Variation in reach-scale bankfull discharge of the Jingjiang Reach undergoing upstream and downstream boundary controls

Abstract

Remarkable channel degradation has occurred in the Jingjiang Reach of the Middle Yangtze River, since the operation of the Three Gorges Project, which has caused significant geomorphic adjustments, including the variation in bankfull discharge. Bankfull discharge is a key indicator of flood-discharge capacity in the Jingjiang Reach, which can adjust in response to the altered water and sediment conditions and local base-level changes. Therefore, it is important to investigate the variation in reach-scale bankfull discharge, because of longitudinal variability in channel geometry and bankfull discharge in the study reach. In this study, a general method to calculate the section-scale bankfull discharge using the simulated stage-discharge relation is outlined briefly, and an integrated method is then presented for estimating the reach-scale bankfull discharge. The post-flood reach-scale bankfull discharges in the study reach from 2002 to 2015 were calculated, using the proposed method, based on surveyed post-flood profiles at 171 sections and measured hydrological data at three hydrometric sections and eight water gauge sections. The calculated results indicate that: (i) the reach-scale bankfull discharge in the Jingjiang Reach ranged between 32,731 and 38,949m3/s over this period, with the average bankfull discharge in the Upper Jingjiang Reach being generally greater than the value in the Lower Jingjiang Reach; (ii) the magnitude of the reach-scale bankfull discharge responded well to the cumulative effect of incoming flow and sediment conditions at the inlet boundary, as well as the change of local base level at the outlet boundary; and (iii) the flood-discharge capacity of the reach was controlled by both the upstream and downstream boundary conditions, and the reach-scale bankfull discharge was expressed by an empirical function of the previous five-year average fluvial erosion intensity during flood seasons at Zhicheng, and the difference between the flood-season average water stages at Zhicheng (upstream) and Lianhuatang (downstream), with a relatively high correlation degree between them being obtained. This relation for the prediction of reach-scale bankfull discharge was verified using the measurements in 2014–2015.