Study On Hydraulic Characteristics In Straight Compound Channel

The natural channel is usually compound channel, it consists of the floodplain and main channel. In the dry season, the flow is mainly concentrated in the main channel; when the flood occurred, the water will flow over the floodplain. Compared with the single section channel, the flow structure in compound channel is more complex. When the over-bank flow occurred, the distribution of depth-averaged velocity, bed shear stress and flow resistance will be changed because of the interaction between flow in main channel and floodplain. In this situation, the formula for single channel is not applicable, thus a new method is needed to describe the flow in compound channel, so it is of great significance to study the hydraulic characteristics of compound channel.Based on the experimental data of SERC-FCF, this paper analyzed the depth -averaged velocity, bed shear stress, Darcy-Weisbach resistance coefficient and Manning coefficient. Based on a large number of references, this paper summarized 12 methods predicting the capacity of compound cross-sections of rivers, including six empirical methods and six theoretical methods.By comparing the calculated discharge by the mentioned methods with the experiment measured discharge, the accuracy of this methods respectively is summarized. Moreover, three numerical models were introduced in this paper, including HEC-RAS, CES and MIKE 21, respectively. The applicability of these methods is verified by experimental data of SERC-FCF. The main conclusions are as follows:(1) In compound channel, the velocity in main channel is higher than in the floodplain; the vertical average velocity has a transverse gradient, especially in the interface between the main channel and floodplain, the gradient comes to its maximum value. Distribution trend of bed shear stress is similar to the distribution of velocity, the variation is related to flow depth and ratio of flow area between main channel and floodplain. The variation of Darcy-Weisbach resistance coefficient and Manning coefficient are related to the bed conditions and the ratio of widths between floodplain, main channel and flow depth.(2) Among mentioned 12 methods, DCM, HDM and HCM have higher accuracy, these methods can be applied to the prediction of the flow capacity in compound channel. However, the SCM have the lowest accuracy, so this method is not applicable to compound channel. Due to the theoretical basis,the applicable of HCM is more universal.(3) The calculated velocity in main channel by HEC-RAS is higher than measured data, and it is also lower for floodplain. Compared the above velocity with HEC-RAS results, the CES and MIKE 21 have better fitting qualities, so it indicates these two models are applicable to the study of flow characteristics in compound channel.