Undersluice functions and design criteria

Why an undersluice is provided

Silt tends to deposit in front of the canal head regulator because the velocity near the pocket is low. An undersluice (scouring sluice) is a set of gated bays at low level beside the head regulator. It is provided to keep a clear, deep channel in front of the regulator so that the canal gets silt-free water.

Major functions

1. Scours out the silt deposited in the pocket in front of the head regulator, by opening the gates (the sluice acts like a drain for the pocket).
2. Maintains a well-defined channel from the river to the regulator, particularly in the dry season when the river flow is low.
3. Passes low floods (and the surplus) without dropping the weir shutters, so the pond level and the supply to the canal are kept.
4. Draws the bottom, silt-laden layer away from the canal (induced curvature, together with the silt excluder).
5. Passes part of the high flood and floating debris, which reduces the afflux and the load on the weir.

Design criteria

• Capacity: at least twice the canal discharge (to give a scouring velocity), the dry-season flow of the river, or 10-15% of the maximum flood, whichever is the greatest.
• Crest level: at the average river-bed level (about 1.2-2.0 m below the canal head regulator crest).
• Waterway: total width about 10-20% of the weir width; gate width 5-8 m with piers 1.5-2.0 m; at least 2 bays.
• Velocity: through the sluices, 2.0-3.0 m/s, enough to carry the bed material.
• Discharge formula: $Q = C\,L\,H^{3/2}$ (free flow, $C \approx 2.0$), or orifice flow when the gates are partly open.
• Floor and protection: designed against uplift (Khosla) like the weir, with u/s and d/s aprons, cutoffs, and a stilling basin for the hydraulic jump.