Drainage Engineering - פל-יל...
Transcript of Drainage Engineering - פל-יל...
P a g e | 1
1. Page | 1
Tahal engineering ltd.e
08
Drainage Engineering Eng. Y. Levy y/l/may 2008
P a g e | 2
Drainage Engineering
1. LAND DRAINAGE:
Optimum agricultural production essentially requires artificial
drainage of most of the soils having high water table, high salt
content, low lying soils and soils at the lower ends of the irrigated
fields.
There are areas where irrigation may not be required during monsoon
season though drainage is necessary. Surplus, water is as harmful to
crops as inadequate water.
The excess water in the field due to over-irrigation or seepage from an
adjacent channel has to be removed to help crop growth. Drainage is
also required in delta areas when irrigation is extended there. The
drainage requirement is determined by the excess moisture on
aeration, soil temperature, structural stability of the soil, soil
chemistry, biological activity and the overall problems of land and
crop management.
Efficient soil and water management can lessen or at times avoid the
need for artificial drainage.
In canal irrigated areas, the implementation of drainage needs to be
taken up simultaneously with the irrigation development so as to
avoid the problems of water logging, and/or salinity development at a
later stage. Thus the drainage as a means of disposal of excess water
is necessitated due to various factor such as i) Water accumulation on
the land surface usually resulting from heavy precipitation and/or
river bank spill combined with the deficiency of drainage capacity,
ii) Excess water on the land surface resulting from water logging or
stagnation in depressions and low areas, iii) Seepage water from
P a g e | 3
canals, tanks and subsoil flow from higher ground. Iv) Rise of sub-
surface water due to excessive irrigation and percolation from other
sources, v) Water stagnation due to obstruction in the natural flow of
drainage such as constr4uction of road, railway line, canal, etc. From
pljant growth standpoint drainage is essential because of the adverse
effects of the excess moisture in the soil such as (i) Damage to roots,
saturated soil results in the stunted crops with yellow leaves. The
plants die if the excess water remains for some time because of
damage to roots caused by reduced supplies of oxygen and
accumulation of carbon dioxide, (ii) Poor aeration. Poor aeration also
results in accumulation of sufficiently high concentrations of reduced
iron and manganese which becomes toxic to the crops, (iii) Reduced
soil temperature. Saturated soil is slow to warm up under a give
amount of heat input. Low soil temperature restricts root
development, depresses biotic activity in the soil resulting in lowered
rate of production of available nitrogen, (iv) Denitrification.
Denitrification occurs because of the competition for nitrogen by the
soil microorganisms that thrive in saturated soil and reduction in
numbers of nitrifying organisms due to lack of aeration, (v) Reduced
uptake of plant nutrients. High moisture level in the soil results in
reduced uptake of plant nutrients because of limited root growth
which restricts the volume of soil from which the plant may draw
nutrients, (vi) Difficulty in tillage operation. The tilling and harvest
with machinery becomes difficult and costlier.
The functional classification of drainage is as under:
1. Surface Drainage: It is the removal of excess water from the
surface of land by providing drainage channel in the area.
2. Subsurface Drainage: It is the removal or control of ground water
and removal or control of salts by means of water
P a g e | 4
2. BENEFITS OF DRAINAGE:
The benefits of drainage are 1) Improvement of the soil structure and
increase in productivity of the soil, 2) Help tillage operations due to
increased soil tilth, 3) Lengthening of growing season, 4) Facilitates
early ploughing and sowing of the crops. Crop period is thus
increased resulting in higher crop yield, 5) More soil moisture is
made available for crop growth due to extension of crop root zone into
the soil, there by ensuring vigorous plant growth, 6) Maintains proper
aeration of upper soil layers, 7) Maintains higher soil temperature.
The soil is kept warmer, 8) Reclamation of water logged lands.
Harmful salts are leached off, 9) Maintenance of water table at a
reasonable depth so that water cannot rise above the natural ground
by capillary action, 10) Improvement in sanitary conditions of the
area, malaria and weed control, and 11) Larger varieties of crops can
be grown.
