The influence of unsteadiness degree on hydraulic characteristics in free surface flow via numerical modeling

In recent years, there have been significant changes in the stream and river hydrographs with the effects, which increasingly more, of global climate change. It has become very important to determine the variation of hydraulic characteristics, at the unsteady flow conditions caused by changing river regimes. In this study, it is aimed to determine the effects on hydraulic characteristics caused by the hysterical effects seen in the free surface flows, via numerical modeling. Therefore, a typical hydrograph with non-permanent discharge time series has been introduced, on a prismatic open channel flow with a wall roughness. Then, a numerical model has been set up which is independent from the flow depth revealed by the aforementioned hydrograph, cross-sectional average velocity, the downstream conditions which gives bed shear stress and any convective term. In the scope of the study, unsteadiness degree (hysterical properties) has been determined by numerical modeling as well as the flow properties such as depth, velocity and bed shear stress, using one-dimensional Saint Vennant equations based on the specified initial conditions. Furthermore, the effects of different parameters on the unsteadiness degree have been analyzed in this study. In order to verify the accuracy of the numerical model; the model results have been compared with the experimental data obtained by Erdog (2013) at the Hydraulics Laboratory of Istanbul Technical University. Due to the fact that the results of the numerical model and the physical model fit well, it is concluded that the existing numerical model can represent the unsteady flow physic successfully. In the scope of the study, hysterical effects have been nondimensionalized in order to observe the fluctuation of hysteresis. Therefore, in the hysterical graphs, the area between water level and the flow curve has been divided by the difference between peak discharge and base discharge and the difference between maximum water level and initial water level. The hysteria has been defined with this non-dimensional parameter. It has been considered that the effectiveness increases as the parameter gets bigger and decreases as the parameter gets smaller. As a result, the plots with large diagrams have big hysteria, whereas narrow diagrams will have ineffective hysteria. According to model results, while the change in unsteadiness degree is negligible up to a value (approx. Γ=10-4), after a certain value it is increasing rapidly. In consequence of analysis, it is revealed that the assumptions for permanent flow conditions are accurate when the unsteadiness parameter is approximately up to 10-4 and the error margin will increase at higher values. On the other hand, it's determined that roughness increases the bed shear stress when the flow is more steady, in the study of roughness effect analysis. However, additional numerical modeling studies have been carried out by expressing roughness in terms of Manning roughness coefficient, while it was previously defined as Nikuradse roughness coefficient in the model. In consequence of the study carried out with Manning roughness coefficient, it has been concluded compatible with the previous study that when the roughness increases the hysterical effects are decreasing. Conclusively, it has been revealed that the decrease in the roughness of the waterways, caused by river restoration projects and urbanization, increases the flow hysterical effects; correspondingly, in smooth waterways flow resistance formulas derived for permanent flow conditions can not be as accurate as in rough waterways.