Lecture Drainage

7/31/2019 Lecture Drainage

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Subsurface drainage Investigations


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Subsurface drainage refers to the removal ofexcess water present below the ground surface.

Agricultural lands affected by high water tablegenerally need subsurface drainage.

While surface drainage removes the excessrainwater before it enters the root zone,subsurface drainage lowers the water table andprovides a better environment in the root zone.

While subsurface drainage problems could benatural, often these problems are manmade.


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It is important to understand the causes and the necessity of theproblem of drainage before discussing the methods of theirreclamation.

Generally the issues of water logging drainage and salinity arevisualised as separate issues occurring independently, andtherefore requiring independent consideration.

However the three problems are intimately connected. In fact

water logging is the main cause which results in the problems ofdrainage and salinity affecting agricultural productivity.

There are various alternative approaches to drainage suiting

different crop and soil hydrological conditions.


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Table 1. Water logging

Norms of water logging

Nomenclature Depth of wate

table b.g.l.

1 Water logged < 2 m

2 Potential area for watelogging

2-3 m

3 Safe > 3 m


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Table 2 Waterlogged and User Areas in Different States

Sl.

No.

State (s) Waterlogged

area (lakh ha)

Usar area

(lakh ha)

1 Uttar Pradesh 8.10 12.95

2 Andhra Pradesh 3.39 2.403 Bihar 1.17

16

0.04

4 Gujarat 4.84 12.14

5 Haryana 6.20 5.26

6 Jammu & Kashmir 0.10 -7 Karnataka 0.10 4.04

8 Kerala 0.61 0.61

9 Madhya Pradesh 0.57 2.42

10 Maharashtra 1.10 5.34

11 Orrisa 0.60 4.0412 Punjab 10.90 6.88

13 Rajasthan 3.48 7.28

14 TamilNadu 0.18 0.04

15 West Bengal 18.10 8.50

16 Delhi 0.10 0.16


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The basic reason for agricultural lands being

affected by water logging and salinity isinadequacy of natural drainage system to handlethe water reaching the land either by natural orartificial means.

When such a situation occurs artificial drainagesystem has to be resorted to.

The earliest drainage method consisted of drainageby open trenches.


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Water logging and salinity was observed in Karnal in 1855.

Systematic efforts on Drainage were made and successfully

demonstrated in Gohana and Sampla at Central Soil Salinity InstituteKarnal around 1983- 1984.

Later under ICAR projects several regional centers for research on

drainage were established in different states.

CSSRI pilot projects under Indo-Dutch schemes on subsurface drainage

were established in different soil hydrological regions in different states,which were quite successful and were picked up well in few states.

No systematic data of total subsurface drainage in the country is

available.

However under RAJAD (Rajasthan Drainage Project) systematic

subsurface drains were laid in about 13000 hectares by fully mechanizedequipments.

In Haryana state also about 3000 hectares of subsurface drains have

been laid by mechanized equipments.

India


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It was also analyzed and calculated that the largestagricultural area was without any water managementsystem.

About 90% of the increase in production had to comefrom the existing cultivated lands and only 19% fromnew land reclamation.

In the rain fed areas without water management waterharvesting and watershed management may improve

livelihood of poor farm families but much increase inproduction is not expected.

Installation of irrigation and drainage systems in areaswithout a system or improvement/modernization of

existing irrigation and drainage system. There has to be a shift of contribution to total food

production 30% for the areas without a watermanagement system, 50% for the areas having airrigation system and 20% in rain fed areas with adrainage system.


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SUBSURFACE DRAINAGE

Subsurface drainage may be defined as the removal or control ofground water and removal or control of salts using water as vehicle.

The source of water may be percolation from precipitation orirrigation leakage from canals, drains or surface water bodies athigher elevation.

Any drain or well designed to control or lower the ground water isconsidered subsurface drainage.