3. ESSENTIAL REQUIREMENTS OF A DRAIN
The essential requi8remnts to be satisfied by a drain are (i) Admit all
the flood discharge from the catchment, (ii) Quick and unobstructed
flow towards the drain from the catchment, (iii) Capacity to carry
away the received water to the outfall, (iv) Ideal outfall conditions, (v)
Stable section with non- silting tendency and capable of avoiding
sloughing of side slopes, (vi) Seepage and/or low discharge does not
spread thin over the entire section, (vii) Low maintenance cost, and
(viii) Low initial cost.
The drainage system of an area is just the reverse of the irrigation
system; the drainage collects water through small drains and outfall
into major drains and ultimately into a river or the sea.
In irrigated areas, the drainage required is of three types;
P a g e | 5
Type I. Are with water table between 0 to 1.5 m. Drain is required
2.5 to 3.0 m. deep. Natural drainage may be deepened so that 0.5 to
0.6 m depth of drain is in pervious strata.
Type II. Area with water table between 1.5 to 3 m and is likely to
rise. Drain is required along the canal to the limit of 1.25 to 1.75 m or
up to the pervious strata which ever is less.
Type III. Water table is below 3 m. Artificial drains are not required.
The natural drainage may be trained to the depth of 1.5 to 1.75 m or
up to the pervious strata.
Drains cater for storm water and seepage water. It is advantageous if
the seepage water collected in the drain is pumped for irrigation in
lower down reaches.
4. CLASSIFICATION OF DRAINS
The drains may be open drains or closed drains with further
classification, as under;
Classification of drains
According to construction According to function
Natural Drains Artificial Drains Open Drains
Surface Seepage Surface-cum- Mole Link Field
Drains Drains seepage drains drains Drains Drains
Classification According to Construction
Natural Drains: These are the lowest valley line between two ridges
Artificial Drains: These are the constructed drains generally aligned
along drainage line, sometimes taken across the valley to reduce length
of the drain or to have proper outfall conditions.
Subsurface Drains
Closed Drains
P a g e | 6
5. CLASSIFICATION ACCORDING TO FUNCTION SERVED
1. Open Drains:
i) Surface Drains, Surface drains are normally used for the
removal of excess surface irrigation water or for the disposal of
storm water. They remove water before it has entered the soil.
Whether constructed for the purpose or not, deep surface
drains with bed level below water table also cater for seepage
water.
ii) Seepage Drains. Seepage drains cater for the subsoil water.
They are made deep enough to allow water table to drop in the
drain and seepage water is carried away. They are of smaller
section compared to surface drains. They help maintain
aeration of root zone depths. Usually these are constructed
along canal bank to drain directly into a natural outfall or into a
carrier drain.
iii) Surface-cum-seepage drains. They serve the dual purpose of
seepage and storm water drain. During rainy season they carry
storm water and seepage water in non-monsoon months. They
have bed level below the water table. A cunnette is usually
provided to cater for the small seepage water.
iv) Mole Drains. Mole drains constitute valuable supplement to
open drainage where they can be used. They are useful in
equalizing water levels between ditches for both drainage and
for sub0irrigatin. Mole drainage is a method of draining soil by
means of mole drains. Mole drains are cylindrical drains formed
in the subsoil by pulling a mole plough of 5 to 10 cm diameter
by a tractor. The plough is pulled along the sloping ground with
P a g e | 7
the mole shoe at a depth of 60cm. They are spaced 3.5 to 5 m
apart. A round channel is formed in the soil with cracks along
the mole. The drainage water enters the mole through these
cracks and is carried along the slope in sub-surface or open
drains. They are suitable in clay and clay loam soils but
impracticable in soils of coarse texture. Durability of the mole
depends on the texture and structure of the soil. In general,
mole drains are not satisfactory.
v) Link and Field Drains. These are branch drains draining sub-
catchment into the outfall drain. These are aligned along
subsidiary valley lines.
Field drains are small drains draining individually or a group of
fields into the link drains
2. Closed Drains:
The sub-surface drains remove water which has entered the soil.
