Silt control in Irrigation Canals The problem of sediment transport and its control

has always been a challenge to designers ofirrigation systems.

Unlined canals can get choked or silted by Unlined canals can get choked or silted bysediment brought by the river water.

Some examples are discussed as to highlight howenormous is the magnitude of silt tonnage is

The River Sutlej transports around 35 millions tons of

sediment per year to Sulamanki Barrage,

River Indus carried a total load of 440 millions tons per

year at Tarbela.

River Jhelum carries 70 millions tons approximately

annually.

The Warsak reservoir on River Kabul built in 1960, had initiallive storage of 23,000 acre feet which in the first ten yearsreduced to a residual minimum of 10,000 acre feet.

Silt control in Irrigation Canals Tarbela reservoir with initial live storage of 9.3

million acre feet when completed in 1975, willreduce to one million acre feet in fifty years.

The Mangla reservoir will loose 30% of its live The Mangla reservoir will loose 30% of its livestorage in same period.

Yangteze River is called Yellow River because of itscolour due to heavy sediment load it carries all theyear averaging 61 lbs per cu.ft of water.

Silt control in Irrigation Canals The construction of the Mangla and Tarbela

reservoirs on the two main Rivers has partlyreduced the silt, but the problem of silting of thecanal system continues to persist to a great extent.canal system continues to persist to a great extent.

The intake of Upper Bari Doab (UBD) canal onSutlej River at Madhopur in India got completelysilted up soon after the construction of apermanent weir in 1870. The amount of siltentering UBD canal has been estimated at 168,000cu ft to 2268000 cu ft per day ( in years of 1939 to1949)

Silt control in Irrigation Canals The Marala Ravi link Canal taking off at Marala

barrage on the river Chenab has silted up to depthof 9 ft (maximum) out of a total design depth of14.5 ft in its upper reach.14.5 ft in its upper reach.

Silt control in Irrigation Canals On contrary the building of High Aswan dam on

the Nile, has created erosion problems in the canalsystem downstream.

The lake of High Aswan dam excludes the silt from The lake of High Aswan dam excludes the silt fromflowing downstream, depriving the farmers of therich soil they used to receive along with theirrigation water, in addition to serious problems ofdeep scouring of the foundations of hydraulicstructures located downstream of the dam.

Silt control in Irrigation Canals Here we will discuss various methods that have

been adopted to exclude and eject silt from canalor to distribute it in such a manner that all the siltentering the canal passes on the fields to add theentering the canal passes on the fields to add thefertility of the soil. There are four possible methods

i) arranging the head works as to exclude the silt asmuch as possible from the canal

ii) To make arrangements to eject the silt whichhas already entered the canal, or properlydistribute it to the off taking distributary

Silt control in Irrigation Canals iii) to design an unlined canal which will produce the

required non silting and non scouring velocity, inother words a design that will ensure that the amountof the silt entering the canal is passed on to the field.of the silt entering the canal is passed on to the field.

iv) to design the outlets and their setting so as to drawan equitable share of silt.

Silt control in Irrigation Canals The following diagram summarizes the approach to

the problem of silt control

By exclusion of silt at the entrance

By ejection of silt from the canal

By proper channel design

By proper design and setting of outlets

Exclusion of Silt at Entrance

Divide Wall and Pocket

Training wall and river bend

Silt Excluder

Divide Wall and pocket A divide wall parallel to the head regulator creates a pocketin front of the canal entrance where silt is deposited in theriver bed due to reduction in velocity.

This measure was first proposed by Kennedy in 1904 when This measure was first proposed by Kennedy in 1904 whenSirhind canal threatened almost to choke the regulator dueto excessive silt.

A divide wall was suggested and along with an undersluiceto clear the deposited silt.

The undersluice gates are closed when the water is flowinginto the canals. The necessary pond level is maintained byworking the gates of normal weir section.

