Journal of Geographical Sciences >
Adjustment of flood discharge capacity with varying boundary conditions in a braided reach of the Lower Yellow River
Cheng Yifei (1997), PhD Candidate; specialized in hydraulics and river dynamics. Email: chengyf@whu.edu.cn 
Received date: 20210301
Accepted date: 20210814
Online published: 20220125
Supported by
National Natural Science Foundation of China(51725902)
National Natural Science Foundation of China(51579186)
Copyright
It is of necessity to investigate the adjustment of flood discharge capacity in the Lower Yellow River (LYR) because of its profound importance in sediment transport and flood control decisionmaking, and additionally its magnitude is influenced by the channel and upstream boundary conditions, which have significantly varied with the ongoing implementation of soil and water conservation measures in the Loess Plateau and the operation of the Xiaolangdi Reservoir. The braided reach between two hydrometric stations of Huayuankou and Gaocun in the LYR was selected as the study area. Different parameters in the study reach during the period 19862015 were calculated, covering bankfull discharge (the indicator of flood discharge capacity), the preflood geomorphic coefficient (the indicator of channel boundary condition), and the previous fiveyear average fluvial erosion intensity during flood seasons (the indicator of incoming flow and sediment regime*Functional linkages at scales of section and reach were then developed respectively to quantitatively demonstrate the integrated effects of channel and upstream boundary conditions on the flood discharge capacity. Results show that: (1) the reachscale bankfull discharge in the predam stage (19861999) decreased rapidly by 50%, accompanied with severe channel aggradation and mainchannel shrinkage. It recovered gradually as the geometry of main channel became narrower and deeper in the postdam stage, with the geomorphic coefficient continuously reducing to less than 15 m^{1/2}*2) The response of bankfull discharge to the channel and upstream boundary conditions varied at scales of section and reach, and consequently the determination coefficients differed for the comprehensive equations, with a smallest value at the Jiahetan station and a highest value (0.91) at reach scale. Generally, the verified results calculated using the comprehensive equations agreed well with the corresponding measured values in 20142015*3) The effect of channel boundary condition was more prominent than that of upstream boundary condition on the adjustment of bankfull discharge at the Jiahetan station and the braided reach, which was proved by a larger improvement in determination coefficients for the comprehensive equations and a better performance of geomorphic coefficient on the increase of bankfull discharge.
CHENG Yifei , XIA Junqiang , ZHOU Meirong , DENG Shanshan , LI Zhiwei . Adjustment of flood discharge capacity with varying boundary conditions in a braided reach of the Lower Yellow River[J]. Journal of Geographical Sciences, 2021 , 31(11) : 1615 1632 . DOI: 10.1007/s1144202119140
Figure 1 Sketch of the Yellow River and the braided reach: (a) the Yellow River; (b) the braided reach 
Figure 2 Calculation of bankfull parameters at JHT: (a) determination of bankfull channel parameters; (b) the stagedischarge curve simulated by the 1D hydrodynamic model 
Figure 3 Temporal variations in the flow and sediment regime entering the braided reach during flood seasons and hydrological years: (a) water volume; (b) sediment amount 
Figure 4 Temporal variations in bankfull discharges at scales of section and reach, and cumulative channel evolution volume (V) in the braided reach 
Figure 5 Temporal variations in the preflood bankfull channel dimensions: (a) geomorphic coefficient; (b) width; (c) depth; (d) area 
Figure 6 Relationships between bankfull discharge and geomorphic coefficient at different sections of: (a) JHT; (b) GC; and (c) in the braided reach 
Figure 7 Responses of bankfull discharge to the preceding fiveyear average fluvial erosion intensity at different sections of: (a) JHT; (b) GC; and (c) in the braided reach 
Table 1 Calibrated parameters in Eq*4) at sections and in the braided reach 
Section/reach  k  m  n  R^{2} 

JHT  4855.8  0.231  0.304  0.55 
GC  1937.4  0.097  0.453  0.77 
Braided reach  5164.9  0.248  0.302  0.91 
Figure 8 Comparisons between calculated and observed bankfull discharges (a) at JHT; (b) at GC; and (c) in the braided reach 
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