IRRIGATION AND DRAINAGE ENG’G

Drainage Engineering a. Open ditches

➢ Ditches used for subsurface drainage
Drainage
may carry both surface and subsurface
➢ is the removal of excess water from the soil or water.
from the land surface. ➢ They have the capacity for a wide range of
➢ Its primary objective is to prevent the flow conditions because of their required
occurrence of excessive moisture conditions depth.
in the rootzone which have (either directly or ➢ Ditches are best adapted to large flat
indirectly) a harmful effect on the growth of fields where lack of grade, soil
crops and at an acceptable economic basis. characteristics, or economic conditions
➢ In arid (irrigated) areas a further objective is to does not favor buried drains.
prevent the accumulation of salts in the
The advantages in using ditches include the following:
rootzone (whenever natural conditions do not
1. They usually have lower initial cost than drains.
provide for sufficient leaching).
2. Inspection of ditches is easier than inspection of
Field Drainage System
drains.
➢ receives the excess water from the fields and
3. They are applicable in some organic soils where
conveys it to the main drainage system which
drains are not suitable due to subsidence.
evacuates the water out of the area:
a. In a subsurface or ground drainage 4. Ditches may be used on a very flat gradient where
system, the water to be removed flows the permissible depth of the outlet is not adequate to
through the soil into the drains either permit the installation of drains having the minimum
horizontally in pipes/ditches or required grade.
vertically in wells.
The disadvantage in using ditches are as follows:
b. In a surface (field) drainage system,
the excess water is evacuated by 1. Ditches require considerable rights-of-way which
flowing over the ground surface (or reduce the area of land available for cropping. This is
sometimes in part through the top soil particularly applicable in unstable soils where flat side
layer, e.g. in mole drains) to an open slopes are required.
ditch system.
2. Ditches usually require more frequent and costly
maintenance than drains.
CLASSIFICATION OF SUBSURFACE DRAINAGE b. Buried drains
➢ Drains refer to any buried type of conduit
❖ Relief drainage
with open joints or perforations which
❖ Interception drainage
collect and/or convey drainage water.
➢ Drains may be fabricated from clay,
**In planning a subsurface drainage system,
concrete, bituminized fiber, metal,
the designer must evaluate the various site
plastic, or other materials of suitable
conditions and decide whether to use relief or
quality.
interception drainage.
➢ Drains, if properly installed, require little
maintenance. They are usually preferred
1. RELIEF DRAINAGE
by landowners as they are buried, and no
➢ used to lower a high-water table which
land is removed from cultivation and
is generally flat or of very low gradient.
maintenance is considerably less than
for ditches.
IRRIGATION AND DRAINAGE ENG’G

The topography of land to be drained and the position, Parallel system direction of the major
level, and annual fluctuation of the water table are all and the desired grade of the lateral
factors to be considered in determining the proper drains is obtained by varying the angle
type of drainage system for a given site. of confluence with the main. This
pattern is used with other patterns in
Relief drainage systems are classified into four
laying out a composite pattern on small
general types: parallel, herringbone, double main,
or irregular areas.
and random.
3. Double – main system
General types of Relief Drainage Systems: ➢ The double-main system is a
modification of the herringbone system
1. Parallel system and is applicable where a depression,
➢ The parallel system consists of parallel which is frequently a natural
lateral drains located perpendicular to watercourse, divides the field to be
the main drain. The laterals in the drained. Occasionally the depressional
system may be spaced at any interval area may be wet because of seepage
consistent with site conditions. This coming from the higher ground. Placing
system is used on flat, regularly shaped a main drain on each side of the
fields and on soils of uniform depression serves a dual purpose; it
permeability. Variations of the parallel intercepts the ground water moving to
system are often used with other the natural watercourse and provides
patterns. an outlet for the lateral drains.

2. Herringbone system
4. Random system
➢ The herringbone system consists of
➢ A random system of drains is used
parallel lateral drains that enter the
where the topography is undulating or
main drain at an angle from either or
rolling and contains scattered isolated
both sides. This system usually is used
wet areas. The main drain, for
where the main or submain drain lies in
efficiency, is usually placed in the
a depression. It may also be used
swales rather than in deep cuts through
where the main drain is located in the
ridges. If the individual wet areas are
Herringbone system Double main large,
the arrangement of submain and lateral
drains for each area may utilize the
parallel or herringbone pattern to
provide the required drainage.
IRRIGATION AND DRAINAGE ENG’G

permeability of soil and subsoil materials is very

important. Design involves anticipating what the
shape and configuration of the cone of depression
will be after pumping. This, in turn, involves spacing
of wells to position their areas of influence properly
and obtain the desired drawdown over the area to
be drained. Usually, it is desirable to install test
wells to determine the drawdown and spacing of
wells. Consultation with a geologist is desirable.

