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Tahal engineering ltd.e

08

Drainage Engineering Eng. Y. Levy y/l/may 2008

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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

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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

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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;

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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

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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

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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.

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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

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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.

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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

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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

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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

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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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 .

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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