Divide Wall and pocket

Divide Wall and pocket The sediment deposited in the pocket is washed away

by closing the canal regulator gates and opening the undersluices gates.

Training wall and River Bend Curved training wall in front of the head regulator on

small canals where flow rate and sediment dischargeboth fluctuate have been used successfully by USBR.

The Woodstone diversion dam on the south fork, River The Woodstone diversion dam on the south fork, RiverSolomon, Kansas is an example.

In order to work the system properly, the structuremust have enough water available for sluicing.

Training wall and River Bend Natural river curvature can be exploited with an

advantage as a silt exclusion device by locating thebarrage on the bend and the canal regulator on theoutside of the curve.outside of the curve.

The heavy load swept inside the curve and thesediment concentration on the outside is lower than atother points.

This effect is due to the spiral flow as explained byThompson.

Silt Excluders The idea of the silt excluder was first presented by

Elsdon in his Irrigation Branch paper No. 5 in 1992. thefirst silt excluder was designed by Nicolson at Khankihead works in 1934.head works in 1934.

The basic idea behind the design is that the lowerlayers of the flowing water carry higher concentrationof silt and therefore of the upper layers of the wateronly can be skimmed into the canal, all the rolling bedsilt and the silt in the lower layers is excluded.

Silt Excluders This is achieved by a silt excluder. This is a diaphragm

slab supported on a number of tunnels. Tunnels areplaced parallel to head regulator and discharge d/sthrough the undersluice.through the undersluice.

The water above the silt excluder slab containing lesssilt is then diverted into the canal. The followingpoints should be kept in mind while designing a siltexcluder.

Silt Excluders

Silt Excluders

Silt Excluders 1. The tunnel discharge through the under-sluice is

recommended to be 20% of the canal discharge.

2. The silt excluder should cover only two bays of the under-sluice as this was found to be more efficient in the under-sluice as this was found to be more efficient in the model studies of Kalabagh barrage than a silt excluder covering four bays.

3. The approach channel need not be lined.

4. The divide wall should be 1.2 to 1.4 times the head regulator length.

Silt Excluders 5. The top of the silt excluder slab should be flushedwith the head regulator crest, i.e. the clear height of thetunnels would be 1/3 the depth of the water minus the slabthickness.

6. The roof slab should be designed to carry a full water 6. The roof slab should be designed to carry a full waterload in case the tunnels are empty.

7. The first tunnel should cover all the head length.

8. The discharge through the tunnels will depend uponthe head measured above the centre line of the tunnel.Tunnels can be treated as box culverts.

9. The velocity in tunnels should be 6 ft/sec to 10 ft/sec.

Silt Ejector It employees the same principle of sediment removal

as the silt excluder except that it is placed in the bed ofthe canal and is located about 1000 yards d/s of thehead regulator.head regulator.

It consists of a horizontal slab a little above the canalbed, which separates out the bottom layers. Under theslab there are tunnels to eject heavy silt laden bottomwater in an escape channel. For designing of siltejector the following points should be kept in mind

Silt Ejector

Silt Ejector 1. It should be located about 1000 yards d/s of the

head regulator.

2. The bed width of the canal is divided into anumber of tunnels. These tunnels curve to right or leftnumber of tunnels. These tunnels curve to right or leftand pass under the canal bank to terminate in aregulator, which is provided with gates to regulate thedischarge

3. The height of the tunnel should be 20 to 25% ofthe design depth of water in canal.

4. The top slab of the tunnels usually project 1.5 ft to2 ft U/S at the entrance.

Silt Ejector 5. 20% of the canal discharge is usually diverted into

the ejector. This means that 20% additional discharge over and above the canal design discharge is allowed to enter the canal at the head regulator.enter the canal at the head regulator.

6. The method of calculating the discharge is the same as that for the silt excluder.

7. Normally a minimum head of at least 2.5 ft is required to operate the ejector.

8. A velocity of 8 ft to 10 ft/sec through the tunnel is adequate to move sand size sediment.