They may be broadly classified in two categories as : (a) Horizontal Drains, and (b) Vertical Drains

(a) Horizontal drainsIt is accomplished by buried pipes or pipe less (mole) drains and also bydeep open ditches.

Pipe drains : They consist of a system of pipes made of baked clayconcrete or perforated plastic pipe or any other materials. The excesswater enters in lateral lines through the joints between two tiles orperforations and flows towards main drains.


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The main objectives of horizontal drains are

(i) In humid region to accomplish aeration in plant root zone to grow uplandcrops like maize, soybean etc and to provide improvement in soilmoisture conditions for operation of tillage, planting and harvesting andthus to increase length of growing season for the next Rabi crop

(ii) In irrigated -less rainfall arid regions, to remove toxic substances likesalts rising in the root zone from saline/alkali water tables by evaporation.

In humid regions generally the water quality is good and drainage is to beprovided only for good aeration in root zone.

The depths of tile drains are generally kept 2.5 ft. to 3.0 ft. In arid regionswith saline soils the root zone of the crops has to be kept free of saltsfrom a saline water table.

In such drainage systems, the depth of tile drains are kept 6.0 ft. to 8.0 ft.

Objectives of HorizontalDrainage


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In humid regions, the spacings are kept 30 ft. to 150 ft. apart and upto300 ft. apart for very permeable soils.

When permeability is very low one may get spacings 30 to 40 ft. Unless the crop is of very high value drainage becomes uneconomical

with such spacings.

In arid regions where depths are more, spacings may range from 300 ft. to600 ft.

(b) Vertical drainage Essentially this consists of a system of shallow tube wells spread in the

area through intensification of minor irrigation works. Through direct extraction of ground water it lowers the water table. The drained water may be used to augment irrigation water supply in the

area. Use of tube wells along with canal water could also be termed as

conjunctive use.


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HORIZONTAL DRAINAGE , Some Clarifications

Can shallow Surface Drains lower water table and leach a Salt affected Profile? Surface drainage removes surface ponding and reduces the recharge on

high water tables and is thus a good supplemental practice, but it cannotbe used to lower water tables and remove salts from salt profile.

For seepage to take place according to Darcys Law there should be somehydraulic gradient between the two points considered.

For lowering of water table and removal of salts the amendments mustpermeate the soil mass .

This is possible only through line sinks created in the form of sub surfacedrains.

With shallow drains only sheet flow over surface can take place providingsurface washing of salts from the surface without any effective leaching ofthe profile.


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Reclamation by Leaching with Gypsum shallow water table areas and reversal ofalkalinity

Leaching has been found to be quite effective in areas with deep water tables.However carrying out reclamation at a heavy cost in areas with shallow water

tables the areas have been found to return to original state of alkalinity.Deep open Drains Functions

Deep open drains say up 80 cm or more depth are theoretically subsurfacedrains and equivalent in performance in tile drains.

Being open at the surface they can perform surface drainage also, besides

subsurface drainage, but they are difficult to maintain as well as to be crossedby men, animal and machinery.

Earlier subsurface drainage works in other countries were carried out by suchopen drains but substituted later by covered drains.

It took about a century to start from subsurface drainage through open drainsstone drains to arrive at the system of closed perforated PVC pipes.


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To reduce or stop seepage is it necessary to line all the earthen canals?

With constraints on funds, it may be infeasible to line all the length of maincanals and distributaries. Highly permeable and low permeable zones

should be identified and as a first priority, only reaches with highpermeability may be lined, which may be gradually extended.

Effectiveness of Limited Combination of Surface and Vertical Drainage

Sometimes a combination of vertical drainage (small tube wells and surfacedrainage (done by farmers) is planned providing all amendments and

fertilizer inputs. A monitoring of water quality in recirculating themoderately sodic ground waters, the continued addition of chemical inputsand its long term projection is desirable, besides the immediate benefitaccrued with such projects.

