Numerical investigation of the cross-section dimensions changes of the regime channel under steady and unsteady over-bank flows

Background and objectives:Floods are one of the natural events that cause significant morphological changes in riverbeds and adjacent lands, which result in financial, human, and other losses. When a flood occurs, the interference of the flow in the main channel and the flood plain causes the transfer of movement and complexity in the flow pattern from the main channel to the flood plain. Due to the dynamic nature of these events, statistical, experimental, and semi-experimental criteria are used to determine the dimensions of stable regime channels. Nowadays, the use of two-dimensional models to study the behavior of the regime channels has a superior advantage over the empirical and theoretical approaches, which are based on one-dimensional. There haven't been many numerical studies done on the flow pattern and sediment transport in alluvial channels, but there have been a lot of experimental studies. In this study, the behavior and hydraulic geometry of the trapezoidal compound channel have been investigated using MIKE21software for type of steady and unsteady overbank flow and Finally, the results of modeling in steady flow conditions showed less difference with real values than in unsteady flow conditions.
Materials and methods:In order to study the behavior of the main channel in the flood flow, the flow has been selected in such a way that the flood plain are in the state before the movement threshold. First, the compound trapezoidal channel section with fully losses boundaries/ with fully mobile boundaries was developed under bankfull flow, and then the behavior and geometry of the developed trapezoidal channel (regime channel) were evaluated under type of steady and unsteady flows. In the first step of permanent steady flow modeling, the flow conditions in the channel at a certain point are constant with respect to time, which means that the speed along the flow and the average depth of the flow are the same. In the second step of the modeling, the unsteady flow along the channel was checked with the same conditions as the first step, with the difference that the input discharge is variable in time.
Results:The results showed that when the cross-section of the main channel is full of water, there was no significant change in the width of the main channel, but the width of the main channel in overbank flows is higher than the full section and increases rapidly. The mean relative differences obtained at upstream and downstream width trapezoidal section, and flow depth in steady flow conditions are0.96, 0.88, and0.98,respectively, and in unsteady flow conditions, the average relative difference obtained at parameters upstream and downstream width trapezoidal section, and flow depth in steady flow conditions are1.12, 1.15and1.05,respectively. According to the sediment transport equations available in the software, the England-Hansen sediment transport relationship chosen as the appropriate relationship for the parameters of the channel dimensions, which can be more predictable to laboratory values.
Conclusion:An stable alluvial channel with a bankfull, when subjected to flooding, tilts towards a new stable channel.Under steady flow conditions, estimating the results of the depth and width of a stable alluvial channel in the MIKE21model is more accurate than the results obtained from unsteady flow. Also, the values of width, depth and sediment concentration predicted by the above software are better estimated than the sustainable channel design methods such as brownlie, vanrajin and WBP. In all models, the width of the channel increases rapidly over the first 3hours, and then approaches the balance stage and the channel erosion rate decreases. As the discharge increases up to 15liters per second, the depth and width of the channel is more severe and then these changes are almost constant.