Study On Evolution Of Wandering Reaches In The Lower Yellow River Under The Operation Of Xiaolangdi Reservoir

The Xiaolangdi Reservoir on the Yellow River is a multi-purpose hydraulic complex with flood control (including ice prevention) as its main function, and accretion mitigation, irrigation, water supply and power generation as its supplementary functions. It is of strategic implication in management and development of the Yellow River. The optimum operational mode of Xiaolangdi Reservoir should be decided by its effect on flood control, accretion mitigation, water resources and ecological environment in the lower Yellow River. After the Xiaolangdi Reservoir being put in use, water and sediment regimes will change, which will induce morphological changes downstream in terms of longitudinal and lateral profiles, river pattern, etc.Performing data analysis and experiment on a physical scale model, the author investigates and forecasts the trend of the evolution of the wandering part of the lower Yellow River in the period when the Xiaolangdi Reservoir is operated to release relatively clear water. Analysis of the data that was measured in the lower Yellow River when the Sanmenxia Reservoir was operated to release relatively clear water leads to in-depth understanding of the downstream channel adjusting process. With this, the author gives qualitative forecasts of the trend of the evolution of the wandering reach in the period when the Xiaolangdi Reservoir is operated to release relatively clear water. Experimental study was performed on a 600 m - long physical scale bed movable model for the 349 km-long reach from the Xiaolangdi dam downstream to Sushizhuang to simulate the evolution of the reach in the first 6 years when the Xiaolangdi reservoir is operated to impound sediment. The analysis of the field data and experimental study lead to the breakthroughs, some of which are given as follows. (l)In the first years after the Xiaolangdi reservoir was operated, the reach evolves following the conclusions that relatively clear flow released from a reservoir erodes the channel downstream in spite of the fact that the wandering reach of the lower Yellow River since the Mid-1980, because of the lasting low-flow regime, is quite different from that when the Sanmenxia was operated. (2) In the first 4-5 years after the Xiaolangdi reservoir was operated, the wandering reach is erodeddeep which should have been under accretion without the operation. Further, in the eroding process, flow tends to run in the main channel, which is narrowing, the bed gradually is under armoring, the previous numerous sand bars disappear or merge into a part of the floodplains, consequently the previous forked wandering reach changes into a non-wandering one with only one main channel. The latter, on one hand, evolves with an ever-increasing curvature, but meanwhile suffers from bank sliding on the other hand, so limiting the process of the evolution. (3)The reach of the interest experiences apparent erosion. Erosion widens the main channel where the river training works functions well, and it mainly deepens the channel where the works doesn' t function well. In the deepening process, the banks of the main channel slides severely, river course shifts frequently and in a large scale and has abnormal bends here and there. So, it is concluded that the wandering reach will not change its wandering pattern if there is not a complete set of training works provided there. The field morphological change of the reach in the recent years is in a good agreement with the concluded above.