Experience with this type of drainage

installation indicates that, in general, pumping
from wells is costly and it is difficult to obtain a
satisfactory benefit-cost ratio. Consideration for
this type of facility should be limited to high-
Pumping system (ground water removal) producing lands with a high return value per acre.

➢ This type of removal applies to deep well Combination system

drainage where the drawdown is extensive and
➢ Combination systems or dual-purpose systems are
does not include shallow water-table control
names that have been given drainage systems that
such as obtained by pumping muck or tide water
provide both surface and subsurface drainage.
areas.
➢ In this type of system any combination of open
**The objective of all subsurface drainage work ditches and buried drains can be used. It is a
is to lower and maintain the water table at some common practice to use drainage field ditches for
level suitable for proper crop growth. This is surface collectors, drains for subsurface
usually accomplished by the installation of collectors, and ditch-type drainage mains and
relatively deep subsurface drains. Water-table laterals for disposal.
levels may also be controlled by pumping from the ➢ In soils of high permeability such as Indiana and
groundwater reservoir to lower and maintain the Michigan sands and some coastal plain soils, a
desired water-table level. field-border ditch for surface water collection is all
that may be needed. Drop structures are required
In some irrigated areas where irrigation water
where surface collectors discharge into deep open
is obtained from wells, the practice of irrigation and
ditches. Buried drains are seldom used to collect or
drainage both may be effected by the pumping of
dispose of surface water. The reasons for this are:
wells. This combination practice is limited to those
(a) surface waters usually carry debris which may
areas with low salinity where it is possible to
lodge in the drain and cause a plug to form, and
maintain a proper salt balance. In salty areas where
(b) surface flows are subject to large variations
pumping is used to effect drainage and where the
which dictate a large and expensive drain.
quality of the drain water is poor, the drain water
usually is discharged into a drainage outlet and not Mole drains
directly reused for irrigation. In some cases it is
➢ Mole drains are unlined, approximately egg-shaped
possible to mix the drain effluent with water of high
earthen channels, formed in highly cohesive or
quality and thereby obtain water suitable for
fibrous soil by a moling plow.
irrigation.
➢ The moling plow has a long blade-like coulter to
The investigations necessary for planning a which is attached a cylindrical bullet-nosed plug,
drainage facility, using pumps to lower the water- known as the mole.
table level, can be quite complex. Detailed ➢ As the plow is drawn through the soil, the mole
information on the geologic conditions and the forms the cavity, at a set depth, parallel to the
IRRIGATION AND DRAINAGE ENG’G

ground surface over which the plow is drawn. vertical drains that seal-up and become
Heaving and fracturing of mineral soil by the coulter ineffective in a relatively short period of time.
and mole leave fissures and cracks which open ➢ Drainage water that is discharged into
toward the mole and coulter slit. These provide underground aquifers usually contains
escape routes through the soil profile and into the pollutants in solution, in addition to the
mole cavity for water trapped at the surface or sediment and debris mentioned above. These
water that has percolated into the soil. pollutants may percolate into other aquifers or
➢ Mole drains, when properly installed in locations areas where wells are used for a domestic
with soils suitable for them, provide drainage for 3 supply. For this reason there is danger of
to 5 years and may, with diminishing effectiveness, contaminating water supplies and most states
provide drainage for as much as 5 years longer. working with the Public Health Service have
➢ Cultivation of moled lands with heavy equipment enacted laws controlling this practice. Some
reduces the effective life of such drains. states forbid the use of drainage wells and
others require that a permit be obtained.
Vertical drains (drainage wells)

