UNIT IV

PART A

1. What is the need for water budget? [N/D-16]

Water budget for a specified water system whether ground water, surfacewater, or the

Complete water system within a geographic area, is a summation of inputs and outputs

to the system.

2. What is the purpose of canal lining? [N/D-16]

Canal lining is the process of reducing seepage loss of irrigation water by adding an impermeable layer
to the edges of the trench.. Canal linings are also used to prevent weed growth, which can spread
throughout an irrigation system and reduce water flow.

3. What is a gravity dam ? [N/D-15]

A gravity dam is a solid structure, made of concrete or masonry, constructed across a river to

create a reservoir on its upstream . The section of the gravity dam is approximately triangular in

shape, with its apex at its top and maximum width at bottom.

4. What are the forces acting on a gravity dam ? [N/D 13]

(i) Weight of the Dam

(ii) Water Pressure

(iii) Uplift Pressure

(iv) Seismic Force

(v) Silt Pressure

(vi) Wave Pressure

(viii) Wind Pressure

5. Write short notes on [N/D 12]

(i) Storage head works

(ii) Diversion head works

(i) Storage head works: when a dam is constructed across a river valley to form a storage
reservoir, it is known as storage head works. The water is supplied to the canal form this reservoir
through the canal head regulator. Again this reservoirs serves the multipurpose functions such as
hydro-electric power generation, flood control, fishery, etc..,
(ii) Diversion head works: when a weir or barrage is constructed across a perennial river to raise

the water level and to divert the water to the canal then it is known as a diversion head works.

The flow of water in the canal is controlled by canal head generator.

6 .What are the points to be remembered while selecting the site for diversion head

works? [N/D 13]

The following points should be remembered while selecting the site for diversion head works:

a. At the site, the river should be straight and narrow.

7. What is a weir? [N/D-15]

Normally, the water level of any perennial river is such that it cannot be diverted to the irrigation

canal. That level of the canal may be higher than the existing water level of the river. In such a

case, weir is constructed across the river to raise the water level from H1 to H2. Then, the water

can be easily diverted to the canal. The surplus water passes over the crest of the weir.
8..What is a divide wall ? [A/M 13]

The divide wall is a long wall constructed at right angles to the weir or barrage, it may be

constructed with stone masonry or cement concrete. On the upstream side, the wall is

extended just to cover the canal head regulator and on the downstream side it is extended up

to the launching apron.

9. .What is a canal head regulator? [A/M 12]

A structure which is constructed at the head of the canal to regulate flow of water is known as

canal head regulator. It consist of a numbers of piers which divide the total width of the canal

into a number of spans which are known as bays.

10 . What are canal falls? [A/M 12]

Generally, the slope of the natural ground surface is not uniform throughout the alignment.

Sometimes, the ground surface may be steep and sometimes it ma be very irregular with abrupt

change of grade. In such cases, a vertical drop is provided to step down the canal bed and then it

is continued with permissible slope until another step down is necessary. This is done to avoid

unnecessary huge earth work in filling. Such vertical drops are known as canal falls or simply

falls.

11. What is the necessity for canal falls? [N/D 12]

When the slope of the ground suddenly changes to steeper slope, the permissible bed slope cannot

be maintained. It requires excessive earthwork in filling to maintain the slope. In such a case falls

are provided to avoid excessive earth work in filling.

In an irrigation project, when the network of main canals, branch canals, distributaries, etc. are

provided, then these canals may have to cross the natural drainages like rivers, streams, etc at

different points within the command area of the project. The crossing of the canals with

such obstacle cannot be avoided. So, suitable structures must be constructed at the crossing point

for the easy flow of water of the canal and drainage in the respective directions. These structures

are known as cross-drainage works.

13. What is the necessity for cross drainage works ? [N/D 13]

The water-shed canals do not cross natural drainages. But in actual orientation of the canal

network, this ideal condition may not be available and the obstacles like natural drainages may be

present across the canal. So, the cross drainage works must be provided for running the irrigation

system. At the crossing point, the water of the canal and the drainage get intermixed. So, far the

smooth running of the canal with its design discharge the cross drainage works are required.

The site condition of the crossing point may be such that without any suitable structure, the water

of the canal and drainage cannot be diverted to their natural directions. So, the cross drainage
works must be provided to maintain their natural direction of flow.

14. What are the common materials used for canal lining ? [N/D 13]

The commonly used materials for canal lining are :

(i) Concrete

(ii) Rock masonry

(iii) Brick masonry

(iv) Bentonite-earth mixtures

(v) Natural clays of low permeability

(vi) Different mixtures of rubble, plastic, and asphaltic materials.

PART B

1. What are cross drainage work? What is necessity of such a work in a canal project, and how does
this necessity is fulfilled by such water? [N/D-16]

What is cross drainage works?

Cross drainage works is a structure constructed when there is a crossing of canal and natural drain, to

prevent the drain water from mixing into canal water. This type of structure is costlier one and needs to

be avoided as much as possible.

