| Significant variation in the water and sediment process had occurred since 1980s in the Yellow River Basin,Causing sediment accumulation that mainly concentrated in the main channel aggravated,and the withered river with apparently decreasing flow-conveying capacity in downstream.The banefull discharge was about 1800m~3/s only around Gaocun,being the reach with the smallest banefull discharge prior to the flood season by In 2002.While,the flow-conveying capacity had somehow recovered,along with the water-sediment adjustment and the reduction of sediment load in Xiaolangdi Reservoir since 2002.Risk events like that once the discharge exceeds the bankfull discharge,and floodplain will occur happen,especially there are lagging of floodplain safety construction in lower yellow river.Thus,maintaining appropriate water channel scale has become one of the most important task in the Lower Yellow River Regulation,studying the constructing riverbed and flow-conveying capacity of medium-size channel in the Lower Yellow River has important theoretical and practical significance.This paper mainly focused on analysis of water-sediment and channel evolution characteristics in lower yellow river.Particulary,according to Measured data in1974~2015,the response relationship between banefull discharge and water-sediment characteristics was discussed while banefull discharge acting index of flow-conveying capacity.Ulteriorly,the mechanism of constructing riverbed would be revealed and flow-conveying capacity would be forecasted through a one-dimensional flow and sediment mathematical model.The main research results are as follows:Firstly,The upper reaches of the lower Yellow River,Gaocun and Sunkou hydrologic station,locating in the lower Yellow River,shared the variation tendency in both water and sediment,that could be seen from the analysis of water and sediment data in 1974~2015.In detail,the annual amount of water and sediment in the lower reaches was greatly reduced since 1987,especially,the tendency aggravated after the operation of Xiaolangdi Reservoir.Additionaly,the annual distribution of water and sediment flowing into the lower reaches had changed,due to the applications of water and sediment regulation in Xiaolangdi reservoir.Before the operation,Water quantity in non-flood season was greater than that in flood season,while,situation reversed after reservoir operation,meanwhile,the proportion of flood season water increased and accounts for a steady proportion in the whole year.There were certain increases in the amount of water and sediment in June and the whole non flood period(from November to June),on the contrary,the corresponding proportion from July to October,the flood season,somehow reduced.Besides,the probability of small flow increased,yet the large flow rate decreased,furthermore,the particle size of suspended sediment was obviously thickening in the lower Yellow River,After the Xiaolangdi reservoir performed.Secondly,the morphological changes of cross section indicated that once floodplain occurred,there would be erosion in channel and sedimentation on floodplain issues,that major trough drifted with water and sediment transformation.Specifically,During the period of 1987~1999,silting-up was the main process in the flood season,while erosion dominated in the non-flood season in the lower reaches,except that flushes and silt alternately still present in Gaocun section during flood and non-flood season.Since 1999,erosion phenomenon occurred in the lower reaches all year round,but situation in Gaocun section still unchanged.Pre-flood-season in 2001,major trough shrunk in Gaocun and Sunkou section obviously.While,after 2002,major trough were gradually increasing in Gaocun and Sunkou section because of the applications of water and sediment regulation in Xiaolangdi reservoir.They recovered at a certain extent pre-flood-season by 2014,and then remained stable.Thirdly,the banefull discharge became smaller along the lower river,according to date from hydrologic stations.Yet the water and sediment regulation of Xiaolangdi reservoir played a role of maintaining the medium-size river channel.In particular,pre-flood-season in2002,banefull discharge was the smallest value,only 1800m~3/s.Major trough relatively shrunk and flow-conveying capacity declined greatly.However,The smallest banefull discharge and flow-conveying capacity were larger since 2002,and discharge that had evolved around Sunkou reached to 4200m~3/s by 2014 pre-flood-season,and since then kept steadily.Fourthly,using measured water and sediment data in 1974~2015 to study influence factors on banefull discharge,and establishing fitting function to reveal the mechanism of medium-size channel constructing riverbed.Results were as follows:Once the annual flow above 2000m~3/s,corresponding flow had a good correlation with banefull discharge.Considering the lagging response,the regression methods were established respectively between moving flow averages of four or five years with the smallest banefull discharge,under a condition that flows were above 2000m~3/s.Thus,interdependencies were greater remarkably,so were Gaocun and Sunkou section.All in all,the characteristic of banefull discharge was caused by the comprehensive effect of water and sediment change.Lastly,by using one-dimensional flow and sediment mathematical model,scenarios of the Yellow River sands with 300 million tons,600 million tons and 800 million tons in the next 50 years were calculated respectively,and prediction of river erosion and deposition and banefull discharge change in the lower reaches were also forecasted.Results were as follows,the lower river channel would basically maintain the equilibrium of erosion and siltation,along with 4000m~3/s banefull discharge under scenario 1.Under scenario 2 and 3,annual average siltations in the lower reaches would be 100 million tons and 159 million tons respectively,with banefull discharge were 3100 m~3/s and 2900m~3/s,correspondingly. |
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