R l f Bl ki f l D i b Hi h R il d C l Th


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Role of Blocking of natural Drainage by Highways Railways and Canals: Thereare large areas (about 15-20 percent) in the country waterlogged because ofblocking of natural drainage to overland flow due to manmade structures likerailways, highways and irrigation canals. These structures provide veryinsufficient size culverts/siphons for drainage crossing to economise their own

projects. Such locations should be identified, indexed after surveying to find thenature and extent of water logging. Depending on funds available, the culvert sizeand drainage ways should be appropriately enlarged in phases.

Role of Toe/Interceptor Drains: Interceptor drains are a good device to reducebank seepage from the canals specially in the canal fill zone. It needs however tobe appreciated that Interceptor drains can only partly reduce seepage from thecanal banks. They cannot reduce seepage from the bed of the canals. Thetoe-drains of canals should be properly constructed and maintained for effectivedrainage of seepage water. Closed drains though costly can be maintained betterthan open drains.


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Borrow pits along Roads Canals and Railways

Borrow pits along the road-side railways and canals should be interconnectedtill they are outletted in the cross drainage works.

Integrated System of Drainage from Field to Outlet

Total effective drainage from a command area needs the essential componentsof field drainage, intermediate link drainage and outlet drainage.

Outlet drains are important but such a system without positive field drainageconstruction may amount only to wastage of money. In effect such a system

would be parallel to construction of main canals and distributaries by onedepartment and requiring establishment of another department of CADA forutilization of irrigation potential. It is desirable that all the three components ofdrainage are coordinated by the same authority for better effectiveness andaccountability.

VERTICAL DRAINAGE


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VERTICAL DRAINAGEVertical drainage and conjunctive use is another good way of lowering watertables and providing subsurface drainage. With good quality ground water it isan excellent practice. On one hand it lowers the water table and on the otherhand through the same process it also provides irrigation for increasing

production of agricultural crops. However, it is important understand differentissues involved with it. One of the issues is whether one can effectivelyimplement conjunctive use in a command area with the presently prevailingdemocratic and socio-economic set up.Conjunctive use of surface and ground waterIt is term used for application of tube wells along with canals for irrigation of

agricultural crops. Essentially this consists of a system of shallow tube wellsspread in the area through intensification of minor irrigation works. Throughdirect extraction of ground water it lowers the water table. The drained watermay be used to augment irrigation water supply in the area. Use of tube wellsalong with canal water could also be termed as conjunctive use.

C j ti th h ti l d i i d lit d t


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Conjunctive use through vertical drainage requires good quality ground water.

Through vertical drainage lowering of general ground waterlevel has to be done through large scale pumping fromshallow tube wells. Only shallow tube wells are useful for

lowering water table. Deep tube wells in confined aquiferswould not be effective to lower the water table.According to Smedema and Zimmer (1994) in Scarpprogramme of Pakistan about 15000 large capacity tube wellswere installed to reduce rising water tables and alleviate

irrigation water shortage. The pumped water was used tosupplement canal supplies at the head of the irrigation watercourses. With success of VD and CU programmes a largenumber of farmers estimated as 300000 farmers started

constructing their own private tube wells. This has been moresuccessful in fresh water zone than in high salinity groundwaters.With private tube wells and non existence of any ground waterlaws it is difficult to implement it in an organised way. Still

encouragement of shallow tube well pumping through minor


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Well point system has Different Objective than Conjunctive Use

Some times, the objective of vertical drainage is assigned as steady statelowering of water tables in the root zone for growing crops due to

overlapping of draw down caused by pumping of closely spaced wells. In other words this is similar to the well known well point system.

Such a system is used for building foundations or other structures inhigh water table areas where rapid lowering of water tables is required forshort periods.

Such a system is costly and would also involve high recurring energycosts and does not seem to be practicable for growing agricultural crops.

It does not seem to be feasible for continued lowering of water tables inroot zone for long periods necessary for growing crops.


