Assessment Method Of Channel Stability And Its Applications In Braided Reaches Of The Yellow River

Braided river is a common type of alluvial rivers,which not only has unique geomorphologic features,but also has complex characteristics of flow and sediment transport and channel evolution.The natural flow and sediment regime entering the Middle and Lower reaches of the Yellow River has changed significantly owing to the continuous changes in nature and the impact of human activities,which has changed the channel stability in the braided reaches.Therefore,it is necessary to put forward the reasonable evaluation index and method to invistigate the variations in channel stability of braided rivers in the long time and short time scale.It is not only beneficial to fully understand and grasp the rule of channel evolution for braided reaches of the Yellow River,but also can provide support for channel improvement and disaster prevention.(?)The time scale for studying channel stability of braied rivers was divided into the long time scale(several decades)and short time scale(a flood or a flood season).When evaluating channel stability in the long time scale,section-scale and reach-scale cumulated bank retreat widths and thalweg migration widths and intensities were put forward to analyze lateral channel stability,based on the measured cross-sectional profiles.Besides,section-scale and reach-scale cumulated depositon thicknesses(scour thicknesses)and bankfull depths were proposed to analyze vertical stability.Approporiate calculaton equations for lateral and vertical channel stability coefficients have been also selected for braided rivers.When evaluating channel stability in the short time scale,the adjustment index in channel geometry and channel evolution volume were used as main evaluation index based on the measured cross-sectional profiles and mathematical models.(?)The processes of bank retreat,the characteristics of thalweg migration,the variation in cumulated depositon thickness(scour thickness)and lateral and vertical channel stability coefficients were investigated quantitatively in the braided reach of the Lower Yellow River from 1986 to 2015.It was found that the phenomenon of bank retreat occurred at 82% of the sections in the braided reach.The average thalweg migration width decreased from 234 m to 123 m,with the lateral stability coefficient and comprehensive stability coefficient increasing after the opertation of Xiaolangdi Reservoir.The factors influencing channel stability indexes were analysed quantitatively in the braided reach of the Lower Yellow River.The results showed that although channel boundary conditions can influence the indexes to some extent,the incoming flow and sediment regime was the most important factor.The key fluvial influencing factor was determined to be a parameter of fluvial erosion intensity,and relations between the channel stability evaluation indexes and the fluvial erosion intensity were established to calculate and predict the variations in channel stability in the braied reach of the Lower Yellow River.(?)The processes of thalweg migration and the variation in depositon thicknesses were investigated quantitatively in the Xiaobeiganliu River during the continuous aggradation period bwtween 1986 and 2011.It is found that the thalweg migration width varied between 210 m and 707 m,and the reach-scale average cumulated depositon thickness and thalweg deposition thickness were both above 1 m.The influence of the incoming flow and sediment regime and base-level changes in the downstream(the varation in Tongguan elevation)were analyzed quantitatively on the channel stability indexes in the Xiaobeiganliu River.The empirical relations were established between the evaluation indexes and comprehensive parameter consisting of fluvial erosion intensity and the variation in Tongguan elevation,and the calculated channel stability evaluation indexes aggred well with the observed data.(?)The influence mechanism of overbank and non-overbank hyperconcentrated floods was analyzed on channel adjustments in the braided reach of the Lower Yellow River,and it is found that overbank hyperconcentrated floods had more influence on channel geometry.Adjustment index in channel geometry at reach scale was proposed as an evaluation index in short time scale,with several factors influencing adjustment index in reach-scale channel geometry being analyzed during hyperconcentrated floods.It indicates that the mean sediment transport rate was a key factor influencing the adjustment index,although pre-flood channel geometry and sediment deposition can also affect the index to some extent.An empirical relationship was developed between the characteristic parameter representing the pre-and post-flood channel geometries and mean sediment transport rate in hyperconcentrated floods.The calculated post-flood characteristic parameter of channel geometry using the empirical relation agreed well with observed data,and the proposed method can be used to predict the reach-scale adjustment of channel geometry during hyperconcentrated floods.A one-dimensional coupled mathematical model was used to calculate the processs of flood routing and channel evolution,and the channel evolution volume can represent the variation in channel stability during hyperconcentrated floods in the braided reach of the Lower Yellow River.