Recent Change Characteristics Of River Bed Resistance In The Lower Reaches Of The Yangtze River

The resistance of alluvial river is the basic problem of sediment motion mechanics,which is closely influence capacity of the discharge and carrying sediment of.Different from the general fixed bed channel flow resistance,the resistance of alluvial river is composed of many parts,including sand resistance,sandwave resistance,sidewall resistance and resistance of channel morphology.The sand resistance and sandwave resistance called riverbed resistance in natural rivers,especially,when width to depth ratio of natural rivers is big,bed resistance is part of most important resistance of alluvial rivers.Meanwhile,the resistance of the river bed reflects the force of the flow to the river bed,which determines the intensity of the motion of the sediment.In recent years,the impact of human activity as the third driving force on the lower reaches of the Yangtze River is increasing.especially the construction of the Three Gorges and the South-to-North Water Diversion Project,lead to a decrease of about 2/3 of the sand.The roughening trend of river bed sediments has a tendency of moving down.This will inevitably lead to an adaptive adjustment of river bed resistance in the lower reaches of the Yangtze River.Therefore,under the new situation,the characteristics of riverbed resistance in the lower reaches of the Yangtze River need to be reconsidered and studied,so as to provide reliable references for channel regulation,shipping safety and flood control.on the other hand,Resistance coefficient of riverbed is an important parameter in numerical simulation,directly controll simulation results and accuracy of hydraulic factors,such as flow,velocity,depth,etc.,is also the key role of accuracy and accuracy of flood evolution prediction in the calculation of water crossing capacity of natural river,open channel and pipeline.The factors affecting the resistance of river bed include roughness,shape,hydraulic radius and depth,water flow pattern and sediment content,etc,to obtain more accurate river bed resistance parameters.In this paper,the sediment,shape,depth and velocity of the bed from Jiujiang to the Changjiang Estuary from 2014 to2016 were collected and analyzed in laboratory,multi-beam sounding system and dual-frequency ADCP were used to measure and analyze the sediment.In order to provide reliable technical and data support for flood control management,the river bed resistance from Jiujiang to Yangtze River Estuary is calculated,and the flood level and discharge process of Nanjing reach of Yangtze River are simulated and analyzed by using Delft3D numerical model.The main findings are as follows:(1)The lower reaches of the Yangtze River Three Gorges Reservoir before the largest from Jiujiang to the Wusong River bed sand resistance of 134 N/m~2,a minimum of 2.2 N/m~2,average sand resistance is 54.6 N/m~2.But after the Three Gorges reservoir,the lower reaches of the Yangtze River from Jiujiang to the largest Wusong River bed sand resistance of 40 N/m~2,a minimum of 5.8 N/m~2,average sand resistance is 20 N/m~2.Before and after the impoundment of the Three Gorges project,the largest bed of sand grain resistance and average grain resistance was reduced by85%and 63%,the overall sand resistance decreases obviously;on the whole,the river(River I-V)process was first increased and then decreased along the two time average sand resistance,but the variation is smaller.(2)Before the completion of the Three Gorges project,little difference in the lower reaches of the Yangtze River Channel of each section of the Sandwaves resistance range,maximum ripple resistance is 0.67 N/m~2,the average sand resistance of 0.27 N/m~2.After the Three Gorges reservoir,little difference in the lower reaches of the Yangtze River Channel of each section of the Sandwaves resistance range,maximum ripple resistance is 0.15 N/m~2,the average sand resistance of 0.05 N/m~2.Each point for the sand grain resistance resistance of less than 1%,indicating the lower reaches of the Yangtze river bed resistance to sand resistance.Two time points,each point of the sand grain resistance resistance accounted for less than 1%,indicating the lower reaches of the Yangtze river bed resistance to sand resistance.(3)Before and after the three Gorges impoundment,the sand wave resistance of each section is less than 1%of the river bed resistance,It shows that the resistance of riverbed in the lower reaches of the Yangtze River is mainly sand resistance.As a whole,the variation trend of average bed resistance from west to east reaches(reach?-?)in two time periods also shows the trend of decreasing along with the first increasing,but the range of change becomes smaller.(4)Before the Three Gorges reservoir impoundment,the river bed resistance in the lower reaches of the Yangtze River is related to the above three factors,from the top to the bottom,followed by particle size,velocity and width to depth ratio,and the corresponding average correlation coefficients are 0.48,0.3 and 0.25 respectively.After Three Gorges reservoir impoundment,the correlation between riverbed resistance and the above three factors in the lower reaches of Yangtze River is from large to small in order of velocity,width to depth ratio and particle size,and the corresponding average correlation coefficients are 0.71,0.41 and 0.3 respectively.But before and after the water storage of the Three Gorges,the main factors affecting the resistance of the river bed are different.(5)The particle size,velocity and ratio of width to depth are also important factors to judge the shape of bed surface.Most of the predecessors used the combination of these discriminant elements,and calculated the Shields number and sand Reynolds number double parameters,and then to judge the bed surface shape.Because of the limitation of the empirical method with two parameters,this paper first tries to judge the bed surface shape with three parameters:Froude number,sand Reynolds number and Shields number,compared with the two parameters of Shields number and sand Reynolds number,the accuracy of the method is improved by35%,Compared with the double parameter discriminant method of gD~3/~2 and sand grain Reynolds number,the accuracy of the method is improved by 30%,Based on Shields number and Reynolds number of sand grains,a discriminant function of bed morphology is established,The error rate of the prediction results of sand ridges is40%,gD~3/~2 and Reynolds number of sand bed form discriminant function based on prediction test bed form for sand prediction results were not successful,It is further explained that Fr plays an important role in the differentiation of bed morphology in low velocity areas,especially in the development of sand ridges.(6)Based on the river bed resistance,the flood of the Nanjing river section is simulated and the change process of the transit flow of the Nanjing hydrological station under different water levels is predicted.The study found that,when the design flood level is 13.5m,the average transit flow of Yangtze River Nanjing reach about113000 m~3/s;when the design flood level is 9 m,the average transit flow of Yangtze River Nanjing reach about 71000 m~3/s;when the design flood level is 8.5 m,the average amount of transit flow in Nanjing reach of Yangtze River is about 68000 to m3/s;to ensure the water level above the lowest navigable water level 0.74 m,upstream flow to ensure that at least about 4300 m~3/s.Moreover,the simulation from different level results Baguazhou the average hydraulic gradient of head to tail section of delta delta hydraulic gradient are much greater than the Nanjing River,and with the increase of design flood,Baguazhou reach hydraulic gradient increases.