They are usually laid 1 to 1.5 m below ground surface and at a
suitable spacing and grade to lower watertable to greater depths.
6. SURFACE DRAINS – DESIGN CRITERIA
These are usually 1 to 1.5 m deep to cater for storm water and
seepage water. They are suitable when (i) Large volume of either
surface or subsoil water from land are to be catered, (ii) Slope is too
slight to permit installation of the tile drains, (iii) plant roots are likely
to clog the tile drain, (iv) There is no satisfactory outfall for tile drain,
(v) Law cost land is traversed. The design considerations are as under:
1. Rainfall: Intensity, frequency and duration of rainfall dictates the
design discharge is a drain. Usually the maximum rainfall of 3
days duration is considered on economic considerations. For drain
section rainfall corresponding to 5 year return, and 10 or 15 years
for higher degree of protection is considered and for the masonry
structures rainfall corresponding to 50 year return is adopted.
P a g e | 8
2. Depth and duration of submergence: The discharge to be catered
also depends on the permissible depth and duration of
submergence which in turn varies from crop to crop. As per
IS;8835-1978 the period of disposal is limited to 7-10 days for
paddy, 3 days for bajra, maize and cotton, 7 days for sugarcane
and 1 day for vegetables.
3. Economic considerations: Economic considerations require
excavated drain section of limited capacity.
4. Environmental Aspects: The negative effects on the environment,
if any, must be considered and remedial measures included in the
drainage scheme.
Other important considerations of design are soil drainage (water
table), crop drainage and salinity drainage.
5. Drain Capacity: In determining drain section, peak rate of runoff,
total volume of runoff as also distribution of runoff are throughout
the year is considered. IS:8835-1978 recommended runoff
coefficient for different soils in plains is 0.7 for plateaus lightly
covered, 0.55 for clayey soils, stiff and bare and clayey soils lightly
covered, 0.4 for loam, lightly cultivated or covered, 0.30 for loam,
largely cultivated and suburbs with gardens, lawns, roads, 0.20 for
sandy soils, light growth and 0.05 -0.20 for parks, lawns,
meadows, gardens, cultivated area.
Boston Society formula Q = CA is also used. The existing drains may
have design capacity Q= CA/5 in forced reaches comprising closed
drainage tracts in which drains cut across high land. In the case of
new drains the capacity is determined by the formulae Q=CA/2 for
C.A 651 sq km and above but bed width is excavated for CA/4 and
balance capacity from CA/4 to CA/12 is provided by deepening the
drains. Here Q = discharge (cumecs), A = catchment area (sq km) and
C= coefficient value 3.5 for areas having annual rainfall 50 cm, 8.4 for
P a g e | 9
rainfall 51-75 and 35 for rainfall 75-100 cm. for areas with rainfall
above 100 cm, every case is considered individually on actual basis.
U.P. and Andhra Pradesh: Q = 0.11 cumec per sq km of catchment
area.
Madhya Pradesh: Q = 0.22 to 0.44 cumec/sq km, 0.44 for C.A up to
13 sq km and 0.22 for C.A. 79 sq km and above.
6. Alignment: Alignment of the drain is required to be such that is
traverses through the lowest contours, i.e. along the drainage line
and length of the drain is minimum consistent with the
requirement to drain off the lowest spots by either directly or
through subsidiary drains. It should not cross irrigation canal or
pass through village habitations as far as possible. Drains aligned
down the slope are usually much more effective than those
excavated normal to the direction of the slope of the ground. The
reduced distances, RDs, are marked along the drain, the zero RD
being at the outfall end and increasing upstream, i.e. just the
reverse of canals wherein RDs increase towards downstream.
7. Water Surface Slope: Water surface slope in the drain is governed
by the general slop of the ground and outfall condition. Non-
weeding velocity which is considered higher than non-silting
velocity is provided. The slope is either kept constant or gradually
decreasing to wards the outfall in keeping with increased discharge
downstream. Slope is generally determined from Lacey’s formula
S = 0.0003 f 5/3 /Q 1/6.