➢ have been used as outlet for both surface and 2. INTERCEPTION DRAINAGE
subsurface drains. They have been used where ➢ is to intercept, reduce the flow, and
gravity outlets were not available or where the lower the flowline of the water in the
cost of obtaining gravity outlets was prohibitive. problem area.
➢ Vertical drains must penetrate a suitable ➢ Interception drains may be either open
aquifer which can absorb the drainage flow. ditches or buried drains. Proper
Investigations for vertical drains must be in location of either type is very
sufficient detail to determine that such an important.
aquifer is present and that it can absorb the ➢ The location and depth required
expected drainage discharge for an indefinite usually are determined through
period. This requires a geologic determination extensive borings and ground-water
made in conjunction with a geologist. It is studies.
usually necessary to make a test boring or a. Open ditches
borings to determine the magnitude, thickness, ➢ The ditch type interceptor may serve
depth, and extent of the aquifer in question. to collect both surface and ground-
Laboratory work may be required to determine water flow. It must have sufficient
the physical and chemical properties of the depth to intercept the ground-water
aquifer material. flow. Such ditches usually have
➢ Vertical drains are wells in which the direction excess capacity at the required depth.
of the flow is reversed. Most of the design The interception ditch is frequently
principles and criteria applicable to water wells used to intercept the surface and
are applicable to vertical drains. The major ground-water flow at the base of a
difference is that relatively clean ground water slope.
is pumped from water wells; whereas,
drainage water discharged into vertical b. Buried drains
drains may contain significant quantities of ➢ Peculiar or unusual subsurface
salt, sediment, and debris. Unless these formations or ground-water
pollutants are removed from the drainage conditions may be responsible for a
effluent before it enters the vertical drain, they high-water table in certain local areas.
tend to plug and seal the drain. Service Likewise, abrupt changes in
experience with vertical drains has been topographic features may cause
disappointing because of the large percent of certain areas to be subject to a high-
IRRIGATION AND DRAINAGE ENG’G

water table. These situations are that it can be crossed with farm
difficult to describe. machinery, if used.

2. Random-ditch system
➢ This system is adapted to areas that have
DRAINAGE METHODS
depressions which are too deep or too
Excess water can be removed from agricultural lands large to fill by land leveling.
by using surface or subsurface drainage systems, or ➢ The surface-drainage ditches may
combination of the two. meander from one low spot to another,
collecting the water and carrying it to an
**For most Asian regions, the use of the surface
outlet ditch.
drainage system is considered feasible and most
➢ Drainage in these areas is improved if the
economical.
entire field is leveled or graded to remove
Types of Surface Field-Drainage Systems: minor depressions and allow the surface
water to flow to the ditches.
❖ the bedding system
➢ In constructing the random ditch,
❖ the random-ditch system
excavations from the ditches can be
❖ the interception system
placed in minor depressions that cannot
❖ the diversionditch system
be drained by these ditches. The ditch
❖ the field- ditch system
must be of sufficient size and depth to
**In addition, pumps are used in many areas to drain off the impounded water rapidly and
transport excess ponded water. completely. The outlet ditch should be 15
cm to 30 cm deeper than the random field
1. The bedding system ditches.
➢ In this system, soil type which determines ➢ On flat, very slow permeable soils, it may
the degree of internal drainage largely be necessary to combine this system
influences the width of the bed to be used. with the bedding system to do an
The following guidelines for determining adequate job of surface drainage. A
the width of three types of soil drainage subsurface system in conjunction with
conditions are shown in Table 21. surface drainage system may also be
considered in a soil which will adapt to
Internal Drainage of the Soil Width of Bed (m) subsurface drainage. If machinery is to be
Very slow 7-12 considered, ditches, ditches should be
Slow 14-16 constructed with sufficient side slopes
Fair 18-28 so that farm machinery can cross them
Table 21. Relative internal drainage of the soil and width of bed easily.
between center to center of dead furrows. 3. The interception system
➢ Also known as cross-slope system,
➢ The furrows drain to collection ditches.
this resembles terracing in that the
Planting, seeding, and cultivating can be
drainage ditches are constructed around
done in either direction. The dead furrows
the slope on a uniform grade according to
must have a continuous grade without any
the land topography.
obstructions that might interfere with the
➢ The interception system is best adapted
flow of water. Outlet ditches should be at
to sloping wet field with a 4% slope and
least 15 cm to 30 cm deeper than
where many shallow depressions hold
collection ditches. This will provide
the water after the rain. The soil profile is
complete drainage of the collection so
such that the collected water cannot
infiltrate into soil. The depressions are
IRRIGATION AND DRAINAGE ENG’G

