Response of channel geometry to flow and sediment conditions in the lower Yellow River

The channel geometry in a fluvial river is significantly affected by the flow and sediment regimes, and the response behavior of channel dimensions usually varies widely to different management strategies from the upstream reservoir. Therefore, it is significantly crucial to investigate the variation of the channel geometry in response to changing flow and sediment conditions and quantify the influence of the latter in the sedimentation reduction and flood releasing in lower reaches downstream of the dam. In this study, three laboratory experiments on the physical model covering the typical braided reach HGK—JHT downstream of the Xiaolangdi Reservoir in the lower Yellow River are carried out, under the discharge of 2000 m³/s, 3000 m³/s, and 4000 m³/s respectively and with the corresponding constant suspended sediment concentration of 8.0 kg/m³. Results indicate that (i) spatially, the erosion and deposition in studied channel reach distributed alternately along the course which performs typical evolution properties of the braided river, corresponding to the total erosion amount of 2.27×10⁶ m³, 10.29×10⁶ m³, and 7.98×10⁶ m³ for three magnitude of discharges; and (ii) four representative adjustment patterns are listed based on the observed cross-sectional geometry after each experiment, including the lateral widening pattern, vertical incision pattern, composite pattern and geometrical stable pattern where sectional geometry rarely changes during the period of experiment; and (iii) the quantity ξ=B^1/2/H where B and H is the width and depth of the main channel zone is selected as the typical indicator to determine the variation of the channel stability. It is discovered that ξ in the reaches upstream of section FJS have rather larger values, implying relatively wider and shallower sectional geometry and lower channel stability which is closely associated with the levee safety. And moreover, the quantity ξ generally has lower values, that is, higher channel stability with the increase of experiment discharge; Besides, through the method of nonlinear regression analysis, the empirical relations for HGK—JHT Reach are developed between the main channel dimensions and incoming flow erosion intensity F=(Q²/S)/10⁶ where Q is the discharge and S is the corresponding sediment concentration. In general, the calculated results are generally consistent with the measured values, as the riverbed degradation and the variation of sectional area increase exponentially with a stronger erosion intensity F.This paper may provide some practical basis for the study of channel evolution in sediment-laden rivers.