Silt Controlling Works

Silt Controlling Works Points : silt controlling works, silt control at headworks, silt excluder, silt extractor or slit ejector Silt Control at Headworks The entry of silt into the canal can be controlled by:

(1) Providing a divide wall in the river at the canal side so as
(a) to create a trap or pocket.
(b) to create the scouring capacity of under sluices by concentrating the current towards them.

(2) Paving the bottom of the approach channel to reduce disturbance.

(3) Installing a silt excluder.

(4) Making entry of clear top water in the canal by
(a) providing raised sill in the canal.
(b) lowering sill level of scouring sluices.

(5) Reducing the velocity of water at the intake by providing wider head regulator.

(6) Avoiding unsteady flow by making the i.mry smooth.

(7) Handling carefully the regulation of weir.
There are two types of special works constructed to control the silt entering into the canal
1. Silt excluder.
2. Silt extractor. Silt Excluder Silt excluder is a device by which silt is excluded from water entering the canal. It is constructed in the bed in front of head regulator. The fundamental principle on which a silt control device acts lies in the fact that in a flowing stream carrying silt in suspension, the concentration of silt charge in the lower layer, is greater than in the upper ones. Hence, the device is so designed that the top and bottom layers are separated without any disturbance. The top water is then led towards the canal while the bottom water containing high silt charge is wasted shows a silt excluder - - (a type used at Khanki weir). The excluder consists of a number of under tunnels resting on the floor of the pocket. The top level of the R.C. roof of the tunnels is kept the same as the sill level of the head regulator. The various tunnels are made of different lengths — (the one near the head regulator) being of same length that of the width of the head regulator and the successive tunnels being of decreasing lengths as shown. This arrangement separates the water into two clear layers. The top layer (above the roof of the under tunnel enters the head regulator, while the bottom layer containing relatively heavier silt charge goes to the under tunnels and discharges to the d/s of the river through under sluices. The capacity these tunnels is kept about 10 % of the canal discharge, and the tunnels are so designed that a minimum velocity of 2 to 3 m/sec is maintained. Knowing the discharge and the scouring velocity the total waterway required for the under tunnels can be determined. Silt Extractor or Slit ejector A silt extractor or silt ejector is a contrivance by which the silt after it has entered the canal is extracted or thrown out. While a silt excluder is a preventive measure, the silt extractor is a curative measure and is constructed on the canal at some distance away from the head regulator. It consists of a horizontal diaphragm placed slightly above the canal bed. The canal bed is depressed slightly below the diaphragm so that the height of the silt tunnels below the diaphragm is about 0.5 to 0.8 metres. The diaphragm thus separates the top water level from the bottom layer containing heavier silt charge. The disturbance at the entry of the tunnel is reduced by proper design. However, to prevent the tunnel from being clogged, the velocity is accelerated quickly by steadily reducing sectional area of the tunnels by stream-lined vanes. The discharge through the tunnels are governed by the gate opening of the escape chamber. The escape channel is given a steeper slope so that the silt is discharged back to the river through the shortest route.