DESIGN OF DEPRESSED WEIR ON PERMEABLE FOUNDATION WITH A DOWNSTREAM CONCRETE CUTOFF

Abstract

A weir is constructed across a river to divert flow into a man made channel satisfying all possibilities of surface and sub-surface - flow considerations. The surface flow consideration decides the crest level, downstream floor length, minimum depth of cutoff /sheet pile for the upstream and downstream end of floor. The maximum depth of cutoff/sheet pile depends upon the design flood. The effect of sub-surface flow is considered in respect of the uplift pressures of the percolating water acting on the bottom of the floor and the exit gradient and hence safety of the structure against piping. The total weir floor length is determined in relation to the downstream cutoff/sheet pile depth in order to satisfy exit gradient criteria. These parameters, cutoff/sheet pile depths and floor length, govern the uplift pressure at different points under the floor. These uplift pressures are counter acted by the floor thickness.