8. Velocity: Drains are designed by Lacey formula. Generally adopted
velocities in the drains for firm loam and clay loam = 0.5–1.0 m/s,
alluvial soil = 0.6-1.25 m/s. IS:8835-1978 has suggested value of n
as 0.025 in computing velocity.
P a g e | 10
9. Side Slope: Side slope of 1:1 is generally adopted for cutting. Usually
considered side slopes are, loose rock and hard soil = 0.5;1, alluvial soil = 1:1,
sandy soil and deep black cotton soil = 2:1, and very sandy soil = 3:1.
10. Bed Width and Depth; Bed width and depth corresponding to
Lacey’s formula, discussed under canals, is generally adopted.
The discharge is divided into suitable reaches, considering that
the flow increases towards the downstream, without taking into
account transmission losses applicable to canals. The full supply
line is generally kept below the natural ground level, say by 0.6m, to that
there is no flooding outside and the drain caters for the drainage effectively but
not higher than 0.3 m above average ground level at the starting point of the
drain.
It is a good practice to dig a cunnette at the centre of the drain to
cater for low flow and seepage. Cunnette section also helps in
preventing weed growth because low flow is not made to spread thin
over the entire section but is contained within the cunnete section. It
also reduces maintenance which for the most period is required for
the counnette portion only.
In Punjab and Haryana States, drain section is dug for the discharge
corresponding to Q = C A/4 to C A/6 and the maximum flood
Q =CA s contained within the banks (Fig. 7.2) In Andhra
H.F. level
Fig. 7.2 Typical Cross Section of drains.
Pradesh, on Nagarjunasagar project, drains are excavated for ordinary
flood discharge only at the lowest average level of the adjoining wet
1.5:1
1.5:1 1.5:1 1.5:1
1.5:1 Natural Surface level
1.1 1.1
6.0
Construction
road Spoil
bank
Inspection
bank Spoil
bank
Bed level
5.0
P a g e | 11
fields while the maximum flood discharge section will have its
maximum flood level at 0.3 m above ordinary flood level with a very
wide berm (fig. 7.3)
Fig. 7.3 Typical drain section (Andhra Pradesh)
Bank Width. The drains, in general, are not banked on both sides. As per
IS:8835-1978 top of embankment is kept 1 m higher than design full
supply level and 1 m minimum berm width. Bank is essentially required
on one side and the side contributing flow may be left un banked. Where
heavy spilling may take place on both sides, continuous embankments
on both sides are provided. Regulated inlets (Fig. 7.4 to 7.6) are provided,
where necessary, to allow the outside water to enter into the drain. In
diversion drains continuous banks on both sides of the drain are
essentially provided.
1.5
:1
1.5
:1 1
.5:1 F.B 1.0. – 1.25 `H.F, Level
0.3 O.F level
1.5 Berm 1.5
1.25
O.F. and M.F. bed level
Full section for M.F.L
Land width
6.0
Non-returnable valve
1.5:1 1.5:1
H.F.L drain
Land side
P a g e | 12
Fig.7.4 Inlet
Fig. 7.5 chute inlet
3.0
3.0
C
Drain patrol bank
Drain patrol bank
Drain bed
1.0
5.0
0.5
2.0
0.3 1:1
1.5:1
Drain bed
0.45
2.0
0.5 N.S.L In 20
0.15
0.15
0.5
0.5
Sec.CD
Section A-A
A
A
B
D 3.0
0.5
1.0
0.5
0.5
1.0
0.5
0.5
1:1
1:1
Sec A
B
P a g e | 13
Fig.7.6 opening through shaped inlet on drain
Minimum bank width is 2.0m; general practice being 3 m width on non-
patrol bank and 6 m on patrol bank side. In large capacity drains a
boundary road 5 m wide is provided (Fig. 7.2).
Disadvantages: (i) Cause wastage of land, i.e. the land brought under the
drain cross section, (ii) Require bridges, etc. for passing drain under
road, railway line, canal, etc., (iii) Require frequent cleaning, (iv) Harbour
and spread obnoxious weeds.
