Influence Of The Three Gorges Dam On Suspended Sediment Concentration In The Delta Front Of The Yangtze

Based on the daily sampled data of suspended sediment concentration (SSC) from April 2002 to October 2005, together with the data of tidal range, sea level, wind direction, wind velocity, the sediment and water discharge of Datong at the same time, the different time scales of variation of SSC and their mechanism were analyzed. The wavelet analysis and the correlation analysis methods were used to reveal the periodical pattern of the suspended sediment concentration. It is shown that: (1) SSC has significant tidal (14.7 days) and seasonal (368 day) variation pattern. On average, the SSC in winter is 2 times than that in summer; and the SSC in spring tide is 2.96 times than that in neap tide. (2) The correlation coefficient of the daily tidal range and SSC at a tide circle is 0.86(at the same day), and it goes up to 0.96 and 0.93 with one day and two days lag of SSC. This reflects that there has a hysteresis of one or two days of SSC to the tidal range. (3) There has a significant negative correlation between the monthly SSC and the water discharge of Datong, especially with a lag of 19 days of the discharge. There also has a significant negative correlation between the monthly SSC and the sediment discharge of Datong. This is affected by the high relationship between the water and sediment discharge of Datong, and the high relationship concealed the real mechanism of the sediment discharge to the SSC (4) The average wind velocity of six hours before the sample, has a positive relation with the SSC. But on the contrary, the monthly wind velocity has no significant affection to the monthly SSC, which reveals that the wind energy has a large affection to the instant SSC, and has no affection to the SSC for a large scale for it is covered by other dynamics. (5) The onshore index and off shore index of wind velocity both has a strong relation with the monthly SSC. (6) The construction of Three Gore and all the projects will reduce the sediment discharge of Datong, which in response will lead to the erosion of the march, the decrease of the SSC, and the project will made a long term affection to the environment.The result is that there have two different time scales of periodical variation of the SSC in Nanhui, i.e. the tidal and the seasonal variation. The former is controlled by the resuspension/settling process of the sediment caused by the tidal dynamic; the later is controlled by the seasonal factors such as the variation of the riverine water discharge, sea level, tidal range, and the wind in one year.