Numerical investigations of flow over side weirs in rectangular and circular channels are reported. Numerical models are obtained from energy principles and solved by a finite difference method. The results are presented in graphic form. Theoretical and experimental equations are presented for the determination of the length of side weirs for rectangular and circular delivery channels. Comparative analyses of side-weir lengths computed for several example cases show that rectangular-channel side-weir procedures generally should not be used for design purposes or for obtaining approximate water-surface profiles along side weirs in a circular channel (except for the special case of a side-weir-height-to-channel-diameter ratio of P/D = 0.66). For all other ratios of P/D, the percent error incurred depends on the basic assumptions involved with the rectangular approximation procedure.