Study On Evolution Law Of Braided Reach Of The Lower Yellow River After The Operation Of Xiaolangdi Reservoir

Braided river is the most unstable among the four alluvial river patterns,which not only has unique geomorphological features,but also has complex water-sand transport and riverbed evolution.The braided reach of the lower Yellow River is densely covered with beaches and intertwined with branches,and the law of riverbed evolution is very complex.With the change of natural conditions and the influence of human activities,especially the operation of Xiaolangdi Reservoir,the conditions of incoming water and sediment entering the lower reaches of the Yellow River have changed greatly,the downstream channel has been severely scoured,and evolution of the channel bars have changed accordingly.Therefore,the study on the evolution law of the beach in the braided reach of the lower reaches of the Yellow River after the operation of Xiaolangdi Reservoir is not only helpful to fully understand the river bed evolution law of the braided river after the operation of the reservoir,but also provide relevant parameters for flood control safety,river regulation and so on.Based on the remote sensing image data of the braided reach of the lower reaches of the Yellow River from 1999 to 2018,this paper calculates the changing process of the number,area and morphological index of the braided reach in the past 20 years,and analyzes the temporal and spatial evolution of the braided reach after the operation of Xiaolangdi Reservoir.The main findings of this paper are summarized as follows:(1)In terms of inter-annual change,the number and area of beaches in the braided reach of the lower reaches of the Yellow River increased at first and then decreased from 1999 to 2018.The number of beaches showed an increasing trend from 1999 to 2011,from 241 in 1999 to 361 in 2011,and gradually decreased to 239 in 2018.The beach area increased from 50.34 km2 in 1999 to 80.04 km2 in 2008 and decreased to 52.16 km2 in 2018.The overall beach shape changes little,and the beach shape index changes in the range of 1.67-1.88.This reflects that in the early stage of the operation of Xiaolangdi Reservoir,due to the severe erosion of the river bed in the braided reach,the water level dropped greatly,which led to more beach exposure,and the erosion capacity of the latter beach was greater than that of the exposed beach due to the drop of water level.According to the change of the year,the number and area of beaches in the braided reach of the lower Yellow River show a very obvious decreasing trend from May to October in the flood season,and an increasing trend in non-flood season,which is mainly affected by riverbed erosion in the flood season.(2)According to the location of the remote sensing map,the braided reach is divided into the upper section(above the ancient city)and the lower section(the Gucheng-Gaocun section)to study the variation law of the beach.It is found that the change range of the number and area of the beach in the Gucheng-Gaocun section is smaller than that above the Gucheng section,the beach shape in the section above the ancient city tends to be wider and shorter,and the beach shape in the Gucheng-Gaocun section tends to be slender.According to the difference of beach area,the beach is divided into four types,and the evolution rules of four types of beach in different reaches are studied respectively.The study found that both the number of beaches and the beach area under 0.3 km2 and 0.3 km2-1.0 km2 are the largest in both the Gucheng upper reach and the Gucheng-Gaocun section,while the very large beaches with an area larger than 2.0 km2 are the least in number and do not change much.During the study period,the change in the number of beaches below 0.3 km2 was the most obvious,followed by the beach from 0.3 to 1.0 km2.(3)The effects of water level,incoming water and sediment conditions and riverbed boundary conditions on the number and area of beach in the braided reach of the lower Yellow River are discussed.It is found that the number and area of beach in the braided reach decreases gradually with the increase of water level.The change of riverbed boundary conditions will have a certain impact on the number and area of beaches in the braided reach,but the incoming water and sediment conditions are always the most important influencing factor.However,there are many factors affecting the number and area of the beach in the braided reach,which is affected not only by the water level,incoming water and sediment conditions and riverbed boundary conditions,but also by other factors.