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Channel geometry adjustments in response to hyperconcentrated floods in a braided reach of the Lower Yellow River

Hyperconcentrated floods with more than 200–300 kg/m3 sediment concentrations often occur in the Lower Yellow River (LYR) during flood seasons, which leads to unique fluvial processes in the braided reach of the LYR. The investigation of channel geometry adjustments in response to hyperconcentrated floods can not only help to gain a better understanding of associated fluvial processes, but also is significant for making flood control strategies in the braided reach. In this study, pre- and post-flood bankfull channel dimensions in the braided reach were calculated based on the observed cross-sectional profiles in 15 years with the occurrence of hyperconcentrated flood events. Adjustments in channel geometry at section- and reach-scales were investigated, with several factors influencing adjustments in reach-scale channel geometry being analyzed. It indicates that the mean sediment transport rate was a key factor influencing the adjustment index, although pre-flood channel geometry and sediment deposition can also affect the index to some extent. An empirical relationship was developed between the characteristic parameter representing the pre- and post-flood channel geometries and mean sediment transport rate in hyperconcentrated floods. Eleven datasets were used to calibrate the parameters in the empirical relation, with the datasets in 1973, 1988, 1995, and 2002 verifying the relation. The calculated post-flood characteristic parameter of channel geometry using the empirical relation agreed well with observed data, and the proposed method can be used to predict the reach-scale adjustment of channel geometry during hyperconcentrated floods in alluvial rivers.