• Excess silt entering into the main canal has to be prevented by all the possible means.
  • If excess silt gets to enter the main canal, it will cause silting of the whole canal system and ultimately.
  • The capacity of the canal will be reduce.
  • The silt entry into the main canal from a river can be controlled by the following measures.
  1. Divide wall in the river creates a quiet pocket behind the head regulator.
  2. The quiet pocket renders top layers of water silt free and only top layers of water are admitt into the canal.
  3. Provid raised crest for the head regulator.
  4. This measure automatically eliminates lower silt-laden layers of water.
silt excluder
silt excluder
  1. Providing a wide head regulator also causes less entry of silt into the canal.
  2. The entry of water into the canal should be smooth.
  3. Smooth entry does not cause any disturbance and as such less silt enters the canal.
  4. By adopting a still pond system of operation of the head regulator.
  5. Installing silt excluders in the river, U/S of a head regulator.
  6. Installing silt ejectors in the head reaches of the main canal to withdraw silt-laden water from the canal.
  7. The two measures ‘still excluders’ and ‘still ejectors’ have discussed herein in detail.

1. Silt excluder.

  • It is a device by which silt-laden bottom layers of river water are separated from the top comparatively clear layers of water.
  • This device is located in the river bed just U/S of the head regulator.
  • It consists of a number of rectangular tunnels, running parallel to the axis of the head regulator and terminating close to under sluices.
  • The top-level of the die roof slab of the excluder tunnels are kept at the same level as the head regulator crest. The silt excluder consists of a number of tunnels each of different lengths.
  • The tunnel near the head regulator is almost of the same length as the width of the head regulator, but each successive tunnel decreases in length.
  • The water which enters the tunnels is ultimately discharging to the D/S side by the under sluices which are kept partially open up to the level of the roof slab on the tunnels.
  • Usually, two to three days of the under sluices are cover by the excluder.
  • The capacity of the excluder tunnels should be about 20% of the canal discharge and the minimum velocity of flow through the tunnels. 2 to 3 m/sec.
  • Total numbers of tunnels can be work out, once the discharge and velocity of flow are decid for the silt excluder.

Silt ejector.

  • It is sometimes also know silt extractor.
  • It is a device by which the silt laden water which has already enter the main canal somehow, is extract.
  • This is a construct in the canal at some distance D/S of the head regulator.
  • It consists of curved tunnels, locate across the canal.
  • Curved tunnels start along the axis of the canal, and then take turn towards a bank.
  • The bed of the canal, where tunnels for silt ejector are to be locate, is lightly depressed.
  • The tunnels are cover by an R.C.C. roofing slab like silt excluder.
  • The top of the roofing slab is kept slightly above the bed level of the canal.
  • The height of tunnels is kept about 60 cm for sandy rivers and 1.20 m for boulder stage rivers.
  • The velocity of flow is maintaine at about 3 m/sec.
  • All the tunnels are provided with gates at the exit end.
  • The silt ejector is locate at a point where the main canal is crossing some natural drainage.
  • The water from the silt ejector is discharge into the drainage, which leads this water back to the river somewhere D/S.
  • In order to accelerate the velocity of flow in the tunnels, the sectional area of the tunnels is reduce by streamline vanes.
  • The radius of bend of the tunnels varies from 10 to 15 times the tunnel width.
  • Bottom layers of heavily silt-laden water of the canal, enter the tunnels and get separat from top layers of water, having comparatively less amount of silt.
  • The silt water entering the tunnels is led out of the canal and discharge into the natural drainage by opening the gates.
  • Silt ejectors are usually design for 20% of the canal discharge.