Quantifying The Influence Of Three Gorges Project On Yangtze Suspended Sediment Flux, Water Discharge And Water Level

Sediment load and water discharge is the basic process to balance the land-ocean. Under the complex human activities and extreme climate, sediment load transport and water discharge of river becomes complicated. As suspended sediment of Yangtze River transport into the sea was decreased sharply in recently years. The environment of Yangtze River month is undergoing significant changes.It still no quantitative assessment about the world-renowned Three Gorges Project on the Yangtze River downstream of the dam, there is a lot of controversy. In this paper, the principle of conservation of water and sediment load quality, according to sediment load transport law, starting simulation the water and sediment load transport from the reservoir to the estuary "without the Three Gorges project run " scenario, compared the simulated value and observed value to evaluate the Three Gorges contribution in water and sediment load into the estuary flux, so that not only enriched the theory of interaction between land and sea, but also for the Yangtze River-River Delta, the coordinated development and management to provide a scientific basis.There was 64%,86% and 84% sediment were intercepted in the Three Gorges reservoir during impoundment PhaseⅠ(2003-2005), PhaseⅡ(2006-2007) andⅢ(2008-2009) respectively, While scouring the riverbed downstream of the dam as part of compensation for the suspended sediment, the flux of suspended sediment at Yichang station dropped 62%, 82% and 84% during impoundment PhaseⅠ(2003-2005), PhaseⅡ(2006-2007) andⅢ(2008-2009) respectively (relative to the same period of without the Three Gorges run, the same below), Hankou station dropped 37%,54% and 56% during impoundment PhaseⅠ(2003-2005), PhaseⅡ(2006-2007) andⅢ(2008-2009), Datong station dropped 28%,40% and 43% during impoundment PhaseⅠ(2003-2005), PhaseⅡ(2006-2007) andⅢ(2008-2009).Since the Three Gorges run, during the flood season of the PhaseⅠ(2003-2005), PhaseⅡ(2006-2007),the amount of suspended sediment import and sediment deposition are 97% of the year, during the flood season of PhaseⅢ(2008-2009 Year) suspended sediment import and suspended sediment deposition are 92% of the year. The suspended sediment import and suspended sediment deposition of PhaseⅢis lower than the PhaseⅠ(2003-2005) and PhaseⅡ(2006-2007). The reason maybe is the water import of PhaseⅢalso lower than the PhaseⅠ(2003-2005) and PhaseⅡ(2006-2007) (sediment is negative relationship with water discharge). Sediment deposition rate of dry season over 90% during PhaseⅠ(2003-2005) and PhaseⅡ(2006-2007), but there is just 68% during PhaseⅢ. This phenomenon maybe result of sediment deposition of beach from dam to Yichang is considered as 0 after 2008 year.Since the Three Gorges dam run, the water discharge of the downstream three stations, take the graduate months consider, the amplitude of month is larger most during the period (September -November). Take the view of year, The biggest amplitude mainly in October 2006, which may be because of 2006 was dry (water discharge is lower than an average of 31%), the Three Gorges reservoir in order to achieve goals, ahead of the schedule of water impounding, increased the water storage capacity. View from the monthly average flow, simulated value is higher than observed value during September to November, same as each other during June to August, simulated value is just a litter lower than observed value during November to May of next year. Because the period time of impounding water is just 2 months, but the period time of draining water is cost near 6 months. View from the daily flow, follow the monthly change rule, but some of the flood season dates will be different, showing the simulate value was significantly higher than measured value, which have some connection with readjusting the peak of flood season on the Three Gorges Reservoir. Value of in the absolute change of the monthly average water level of downstream Three stations shows the various decreasing trends from Yichang to Datong, the water level changes in response to Three Gorges is similar as changes in flow in the three stations. This law also present in monthly average water level change in 2004-2009. The amplitude of date Water level changes on the (04-09) is presented in the whole Yichang, Hankou and Datong is increasing, which is similar as the magnitude of monthly water level, on the part of the Date, the amplitude of Hankou is higher than Datong in 2007-2009 (PhaseⅡ,Ⅲ). That indicating the reposing of three stations to impounding is different, probably relate to the sand mining,water in taking and other human activities after 2007. Compared With water prior to 7 years (1996-2002), the sediment import of upstream was reduced by about 50% during 7 years of impounding. Therefore, during the period (2003-2009) of Three Gorges impounding, the observed suspended sediment flux of Yichang, Hankou and Datong was decline (decreased by 341,180,175×109 kg/a, respectively),where the contribution rate of the Three Gorges Reservoir (decreased by 153,102,78×109 kg/a respectively) only 45%,57% and 45% respectively, the rest of the contribution rate from other human activities of River basin (e g, built reservoirs on the tributaries, the dam built on the Jinshajiang River and the operation Such as the closure of Xiluodu Hydropower Station on the upstream of Yangtze River in November 2007, increasing water use of industry along the river basin, projects about conservation of water and soil. Inter basin water transfer, etc) and Basin drought events.Studies have shown that sediment trapping owing to dam is significant, the dam will intercept most of the suspended sediment of the upstream. Because of the reservoir has little effect on runoff, suspended sediment concentration decreased sharply after the massive water throughout the dam, then caused the river bed (mainly dam - Hankou section) occur more severe erosion. However, this erosion is hardly enough to compensate for interception in the dam, therefore, the suspended sediment to sea is decline significant. In addition, after the Three Gorges Reservoir, the sediment from the River into Dongting Lake was reduced significant, that cause the sediment depositing was slow in the Dongting Lake. The phenomenon of sediment deposition cause by dam construction is very significant, impaction in large spatial scale, the extent of the impact is different in different spatial location. Affection of dam always intertwined with other natural reasons and human activities, in order to scientific and objective understanding of the impact of dams, need to quantify as much as possible stripped.The water impounding of Three Gorges Reservoir was completed in 2009, influence of downstream suspended sediment and water level after the Three Gorges dam full operation will further increase over time. However, suspended sediment and water level on the main stream of the Yangtze River water also will be affected by other natural and man-made factors become complex and difficult to predict accurately, it is worth to pay attention to this continually.