too numerous and slope too great for between ditches. All farming operations
success bedding and subsurface drains should be perpendicular to the ditches.
to be practical and feasible. 5. Field-ditch system
➢ The cross-slope ditch or terrace should ➢ Field-ditch systems are suitable for
be constructed across the slope as sandy soils, mineral soils, and organic
straight and parallel as the topography soils like peat and muck. Maximum
permits. There should be limited cutting recommended spacing of the parallel
through the ridges and humps. The filed-ditches are 200 m for sands, 100
spacing between ditches should be m for mineral soils (except sand), and
about 30 m on a 4% slope and 45 m on a 60 m for peat and muck soils.
5% slope. The ditches are built with little Recommended cross sections for
or no ridge on the down-slope side of the each kind of soil are also different.
ditch. This reduces damage caused by ➢ Spoils from the ditches should be
overflow and makes it easy for farm spread in depressions over the field
machinery to cross ditches. uniformly. All humps and back furrows
➢ The excavated spoil material from the should be removed by land smoothing
ditches can be placed in the depression ar grading so that surface water can
areas between the ditches. Any flow to ditches unobstructed,
excavated material not used in this depressions too large to be filled
operation should be spread out on the should be drained separately to the
down-hill side of the ditch. The ridge field ditch by a shallow surface ditch.
should not be over 1.2 cm above the All farming operations should be
natural ground surface. The area carried out parallel to the field ditches.
between ditches should be graded to
eliminate all minor depressions or
humps. In the layout of any drainage system, the following
➢ The success of this type of system general rules should be observed as much as
depends upon the elimination of the possible:
depressions between the ditches. All
1. Place the main outlet at the best possible
farming operations should be parallel to location.
the ditches.
2. Provide as few outlets as possible.
4. Diversion-ditch system
3. Use short main ditches and long lateral
➢ Also known as the parallel ditch
ditches.
system, this is suitable on flat, poorly 4. Use available slopes to the best advantage.
drained soils that have numerous 5. The main ditch should follow the general
shallow depressions. In general, the
direction of natural waterways.
ditches are 185 m to a maximum of 6. Avoid locations that result in excessive cuts.
370 m apart (not necessarily 7. Avoid crossing waterways wherever possible.
equidistant); and the land between the If waterways must be crossed, use as near a
ditches is sloped and smoothed to
right-angle crossing as the situation will
eliminate any minor depressions or permit.
obstructions to the overland flow of
8. Where possible, avoid soil conditions that
the water. increase installation and maintenance cost.
➢ The outlet ditch should be 0.30 m deeper
than the parallel ditches. A minimum
grade of 0.5% should be established
IRRIGATION AND DRAINAGE ENG’G

Summary of Limitations (so that the method which is In irrigated areas where there is insufficient
most nearly applicable may be applied) experience to establish acceptable drainage
coefficient for general use, they can be computed
1. Horizontal flow theory (ellipse equation)
from the following formula based on irrigation
➢ used where the flow is largely
application.
horizontal as for shallow drains
compared to their spacing with all 𝑃+𝐶
( 100 )𝑖
impermeable layers at or close to the 𝑞=
24𝐹
bottom of the drain.
2. Radial flow theory
➢ applied to homogeneous isotopic soil where:
of great depth, with flat or nearly flat
𝑞 = drainage coefficient, iph
water table.
3. Combined horizontal and radial theories 𝑝 = deep percolation from irrigation including leaching
(as Hooghoudt) requirement, percent (based on consumptive-use
➢ used for situations where the studies)
impermeable layer is either shallow or
𝑐 = field canal losses, percent
deep by using Hooghoudt’s “
equivalent depth” (de) or the 𝑖 = irrigation application, inches
nomograph published by Visser.
4. Van Deemter’s hodograph analysis 𝑓 = frequency of irrigation, days
➢ applicable only to the drains running
just full or to problems where the
water table stands immediately above Drainage coefficient for subsurface drainage:
drains.
➢ related to the source of the excess H2O, to the
Design criteria: rate of flow of the excess H2O through the soil
and to the tolerance of crops in the cropping
a) The rate of water removal necessary to provide system to excess H2O.
a certain degree of crop protection. ➢ usually, are much smaller than surface
b) Optimum depth to the water table. drainage coefficient (because of the slower
rate of flow through soils as compared to that
DRAINAGE COEFFICIENT on overland).

➢ rate of water removal Drainage coefficient for pumping plants:

➢ may be expressed as a certain depth of water ➢ considers the characteristics of flow to the
to be removed from the watershed per day, or pumping plant (whether surface or
as a rate of flow per unit area as ft.3 per second subsurface) in determining pump capacity.
per square mile.
Drainage coefficient for watershed protection:
Drainage coefficient for surface drainage:
➢ considers the whole watershed (conditions for
➢ should consider the characteristics of plant growth protection against excess
precipitation in the area as well as other surface water, control of moisture content).
climatic factors, topography, crop tolerance to
excess H2O, soils, and irrigation. Depth to water table:

