Analysis And Calculation Of Channel Adjustment Process In The Lower Yellow River And The Yellow River Estuary

Owing to the intense human activities and climate changes,the drastic morphological adjustments have frequently occurred in the lower Yellow River(LYR)and the Yellow River Estuary(YRE),especially the braided reach.Therefore,investigating the channel adjustment characteristics of the LYR and YRE,establishing the empirical relations between the channel incision and bank retreat and developing the longitudinal and lateral deformation model for the braided reach help to predict the channel evolution in the future,and provide relevant parameters for river regulation works and flood control management.In order to study the channel evolution process in the LYR and YRE,the geomorphic adjustments in planform and cross-sectional geometries were investigated quantatively based on the observed cross-sectional profiles and remote sensing images.For better understanding the relationships between the longitudinal and lateral channel deformation downstream of the Xiaolangdi Reservoir(XLD),the empirical formulas between the channel incision and the bank retreat were established firstly.Then a 1D longitudinal and lateral deformation model(LLDM)was developed by integrating the bank retreat model with the 1D steady flowsediment model(SFM),which can simulate both of the longitudinal and lateral channel evolution during the flood seasons in 2002 and 2005.The conclusions include the followings:(1)After the operation of the XLD,the whole reach of the LYR,especially the braided reach,suffered remarkable channel scouring owing to the decreasing sediment load,and some sections in the braided reach experienced severe bank retreat.The maximum increase of the bankfull depth and width was up to 178% and 46%,respectively.The channel incision was dominant in the transitional reach and meandering reach,with a slight adjustment in bankfull width.According to the soil test,the banks of the braided reach are composed of cohesive soil,with the unit weight,cohesion and internal friction angle of the soil closely related to the moisture content.In general,the moisture content increases,the unit weight increases,and the cohesion and internal friction angle decrease,which results in the decrease of the shear strength.(2)Remarkable channel adjustments have occurred in the YRE due to severe human activities such as reservoir operations and artificial avulsions.The processes of the channel evolution can be divided into four phases,following the sequences of “rapid aggradation—channel scouring—channel shrinkage—severe degradation”.The variations in channel geometry indicated that:(i)during the first period from 1976 to 1980,the study reach underwent drastic aggradation,with the increase rate of the channel length being from 75.1km to 93.2km;(ii)numerous branches near the river mouth gradually coalesced into a single channel directed to the southeast in 1980-1984,and channel incision was dominant during this period;(iii)After 1984,the bankfull area decreased by 61% due to the severe shrinkage caused by the operation of the Longyangxia Reservoir.The thalweg migrated frequently with the migration intensity and distance of 0.17 and 102m/a,respectively;and(iv)the bankfull depth increased by 41% during the period from 1999 to 2016,and the migration width and intensity decreased to 37m/a and 0.09,respectively.In addition,empirical relationships between the reach-scale bankfull dimensions,the previous 4-year average fluvial erosion intensity and the water stage differences during flood seasons were developed,with the correlation degrees of over 0.85.(3)The channel downstream of a dam usually undergoes severe degradation and significant bank retreat processes due to the altered flow and sediment regime after the operation of a reservoir.Results indicate that the main adjustment in channel geometry was characterized by channel incision,with bank retreat being remarkable in local regions.Empirical relations were developed between the cumulative reach-scale bank retreat width and channel incision depth in different reaches,with the correlation coefficient above 0.85.Quantitative equations between channel incision depth and bank retreat width were derived in different reaches based on 1D continuity equations of flow and sediment,and the channel incision depth is in close connection with the rate of bank erosion and the initial channel depth.In addition,the widths of cumulative bank retreat calculated by two methods were both in close agreement when the channel degradation approached a certain depth,which indicates the proposed two relationships can be used to estimate the tendency of fluvial processes in the braided reach of the LYR.(4)The traditional SFM can only calculate the channel incision,and can not simulate the variations of the bank geometries.However,the LLDM can simulate the channel incision and bank retreat synchronously.For better simulating the longitudinal and lateral channel deformation processes in the braided reach,a LLDM was developed by combining the bank retreat model with the SFM.After simulating the channel evolution in the braided reach during the flood seasons in 2002 and 2005,it was found that the cross-sectional geometries calculated by the LLDM are more similar to the measured values than the values calculated by the SFM,especially bank geometries.The deviations between the measured bankfull widths and the values calculated by the LLDM were only 2.99% and 3.13%,which were much smaller than the deviations between the measured values and the values calculated by the SFM.In addition,the bankfull depths calculated by the two models only differ slightly.It indicates that the LLDM can be used to simulate the channel incision and the bank retreat of the braided reach in the LYR,and the results of the model are more accurate than the results of the SFM.