Quantifying the Effects of Near-Bed Concentration on the Sediment Flux after the Operation of the Three Gorges Dam, Yangtze River

Abstract

Abstract:The regime of sediment transport in the Jingjiang Reach has significantly changed from quasi-equilibrium to sub-saturation since the impoundment of the Three Gorges Dam (TGD), and vertical profiles of suspended sediment concentration (SSC) have changed accordingly. Vertical profiles of SSC data measured at three hydrological stations in the Jingjiang Reach (Zhicheng, Shaishi, and Jianli), before and after the impoundment of TGD, were collected and analyzed. Analytic results indicate a remarkably large concentration in the near-bed zone (within 10% of water depth from the river-bed) in a sub-saturated channel. The maximum measured concentration was up to 15 times the vertical average concentration, while the ratio in quasi-equilibrium channel was less than four times that. Concentrations normalized with reference concentration at the same height, and may decrease with increasing values of suspension index (settling velocity over shear velocity). In addition, concentration near the water surface may be larger than concentration in the near-bed region when the suspension index is smaller than 0.01. Sediment flux transported in the near-bed zone may be up to 35% of the total sediment flux in unsaturated flows. The relationship between deviations of estimating sediment flux when ignoring the near-bed concentration and discharge in flood season and non-flood season are different in unsaturated and quasi-equilibrium channels. Analysis indicates that, in the quasi-equilibrium channel, more attention should be paid to near-bed concentration during non-flood season, the same as measurements during flood season with larger discharge.

Evaluating Regime Change of Sediment Transport in the Jingjiang River Reach, Yangtze River, China

“…For profiles measured at Shashi station, measured flow depths range from 2.09 m to 19.7 m, and the normalized depths of these two near-bed points are approximately (0.02-0.2)H. For profiles measured at Jianli station, measured flow depths range from 2.13 m to 21.2 m, and the normalized depths of these two near-bed points are approximately (0.0047-0.047)H. Table 1. Statistics of data measured at the JJRR (following [17]).…”

“…Zhicheng 1996, 1998, 200255 Shashi 1996, 1998, 2010130 Jianli 1986, 1998, 2002, 2010 Equipment adopted in the field survey includes global navigation satellite system (GNSS, antenna, and receiver), total station, vessel-mounted acoustic Doppler current profilers (ADCP), digital level, GPS, and laser particle size analyzer (LPSA) [17]. All of this equipment has been strictly examined by professional surveyors.…”

“…Therefore, except a slowed down settling velocity of the sampler, the near-bed concentrations may be double-checked after sampling and grain-size analyses. The assumption is that the measured concentration is reliable only if the vertical concentration profiles of particles finer than 0.062 mm have no abrupt changing point in the near-bed region [17]. Thus, bed materials can be distinguished from suspended sediments, which may be caused by disturbance during the sampling processes.…”

“…The cross-sectional sediment flux can be estimated with all measured vertical profiles. The delta-shaped area between the left bank and the first vertical line from the left bank is also considered [17]. The delta-shaped area between the right bank and the last vertical line from the left bank is not considered due to the difficulty of identifying the right bank [17].…”

“…Thus, it is also necessary to analyze the changed sediment regime in the local scale. The contribution of the remarkable, large, near-bed concentration on the sediment flux in the unsaturated channel has been analyzed [17]. Here, this paper aims to evaluate the changed sediment regime of the JJRR from both a local-scale view (vertical concentration profiles) and a reach-scale view (sediment flux).…”