Preliminary Study On Sediment Deposition Law And Prevention Of Confluence Reach In Reservoir

As one of the most typical landform units in river system, confluence river holds a vital role in the development of human society and economy. Accompanied with hydro-project construction, confluence river which used to be fluvial equilibrium state begins to change, due to the altered condition of flow-sediment and channel boundary. The change of sediment transportation and river evolution is especially prominent on confluence reaches in reservoir, whose erosion basis and flow-sediment will be greatly changed by cascade reservoirs. At this point there could be further more complicated aggradation and degradation, sediment hazards take place. Therefore the research on sediment deposition law of Confluence reaches in reservoir area owns the important theoretical significance in the reservoir sediment research and hazard control field.Field investigation, theoretical analysis and generalized model test were adopted to conduct the research. Different sediment deposition types in confluence reach in reservoir, as well as the main influence factors for sediment deposit were discussed, then the hydrodynamic character and siltation law were systematically studied. At last, since serious problems could be caused by sediment deposit of confluence reach in reservoir, the efficiency of a new siltation prevent measure was tested.The following conclusions were achieved in this dissertation(1) Respectively on the view of deposit longitudinal morphology, deposit distribution, and deposit source, the sedimentation was classified. There was connection among the deposit distribution, sediment source and their longitudinal shape. The accumulated sandbar could appeared in mainstream channel and tributary channel (back-slope cone shape in longitudinal) both depending on different condition.(2) According to analysis, the influence factors such as runoff-sediment condition, reservoir water level which is called reservoir characters, and confluence ratio, confluence angle, and confluence slope called the confluence river characters, could affect the sediment deposition of confluence reaches in reservoir. But the main influence factors are runoff-sediment condition, reservoir water level, and confluence ratio.(3) There always existed a turning point in confluence region for flow velocity from mainstream channel to the dam, first rise then fell. While from tributary to dam, the same turning point only appeared when R was less than0.5. Meanwhile the sediment carrying capacity also showed increased firstly then decreased, no matter from mainstream or tributary. The turning point could be the formation mechanism for accumulated sandbar. When R ascended, the velocity increased more significantly. As to the water level, the flow velocity in the whole area decreased when it rase, as well as the decreaseing rate of velocity and sediment carrying capacity along the channel.(4) Reservoir water level, runoff-sediment condition from mianstream and backwater effects (including intensity and perdurability) apparently determined silting volume and distribution of confluence reach in reservoir. Results indicated that severe siltation usually occurred in the position where the sediment-laden flow passed, and when reservoir water level went up, sand came from tributary would silt in tributary channel, so did the particles from the mainstream. Finally the silting difference was weakened.Based on the synchronous floods tests, small R meant flow from tributary was resisted by mainstream, and the sediment from tributary primarily deposited in tributary channel, where the back-slope cone appeared when larger volume of sediment load came from mainstream under this circumstance. While long-time resist effect in mainstream by tributary flow resulted in the cumulative deposition, whose area ranged from tributary channel to confluence region, indeed some particles could entered the mainstream channel by means of density flow, accumulated sandbar formed.Based on the asynchronous floods tests, continuous tributary floods caused silting volume raised up in tributary channel, but there was a critical silting ratio, when the silting volume reached this critical ratio, extra tributary floods was meaningless to the enhancement.(5) Defined several integrated confluence ratio (ICR), respectively were Rave,Rall ave, Rmax and Rana. On this basis evaluated siltation distribution. Result showed during the flood period, all these ICRs could reflect the mutual effect between mainstream and tributary that decided the deposit distribution. When the integrated confluence ratio grew larger, the siltation ratio in confluence region and tributary region would ascend, while the siltation ratio in mainstream region climbed up as the ICRs went down. Rail ave indicated better relationship between deposit distribution ratio and the mutual effect between mainstream and tributary.(6) Discussed the various hazards caused by sediment deposition of confluence reaches in reservoir, and the necessity of relevant prevention. The dissertation proposed a prevention measure by transforming the siltation distribution, and its desilting efficiency was tested in generalized model. Result indicated that with the appropriate siltation district planning, the siltation volume in the whole confluence reaches can be reduced, as well as the siltation ratio in confluence region, simultaneously the formation of accumulated sandbar was contained. It was more efficient especially when applied on those confluence reaches with larger flood-dry ratio and flow concentrated in flood season.