Medium-sized channel shaping in the lower Yellow River due to water-sediment regulation

The Yellow River (YR) is famous for its 'hanging river' due to high sedimentation rate in the lower reach. The reservoir sediment retaining (RSR) and water-sediment regulation (WSR) of the Xiaolangdi Reservoir (XLDR) contribute to mitigating the siltation and adjusting the bankfull channel geometry in the lower Yellow River (LYR). However, few studies have been conducted to quantify the contribution of RSR and WSR to the medium-sized channel shaping. In this paper, we analyzed the temporal and spatial variations of bankfull discharge in the LYR, established empirical expressions between the bankfull discharge and the previous five-year average discharge above 2,000 m(3)/s and the incoming sediment coefficient during flood seasons, and constructed three different water and sediment scenarios to quantify contribution of WSR of the XLDR to the medium-sized channel shaping in the LYR. Calculated results indicated that the contribution of WSR to the recovery of medium-sized channels increased with time and became stabilized eventually. Along the LYR, such contribution also increased and eventually became stabilized in narrow reaches, reaching over 90%. The results will lay a solid technical foundation for the WSR of the XLDR, river regulation downstream of the reservoir, and sediment disposal of the YR.