7. CLOSED DRAINS – DESIGN CRITERIA
Closed drains are underground drains laid deep in the ground and
then covered. Their use is indicated in high cost land because they do
not occupy surface land and also cause no hindrance to the
agricultural operations. They are located at a suitable depth and
grade below the ground surface depending on the topography, they
are located at a suitable depth and grade below the ground surface
depending on the topography, existing water table and the extent of
depression of water table required fig (7.7).
Table
Water
Pipe Drain
Water
Graded filter
10-15 cm
P a g e | 14
Fig. 7.7 Tile Drain (without graded Fig. 7.8 Tile drain (with filter
Filter) in a previous soil. In a previous soil
They are placed in permeable stratum. In low permeable stratum the
drain is shrouded with filter material of high permeability (Fig. 7.8) to
ensure effective drainage. They help lower the full saturation line
adequately below the ground surface.
Advantages: (i) No hindrance to farming operations, being underground,
(ii) Occupy no surface land, thus no area is put out of cultivation, (iii)
Give root zone greater depth, (iv) Lower water table to greater depth, and
(v) Permanent reclamation of saline and saline- alkali soils.
Tile drains
Closed drains are commonly designated as tile drains. Tile drains
constitute the most efficient and permanent type of sub-surface drainage
for the irrigated areas where the water table has permanenty risen close
to the ground surface. They are located at a suitable depth, usually 1 to
1.5m, below the ground surface and at a suitable spacing and grade
depending on the soil, _________ and topography of the area. The centre
of the tile drain is usually 0.3 to 0.6 m below the level up to which the
water table is desired to be lowered below the root zone of the plants
(Fig. 7.9) ____ water enters through the open joints of the tiles. The water
drained by tile drain
Water table after
drawdown
Impervious layer
Tile drain
O
R
IG IN AL WAT ER TABLE
P a g e | 15
Fig. 7.9 Drawdown curve with a single tile drain
S disposed of by gravity into deep surface drain or pumped out
depending on outfall conditions. In the system, laterals remove water
from the soil, sub-drains collect water from a group of laterals and
empty into mains for disposal into outfall.
Tile spacing and depth. The drain depth is reckoned from the ground
surface to the bottom of the tile. The deeper the drain the more is the
drainable area per drain line and farther is the spacing of drains. The
drains are closely spaced in clay soils and far apart in sandy soils,
IS:10970-1984 recommended drains placed about 1.25m deep are
given in Table 7.1.
Table 7.1 Drain spacing in humid areas
Soil Hydraulic conductivity Spacing (m)
Clay and clay loam Very slow (<1.3 mm/h) 9 to 21.5
Silt and silty clay loam Slow to moderately
Slow (13.2 mm/h)
18.5 to 30.5
Sandy loam Moderate to rapid (20-
250 mm/h)
30.5 to 91.5
Hooghoudt formula for spacing is as follows (Fig 7.10)
Where, S = spacing between tile drains, V = rate of discharge or
rainfall per unit area of land surface, d = depth of bottom of drain
above impervious layer, h = depth of water in tile drain, H = maximum
height of water table above drain bottom, k = coefficient of
permeability.
P a g e | 16
For practical purpose S2 = 4KH (2d + H)/V, considering drain is
empty and h = 0
Fig. 7.10 Hooghoudt tile drain
v
hddhHKs
24 222
Design of Tile Drains
The capacity of the drain flowing full is determined from the
Manning’s velocity formula taking roughness coefficient, n= 0.018. the
diameter of the drain is computed by equating the capacity of the tile
drain to the design runoff for the area. The tile diameter is given by
the formula, D = 0.1635 D c 0.375 x S -0.1875 x A 0.375
Where, d = internal diameter of tile (cm), Dc = drainage coefficient (cm
per day), A = drainage area (m2), S = drain slope.
