Contents preface (VII) introduction 1—37

Fig. 13.7 Weir profile for Example 13.1
13.8. DIVIDE WALL
The divide wall is constructed parallel (or nearly parallel) to the canal head regulator. It separates the main weir bays from the bays of the undersluice as shown in Fig. 13.1. The wall extends on both sides of the weir. Extension of the divide wall towards the downstream of the weir avoids cross-flow in the immediate vicinity of the structure which, otherwise, may cause objectionable scour. The divide wall is usually extended up to the end of either the impervious floor or the loose apron on the downstream side. The divide wall serves the following purposes:
(i) It isolates the canal head regulator from the main river flow and creates a still pond of water in front of the canal head regulator. This results in deposition of sediment in the pocket and entry of relatively sediment-free water into the offtaking canal. It also improves scouring of the undersluices by ensuring straight approach.
(ii) It separates the weir floor from the floor of the undersluices which is at a lower level than the weir floor.
(iii) If the main current has a tendency to move towards the bank opposite to the canal head regulator, the weir forces the water towards the canal head regulator. This causes cross-currents which may damage the weir. Under such adverse flow condi-tions, additional divide walls at equal intervals along the weir are provided to keep the cross-currents away from the weir.

CANAL HEADWORKS

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When only one canal takes off from a river, the length of the divide wall should be half to two-thirds the length of canal regulator (1). When more than one canal takes off from the same bank, the divide wall should extend a little beyond the upstream end of the canal farthest from the weir (1). Some experimental studies have shown that a slight divergence of the divide wall from the regulator improves its efficiency. This divergence should not exceed 1 in 10. To reduce the scour at the nose of the divide wall, the nose end of the wall is given a slope of 3(V):1(H).

The divide wall is generally constructed as a strong masonry wall with a top width of about 1.5 to 2.25 m and checked for safety for the following two conditions:
(i) For low stage of the river, the water levels on the two sides of the walls are the same but the silt pressure is assumed to correspond to the sediment deposit up to full pond level on the pocket side.
(ii) For the high stage of the river, the undersluices are discharging. At this condition, the water levels on the two sides are assumed to be different; the weir side level being higher by about 1.0 m.
It the river curvature is not favourable to sediment-free entry of water into the offtaking canal by inducing convex curvature opposite the head regulator, a second pocket of river sluice adjoining the undersluices improves flow conditions considerably. Such provision is useful in case of wide rivers to guide the river to flow centrally, minimising cross-flow, and prevent shoal formation in the vicinity of the head regulator. The location and layout of the river sluice should be decided by model studies for satisfactory performance (1).