Research On Rules Of The Evolution Of Lacustrine Delta And Its Channel System

Lacustrine deltas widely exist in the area where the river enters the lake as a kind of common landform.It is an important research subject and have long been focused on by the field of river evolution and geomorphology.However,due to the particularity of formation conditions and the complexity of influencing factors,the understanding of the evolution rules of lacustrine delta and its channel system is not very clear and needs to be improved.At present,the research at home and abroad is mainly carried out from aspects of morphology,sedimentary structure and influencing factors of lacustrine delta,and many positive results have been achieved.However,there are still some deficiencies after summarizing the existing research results.First of all,the previous studies seldom recognize the formation mechanism of the channel system of lacustrine delta and classify it from the perspective of the characteristics of sediment laden flow and the process of erosion and deposition;secondly,the quantitative physical experimental study on the influence of upstream and downstream factors on the evolution of lacustrine delta and its channel system is lacked;Thirdly,a theoretical mode for the formation and evolution of lacustrine delta which is combined with existing mature theories is lacked.Therefore,how to overcome the shortcomings of the existing achievements and improve the understanding of the formation and evolution of lacustrine delta and its channel system is an important subject to be studied.In this paper,the evolution process of lacustrine delta and its channel system has been studied in detail,and the following results have been obtained:(1)Based on the resistance law,a new river pattern discriminant method using threshold curve function is proposed.Firstly,the energy analysis of the whole river is carried out in order to make the resistance during the process of flowing fully considered;Secondly,the bed and bank activity indicators are defined and the calculation formula is determined through analysis,the introduced morphology parameter is related to bed and bank activity indicators;Thirdly,the discriminant threshold curves of sandy and gravel river are obtained by regression analysis of natural alluvial river data,and the effectiveness of the discriminant method is verified by classical data set.(2)Taking river pattern discriminant method as a pointcut,the activity indicator of delta channel is calculated by taking the effect of sediment contained in flow on the interaction between it and bed surface into account.Firstly,the influencing factors of limit shear stress of muddy water are summarized and the calculation formula is given;Secondly,the influence degree of factors on the limit shear stress of muddy water is judged through the analysis of experimental data;Thirdly,the calculation formula of the activity indicator of delta channel is obtained by regression analysis,and the calculation results of natural delta channel systems are consistent with channel fractal dimension results,which proves the effectiveness of the discriminant method.(3)Based on the jet theory and boundary layer theory,the plane jet theory mode of the initial section of lacustrine delta are established.Firstly,according to characteristics of the initial section formation of lacustrine delta,the basic equations are simplified reasonably by dimensional analysis;Secondly,the flow field distribution is obtained with the similar solution method;Finally,the theoretical expression of the morphological characteristics of lacustrine delta initial section is derived in terms of general mathematical model of river bed evolution.(4)The evolution experiment of lacustrine delta and its channel system is designed and completed,the phenomena and rules have been recorded and summarized.First of all,according to the evolution characteristics of the lacustrine delta,the evolution process under each condition is divided into six stages;Secondly,the calculation method of flow regime transition period is presented,and it can estimate the flow regime transition period through several easily available variables,and the channel system has been graded and its evolution rules have been analyzed;Thirdly,the general model of delta sedimentation under distinct sediment supplied rates and lake levels is summarized through comparative experiments;Finally,the stochastic expression of delta morphology has been shown through the comparison between control and repeated group,and the influencing factors have been explored.(5)The physical experiments are designed to explore the periodic evolution rules of lacustrine delta based on the background of Ganjiang Delta with the phenomena at each stage being recorded,and staged characteristics of erosion and depositon and channel evolution are summarized based on experimental data.Firstly,experimental conditions are designed according to the characteristic of coming water and sediment of Ganjiang Delta and the fluctuation of water level of Poyang Lake in one year;Secondly,the phenomena occured during the the formation and evolution process of eight stages are recorded;Thirdly,the evolution characteristics of channel system in each stage are compared,and evolution rules are summarized;the characteristics of deposit growth in longitudinal,transverse,vertical directions and evolution rules of coastline outline are analyzed;Finally,the evolution process of Ganjiang Delta in one year is divided into four stages by analyzing evolution rules of Ganjiang Delta in one year,and the evolution process of the laboratory scale delta and Ganjiang Delta is compared and analyzed,and the two show similar evolution rules in the same stage.