| Large water conservancy construction had many great influences on downstream river. The most important one was the change of sediment movement characteristic. Sediment was intercepted and settled in the reservoir by dam. This had directly reduced the sediment concentration of downstream river. It had made the resource of bed material load in downstream river change to the bed surface along the river. These changes had caused downstream river regime have a complex evaluation. However, the river regime evaluation process was based on adjustment and balance of water and sediment. It was also achieved by exchange of suspended load and bed material load.Based on hydrological and topography data before and after three Gorges reservoir impoundment and other reservoirs evolution, sediment exchange law and influencing factor had been analysed to predict the trend of sediment transport, variations of riverbed topography and shoal evaluation. From a shipping perspective, a new river regulation strategy on the condition of new silt-discharge condition was proposed.The main contents of this paper were as follows:(1) After analyses of sediment discharge before reservoir impoundment, erosion and deposition processes had been always go together with reciprocating adjustment of bed sediment diameter composition and riverbed topography within the year. It was a short term mechanism that adapted to water and sediment conditions of upstream. Sediment discharge more big, variation within the year more great, conversely, is variation more softly. The phenomenon reflected intuitively to this mechanism variation intensity was variation of sediment transport capacity. As sediment transport capacity variation and incoming sediment process was unsynchronized, the river balance concept is not applied to short period variation. However, variation within the year was the base of secular variation. Since periodic hydrological conditions, the incoming water and sediment process of every year couldn’t be entirely similar. For shoal evolution, there was a good or bad impact of water and sediment condition. The essence of it was discrepancy within the year caused by periodic variation of incoming water and sediment.(2) Sediment transport process was also the exchange process of suspended load and bed material load. Sediment diameter composing had an important influence on sediment transport before and after the reservoir impoundment. After the reservoir impoundment, sediment was intercepted and settled in the reservoir by dam. This had directly reduced the sediment concentration of downstream river. It had made the resource of bed material load in downstream river change to the bed surface along the river. Thus, from the sediment diameter composing condition, we could analyze the bed material load recovery state along the river.Sediment completely recovery distance in downstream was short. Beyond the distance, suspended load and bed material load exchange had made river bed diameter become bigger even if the river bed contained a lot of fine sand. This work had analyzed the cross section and longitudinal section shape evaluation of sand reach after reservoir impoundment. Based on these analyses, we found that bed erosion had been limited by sediment grain size coarsening and shear stress changes. Thus, in consideration of sediment diameter composition, erosion and deposit within the year and flow condition change, it could be sure that bed elevation changing trend in downstream river would not be the long-time average annual value before reservoir impoundment, but the value that adjust to new water and sediment condition. Thus, it was meaningless to decide the future river evolution trend by reduction of bed material load transport quantity. The bed material load transport quantity decrease would continue.(3) Though reduction of bed material load transport quantity is different, there effects on river channel were coincident. For channels within sediment completely recovery distance, obvious reciprocating changes would impossible to appear. Effects of sediment transport quantity differences on river evolution among years would decrease. The effects would be reflected in flow process differences among years. For channels beyond sediment completely recovery distance, bed material load transport quantity decreased for all channels, and the reduction was significant. Thus, effects of sediment transport load differences on river evolution would be also reflected in flow process differences among years. As the effect of dominant role of the main flow, flow process difference would induce the difference of bed evolution. From the energy point of view, turbulence near bed surface made by main flow was more than that by other flows. The upward sediment flux was more than other areas and downward sediment flux was less than other area. For this cause, main flow area had become the main erosion area. The erosion degree was limited by low flow channel for the large depth and coarse grain size of sediment. However, when low flow channel had an obvious diverge from flood flow path, bar on flood flow path would have a large deformation. The erosion of beach would also cause the channel broadening and shifting for median flow channel and low flow channel. Then, the channel navigation condition would become worse. Thus, the income sediment reducing after Three Gorges reservoir impoundment was the key reason for main flow shifting in most of downstream channels. Based on this analysis, bar in middle and lower Yangtse River had become from periodically type with water and sediment condition to runoff type beach. This work praised that, for restraining runoff process in specific discharge level, the narrow-deep channel could stay stable. However, when the flood process that could dispense this constraint happened, this stable channel that had a large adjustment was almost inevitable.(4) This work had explored countermeasures of sandy river in middle and lower Yangtse river. It states that the income flow process was unpredictable, while erosion in main flow was predictable. In eroding state, erosion process was limited by sediment transport capacity and channel adjustment rate is slow. This was obvious different from that large scale deposit in flood period could lead to low flow channel trending to wander. This predictability had provided a preparation time for engineering implementation. Thus, the adverse effects caused by Three Gorges reservoir impoundment were controllable. Flowing route were often different in different discharge level, but the low flow channel was always chosen to be navigation way. Based on this, a protecting and controlling type measure had been put forward in this work. That is, taking protecting and controlling measures to stabilize main flow in middle and low flow channel, to protect bars and branch channel division capacity in low flow period from flood erosion. Though channel erosion was weak in middle and low flow channel, but it sustained longer more than that in flood period. This cause the cumulative eroding effect is significant. The advantages of this waterway regulation strategy was that, it could not only well achieve the waterway regulation target, but also could reduce the effects of waterway regulation on other river functions.(5) This work also had researched the problem of sand-gravel river. River-bed erosion was the main reason for water level decrease in sand-gravel river. The outlet water level decrease was caused by lower reach bed erosion. River-bed erosion had affected bed resistance, which finally affected water level in low water period. Sediment grain size coarsening of river bed had a positive effect on inlet water level stability. Yet it only happened before the formation of armouring layers. Erosion position had an obvious effect on relation between erosion quantity and water level changes. Variation of water level was very sensitive to erosion in main flow and river node. However, in slack water area and deep groove, effect of erosion on water level was weak.Lujiahe channel was located at the end of sand-gravel river. Its sediment deposit amount in inlet area had decreased after Three Gorges reservoir filling. However, sediment deposit amount had increased in recent years. The main reason included upper reaches regime change, Shihong channel development, the income sediment coarsening, recession hydrograph speed increasing. This phenomenon had appeared in most of reachs in Yichang to Changmenxi channel. It appeared in Zhicheng reach when the channel bed load transport rate reducing to less than5%of it before reservoir impoundment. Reason for this happened was that bed sand coursing had lead to parts of channel bed difficult to erode but easy to be deposit. This evolution law was universal in middle and lower Yangtze river. |
