| The rare happened river evolution process occurred in the Lower Yellow River during the periods from1980s to1990s, which resulted in channel shrinkage, heavy silting, and significant decreasing of the flow area and flood-sediment carrying capacity, which threatened to the safety of flood protection, consequently. Therefore, the channel shrinkage is becoming a complicated problem and is urgent need to be studied in depth in the Lower Yellow River. In this thesis, combined with the data analysis and mathematical modeling, the characteristics of the variation of water and sediment and the phenomenon of channel shrinkage in the Lower Yellow River are analyzed; the mechanisms of channel shrinkage and regulation critical index are discussed. Furthermore, the roles of different water-sediment series on forming the channel for water-sediment transport and their influencing factors are studied, and the rational scale and comprehensive regulation measurements to form and maintain the channel for water-sediment transport are proposed, in aim to support the construction of the water-sediment regulation system in the Yellow River and to regulate the shrunk channel in the Lower Yellow River in technique. The main contents and achievements are as follows:(1) The amounts and processes of incoming runoff and sediment in the Lower Yellow River showed a decreasing trend since the1950s. Especially, these trends had undergone significant variation after1986, which can be characterized as reduction of the amount of runoff and sediment obviously, un-conformity of water-sediment relationship, reversion of water allocation between the flood and dry seasons, and significantly decreasing occurrence of large floods.(2) The Channel Shrinkage in the Lower Yellow River is one of river bed evolution processes under the particular incoming water-sediment conditions. Its main features show serious siltation, strongly reduction of cross-section, significant descent of flood carrying capacity, and always accompanied with the developing of special phenomenon, including abnormal river bends, channel hump, and so on. The channel shrinkage in the Lower Yellow River is principally caused by lower flow rate and higher sediment concentration in continuous years. Both the extending duration of small and medium flow and the frequently happened hyper-concentration floods promote the channel shrinkage, the former results in the serious siltation in the main channel, and the later increases the sedimentation in the newly-formed floodplain. the comprehensive conditions and water-sediment critical relationships were proposed based on the data from1986to1999. The instability of the river regime and the configuration of cross sections show that the shrunk channel can recover as long as the incoming water-sediment conditions are reasonable.(3) Analysis results show that the reach from Huayuankou to Lijin is in relatively balance of scoring and silting when the incoming sediment coefficient is about0.012kg.s/m6and the yearly-averaged discharge is about1850m3/s at Huayuankou; the reach from Xiaolangdi to Lijin will not silt when the flood incoming sediment coefficient is about0.012kg.s/m6. The bankfull area of the typical cross-sections in the Lower Yellow River increases with the maximum flood peak discharge during the year, the ratio of bankfull width to depth increases with incoming sediment coefficient during the flood season. However, the configurations of cross-section tend to be stable when the flood incoming sediment coefficient is larger than0.04kg.s/m6. The maximum flow area of each typical cross-section is getting larger with increasing of peak discharge. The required flow area to transport the non-overbank flood, with the peak flow of4000m3/s and maximum sediment concentration of45kg/m3, at Huayuankou and Lijin are1987m2and1603m2, respectively. The minimum ratio of width to depth at Huayuankou decreases with the flood-averaged sediment coefficient, and vise verse at Gaocun, Aishan and Lijin. The variations of minimum ratio of width to depth are slight when the flood-averaged sediment coefficients are larger than0.04kg.s/m6,0.035kg.s/m6,0.03kg.s/m6,0.025kg.s/m6at Huayuankou, Gaocun, Aishan and Lijin stations, respectively. The bankfull discharge in the Lower Yellow River will increase with the annual runoff (or runoff during flood season) and peak flood flow, and it is possible to keep the bankfull discharge of about4000m3/s in the future, if the incoming annual runoff gets to the averaged value from1986to2010by Xiaolangdi reservoir operation.(4) The multi-annual operation of Xiaolangdi reservoir make it possible to form the channel for water-sediment transport with bankfull discharge of about4000m3/s in the Lower Yellow River. Both forming and maintaining the basic channel for water-sediment transport are the results of the loss of some sediment storage capacity. The optimal procedures to form and maintain the basic water-sediment transport channel can be concluded as:to form the channel with bankfull discharge of about4000m3/s by retaining sediment in the early5to8years after Xiaolangdi reservoir operation and controlling the sediment delivery ratio less than0.3; and then, the multi-annual operation of regulating flow and sediment and discharging sediment at proper times can be adopted, the sediment deliver ratio can increase to about0.7to maintain the water-sediment transport channel, and to avoid the channel continuous expansion firstly and shrinking after then. This optimal reservoir operation is benefit to extend the service life of sediment storage capacity and to keep the basic water-sediment transport channel in stable.(5) Trapping the coarser and delivering finer sediment particles in Xiaolangdi reservoir are in favor of increasing the water transport capacity and making the basic water-sediment transport channel narrow and deep in the Lower Yellow River. However, the river bed armoring brings forth higher requirement for incoming runoff and sediment conditions. Otherwise, the formed channel becomes wide and shallow. The existing river training works in the Lower Yellow River play less role on forming the water-sediment transport channel. The matching of production dikes and river training works is in need to form the effective flood draining and sediment transport channel. The engineering of dredging, flood-plain protection and river works should also be planed integrated to keep the formed water-sediment transport channel in stable.(6) It can be realized to form and maintain the basic water-sediment transport channel in the Lower Yellow River by regulating the delivering process of water and sediment of Xiaolangdi reservoir and co-operating with the comprehensive river training measurements in the near future. Furthermore, to maintain the channel in the medium and long term, it can be achieved by the combined water-sediment regulation of Xiaolangdi reservoir, planed large-scale hydraulic projects in the middle reach of Yellow River and inter-basin water diversion engineering. According to practical flow and sediment process in the Lower Yellow River, it is possible and advisable to form and maintain a basic water-sediment transport channel with bankfull discharge of about4000m3/s. |
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