Contents preface (VII) introduction 1—37

13.5. UNDERSLUICES
The construction of weir across a river results in ponding up of water and causes considerable sediment deposition just upstream of the canal head regulator. This sediment must be flushed downstream of the weir. This is done by means of undersluices (also called sluice ways or scouring sluices). A weir generally requires deep pockets of undersluices in front of the head regulator of the offtaking canal, and long divide wall to separate the remaining weir bays from the undersluices. The undersluices are the gate-controlled openings in continuation of the weir with their crests at a level lower than the level of the weir crest. The undersluices are located on the same side as the offtaking canal. If there are two canals each of which offtakes from one of the banks of the river, undersluices are provided at both ends of the weir.
The undersluices help in keeping the approach channel of the canal head regulators relatively free from deposition of sediment, and minimise the effect of the main river current on the flow over the head regulator. In addition, the undersluices are also useful for passing low floods, after meeting the requirements of the offtaking canal, without having to raise the gates or drop the weir shutters. With the provision of the undersluices, the weir shutters have to be dropped (or the gates have to be raised) only to pass the high floods which occur only during the monsoon. The shutters are again raised (or the gates are lowered) at the end of the monsoon.
The level of the crest of an undersluice is related to: (i) cold weather river bed level at the site of the weir, and (ii) the crest level of the head regulator. The crest of the undersluices generally coincides with the lowest cold weather level of the river bed at the site of the weir. Also, the crest of the undersluices should be kept at least 1.20 m below that of the head regulator so that the sediment deposited upstream of the undersluices (and the head regulator) does not enter the offtaking canal and can be carried to the downstream of the undersluices. If a sediment excluder (a structure to reduce entry of sediment into the canal) is to be provided, it becomes necessary to lower the crest of the undersluices to about 2.0 m below the crest of the head regulator. Alternatively, the crest of the regulator is raised.
The discharge capacity of the undersluices is kept equal to the maximum of the values given by the following three considerations (2):
(i) To ensure sufficient scouring capacity, the capacity of the undersluices should be at least twice the canal discharge.
(ii) To reduce the length of the weir, the undersluices should be capable of passing about 10 to 20 per cent of the maximum flood discharge at high floods.
(iii) The undersluice should possess enough capacity to pass off low floods with the water surface in the reservoir at pond level so that the need of lowering the weir shutters (or raising the weir gates) does not arise.

CANAL HEADWORKS

441

The longitudinal section of the undersluices will be similar to that of a weir and, hence, the design procedure is the same as that of a concrete weir. However, because of the larger discharge per unit length, the undersluices would need much heavier protection downstream. When sediment excluders are provided, the width of undersluices is determined by the velocity required to induce siltation. On all major headworks, the design of undersluices and divide wall is finalised on the basis of model investigations.