Necessity of Cross Drainage Works

The water-shed canals do not cross natural drainages. But in actual orientation of the canal network,
this ideal condition may not be available and the obstacles like natural drainages may be present
across the canal. So, the cross drainage works must be provided for running the irrigation system.
At the crossing point, the water of the canal and the drainage get intermixed. So, for the smooth running
of the canal with its design discharge the cross drainage works are required.

The site condition of the crossing point may be such that without any suitable structure, the water
of the canal and drainage cannot be diverted to their natural directions. So, the cross drainage
works must be provided to maintain their natural direction of flow.

Cross drainage works can be avoided in two ways:

 By changing the alignment of canal water way

 By mixing two or three streams into one and only one cross drainage work to be constructed,
making the structure economical.

Types of Cross Drainage works:

There are three types of cross drainage works structures:

Type – 1: Cross drainage work carrying canal over the drain

The structures falling under this type are

 Aqueduct

 Syphon Aqueduct

Type – 2: Cross Drainage work carrying Drainage over the canal

The structures falling under this type are

 Super passage

 Canal Syphon

Type –3: Cross drainage works admitting canal water into the canal
The structures falling under this type are

 Level Crossing

 Canal inlets

Type – 1: Canal over drainage [HFL < FSL]

Aqueduct:

In an aqueduct, the canal bed level is above the drainage bed level so canal is to be constructed above

drainage.

A canal trough is to be constructed in which canal water flows from upstream to downstream. This canal

trough is to be rested on number of piers. The drained water flows through these piers upstream to

downstream.

The canal water level is referred as full supply level (FSL) and drainage water level is referred as high

flood level (HFL). The HFL is below the canal bed level.

Aqueduct is similar to a bridge, instead of roadway or railway, canal water are carried in the trough and

below that the drainage water flows under gravity and possessing atmospheric pressure.
 Fig (a) Aqueduct

Syphon Aqueduct:

In a syphon aqueduct, canal water is carrier above the drainage but the high flood level (HFL) of

drainage is above the canal trough. The drainage water flows under syphonic action and there is no

presence of atmospheric pressure in the natural drain.

The construction of the syphon aqueduct structure is such that, the flooring of drain is depressed

downwards by constructing a vertical drop weir to discharge high flow drain water through the

depressed concrete floor.

Syphonic aqueducts are more often constructed and better preferred than simple Aqueduct, though

costlier.
 Fig (b) Syphon Aqueduct

Type – 2: Drainage over canal (HFL > FSL)

Super Passage:

Super passage structure carries drainage above canal as the canal bed level is below drainage bed

level. The drainage trough is to be constructed at road level and drainage water flows through this from

upstream to downstream and the canal water flows through the piers which are constructed below this

drainage trough as supports.

The full supply level of canal is below the drainage trough in this structure. The water in canal flows

under gravity and possess the atmospheric pressure. This is simply a reverse of Aqueduct structure.
 Fig (c) Super passage

Canal Syphon:

In a canal syphon, drainage is carried over canal similar to a super passage but the full supply level of

canal is above than the drainage trough.so the canal water flows under syphonic action and there is no

presence of atmospheric pressure in canal.

When compared, super passage is more often preferred than canal Syphon because in a canal Syphon,

big disadvantage is that the canal water is under drainage trough so any defective minerals or sediment

deposited cannot be removed with ease like in the case of a Syphon Aqueduct.

Flooring of canal is depressed and ramp like structure is provided at upstream and downstream to form

syphonic action. This structure is a reverse of Syphon aqueduct.

Type –3: Drainage admitted into canal (HFL = FSL)

In this case, the drainage water is to be mixed up with canal water, here the cost of construction is less

but silt clearance and maintenance of canal water becomes really difficult. So the structures falling

under this category are constructed with utmost care.

Level Crossing:

When the bed level of canal is equal to the drainage bed level, then level crossing is to be constructed.

This consists of following steps:

1. Construction of weir to stop drainage water behind it

2. Construction of canal regulator across a canal

3. Construction of head regulator across a Drainage

Functioning of a level crossing:

drainage water from that of canal for certain time interval. Once the drainage is cleared, the head

regulator is closed down. Anyhow, cross regulator is always in open condition throughout year to supply

canal water continuously.

Canal inlets:

In a canal inlet structure, the drainage water to be admitted into canal is very less. The drainage is taken

through the banks of a canal at inlet. And then this drainage mixed with canal travels certain length of

the canal, after which an outlet is provided to create suction pressure and suck all the drainage solids,

disposing it to the watershed area nearby.

There are many disadvantages in use of canal inlet structure, because the drainage may pollute canal

water and also the bank erosion may take place causing the canal structure deteriorate so that

maintenance costs are high. Hence this type of structure is rarely constructed.
Figure (f) Canal inlet plan and cross sectional views (a) pipe type (b) open cut type
2. Draw a neat sketch of a typical cross section of a canal carrying a discharge of 60 m3/sec and lined
with brick in cement motor. Mark the salient features on the sketch. [N/D-16]

What is canal lining?