➢ considers the quality of water so that if it is free

from salts, the water table needs only to be as
deep as required to provide sufficient root
IRRIGATION AND DRAINAGE ENG’G

zone depth for supports of plants to be grown

and to support tillage equipment.
➢ approximately about (1) ft. more than the
depth of root penetration desired.
➢ Where salts are present, it must be deep
enough to prevent capillary flow from bringing
dissolved salts up to the root zone. ▪ This equation neglects the curvilinear flow due
to the drawdown shape.
PUMPED-WELL DRAINAGE ▪ The error is not large if r1 and r2 are sufficiently
➢ Versatile and may have economic advantage large that the curvature is negligible.
over other methods of lowering and ▪ Used to predict drawdown curve and radius of
maintaining a desirable water table level. effective influence.

Basic Principles: 2. Confined aquifers or artesian wells

1. A pump well, like any other forms of artificial ➢ remove water from a fully saturated aquifer
drainage, increases the flow energy gradient which is confined by impermeable layers.
by creating a sink within a saturated zone. ➢ Dupuit equation for confined aquifers:
2. Energy which the well makes available to the
ground water flow system is derived from the
motor which lifts the water from the sink.
3. The increased gradient must extend to the
crop root zone to such degree as to control the
water table within the desired area and to the
desired level. Basis for Design of Pumped-Drainage Wells:
4. The increased energy gradient may be in the
form of a drawdown, i.e. water table slope 1. Capacity should be sufficient to lower the
towards the well; or it may be in the form of a water table after irrigation, heavy
pressure gradient where the groundwater is precipitation, or other influent seepage, in a
confined. In either case, at a given point in a relatively short time to avoid crop damage.
saturated zone, the quantity. P2 is decreased 2. Capacity should be sufficient to remove at
by the expression. least the seasonal net replenishment, which
is the groundwater replenishment less
depletions from causes other than the
pumped well in question.

Thus, increasing the hydraulic gradient towards Advantages of Pumped-Well Drainage:

the well. 1. The water table may be lowered to much
greater depths.
2. Deep strata may be much more permeable
WATER TABLE than those nearer the surface.
3. Productive land which would be occupied by
1. Water table wells
open drains is saved.
➢ remove water directly from the free
4. Maintenance costs are less than for open
groundwater, creating a drawdown surface in
drains and may be less than for closed
the water table - the approximations for the
ditches.
Dupuit are the basis for the equation.
5. Pumped-well may be a valuable supplement
to the irrigation water supply.
IRRIGATION AND DRAINAGE ENG’G

LAND FORMING 2. Profile Method

➢ ground profiles are plotted, and a
➢ mechanical changing of the land surface to
grade is established that will provide
drain H2O.
an approximate balance between cuts
➢ may be done by smoothing, grading, bedding,
and fills as well as reduce haul
or leveling.
distances to reasonable limits.
a. Land smoothing
3. Plan-Inspection Method
➢ eliminates minor differences in field
➢ The grid point elevations are recorded
elevation including shallow
on the plan and design grade elevation
depressions.
are determined by inspection after a
➢ permits more efficient operation of
careful study of the topography.
farm equipment, reduces the costs of
4. Contour Adjustment Method
ditch maintenance.
➢ a contour map is down, and the
➢ soils to be smoothed must have a
proposed ground surface is shown on
profile which will allow small cuts
the same map by drawing new contour
without exposing layers that will
lines.
hinder equipment operation or plant
➢ the proper balance between cuts and
growth.
fills are estimated graphically.
b. Land grading
➢ consists of shaping the land surface
by cutting.
Subsurface Drainage Design:
➢ does not require shaping of the land
into plane surfaces with uniform Benefits:
slope.
➢ emphasis in planning is given in filling 1. Aeration of the soil for maximum
depression with soil from adjoining development of plant roots and desirable soil
ridges or mounds. microorganisms
2. Increased length of growing season because
Factors influencing design: of earlier possible planting dates.
3. Decreased possibility of adversity affecting
▪ Slopes, cuts, and fills are influenced by soil,
soil tilth through tillage at excessive moisture
topography, climate, crops to be grown,
levels.
methods of irrigation and drainage.
4. Improvement of soil moisture conditions in
▪ For irrigation purposes, the largest floor
relation to the operation of tillage, planting
occurs at the upper end of the slope, while for
and harvesting machines.
the drainage, rainfall enters along the entire
5. Removal of toxic substances such as salts,
slope length with the highest runoff at the
that in some soils retard plant growth.
lower end.
6. Greater storage capacity for water, resulting in
Methods: less runoff and a lower initial water table
following rains.
1. Plane Method
7. Enhances farm productivity by:
➢ assumes that the area is to be graded
a. Adding productive acres without extending
to a true plane. The average elevation
from boundaries.
is determined, and this elevation is
b. Increasing yield and quality of crops.
assigned to the centroid of the area.
c. Permitting good soil management.
d. Assuring that crops are planted and
harvested at optimum dates.
IRRIGATION AND DRAINAGE ENG’G

