Design steps of a silt excluder

A silt excluder is a set of tunnels built under the canal head regulator in the river pocket that passes the silty bottom water to the river through the undersluices.

Design steps

1. Fix the crest levels. Undersluice crest level (river-bed level) and the head-regulator crest level, normally 1.2-2.1 m higher. The difference gives the available tunnel height (minus roof slab thickness 0.2-0.3 m).
2. Discharge through the tunnels. Take 20% of the canal discharge (Garg), or the silt-carrying capacity needed.
3. Tunnel velocity. Fix 2.5-3 m/s (non-silting, non-erosive). Required area $A = Q_t/V$.
4. Height of tunnels $h =$ (regulator crest level $-$ undersluice crest level) $-$ roof slab thickness. Total tunnel width $= A/h$.
5. Number and width of tunnels. Adopt 2-4 tunnels with 0.5-0.75 m thick piers so that the total width equals the bay of the undersluice.
6. Length of tunnels equals the width of the head regulator plus the transition, with a gently sloping floor to improve flow.
7. Hydraulic checks. The head causing flow comes from the pond to the tailwater in the river (difference of levels), checked by the orifice formula $Q = C_dA\sqrt{2gH}$.
8. Structural design. Roof slab, piers and floor are designed for water load; the tunnel floor, piles and cut-offs are checked for uplift.
9. Entrance and exit. Wing walls with smooth transitions, so that the silt does not enter the canal; d/s outlet to the sluiceway with an apron.

 SECTION
  pond level ~~~~~~~~~~~~~~~~~~~~~~~~~~~
  regulator crest ____________  (roof of tunnel)
  ---> clear water  |  roof slab  |--> canal
  silty water ---> [  tunnel  ]---> to river
  undersluice crest level  ____________