Canal Linings – Various types of canal lining - their
related advantages and disadvantages
Canal Linings are provided in canals
to resist the flow of water through its bed and sides. These can be constructed
using different materials such as compacted earth, cement, concrete, plastics,
boulders, bricks etc. The main advantage of canal lining is to protect the
water from seepage loss.
What
is Canal Lining?
Canal Lining is an impermeable layer
provided for the bed and sides of canal to improve the life and discharge
capacity of canal. 60 to 80% of water lost through seepage in an unlined canal
can be saved by construction canal lining.
Fig 1 Canal Lining
Types
of Canal Linings
Canal linings are classified into
two major types based on the nature of surface and they are:
- Earthen type lining
- Hard surface lining
1.
Earthen Type lining
Earthen Type lings are again
classified into two types and they are as follows:
- Compacted Earth Lining
- Soil Cement Lining
Compacted Earth Lining Compacted earth linings are preferred for the canals when
the earth is available near the site of construction or In-situ. If the earth
is not available near the site then it becomes costlier to construct compacted
earth lining. 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.
Fig 2: Compacted Earth Lining
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 minimum 2-8% of the soil by volume. However, larger cement
contents are also used. In general, for the construction of soil-cement linings
following two methods are used.
- Dry-mix method
- 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 50 mm 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.
Fig 3: Soil Cement Lining
2.
Hard Surface Canal Linings
It is sub divided into 4 types and
they are
- Cement Concrete Lining
- Brick Lining
- Plastic Lining
- Boulder Lining
Cement Concrete Lining Cement 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 fulfill
every purpose of lining. There are several procedures of lining using cement
concrete
- Cast in situ lining
- Shortcrete lining
- Precast concrete lining
- Cement mortar lining
Fig 4: Cement Concrete Lining
Brick Lining In case of brick lining, bricks are laid using cement
mortar on the sides and bed of the canal. After laying bricks, smooth finish is
provided on the surface using cement mortar.
Fig 5: Construction of Brick Canal
Lining
Plastic Lining Plastic lining of canal is newly developed technique and
holds good promise. There are three types of plastic membranes which are used
for canal lining, namely:
- Low density poly ethylene
- High molecular high density polythene
- Polyvinyl chloride
The advantages of providing plastic
lining to the canal are many as plastic is negligible in weight, easy for
handling, spreading and transport, immune to chemical action and speedy construction.
The plastic film is spread on the prepared sub-grade of the canal. To anchor
the membrane on the banks ‘V trenches are provided. The film is then covered
with protective soil cover.
Fig 6: Plastic Canal Lining
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. When
roughly dressed stones are used for lining, the surface is rendered rough which
may put lot of resistance to flow. 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.
Fig 7: Canal Lined with Boulders
Advantages
of Canal Lining
- Seepage Reduction
- Prevention of Water Logging
- Increase in Commanded Area
- Increase in Channel Capacity
- Less Maintenance
- Safety Against Floods
1.
Seepage Reduction
The main purpose behind the lining
of canal is to reduce the seepage losses. In some soils, the seepage loss of
water in unlined canals is about 25 to 50% of total water supplied. The cost of
canal lining is high but it is justifiable for its efforts in saving of most of
the water from seepage losses. Canal lining is not necessary if seepage losses
are very small.
2.
Prevention of Water Logging
Water logging is caused due to
phenomenal rise in water table due to uncontrolled seepage in an unlined canal.
This seepage effects the surrounding ground water table and makes the land
unsuitable for irrigation. So, this problem of water logging can be surely
prevented by providing proper lining to the canal sides.
3.
Increase in Commanded Area
Commanded area is the area which is
suitable for irrigation purpose. The water carrying capacity of lined canal is
much higher than the unlined canal and hence more area can be irrigated using
lined canals.
4.
Increase in Channel Capacity
Canal lining can also increase the
channel capacity. The lined canal surface is generally smooth and allows water
to flow with high velocity compared to unlined channel. Higher the velocity of
flow greater is the capacity of channel and hence channel capacity will increase
by providing lining. On the other side with this increase in capacity, channel
dimensions can also be reduce to maintain the previous capacity of unlined
canal which saves the cost of the project.
5.
Less Maintenance
Maintenance of lined canal is easier
than unlined canals. Generally there is a problem of silting in unlined canal
which removal requires huge expenditure but in case of lined canals, because of
high velocity of flow, the silt is easily carried away by the water. In case of
unlined canals, there is a chance of growth of vegetation on the canal surface
but not in case of lined canals. The vegetation affect the velocity of flow and
water carrying capacity of channel. Lined canal also prevents damage of canal
surface due to rats or insects.
6.
Safety against Floods
A line canal always withstand
against floods while unlined canal may not resists and also there is chance of
occurring of breach which damages the whole canal as well as surrounding areas
or fields. But among the all concrete canal linings are good against
floods or high velocity flows.
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