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Silt excluder, silt ejector, sediment control

Short answer

A silt excluder is placed in the pocket before the head regulator and diverts the bed-load layer through tunnels back to the river, preventing silt entry. A silt ejector is located in the canal downstream and extracts already-entered bed silt via bed tunnels, returning it to the river.

Differentiate between a silt excluder and a silt ejector (extractor). Explain, with sketches, how they control the bed load (sediment entry) into the irrigation canal at the headworks.

Silt excluder and silt ejector

A silt excluder prevents silt from entering the canal; a silt ejector removes silt that has already entered the canal.

PointSilt excluderSilt ejector (extractor)
LocationIn the river pocket in front of the head regulator, at the undersluice floor levelIn the canal, a short distance downstream of the head regulator
ActionDiverts the silt-laden bottom layer to the river before it enters the canalExtracts the bed silt from the canal water and returns it to the river
PrincipleTunnels below the regulator crest; the clearer upper layer enters the canalTunnels on the canal bed under a diaphragm slab: the upper water goes on, the bottom layer passes through the tunnels
Position of regulator crestRaised above the sluice crest so only the upper water is drawnNormal regulator crest; silt is already in the canal
EfficiencyBetter (keeps silt out)Less (deals with silt already in the canal, and the canal loses water)
Water usedAbout 20% of the canal discharge passes through the tunnels to the riverAbout 10-20% of the canal flow is escaped back to the river
CostModerateMore, and needs a good outfall into the river

Working and sketches

Silt excluder (plan and section through tunnels):

PLAN
 River flow -->
 ____________________________________
 |  under-    | tunnels ->   |  river
 |  sluice    |==============|  (d/s)
 |  pocket    |==============|
 |____________|  head        |
              | regulator    |-> canal
 -------------------------------------
SECTION (through the pocket)
 water level ~~~~~~~~~~~~~~~~~~~~~~~~~
   regulator crest (high)  +-----+
 ------------------------  |     |-> canal
   silty bottom layer -->  [==tunnel==]-> river
 undersluice bed level ____|_________

The tunnels have their roofs at the level of the canal regulator crest. Clear water from the upper layers flows over the tunnel roof into the canal. The bottom layer, which carries the bed load, passes through the tunnels and is released through the undersluices into the river.

Silt ejector (on the canal):

 canal -->  [  ejector at   ] -->  canal continues
            |  ===tunnels===|--> escape to river
 section: bed silt enters the tunnel because the
 tunnel is placed on the canal bed, with a lower
 level outfall into the river.

The canal water passes over the tunnel roofs; the silt-laden bottom layer enters the tunnels (formed below a horizontal diaphragm slab a little above the canal bed) and goes through an escape channel back to the river. The tunnels are designed for about 10-20% of the canal discharge, with a velocity of about 2.5-3 m/s so that the silt keeps moving.

Control of bed load

  • The undersluices are kept at a low level so that the flow in the pocket scours the deposits and sends the bed silt downstream.
  • The canal head regulator crest is placed about 1.2-2.0 m above the sluice crest, so mainly the upper layer, with less silt, is drawn.
  • The silt excluder diverts the bed load in the pocket, and the ejector removes what still enters, so silting of the canal is reduced and the sediment is returned to the river.

✓ Answer checked by Claude Opus 5.5 (high effort)

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