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Flooding of rivers is accompanied with a threat on the population living on their floodplains and on the neighboring settlements. Accurate modeling of such flows is thus imperative to assess flood risks, perform real-time flood routing, or estimate the impact of a mitigation schema.The converging of the compound channel causing the flow become even more complicated. The flow patterns in converging compound channel and free-surface profile has been simulated by using RSM turbulence model and VOF method, respectively. The comparison of the experimental results including longitudinal free surface profiles, depth-averaged velocity distribution and the ratio between floodplains and total discharge confirmed that the numerical simulation can be used to model the flow pattern in converging compound channel. Furthermore, Absolute Percentage Error (APE) for each of these parameters was amounted to 3.25%, 4.66% and 9.72%, respectively. Respect to the numerical simulation capability in anticipating the flow field parameters, we investigated the effect of relative depth on the flow patterns in a converging compound channel. Moreover, the flow parameters including velocity distribution, depth-average velocity, secondary flows, ratio between floodplain and total discharge, bed sheer stress and energy dissipation were investigated in different relative depths h* (0.1, 0.2, 0.3, 0.4 and 0.5). By evaluating and comparing the flow results in the different relative depths, we came to conclusion that the longitudinal-average velocity in the main channel increased as the cross section was narrowed. However, the longitudinal-average velocity in the floodplains decreased in the relative depths of 0.1 and 0.2 as the cross section was narrowed. In contrast, this parameter increased in the relative depths of 0.3, 0.4 as well as 0.5 the floodplains narrowed. Velocity gradient between the main channel and floodplains in the relative depth of 0.1 was strong and in the relative depth of 0.5 was insignificant. In the smaller shallow depth, this velocity gradient has been resulted in secondary flow in the cross section of converging compound channel. Accordingly, in the relative depth of 0.1 and 0.2 four cells of secondary flows were formed and in the relative depth of 0.3 just two cells of secondary flows were formed. The secondary flows in the relative depths of 0.4 and 0.5 was eliminated. Convergence in the length of main channel get the discharge conveyance capacity of floodplain to decrease. furthermore by decreasing the relative depth the capability of floodplains to conveyance the discharge was significantly decreased. This decrease was evident in the depth of 0.1 in which the ratio between floodplain and total discharge was amounted to 2.52%. Sheer stress in channel bed increased when the relative depth of the main channel increased and the maximum amount of bed shear stress was happened at end of the channel. On the other hand, in the floodplains, this parameter decreased along with the converging in the lower relative depths (0.1, 0.2) and for the other relative depths the bed sheer stress increased along with converging. In channel inlet the maximum and minimum amount of energy dissipation was resulted at the relative depth of 0.1 and 0.5 respectively.

Keywords: Converge, Numerical simulation, Relative depth, Floodplain

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