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Suitable Conditions For Conjunctive use de Ridder (1983), Attia and Twinhoff (1989) recommend technical feasibility of

tube well drainage only based on geometry of aquifer hydraulic parameters,thickness and hydraulic resistance of clay cap, ground water quality and rate

of recharge. According to them well drainage enables the ground water to be lowered to a

much greater depth than gravity drainage. Where deeper layers of substrata are more pervious than layers near the

surface pumping from these layers may reduce the artesian pressure that isoften present creating a vertical downward flow through the upper layers.

According to CWC and USAID the feasibility of tube well drainage isrecommended for large areas of flat lands with high water tables, thickaquifers with good hydraulic conductivity, areas having high infiltration rates,ground water under artesian pressure, ground water with good quality, wherepipe drainage is feasible but costly because of inadequate outlets, whereground water lowering is desired beyond 2 to 5 m.


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BIO-DRAINAGEUtility of Bio drainage Bio-drainage is proposed as a good method of subsurface drainage. However, it is important to identify appropriate situations where this method

can be effectively and usefully utilized. It is unfair to assign it an objective which it can not take care and then

misinterpret the approach. It is a good method to reduce bank seepage in canals and have been used

for this purpose in many canal commands It is not only an economic method of drainage, it also improves the ecology

of the area and is environment friendly. It provides costly wood useful for multifarious purposes and also various

range of bio-mass. It can transpire water from ground water table in good amounts. A large

range of crops tolerant to salinity can be grown in salt affected lands. It is a good method for economical exploitation of a waste land.

Bio drainage useful for long term water table lowering but not for growing


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Bio drainage useful for long term water table lowering but not for growingAgricultural Crops

Agricultural drainage requires frequent and rapid lowering 1 to 2 m of watertable in 2 to 3 days after every recharge due to irrigation/rainfall during the cropgrowth period.

trees can lower water table by 1 m in 1 to 2 years what will happen to cropsduring this period. This, lowering also is possible only if there is no recharge inthe tree cropped area. If there is frequent recharge due to irrigation/rainfall itdoes not appear feasible that the water table can be lowered by tree crops.


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Bio drainage utility in old or new commands

The trees have to be planted till they become affective in providingevapotranspiration. This is possible only in new commands not yet

water-logged. In water-logged and salt affected commands it is not practical toplant and establish trees. if they can be planted and grown over a time seepageprocess from the canal would require to wait for their becoming old enough toprovide evapotranspiration.

Bio drainage can remove only water and not the salts from a

saline soil?There are no evidence of systematically controlled experiments to demonstrateremoval of salts from a saline high water table soil profile. Growing salt tolerantcrops does not imply removal of salts from soil profile and enabling it for growthof agricultural crops. The scope of bio-drainage seemed to be more favourable

in arid zone where drainage surpluses are small in relation to evapotranspirationrates (1 to 2 mm/day vs 10 mm/day).


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POLICY ISSUES FOR PROMOTING AGRICULTURAL DRAINAGE

Periodic monitoring and reporting of Water tables

It is necessary to determine pre monsoon and post monsoon water tablesevery three years in order to determine the increase in the extent of water

logging with progress of time for the pre-monsoon and post-monsoonseason.

Only a periodic comparison of successive pre-monsoon water levels orsuccessive post monsoon water levels for every three years can provide thisinformation.

Long term canal rostering and conjunctive use in waterlogged area

In high water table areas with good water quality water, conjunctive usethrough intensification of subsidies on minor irrigation works and lesserrelease of water in main canals and distributaries should be properly plannedand encouraged.


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In the canal commands, irrigation should be reduced by long durationrostering of canal networks considering ground water levels in the areas.

If the water level is less than 5 m b.g.l., canal irrigation should bediscouraged.

For paddy cultivation, canal irrigation should be given preference butover-irrigation should be discouraged.

Where canal irrigation is in vogue it is strongly recommended to go for aconjunctive use of surface and groundwater and to use adequatequantity of groundwater from tube well irrigation so that thepre-monsoon groundwater level should be around 10 m below land

surface. Further rostering of the canals for long periods and use of groundwater

for irrigation will lower the water table and keep it in control and, thereby,increase the agricultural production.

