The hyperconcentrated flood of the tributaries of the desert and wide-valley reaches of the Upper Yellow River formed sandbars in the confluences blocking the main stream several times. Based on field and experimental data, the properties of hyperconcentrated flood, the process of blocking and the sandbar morphology in the confluence were analyzed. The results indicate that the hyperconcentrated floods from the tributaries feature steep rising and dropping, short duration, high peak, large flood volume, high sediment concentration. The evolution process of the sandbar included three stages, the forming, the stability and the scouring. The larger the magnitude of the flood, the greater the scale of the sandbar, and the longer the time of sandbar scouring. The hyperconcentrated flow confluence can be divided into five hydraulic regions, the backwater zone, the separation zone, the maximum velocity area, and the shear layer. The bed morphology consists of four basic elements, the sandbar in the backwater zone, the bar in the separation zone, the bars in the reach downstream of the separation zone, and the channel for flow conveyance and sediment transport. Based on field and experimental data, the inequations were established for discriminating the magnitude of sandbar blocking caused by hyperconcentrated floods of the tributaries.
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