Numerical Study of stability of alluvial compound channels using empirical results

The first step in design and optimal management of flood control projects is appropriately identifying behavior of the river during the flood. When channel is subjected to overbank flow، interaction between the main channel and flood plains and momentum transfer at the interface between the two partial cross sections results in energy loss and cause the cross-channel to become more complex. Also the main distinction between compound and ordinary sections is due to this phenomenon. The purpose of present paper is to study geometric parameters and dynamic stability approaches of simple and compound alluvial channels. To this end a laboratory channel has been modeled with GSTARS4. This mathematical model performs based on theory of total stream power minimization. The calibration of this model was based on water surface elevation، cross section geometry، sediment transport equations and friction factor. Some of the data not used in the calibration were used to verify the model. The results show that bankfull channels changed initial hydraulic geometry and reached a stable state. Then stable channels subjected to overbank flow، they adjusted their conditions to achieve a new stability so that Increase in the longitudinal slope resulted in increase in relative depth and decrease in relative width with time. Also in experienced range of relative depths، show that discharge calculations as a single channel is more appropriate than rate of decomposition for Developed compound channel. Proportion of Apparent to average shear stress ratio in low relative depths was 4 times higher than greater relative depths.