e. Eliminating inefficient machine operation Definition of the Drainage Problem:

caused by net areas.
In order that a reliable solution to the drainage
problem can be explored, a thorough field
investigation should first be undertaken which among
Drainage Requirements and Plant Growth:
other things include the following:
➢ determined largely by the volume and content
1. The water table
of the soil and air.
➢ the upper limit of a waterlogged
➢ these needs depend on the type of crop, the
condition - it can be determined by
soil, availability of plant nutrients, climatic
digging a hole in the soil and observing
conditions, biological activity and soil and
the height to which the hole fills with
crop management practices.
water.
** *Rooting depth and tolerance to excess H2O are the
Factors affecting the water table height:
most important crop characteristics.
a. Irrigation water and/or rainfall
**soil and air temperatures affect O2 diffusion rates as
➢ For water to be added to the soil
well as soil organisms and the biological processes in
surface to affect the water table
the plant.
height, it must first percolate into the
water-logged zone.
b. Lateral seepage from canals and rivers
SUBSURFACE DRAINS ➢ If the water table responds to changes
a. Pipe Drains in river stages or to changes in water
➢ include concrete and burned clay tile, levels in canals, then seepage from
corrugated plastic tubing or other the channels is contributing to the
perforated conduit. drainage problem.
➢ corrugated steel pipe with high structural c. Upward seepage
strength is suitable to withstand high soil ➢ Seepage from shallow artesian layers
loads to cross unstable soils that require contributes to the rising of the water
the rigidity of a long pipe and to provide a table. However, if plants are growing in
stable outlet into open ditches. the area, the water table fluctuates
b. Mole Drains daily during the cropping season and
➢ cylindrical channels artificially produced builds up only when the crop is
in the subsoil without digging a trench harvested.
from the surface. ➢ During the day, evapotranspiration
➢ like pipe drains except that they are not causes the water table to decline
lined with tile or other stabilizing since the rate of evapotranspiration
material. exceeds the upward seepage.
➢ temporary method of drainage. - fail d. Lateral seepage from adjacent areas
because the soil is not sufficiently stable ➢ On sloping, rolling land, the seepage
to maintain a channel. from higher elevation will affect the
• depth varies from 0.5-1.2m water table downslope.
• spacing ranges from 1.0-10m e. Deep seepage
• length is usually < 500m ➢ The perched water table are caused by
• grade ranges at about 5% a shallow layer of soil having a low
hydraulic conductivity. If slow
downward seepage through less
permeable layers is not sufficient to
IRRIGATION AND DRAINAGE ENG’G

provide drainage for farming NOTE!!

operations, then use artificial
The designer may need to present to the landowner
methods.
alternate methods or intensities of drainage, so the
f. Evapotranspiration
owner may make the final decision.
➢ For this to happen, the soil water in the
surface layers must be favorable for Types of Drainage Problems:
the germination of the plants and the
development of a root system. 1. Surface Drainage Problems
g. Evaporation and lateral seepage outside farm ➢ Flat and nearly flat areas of land are
boundaries subject to ponded water caused by:
a. Uneven land surface with pockets and
ridges to prevent or retard natural
runoff. Slowly permeable soils
2. Artesian conditions
magnify the problem.
➢ Artesian water is one which is
b. Low-cap disposal channels within the
confined under pressure in an aquifer.
area which remove H2O so slowly that
➢ if the aquifer is perched with a pipe,
the high water level in the channels
water will rise in the pipe even
causes ponding on the land for
probably above ground surface.
damaging periods.
➢ the purpose of the piezometer pipe is
c. Outlet conditions which hold the
to measure the soil water pressure
water surface above ground level,
which is represented by the elevation
such as high lake or pond stages, or
of the water in the pipe.
tide water elevation.
3. Soil information
➢ it is essential to have complete soil **Surface drainage methods such as land grading or
survey made of the area if drainage smoothing and field ditches are used on fields to
design constitute a large project. collect and convey surface water to natural channels
or constructed disposal systems.
**For reconnaissance purposes, scale
should be: 2. Subsurface Drainage Problems
1:50,000 Types:
**For adequate design purposes: a. Basin-type-free water table
b. Water table over an artesian
1:5,000
➢ groundwater may be confined in an
The following should be indicated in soil aquifer so that its pressure surface
maps: (elevation to which it would rise in a
well tapping the aquifer) is higher than
1. Soil texture as determined in the field
the adjacent free-water table. - the
2. Depth to the permeable layer
present surface may or may not be
3. Waterlogged conditions
higher than the ground surface (such
4. Outflows from drainage systems
ground water is termed artesian).
5. Topography
c. Perched water table
Elements of Drainage Design: ➢ subsurface drainage problem may be
caused where excess water in the
1. Crop requirements normal root zone is held up by layer of
2. Site investigation low permeability so that the perched
3. Design criteria water is disconnected from the main
4. Plans and specifications body of the groundwater.
IRRIGATION AND DRAINAGE ENG’G

