Contents preface (VII) introduction 1—37

Section through a tunnel
Pond level
Crest of head regulator

Section A-A
Fig. 13.11 Typical layout of a sediment excluder
13.11.1. Sediment Excluders
The sediment concentration is maximum in the bottom layers of a stream. Tunnel-type sediment excluders (Fig. 13.11) prevent these bottom layers from entering the offtaking canal and allow only the top layers of the stream, containing relatively less sediment, to enter the offtaking canal. Such an excluder was first conceived by Elsden in 1922 and was first constructed by Nicholson for the Lower Chenab canal at Khanki headworks in Punjab. It was followed by another at Trimmu headworks. The tunnel-type sediment excluder, provided in front of the Nangal hydel channel, has been divided into two chambers – upper and lower – such that the heights of the upper and lower chambers are in the ratio of 2 : 1, and each of them has a separate control. The upper chamber is operated only during very high floods. This reduces the escape discharge at low flows.
The present design procedure for designing a sediment excluder is based on thumb rules evolved from past experiences on such structures. The minimum discharge passing through

458 IRRIGATION AND WATER RESOURCES ENGINEERING
the tunnels of the excluder is kept around 20 per cent of the canal discharge. The self-flushing velocity in the tunnel ranges from 1.8 to 4 m/s depending upon the sediment size.
Usually, 2 to 6 tunnels are provided in an excluder. The tunnels are to be accommodated in the space between the undersluice floor and the crest of the head regulator. The height of the tunnels is, thus, determined keeping in view convenience for inspection and repair, as well as self-flushing velocity. One can now estimate the width of waterway required for tunnels. This width is divided into a suitable number of tunnels such that a whole number of tunnels are accommodated in one undersluice bay. These tunnels are usually of rectangular cross-section and are bell-mouthed at the upstream end.
The water and sediment discharge carried by all the tunnels of an excluder should be the same. Also, the depth of all the tunnels are kept the same. Therefore, in accordance with the resistance and continuity equations, the width of the shorter tunnel will be smaller than that of the longer tunnel so that the head losses are the same in all tunnels.
The length of the tunnel nearest the head regulator must be equal to the length of the head regulator. Other tunnels are successively shorter in length such that the mouth of the one nearer the head regulator comes within the suction zone of the next tunnel so that no dead zone is left between the adjacent tunnels to cause sediment deposition. The excluder designed in this manner is usually tested through model study and is suitably modified, if required.
The design procedure outlined above takes into account the concepts of minimum energy loss and self-flushing velocity. It does not consider the actual sediment transport capacity of the tunnels for given set of conditions. It would, obviously, be more logical to compute the sediment load coming into the undersluice pocket and then design the excluder such that it is capable of carrying that sediment load without causing objectionable deposits of sediment. Garde and Pande (8) have suggested the following design procedure which takes into account the actual sediment transport capacity as well.
The velocity in the excluder tunnels, U_ex is chosen to lie between the critical velocity U_c (at which sediment starts moving) and the limit deposit velocity U_L (at which no sediment will be deposited) for the maximum sediment size, d_m. The critical velocity U_c is given as (9)