DISCHARGE PREDICTION IN STRAIGHT COMPOUND CHANNELS USING THE MIXING LENGTH CONCEPT

A method for predicting the depth-discharge relationship in a compound channel is developed and applied to two different sets of experimenta l results. The method uses a mixing length formulation to account for the turbulent interaction between the main channel and the floodplain and the resulting momentum exchange. This momentum transfer tends to r educe the discharge in the main channel and increase the discharge on the flood-plain. The net effect is a reduction in the overall discharg e capacity of the compound channel. As a result, practical methods whi ch can allow for the interaction effect are needed. In this formulatio n a variation of Prandtl's mixing length hypothesis is applied to calc ulate the apparent shear stresses, indicative of the turbulent interac tion, on the sides of small vertical elements which comprise the compo und channel cross-section. The approach suggested is to use the mixing length approximation to calculate the correction for the momentum int eraction effects that are neglected when the traditional divided chann el approach is used. The traditional divided channel approach referred to herein typically divides the compound channel into three large sub -areas: main channel, left floodplain and right floodplain using a ver tical division at the edges of the main channel.