Study On Cause Of River-bed Scouring And Siltation Of Yangtze River Nantong Reach At Xinda Dock Waters

Nantong Reach of the Yangtze River is an anabranched stream that is influenced by tides. Study on cause of river-bed scouring and siltation of tidal reaches is a disciplinary problem. Based on abundant in-situ data, this paper analyzes the recent fluvial process of Nantong reach and the river-bed scouring and siltation at Xinda dock waters by applying the basic principle of fluvial process, and the cause of river-bed scouring and siltation at Xinda dock waters is concluded from construction impact, runoff impact, river upstream evolution processing impact and tide impact.The analysis of recent fluvial process has divided the Nantong reach into five geomorphic units, Fujiang sand channel, Rugao sands channel, Tongzhou sand channel, Langshan sand channel and Xinkai sand channel, which are analyzed respectively. The results indicate that the river-bed of Nantong reach is stable on the whole and Xinda dock waters will serve as a main channel over the long term.The analysis of river-bed deformation at Xinda dock waters is composed of two parts. One is the analysis of cross section and river-bed in the range of 2 km upstream to 2 km downstream of Xinda dock waters. The other is the analysis of river-bed deformation in the range of 200m upstream to 200m downstream of Xinda dock waters by using several years of 1:1000 underwater topography data. Scouring and siltation volumes before and after four times of dredging are calculated by applying the scouring and siltation quantitative methodology on the basis of TIN model. The results indicate that project waters substantially appear stable with a little deposition, and the deposition thickness of dock basin is about 2.5m/a. During a very long term in the future, the dock basin must be dredged for the berth of sea crafts.The analysis of the cause of scouring and siltation of the Project waters is concluded as follows, a) Local scouring and siltation resulted from the projects is substantially ended, b) Yangtze River water-rich runoff and tide of the outer side of Xinkai sand are favorable in terms of maintaining the water-depth condition at the outer side of the dock, c) The increase in the deflecting pressure of upstream Longzhao Rock and the siltation at the entrance of the Xinkai sand channel are adverse factors that cause the project waters siltation, and the former is the main factor.