Necessity of a separate body to look after Drainage provision and


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Necessity of a separate body to look after Drainage provision andmaintenance There should be an independent investigation and planning organization

in Irrigation department for collection of drainage data, drainage surveys,storage and processing of data base and preparing and dovetailing of

drainage project in an integrated manner. This needs being implemented inright spirit.

Integrated approach of drainage starting from drainage link drains maindrains and natural waterway

For appropriate agricultural drainage, it is necessary that field drains linkdrains and main drains should be constructed and made to function in anintegrated manner.

Most of field drains constructed by rural development projects have beenencroached by farmers by putting bunds across drain for irrigation orputting it to cultivation.

It is necessary to establish responsibility of repairs and maintenance onsome institution which presently does not exist.


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In highly waterlogged areas where drainage is difficult and costly WaterProductivity concepts may be used as developed in different Researchcomplexes and Water Technology Centers.

Simultaneous Planning and sanction of Irrigation and Drainage butimplementation of Irrigation woks carried only for Irrigation.

The drainage projects components are simultaneously provided alongwith the irrigation project.

But generally irrigation component of the projects are completed

expeditiously, whereas, drainage component keeps lagging.

Such disbalance in project construction should be properly monitoredand discouraged.


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Drainage Maintenance Norms

The norms of maintenance of drains recommended by expert

committee, Govt. of India (1981) should be appropriately followed. Only 35 percent of cost of maintenance of drains being met by state

government is insufficient and should be increased for effective results.

The various provisions of North India Canal and Drainage Act (1873) andits modification requiring Gaon Sabha to construct repair and maintain

the water courses below the outlet should be suitably amended toappropriately define the responsibility of Gaon Sabha and variousdepartments.

Similar modifications needs be done in respect of Panchayat Raj Act(1947).


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BENEFITS OF SUBSURFACE DRAINAGE1.Aeration of root zone for maximum development

of the plant roots.2.Opportunity for desirable soil micro organisms todevelop through aeration and higher soiltemperatures.

3.Availability of the soil for early cultivation and thusincreased crop growth period.

4. Improvement of soil moisture conditions foroperation of farm machinery.

5.Removal of undesirable salts from the root zone.6.Greater storage of rainwater in the root zone

because of a low initial water table before therains.


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INVESTIGATION FOR SUBSURFACE DRAINAGE

1.Topographic map of the area.2.Data on soil salinity and alkalinity, drainable porosity

etc.

3.Position and fluctuations of water table levels relative

to the ground surface and artesian pressures.4.Ground water quality.

5.Hydraulic conductivity measurements.

6.Crops proposed to be grown and their drainage

requirements.7. Irrigation practices and requirements.


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The topographic map gives the details of

land slope, possible outlets, existingdrainage pattern, undulating land areasetc., and serves as the base map for

preparing the water table contour maps.

Information on soil salinity and alkalinity

is needed if surface drainage systemsare to be planned along with reclamationof such soils.


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Hooghoudts steady state equation


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DRAINABLE POROSITYDrainage porosity is the volume of waterreleased from a known volume ofsaturated soil under the force of gravity

and inherent soil water tensions. It isexpressed as a percentage of the totalvolume of saturated soil. It is also frequently

referred to as the specific yield.


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DRAINAGE COEFFICIENT orDRAINAGE DESIGN RATE

Drainage coefficient or drainage

design rate with reference to subsurfacedrainage system is the design value at

which water is to be removed per unit of

time. It is also sometimes referred to asdrainage modulus.


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DRAINAGE EFFICIENCY

Drainage efficiency is the ratio of thevolume of water discharged by the drains

during a certain period to the precipitation

generated in that period.

STEADY FLOW

Steady is one in which the volume of waterpassing a given point per unit of time remains

constant.


