عنوان مقاله [English]
Background and objectives: The first step to predict reaction of alluvial rivers against natural and man-made changes is to know and to predict stable geometry of rivers. One of the most important problems in civil engineering in relation to irrigation projects, river engineering and hydraulic projects is determining of stable cross section geometry, slope and stability criterion for alluvial channels or rivers. The purpose of present paper is to study geometric parameters, equilibrium criteria and stability approaches for stable alluvial channels.
Materials and methods: A laboratory channel has been modeled with GSTARS4.0, which is developed based on theory of total stream power minimization. This mathematical model using concept of stream tubes is able to simulate sedimentation pattern in the rivers quasi two-dimensionally. For hydraulic calibration, the mathematical model is run for different discharges and Manning roughness coefficients. Then, the computational water surface profile is compared with the actual water surface elevation on the channel. Using the water surface elevations for different discharges, the Manning roughness coefficients of the channel are selected such that the channel dimensions calculated by the model are consistent with the actual value for certain discharge and water level. Comparisons demonstrate a good agreement between computational and observed water surface elevations for the Manning roughness coefficients of n=0.014. For sediment calibration of this mathematical model, cross section geometry has been used. For this purpose, the stability angle of the bed materials is considered equal to the experimental value (φ=33 º). Then, the changes in laboratory channel cross-section are simulated for different empirical correlations in GSTARS mathematical model. The simulation results are compared with the experimental cross section. Yang’s sand sediment transport equation (1979) is in good agreement with experimental observations as compared with other correlations. Some of data that had not been used for calibration is used to verify the model. In the final step, the main parameters of top and bottom width of channel are predicted by correlation coefficients of 0.975 and 0.942 respectively.
Results: The alluvial channels adjust the hydraulic geometry to make a balance among flow and sediment. In the first 2 to 3 hours duration of experiment, aggradation and degradation of the channel banks and bed was high and consequently a considerable value of sediment transport and widening rate was observed, then it eventually declined. Increasing the width of the top, decreasing the width of the bottom, and finally raising the side slope at all models can be seen. Major changes, consist of adjustment of the slope of the channel bed and also holding constant uniformity of profile of water surface, happen in the first 2 hours of experiment. In order to distinguish between straight channels, straight channels with shoals and meandering channels, presented models are plotted against Ackers-Charlton and lane lines. The channels are located below the lines and are straight.
Conclusion: The research shows capabilities and good performance of GSTARS4.0 mathematical model to forecast longitudinal and transversal changes of channel regime bed. Afterwards, stability criteria such as width adjustment, water surface adjustment (straight energy slope) and sediment transport rate uniformity which presented by various researchers were studied. It was concluded that all of the mentioned criteria had occurred in all of the models. Therefore a combination of these criteria proposed for stability of regime channels.