➢ lateral percolation is too slow to drain 4. High water table limits root penetration.
the perched water table naturally. ➢ on the other hand, the water table
should not be lowered so far that a
**Drainage systems for perched water tables are
severe water deficiency occurs and a
based on conditions:
large amount of irrigation water is
▪ usually, they consist of relief drains (but an required.
interception drain may be effective in cutting
Two limiting depths of the water table to be
off lateral seepage into the wet area).
considered:
▪ theoretically, perched water could be drained
downward by drilling vertical drains (wells) a. upper limit dictated by the aeration
through the restrictive layer (this might be demanded by roots.
impractical outlets for economic of other b. lower limit that will supply adequate
reasons). water to the roots so as to prevent
▪ may be subject to control by reducing death or reduced plant production.
seepage from canals, by improving irrigation 5. Soil structure is adversely affected.
practices, or by providing adequate surface 6. Salts and alkali, if present in the soil or
drainage. groundwater, tend to be concentrated in the
d. Lateral groundwater flow problems rootzone or at the soil surface.
➢ characterized by more or less 7. Wet spots in the field delay farm operation or
horizontal groundwater percolation prevent uniform treatment.
within or toward the crop-root-zone.
Drainage Requirements Determined by Crops:
➢ the flow pattern is strongly influenced
by soil stratification and other natural 1. Maximum duration and frequency of surface
barriers to flow (eg. Subsurface soil of ponding
low permeability like clay tenses, clay 2. Maximum height of water table
bars, formed in geologic past). 3. Minimum rate at which the water table must
➢ the depth, orientation and inclination be lowered
of the strata determine the drainage
method and location. Classification of Drainage Theories by Basic
Assumptions:

1. Horizontal Flow Theories

CROP REQUIREMENTS Assumptions:
a. that all streamlines in a gravity flow system
Effects of excess water on crops:
are horizontal
Poorly drained soils depress crop production in b. that the velocity along these streamlines is
several ways: proportional to the slope of the free-water
surface, but independent of depth
1. Evaporation, which takes heat from the soil,
lowers soil temperature. Also, wet soils Restrictions to which these assumptions
requires more heat to warm-up than dry soil apply:
due to the high spec. heat of water as
a. Open ditches that are shallow compared
compared to that of soil. Thus, the growing
to their spacing and that can penetrate to
season is shortened.
or are close to an impermeable layer.
2. Saturation or surface ponding stops air
b. Open ditches that are excavated in
circulation in the soil prevents bacterial
stratified materials.
activity.
3. Certain plant diseases and parasites are
encouraged.
IRRIGATION AND DRAINAGE ENG’G

c. Buried drains under conditions a and b ➢ applies only to the running full (tile
particularly if the backfilled trench is more drainage)
permeable than the undisturbed material.
Design Criteria:
**One expression of the horizontal flow theory is the
a. The rate of water removal necessary to provide
ellipse equation of which the tile-spacing formula
a certain degree of crop protection.
developed by Donnan is one form.
b. Optimum depth to the water table

*The rate of water removal, often referred to as the

drainage coefficient may be expressed as a certain
depth of water to be removed from the watershed per
day, or as a rate of flow per unit area as cubic feet
second per square mile.