An impermeable layer is provided at the bed and sides of canal to improve the life and discharge

capacity of canal known as canal lining. Generally seepage can result in losses of 30 – 40 % of irrigation

water in canals, so we can reduce the effect of seepage by providing lining to the canal.

Advantages of Canal Lining:

An important reason for lining a canal can be the reduction in water losses, as water losses in unlined

irrigation canals can be high. Canals that carry from 30 to 150 liters/sec can lose 10 to 15% of this flow

by seepage and water consumption by weeds. Lining a canal will not completely eliminate these losses,

but roughly 60 to 80% of the water that is lost in unlined irrigation canals can be saved by a hard-
surface lining. 42 Canal lining Minimizing water losses is very important, and especially so in schemes

where irrigation water is pumped. Reduced water losses means less water to pump and thus a

reduction in pumping costs.

Seepage reduction:

If canal banks are highly permeable, the seepage of water will cause very wet or waterlogged

conditions, or even standing water on adjacent fields or roads. Lining of such a canal can solve this

problem, since the permeability of a lined canal bank is far less than that of an unlined bank, or may

even be zero, depending on the lining material.

Reduced canal dimensions:

The roughness – resistance to flow – of a lined canal is less than that of an unlined canal, and thus the

flow velocity will be higher in the lined canal when the canal bed slope is the same. Moreover, the hard

surface of the lining material allows a higher velocity compared to an earthen canal surface as it is not

so easily eroded. As discussed earlier, canal discharge is the product of the cross-section of a canal

and the velocity of the flow. Therefore, with the higher velocity allowable and obtainable in lined canals,

the canal cross-section for a lined canal can be smaller than that of an unlined canal.

Reduced maintenance:

A surface lining, such as concrete, brick or plastic, on the canal prevents the growth of plants and

discourages hole-making by rats or termites, and so the maintenance of a lined canal can be easier and

quicker than that of an unlined canal. Moreover, the higher velocity that can safely be allowed in the
lined canal prevents the small particles of soil carried in the water from settling out, accumulating and

causing siltation.

Above mentioned are the major uses of canal lining and some other uses are:

 To avoid piping through and under channel banks

 To avoid water logging of adjacent land

 To reduce movement of contaminated ground water plumes

 To control vegetation in canals

3. List the different types of canal lining in common use. [N/D-16]

Types of canal linings:

Mainly there are two types of canal linings:

1. Earthen type lining

2. Hard surface lining

Earthen type canal linings:

It is sub-divided into 2 major types:

Compacted earth lining

If suitable earthen material is available near the site of construction, or is in-situ, a lining of compacted

earth is an inexpensive and efficient means of controlling seepage. Compaction reduces soil pore sizes

by displacing air and water. Reduction in void size increases the density, compressive strength and
shear strength of the soil and reduces permeability. This is accompanied by a reduction in volume and

settlement of the surface. Proper compaction is essential to increase the stability and frost resistance

(where required) and to decrease erosion and seepage losses.

Soil cement lining

Soil-cement linings are constructed with mixtures of sandy soil, cement and water, which harden to a

concrete-like material. The cement content should be from 2-8% of the soil by volume. However, larger

cement contents are used. For the construction of soil-cement linings two methods are in general use:

(1) the dry-mix method and (2) the plastic mix method. For erosion protection and additional strength in

large channels, the layer of soil-cement is sometimes covered with coarse soil. It is recommended the

soil-cement lining should be protected from the weather for seven days by spreading approximately

50mm of soil, straw or hessian bags over it and keeping the cover moistened to allow proper curing.

Water sprinkling should continue for 28 days following installation.

It is sub divided into:

Cement concrete lining :

Concrete linings are widely used, with benefits justifying their relatively high cost. They are tough,

durable, relatively impermeable and hydraulically efficient. Concrete linings are suitable for both small

and large channels and both high and low flow velocities. They fulfil every purpose of lining.

There are several procedures of lining using cement concrete:

 Cast in situ lining

 Shotcrete lining

 Precast concrete lining

 Cement mortar lining

The canal is said to be lined with bricks when the sides and bed are protected with brick surfacing laid in

cement mortar.

Plastic lining:

As a modern technique use of plastics in canal lining holds good promise. There are three types of

plastic membranes which are used in canal lining, namely:

 Polyvinyl chloride

The plastic as a material for canal lining offers certain characteristic advantages like negligible weight,

easy for handling, spreading and transport, immune to chemical action and speedy construction. The

plastic film is spread on the prepared subgrade of the canal. To anchor the membrane on the banks ‘V

trenches are provided. The film is then covered with protective soil cover.

Boulder lining:

This type of lining is constructed with dressed stone blocks laid in mortar. Properly dressed stones are

not available in nature. Irregular stone blocks are dressed and chipped off as per requirement. It makes

the type costly. When roughly dressed stones are used for lining, the surface is rendered rough which

may put lot of resistance to low. Technically the coefficient of rugosity will be higher. Thus the stone

lining is limited to the situation where loss of head is not an important consideration and where stones

are available at moderate cost.