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The movement of water into subsurface drains is

influenced by the following factors:

1.The hydraulic conductivity of the soil horizons;

2.The configuration and location of the free watersurface, and the presence and magnitude of

artesian pressure or of the back pressure in the

drains;

3.Depth of drain below ground surface and location of

drain with respect to various soil horizons;

4.The horizontal distance between individual drains;

5.The diameter of drain;6.The tile joint spacing/ diameter and spacing of holes,

in case of PVC pipes and

7.The depth to impervious layer below the ground

surface.


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Let,

S = Spacing of drains, md= depth of impermeable layer below thedrain axis, m

H = height of water table at mid point betweenthe drains above drain axis, m

R = Constant rate of recharge due to rain/irrigation, mm/hour

K = Hydraulic conductivity of the soil,mm/hour

i = Hydraulic gradient = dh/dx


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Now assume a vertical section A-A at adistance x from the drain axis and let h be

the height of water table/ phreatic lineabove drain axis,Using Darcys law, the flow across section

A-A towards the drain per unit length canbe expressed as,

qx=Kh (dh/dx) (1)

In which qx is the discharge per unit lengthof drain at a section x distance away fromthe drain,


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The recharge per unit length of drain will be given by,qx = (S/2 - x) . R (2)

Equating equation (1) and (2), we get,

dx

dhhKRx

S..

2(3)

dxxS

K

Rdhh

2

...(4)

Re-writing (3),


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Hd

d

s

dxxS

K

Rdhh

2/

02

.

2/

0

22

22

.

2

sHd

d

xxS

K

Rh

On integrating (4) between limits, we get,


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842

2222

SS

K

RdHd

5.0

24

Hd

R

KHS

.(5)

Re-writing (5) for S, we get,


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In case if the drain is assumed to be placedat the junction of two layered soil with K1 and

K2 as hydraulic conductivity for the top andbottom layer than,5.0

2

1248

R

hKHdK

S

In case of pipe drains the term d is to be

replaced by, d(e), called equivalent depth tomoderate the effect of raising water table dueto the presence of impermeable layer at

shallow depths.


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Random drainage - herringbone -

grid iron types


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Pipe materials - tile, plastics cement -

Envelope materials. Load factors -

blind inlet - filters - mole drains,drainage wells


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Mole drains : These are cylindrical channels artificially produced in the

subsoil by a mole plough.

In principle and hydraulics they are similar to pipe drain exceptthat they are not lined with tiles or plastic pipes.

Moling is considered as a temporary method of drainage asmoles deteriorate and have to be reconstructed foreffectiveness.

Deep open ditches :

If closely spaced, they also work similar to pipe drains or moledrains for lowering water table and removing excess water.

However, they are difficult to maintain and provide lot ofinconvenience for crossing of men, animals and farmequipments.


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Pipe materials satisfy the following conditions:1.The pipe materials should withstand various

pressure and stresses like tensile, Compressionand hoop under water hammer condition.

2. It should be resistant to corrosion and abrasioncaused by the water.

3. It should be durable having sufficient strength tobear the external loads coming over it.

4. It should be structurally safe.5. It should have minimum possible weight.

6. It should be economical and uniform in size andshape.

7. It should be capable of easy hoisting and handlingat site.


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CONSTRUCTIONS

1. Pipe inlet laid below the groundsurface.2. Tile line section placed over the

inlet3. Coarse materials are filled on theinlets.

4. Size of the materials is becomeless towards the surface

5. Finally back filling with sand.


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LOAD FACTORThe load factor is the ratio of the

strength of a rigid conduit under given

bedding conditions to its strength asdetermined by three edge bearing test.

Generally it ranges from 1.2 to 1.5 for

drainage pipe laying conditions.


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DRAINAGE WELLThe use of wells for the purpose of

draining land is called drainage well.The soil permeability plays an important

role in determining the feasibility of well

drainage.


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Lecture Drainage

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