The discharge formula of side-weir constructed beside of a rectangular open-channel flow was obtained by De Marchi. However, such a formula which is able to apply in trapezoidal open-channel flows has not been suggested as yet. In this study, the discharge formula of side-weir constructed beside of a rectangular open-channel flow can be obtained under the condition that the specific energy is constant in the flow direction and also that the friction and bed slope are negligible small. Rather, the discharge coefficients were obtained from the present experiments, in which the Froude number, relative flow depth, relative weir height, relative weir length and side-wall slope were changed. A new discharge formula was suggested. It was conformed that the accuracy of the suggested formula is high as compared with that of previous one.
