Response of flood discharge capacity to the incoming flow and sediment regime and channel geometry in the braided reach of the Lower Yellow River

The flood discharge capacity of an alluvial river is related to its channel geometry, which also depends on the upstream flow and sediment regime. It is important to analyze the relationship between flood discharge capacity and channel geometry and flow and sediment regime, in order to investigate the channel evolution of the braided reach in the Lower Yellow River. The relationship was developed between bankfull discharge and flow-sediment regime, represented by incoming sediment coefficient and fluvial erosion intensity, and the channel geometry, represented by the pre-flood hydraulic geometry from 1986 to 2015 in the braided reach at section- and reach- scales. Results indicate that:① Serious deposition and main channel shrinkage occurred in the study reach from 1986 to 1999, with the flood discharge capacity declining rapidly, and the main channel became narrower and deeper continuously owing to significant channel degradation after the operation of the Xiaolangdi Reservoir, with the flood discharge capacity recovering gradually from 1999 to 2015;② Empirical formulas (power functions) were developed between section- and reach- scale bankfull discharges and the integrated effects of flow-sediment regime and channel geometry respectively, whose correlations are greater than 0.50, but the correlation coefficient for the reach scale improved by 17% at least, as compared with the section scale; and ③ A higher correlation (0.94) was obtained between the reach-scale bankfull discharge and the previous five-year average fluvial erosion intensity during flood seasons and the post-flood hydraulic geometry in the last year, and the corresponding formula can well reflect the variation in bankfull discharge of this study reach, which also provides a method to analyze the variation in bankfull discharge of other reaches.