Contents preface (VII) introduction 1—37

456 IRRIGATION AND WATER RESOURCES ENGINEERING
13.11. SEDIMENT CONTROL IN CANALS
Sediment entering into an offtaking canal, if excessive, causes silting, and thus, reduces canal capacity. Even the fine suspended sediment, in power canals, would cause damage to the turbine blades and, therefore, needs removal prior to the canal water entering the power plant. As such, it is important to control amount of sediment entering into the offtaking canal. In all diversion structures, therefore, provision of adequate preventive or curative measures for sediment control is essential. If a canal offtakes from the outer side of a curved reach of a river, it draws much less sediment than the one offtaking from the inner side. This is due to the secondary flow which develops along the curved reach of a river.
Entry of sediment into the offtaking canal can be controlled by one of the following three methods of barrage regulation:
(i) Still pond method,
(ii) Semi-open flow method, and (iii) Wedge-flow method.

In the still pond method of the barrage regulation the undersluices are kept closed while the canal is taking its supplies, and the surplus water, if available, flows through some weir bays. This causes considerable reduction in the velocity of flow in the undersluice pocket which results in deposition of coarse sediment in the pocket and water containing much less (or no) sediment is drawn by the offtaking canal. However, with increasing amount of deposition of sediment in the pocket, the offtaking canal may start withdrawing sediment as well. At this stage, the canal is closed and the deposited sediment is flushed downstream of the undersluices by opening the undersluices. This methods has been found satisfactory but requires closing of canal at some regular interval.

Alternatively, the undersluice gates are kept partially open while the canal is withdrawing its supplies. This semi-open flow method of barrage regulation results in continuous flushing of sediment through the undersluices while the canal is withdrawing top layer water which contains much less sediment. Besides requiring surplus water, this method results in two streams – one entering the canal and the other entering the undersluices which may generate enough turbulence in the pocket upstream of the head regulator and thus bring sediment into suspension. The suspended sediment may enter the canal. Also, the method would not work satisfactorily if there is no surplus water. This method is, therefore, not suitable except during floods.
In the wedge-flow system of barrage regulation, the undersluices near the divide wall are opened more while those near the head regulator are opened less. This results in wedge-like flow cross-section which causes favourable curvature of flow in the undersluice pocket, and thus, reduces the amount of sediment entering the canal.
When the stream is carrying high sediment load, the sediment entry into the offtaking canal can be best checked by closure of the canal itself.
One of the most commonly used preventive measures is the sediment excluder (also known as silt excluder). The excluder is constructed in the river bed in front of the canal head regulator to prevent, as far as possible, excess sediment entering into the offtaking canal. Figure 13.11 shows a typical layout of a tunnel-type sediment excluder. The tunnels of the excluder, used for flushing the sediments, are parallel to the axis of the canal head regulator and are of different lengths, and the tunnels terminate at the end of the undersluice bays. Some kind of sediment control devices, such as skimming platform (6) and curved wings with

CANAL HEADWORKS

457

sediment vanes (7) are provided in case of channels offtaking from main canals or branch canals for proportionate distribution of sediment. If the offtaking canal has already drawn more sediment, curative measures, such as construction of sediment ejectors, are adopted. A sediment ejector (or extractor), also known as silt ejector, is a curative measure and is constructed in the offtaking canal downstream of the canal head regulator to remove the excess sediment load which has entered the canal. Alternatively, a settling basin can be constructed in the offtaking canal for the purpose of sediment ejection.

Weir axis




A

Excluder width, B_ex + wall thickness

A
Axis of head regulator
Plan
Pond level




Undersluice gate
Tunnel depth

Glacis of undersluice