Drainage Coefficient (Cypress Cree Formula)

5⁄
2. Radial Flow Theories 𝑄 = 35 𝑀 6

Assumptions: 𝑄 = rate of RO at any pt. in a system from the drainage

➢ a tile drain may be thought of as a area above the pt. (ft3 /sec)
horizontal well, with water
approaching the tile along radial 𝑀 = drainage area (miles)
streamlines.
35 = RO coefficient
a. a homogeneous isotopic soil of infinite
depth Drainage Coefficient for Surface Drainage
b. a flat water table
➢ should consider the characteristics of
➢ this method can give a good
precipitation in the area as well as other
approximation of actual flow
climatic factors, topography, crop tolerance to
conditions if the curvature of the water
excess H2O, soils, and irrigation.
table is small (as with a low rainfall rate
and relatively high permeability) and if *In irrigated areas, where there is insufficient
in the drain there is no layer of greatly experience to establish acceptable drainage
reduced permeability. coefficient for general use, they can be computed
3. Combined Horizontal and Radial Flow from the following formula based on irrigation
Theories application.
➢ Hooghoudt and Ernst have developed
𝑃+𝐶
solutions of the flow problem by ( 100 )𝑖
combining the radial and horizontal 𝑞=
24𝐹
flow hypothesis.
➢ these solutions correct the major where:
shortcoming of the ellipse equation 𝑞 = drainage coefficient, iph
(neglect of convergence of flow near
the drain). 𝑝 = deep percolation from irrigation including leaching
4. Van Deemter’s hodograph analysis requirement, percent (based on consumptive-use
➢ mathematical analysis involving the studies)
solution of certain differential 𝑐 = field canal losses, percent
equations to satisfy the boundary
conditions. 𝑖 = irrigation application, inches

𝑓 = frequency of irrigation, days

IRRIGATION AND DRAINAGE ENG’G

Drain spacing: at midpoint between the drains, x = L/2 and y = H.

Substituting these values in equation (3) yields,

but 𝑄 can be interpreted as 𝐿𝑞 (𝑞 = drain discharge

rate per unit surface area)
4𝐾 4𝐾
 𝐿 = 𝐿𝑞
(𝐻 2 − 𝐷 2 )  𝐿2 = 𝑞
(𝐻 2 − 𝐷2)
Assumption: Constant rate of rainfall is removed
Or
equally well at all distances from the drain.
4𝐾(𝐻 2 −𝐷 2 )
Then, (1) 𝑞= 𝐿2
→ Ellipse Eq’n

But

(𝐻 2 − 𝐷 2 ) = (𝐻 + 𝐷)(𝐻 − 𝐷)
let ℎ = (𝐻 − 𝐷) 𝑎𝑛𝑑 (2𝐷 + ℎ) = (𝐻 + 𝐷)

therefore,
4𝐾(ℎ)(2𝐷+ℎ) 8𝐾𝐷ℎ+4𝐾ℎ 2
𝑞= 𝐿2
= 𝐿2
Hooghoudt equation

Where,
From Darcy’s law and Dupuit assumption that the
velocity is proportional to the water table slope, (2) 8𝐾𝐷ℎ
→flow below drain and
4𝐾ℎ 2
→ flow above drain
𝐿2 𝐿2
𝑑𝑦
𝑞𝑥 = − 𝑦𝑉𝑋 = 𝐾𝑦
𝑑𝑥
Where: Hooghoudt’s Equivalent depth “d”

𝑉𝑥 = velocity at x Recall: if horizontal flow above drain level is neglected

and q = R (rate of recharge)
𝐾 = hydraulic conductivity

(1) = (2)
IRRIGATION AND DRAINAGE ENG’G

*Instead of working with the (2) equations above,

Hooghoudt considered it more practical to have a
similar formula, but considering for the extra
resistance caused by the radial flow by introducing a
reduction of depth D to a smaller equivalent depth d.

*Hence, the flow pattern is replaced by a model with

horizontal flow only:

Problem:

For the drainage of an irrigated area, drain pipes with

a radius of 0.1 m will be used. They will be placed at a
depth of 1.8 m below the soil surface. A relatively
impermeable soil layer was found at a depth of 6.8 m
below the soil surface. From auger hole tests, the
hydraulic conductivity above this layer was estimated
at 0.8 m/day .

Suppose that irrigation is applied approximately once

in (20) days. The average irrigation losses, which
recharge the already high groundwater table amount NOTE!!
to 40 mm per 20 days so that the average discharge of
In the equation 𝐿2= 1920𝑑 + 576, the term 576 is
the drainage system amounts to 2 mm/day. comparatively small.
What drain spacing must be applied when an average Neglecting it, one obtains
water-table depth of 1.2 m below the soil surface is to
be maintained? 𝐿2= 1920𝑑  𝐿=83m (which is close to 87m)

The same problem can, however, be solved using the

equation for the equivalent depth “de” in the absence
of tables and still, the same spacing could be
obtained.