| The Three Gorges Reservoir has began to store water since 2003 and a lot of upstream sediment was blocked in the reservoir,Discharge increased in dry season and reduced in flood season, result to that the relative equilibrium between water and sediment has been undermined in the downstream river which has been erosed and the bed has been coarsened. River channel evolution characteristics in downstream river of the Three Gorges Reservoir (TGR), the erosion and coarsening phenomena from Yichang to Zhicheng witn different TGR operation modes are studied in this paper. The main findings and conclusions are summarized as follows:(1) The characteristic of riverbed and water with sediment in the Jingjiang River are analysised. The results show that the annual runoff of the major hydrological stations from 2003 to 2013 has little difference with the average discharge from 1955 to 2002 in Jingjiang River. The average annual sediment load from 2003 to 2013 is reduced significantly when compared to the data from 1955 to 2002 and the reduced rate decreases along the river. The annual average sediment concentration significantly reduced and reducing amplitude decreases along the river. The diversion ratio in the three tributaries of Jingjiang River from 2003 to 2013 is 12%and the data from 1999 to 2002 is 14% and larger diversion ratio occurs from May to October. The siltation volume of Zhijiang reach and Shishou reach from Oct.2002 to Oct.2012 is nearly equality in the four conditions of the low flow channel, average channel, flat beach channel and flood channel, and the siltation volume in Gongan reach is smaller than Shashi reach. The erosion amount in Jianli reach is differ quite a bit under that four conditions and the erosion amount is the greatest in flood channel. Portrait thalweg mainly scour from Oct.2002 to Oct. 2012 in Jingjiang reach.(2) The changeable process of cross sections and riverbed gradation in the reach from Yichang to Zhicheng during 2003-2013 is studied in this chapter. The results show that the section shapes in Yichang to Zhicheng reach have been basically stable, and the bed material coarsens significantly since 2010.(3) Summarizing Han Qiwei,He Wenshe,Qin Rongyu and Yin-Yang methods and applying these four methods to studying the coarse scouring phenomenon in Huangjiagang to Guanghua reach of Han river downstream the Danjiangkou Reservuir. It conclued that the results used by Han Qiwei method are more accurate than other methods when compared to the actual scour depth and riverbed gradation and on this basis, a scour rough model is established. The model is proved to be feasible to studying the erosion phenomena in Yicang to Zhicheng reach through practical examples.(4) Reducing the natural runoff at the dam site of Three Gorges Reservoir after it impounded water and Calculating the fill rates from 1950 to 2013 with the original storage project and other programs with different storage time and from water level. It is concluded that the fill rate is 81.2% with the original water impoundment program. When the storage time is no later than September 10 and the water level is unchanged, the reservoir fill rate increases significantly to 93.8%. If the storage time is in prior to September 20 and the water level is150m, the fill rate could reach more than 95.3%. If the storage time is in prior to September 20 and the water level ranges from 155m to 160m, the fill rate could increase to 95.3%. Considering the safety of flood control and comprehensive benefits, it is appropriate to store water with the program that the water level is 150m and the storage time is September 10, especially when it forecasts that the upstream water is much more than the normal runoff, the storage time can be postponed to September 20.(5) The correlation of average daily discharge between Yichang station and Zhutuo station, Wulong station,Beibei station in the upstream reaches is analysised. It leads to the restore relation about the average daily discharge at Yichang station during the non-storage months and restoring the average daily discharge at Yichang station from 2003-2013. It is concluded that the representative bed flow is 55000m3/s with the recommended operation mode of the Three Gorges Reservoir.(6) Yichang to Zhicheng reach will continue to scour with the representative bed flow compared to the scour depth along the reach which is relatively stable state currently. Forecasting the scour depth and the bedsand variation with the scour rough model. The results show that the smallest limit scour depth is 0.84m at Chang 14 section downstream Yichang station 6.0km. The sections Yi 69,71,73,75 at which the scour depth is deeper than 6m respectively downstream Yichang station 40.6km,43.0km,45.5km and 48.6km. The smaller scour depth is 6.75m at Yi 75 section and the deepest scour depth is 10m at Yi 71 and 73 sections. With the representative bed flow,Yichnag to Zhicheng reach will continue to scour 2.0m averagely compared the average scour depth 2.56m in relatively stable state currently and the bed material coarsens significantly compared to 2010 level. |
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