The minimum size of tile is 10 to 15 cm. maximum velocity prescribed
by IS:10907-1984 is 1.1 m/s for sand and sandy loam, 1.5 m/s for
silt and silt loam, 1.8 m/s for silty clay loam, 2.1 m/s for clay and
S2
d S
h
Formation Imperviouse
Final water level
Ground surface
Rain fall, V
P a g e | 17
clay loam and 2.7 m/s for coarse sand or gravel. A suitable
longitudinal slope, depending on the slope of the ground and depth of
outlet, is provided. IS:10907-1984 has indicated chart for determining
discharge for a given area. Tiles laid to little grade tend to fill up
readily while those with steep grade cause high velocities of flow. The
desirable range of working grade along the tile drain is 0.1 to 0.2%.
The grade for 10 cm tile is 0.10%, for 13 cm tile is 0.07 % and for 15
cm tile is 0.05 %. The drainage coefficient with no surface water
admitted directly into the drain is 5 to 10 mm/day, recurrence
interval 5 years is recommended by IS:10907-1984.
The laying of tiles begins at the lower end of the line and progresses
up grade. The tiles are laid true to the line of the trench and firmly
bedded in the bottom of the trench and on grade. Joints between the
tiles are kept open, shrouded with filter, to admit drainage water into
the line. A gap of 3 mm in the case of silt, loam and clay soils and 6 to
10 mm for peat and muck is usually allowed. An approximate method
for designing tile drain (refer Fig 7.11) is as under:
Land Surface
Original Water table
D 0.6 to 2.0m Final water table
0.3m
b y
s
a
Fig. 7.11 Spacing of tile drains
Where, S = spacing of tile drain (m), a = depth of impervious layer
from the centre of the drain, b = maximum height of the drained water
table above the impervious layer, x,y = x is any distance from the
centre of the drain where height of water table above the impervious
Impervious layer
P a g e | 18
layer is y, and V = rate of rainfall or discharger per unit area of land
surface.
It is assumed that the hydraulic gradient at a distance x from the
drain is dy/dx, flow lines are parallel and cross sectional area of flow
at a distance x is yx1 = y, and discharge q towards the drain is
inversely pe3oportional to the distance from the drain, and Q is the
total discharge per unit length carried by the drain so that 2
1Q enters
the drain from either side.
According to Darcy’s law
Q = KIA
Or dx
dyKyq i
Where, q is discharge per unit length of drain passing through y. K is
coefficient of permeability of soil.
Now Qq2
1 when x = 0
And q = 0 when 2
Sx
Therefore, 2/2
1
S
xQq
Or = )2(2
SSS
Q (ii)
Equating equations (i) and (ii)
)2(2
xSS
Q
dx
dyKy
Or ydydxxSKS
Q )2(
2
Integrating, we have
P a g e | 19
When a
acayx
2
,,0
)(
)(2
22
xSx
aySKQ
When byS
x ,2
Hence S
abKQ
22(4 (iii)
Or Q
abKS
22(4 (iv)
Also )(2 22 abv
kS Dupuit formula
Q depends on infiltration into the ground and is usually assumed as 1
% of the average annual rainfall in 24 hours.
Equation (iii) and (iv) give discharge and spacing of the tile drain.
Advantages: Advantages of tile drains have been enumerated under
closed drains.
Disadvantages, (i) High initial cost, (ii) Limited drainage capacity,
(iii) |Not open to inspection, being underground,
(iv) Repairs costly and inconvenient, and (v) Requires
steeper slope.
8. TILE DRAINAGE SYSTEM
Drainage system comprises a main drain, its branches and subsidiary
drains. Tile system is so devised as to cater for all the wet areas that
P a g e | 20
could eventually be drained into one main drain. Various tile drainage
systems are briefed as follows:
1. Natural system. It consists of a system of drains, similar to
trunk system, but covering a much larger area (Fig. 7.12). The
main drains are located along the depressions or low spots to
conform to topography. Natural system is generally installed in
areas of rolling or broken topography where drainage of isolated
tract is required.
