Contents preface (VII) introduction 1—37

13.7.1. Vertical Cutoffs
Vertical cutoff (or sheet pile lines) at the upstream and downstream ends of a weir are always provided to guard against scouring at the upstream and downstream ends and the piping effects at the downstream end. Intermediate cutoffs, provided at the ends of the upstream and/ or the downstream slopes of the impervious floor, are useful in holding the main structure, i.e.,

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the weir, in case of failure of the upstream and/or the downstream cutoffs. The depth of cutoffs should be such that its bottom is lower than the level of possible flood scour at that section. The downstream cutoff, in addition, should also be sufficient to reduce the exit gradient within safe limits which is decided by the subsurface conditions. The depth of scour below HFL, R is given by Lacey’s equation, Eq. (8.33),

F q2 I ^1/3

R = 1.35 ^{G J} (13.3)

H f1 K

For fixing the depths of cutoffs, the scour depth R should be calculated with discharge intensity q taking into account the concentration factor. The concentration factor is a factor by which the discharge per unit length of a weir, assuming uniform distribution across the river width, is required to be multiplied to obtain the design discharge per unit length for designing stilling basin and the cutoffs of the weir (2). The concentration factor accounts for the nonuniform distribution of the flow along the waterway during the operation of weir bays.
Equation (13.3) computes scour depth R for the regime condition of flow existing during high floods. Other features, such as bends, increase the depth of scour. The extent of scour (below water level) in a river with erodible bed material varies at different places along a weir as shown in Table 13.2.