EXPERIMENTAL STUDIES ON FLOW CHARACTERISTICS IN A COMPOUND CHANNEL

Abstract

A compound channel consists of a deep main channel and shallow flood plains on one or both side of it. Flow in a compound channel is not found to have the some flow characteristics as those found in the channels with simple cross-section. The characteristics of flood plain and deep main channel are also found to vary with the relative depth of flow in the compound channel. As a result, the velocity, discharge, shear force and shear stress distributions are different in the main channel and in the flood plain. Thus, the above parameters cannot be computed for a compound channel using the procedures applicable to the simple channels. Morning's and Chezy's equations are thus not possible to be used in these channels directly for the computation of flow velocity and discharge. An experimental investigation was, therefore, taken-up in the present study to study the flow characteristics in a compound channel. The compound channel having main deep channel of width 0.25 m and depth 0.14 m with flood plains of 0.25 m width on either side of the main channel was used for the experimental programme. The maximum height of the flood plain was 0.11 m. An Acoustic Doppler Velocity meter was used for velocity measurements at different section in the compound channel. The study has reveled that the traditional modeling practice is inadequate for computing the flow carrying capacity of a compound channel. Consideration of compound channel as a single channel underestimates its carrying capacity whereas consideration of sub-sections of a compound channel over-estimates the carrying capacity. Two approaches have been proposed in the present study for the computation of flow characteristics in a compound channel. In the first approach, the velocity at the iii interface of the flood plain and the deep channel is ignored while in the second approach the velocity at this interface has been taken into account. The results of the present study have shown that the considerations of velocity at the interface of compound channel produce more accurate estimates of the flow parameters. The study has revealed that significant transfer of momentum occurs in a compound channel for relative depth values lying between 0.2 and 0.3. All the flow parameters viz. discharge, shear stress etc. get considerably affected because of this reason both in the main deep channel as well as in the flood plains. The appropriate methods are identified in the present_ study for-the computation of How parameters in deep channel and flood plains of a compound channel.