Channel-form adjustment of an alluvial river under hydrodynamic and eco-geomorphologic controls: insights from applying equilibrium theory governing alluvial channel flow

Alluvial rivers commonly are subject to the integrated effects of hydrodynamic and eco-geomorphologic controls and there has been a lack of suitable methods to measure the effects. By taking the alluvial reach of the Yellow River over the Yinchuan Plain as a suitable example, this study evaluates the degree of hydrodynamic control in the channel-form adjustment of different channel patterns in light of the advances in equilibrium theory governing alluvial channel flow. In response to the significant variations in flow regime and channel forms, the non-dimensional number (Formula presented.) that measures the equilibrium state of alluvial channel flow varies in the ranges of 0.23–0.65, 0.047–0.17 and 0.0012–0.0024 respectively in the anabranching, meandering and braided reaches during 1993–2015. The significant differences among the H-ranges are mainly because the individual anabranches have neither very narrow nor very deep cross-sections, while the single-thread channels in the meandering and braided reaches take moderately and significantly wider and shallower cross-sections, respectively. These results demonstrate that the (Formula presented.) number is a good discriminator of river channel patterns, and the relatively small variability of (Formula presented.) within each channel pattern implies that the channels in the study reach are resilient to significant change in flow regime but yet hydrodynamic control is only partial. While the (Formula presented.) number is capable of embodying the outcome of the complex integrated effects of multiple localized eco-geomorphic controls with flow dynamics, more studies are required to define its specific varying ranges for different river channel patterns and differing eco-geomorphic controls.