths normally ranging from 1 to 3 m (Bos
and Boers, 1994). Excess groundwater enters the perforated field drains and
flows by gravity to an open or closed collector drain. A detailed discussion of
subsurface drainage technique is provided in Lesson 4.

2.3.3 Vertical
Drainage

Vertical drainage or tubewell drainage can
be defined as the ‘control of an existing or potential high water table or
artesian groundwater condition’. It is accomplished using shallow or deep
tubewells; sometimes open wells are also used. Most tubewell drainage systems
consist of a group of wells spaced with a sufficient overlap of their cones of
depression so as to control the water table at all points in an area. When
draining newly-reclaimed clay soils or peat soils, the drainage engineer has to
estimate land subsidence due to drainage of these soils, because this will
affect the drainage design (Bos and Boers, 1994). The problem of land
subsidence can also occur in the areas drained by tubewells. A detailed discussion
of vertical drainage technique is provided in Lesson 12.

Irrespective of the technique used to
drain a given area, it is evident that the drainage technique must fulfill the
local need to remove excess water. These days, the ‘need to remove the excess
water’ is strongly influenced by a concern for the environment. The design and
operation of all drainage systems must ensure sustainable agriculture in the
drained area and must minimize the pollution of rivers and lakes from
irrigation return flow or drainage effluent (Bos and Boers, 1994). The quality
of drainage effluent is generally inferior because it often contains
significant amounts of sediments, agricultural chemicals (fertilizers and
pesticides) and other contaminants. Therefore, proper disposal of drainage
effluents is a serious concern in most canal commands of the world, especially
in developing countries. 

References

ASAE (Surface Drainage Committee) (1979).
Design and Construction of Surface Drainage Systems on Farms in Humid Areas.
Engineering Practice EP 302.2, American Society of Agricultural Engineers
(ASAE), Michigan.

Bos, M.G. and Boers, Th.M. (1994). Land
Drainage: Why and How? In: H.P. Ritzema (Editor-in-Chief), Drainage Principles
and Applications, International Institute for Land Reclamation and Improvement
(ILRI), ILRI Publication 16, Wageningen, The Netherlands, pp. 23-31.

Michael, A.M. and Ojha, T.P. (2006).
Principles of Agricultural Engineering. Volume II, M/s Jain Brothers, New
Delhi, India.

Ritzema, H.P. (Editor-in-Chief) (1994).
Drainage Principles and Applications. International Institute for Land
Reclamation and Improvement (ILRI), ILRI Publication 16, Wageningen, The
Netherlands.

Smedema, L.K. and Rycroft, D.W. (1983).
Land Drainage. Batsford Academic and Education Ltd., London.

Suggested Readings

Murty, V.V.N. and Jha, M.K. (2011). Land
and Water Management Engineering. Sixth Edition, Kalyani Publishers, Ludhiana,
India.

Michael, A.M. and Ojha, T.P. (2006).
Principles of Agricultural Engineering. Volume II, M/s Jain Brothers, New
Delhi, India.

Schwab, G.O., Fangmeier, D.D., Elliot,
W.J. and Frevert, R.K. (2005). Soil and Water Conservation Engineering. Fourth
Edition, John Wiley and Sons (Asia) Pte. Ltd., Singapore.