2. Parallel system. It consists of a system of drains with long
parallel laterals emptying into a single main drain (fig. 7.13). it
is used in poorly drained soils having uniform texture and little
slope.
0
3. Herringbone system. This system of drains consist of a main or
submain along the depression with parallel lines of field drains
sloping towards the main drain and joining it at staggered intervals
(Fig.7.14). It is used for lands lying on both sides of a narrow
depression and laterals must enter from both sides. It is less
economical on account of double drainage occurring where the
laterals and main join.
4. Grid iron system. In this system, the field drains are constructed in
parallel lines along the direction of slope and join the main drain at its
bottom (fig 7.15). It is used for flat land with a uniform slope.
102
101
100
Main
Lateral
Lateral
Main
100
101
102
103 104
Lateral
Main
Lateral
Main Sub Main
Fig. 7.12 Natural system Fig. 7.13 Parallel system Fig. 7.14 Herringbone
system
Fig. 7.15 Gridiron system
P a g e | 21
5. Double system. It is a system of drains similar to that of herringbone
system except that there are two main drains in this system on each
side of the depression (Fig. 7.16). It is used when the bottom of the
depression is wide. It is not generally used as conditions which it
requires are seldom met with in practice.
6. Grouping system. It is similar to the natural system except that a few
laterals are provided in wet areas or ponds along the system (Fig.
7.17). It envisages collection of water from the topography and
wetness on the field vary and pattern of drainage must be changed to
fit the different conditions.
7. Random system. In this system drains are laid more or less at random
to drain the wet areas. (Fig. 7.18). The main is located at natural
drainage line and individual wet spots are connected through
submains and laterals. Where wet spots are large the arrangement of
the submains and laterals for each wet place may utilize one or more
of the parallel systems to provide the required drainage. It is used in
rolling areas that have scattered wet areas slightly isolated from each
other.
Main
Late
rals
Late
rals
Fig. 7.16 Double main system Fig. 7.17 Grouping system
Main
Stream
20cm tile 12.5 20 cm tilt
cm tilt
100
95
90
85 Low ground
High land Drain
Bottom land
P a g e | 22
8. Intercepting system. In this system, tiles are
placed along the hillside to intercept the seepage
water that follows the upper surface of an
impervious subsoil to prevent it from reaching
the bottom land (Fig. 7.19) It is used for draining
seepage along hillsides.
9. composite system. It is a combination of systems
of tile drain arrangement such as the
herringbone and grid iron systems. (Fig. 7.20)
10. Sink hole drainage system. It is a system of
drainage used to intercept seepage water, but
has in addition wells dug at regular intervals to
let the water come up from a lower stratum and
enter the drain (Fig. 7.21)
LATERALS
Su
b m
ain
Fig. 7.20 composite system
Fig. 7.22 Zigzag system.
Fig. 7.17 Random system
Fig. 7.21 sink hole system .
P a g e | 23
11. Zigzag system. In this system field drains as
well as mains are constr4ucted zigzag (Fig. 7.22)
to reduce high velocities.
9. OPERAION AND MAINTENANCE OF DRAINS
The drains designed on regime theory applicable to canals inherent
maintenance problems, since these are not stable. Regular maintenance
is required to keep them functional as designed. Various factors on
which the frequency and degree of maintenance depends are amount of
rainfall, climate and ground water conditions. In general, open drains
require maintenance after a heavy storm. They suffer from operation and
maintenance problems such as (i) They carry variable flow, unlike
irrigation channels; maximum during heavy rains and normally very
small discharge and as such as susceptible to erosion and silting up and
do not maintain their section for long, (ii) They are infested with weed
growth as these are run with low velocities for most of the time which
choke their waterway, (iii) Repairs and maintenance difficult because of
their location far away from roads and other means of communication,
(iv) Rarely inspected since they are situated away from irrigation
channels and generally not provided with inspection bank, (v) Roads,
railroads and canals often caused obstruction to drainage as they cause
an afflux in water level and create congestion, and (vi) cross bunds are
often put up across the drains to divert or pump out water for irrigation
and not entirely removed after
10. DF