CANAL DESIGN ASPECTS

ISSUES IN CANALS Shape of Cross Section –

Hydraulically efficient section minimizes the area required to convey a specified discharge. However, the area which required to be excavated may be much larger if one considers the removal of the over burden.

If the channel is to be lined, the cost of the lining may be comparable with the cost of excavation. Excavation Cost – Material to be removed, disposal of the material removed

Side slope of the canal - IS 4701 (1982)

Longitudinal Slope Topography, the head required to carry the design flow. Slopes vary from 1 : 4000 to 1 : 20000

Permissible Velocities - Minimum and Maximum "Minimum permissible velocity" refers to the smallest velocity which will prevent both

sedimentation and vegetative growth in general. Maximum permissible velocities entirely depend on the material that is used and the bed slope

of the channel. Maximum permissible velocity is for the normal canals built for irrigation purposes and Power

canals in which the energy loss must be minimized

Resistance to the flow Rate of flow is inversely proportional to the surface roughness

Freeboard The vertical distance between the top of the channel and the design flow at normal depth. To prevent the overtopping of either the lining or the top of the channel fluctuations in the water

surface Wind - driven waves Tidal action Hydraulic jumps Super elevation of the water surface as the flow goes round curves at high velocities Interception of storm runoff by the channel Occurrence of greater than design depths of flow caused by canal sedimentation or an

increased coefficient of friction Temporary mis-operation of the canal system For semi-circular channels, when the velocities < 0.8 times the critical velocity then 6% of the

diameter as free board.

FREE BOARD

freeboard for an unlined channel can be obtained from USBR (United States Bureau of Reclamation) formula.

= Freeboard in feet, y = design depth of flow in feet,

C = coefficient, (1.5 for Q = 0.57 m3/s to C = 2.5 for Q = 85 m3/s)

CyFB

BF

LINING OF CANALS (a) Reduction of seepage losses resulting in saving of water which can be utilized for additional irrigation.

(b) Prevention of water logging by reducing seepage to water-table.

(c) Reduction in area of cross-section (and there by saving in land) due to increase in permissible velocity by reduction in the value of rugosity and availing of steeper slope, where available.

(d) Improvement of discharging capacity of channels.

(e) Improvement of operational efficiency.

(f) Prevention of weed growth.

(g) Reduction of maintenance cost.

(h) Long economic life

(i) Insure Cross section stability from scour, low flow conditions etc.

BERMS FOR CANALS• Berms are provided at canal banks in deep cutting

• Serve as a road for inspection vehicles

• Help to absorb any soil or rock that may drop from the cut-face of soil or rock of the excavations.

• Berm width may be kept at least 2m. If vehicles are required to move, then a width of at least 5m may

be provided.

• Berms run parallel to the main channel

• Berms have the same gradient as the gradient of the main channel;

DIVERSION HEAD WORKS

DIVERSION HEAD WORKS

Diversion Head Works:

Constructed at the head of the canal to divert the river flow water towards the canal and to regulate continuous supply of silt-free water with a certain minimum head.

Storage Head Works

Stores water in addition to diversion into the canal

Consists of dam constructed across a river to create a reservoir

Water will be supplied from the reservoir according to the demand.

Diversion Head Works

Rises the water level in the river to increase the command area

Regulates the supply of water into the canal

Controls the entry of silt into the canal

Reduces fluctuations in the level of supply in the river

COMPONENTSWeir: •Structure constructed across a river to rise its water level and divert the water into the canal.

•Aligned at right angles to the direction of flow of the river.

•It may be provided with small shutters (gates) on its top.

•Most of the raising of water level or ponding is done by the solid weir wall and little with by the shutters.

•Initial cost of the weir is very low

Barrage:

•A barrage has a low crest wall with high gates.

•As the height of the crest above the river bed is low most of the ponding is done by gates.

•During the floods the gates are opened so afflux is very small

•Will have a good control during floods, the outflow can be easily regulated by gates.

•Initial cost of the Barrage is quite high

DIVIDE WALLA divide wall is constructed parallel to the direction of flow of river to separate the weir section and the under sluices section to avoid cross flows.

Divide the river width into two portions; Weir portion and under sluice portion

If there are under sluices at both the sides, there are two divide walls

DIVIDE WALL•The divide wall is a long wall constructed at right angles in 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.

•It helps in providing a comparatively less turbulent pocket near the canal head regulator, resulting in deposition of silt in this pocket and, thus, to help in the entry of silt-free water into the canal.

•It helps to keep cross-currents away from the weir.

•The top width of the divide wall is about 1.5 to 2.5 m.

•Designed as cantilever retaining walls subjected to silt pressure and water pressure from the under-sluice side.

UNDER SLUICES OR SCOURING SLUICES•openings provided at the base of the weir or barrage, provided with adjustable gates.

•The crest of the under sluice portion of the weir is kept at a lower level (1 to1.5 m) than the crest of the normal portion of the weir.

•The crest level of the canal head regulator is kept high than the crest level of the under-sluices, so that only silt free water is admitted into the canal through the head sluices.

The main functions of under-sluices are:

o To maintain a well defined deep channel approaching the canal head regulator.

o To ensure easy diversion of water into the canal through the canal head regulator even during low flow.

o To control the entry of silt into the canal

o To help scouring and of the silt deposited over the under-sluice floor and removing towards the downstream side.

o To help passing the low floods without dropping the shutters of the weir.

FISH LADDER A narrow opening including suitable baffles or staggering devices in it is provided adjacent to the divide wall.

The fish ladder is provided just by the side of the divide wall for the free movement of fishes. Rivers are important source of fishes.

In a fish ladder the head is gradually dissipated so as to provide smooth flow at sufficiently low velocity.

In the fish ladder, the fable walls are constructed in a zigzag manner so that the velocity of flow within the ladder does not exceed 3 m/sec

The width, length and height of the fish ladder depend on the nature of the river and the type of the weir or barrage.

CANAL HEAD REGULATOR A structure which is constructed at the head of the canal to regulate flow of water is known as canal head regulator.

Consists of a number of piers which divide the total width of the canal into a number of spans which are known as bays. The piers consist of number tiers on which the adjustable gates are placed. The gates are operated form the top by suitable mechanical device. A platform is provided on the top of the piers for the facility of operating the gates.

CANAL HEAD REGULATOR

Functions of Canal Head Regulator:

o It regulates the supply of water entering the canal

o It controls the entry of silt in the canal

o It prevents the river-floods from entering the canal

The water from the under-sluice pocket is made to enter the regulator bays, so as to pass the full supply discharge into the canal.

The entry of silt into the canal is controlled by keeping the crest of the head regulator by about 1.2 to 1.5 meters higher than the crest of the under-sluices.

If a silt-excluder is provided, the regulator crest is further raised by about 0.6 to 0.7 meter.

Silt gets deposited in the pocket, and only the clear water enters the regulator bays.

The deposited silt can be easily scoured out periodically, and removed through the under-sluice openings.

CANAL HEAD REGULATOR

RIVER TRAINING WORKS Required near the weir site in order to ensure a smooth and an axial flow of water, and thus, to prevent the river from outflanking the works due to a change in its course.

The river training works required on a canal headwork are:

Guide banks

Guide banks are provided on either side of the diversion head works for a smooth approach and to prevent the river from outflanking.

Marginal bunds

Marginal bunds are provided on either side of the river u/s of diversion head works to protect the land and property which is likely to be submerged during ponding of water in floods.