| The stable meandering rivers often can be seen in the area with high riparian vegetation coverage and developed root system in the source region of the Yellow River.The well-grown riparian vegetation can resist current scour,decrease bank failure,increase bank stability,maintain and recover the ecological health of water body.This study takes the Lanmucuoqu as the research area,the tributary of the Yellow River,is located in Henan County,Huangnan State,Qinghai Province.In order to study the function of the riverside vegetation in protecting riverbank,the effects of alpine meadow vegetation on river bank stability are analyzed.Based on the investigation of climate,terrain and soil characteristics,the main contents of this study are made as follows:biological characteristics of riparian vegetation,mechanical characteristics of the dominant vegetation root systems and the soil-root composite,collapse mechanism and stability analysis of meandering riverbank,lateral fluvial processes of meandering river channel.Some experimental studies are carried out at the typical meandering reach of Nanmucuo river which belongs to the source region of the Yellow River.The field experiment of stationing and measuring,quadrat survey,in situ shear test of soil-root composite and field situ pull-out test,remodeling soil-root composite shear test,freeze-thaw shear test and indoor single root extension test are conducted in this research.The main aim is to make a quantitative analysis of the shear reinforcement of river bank by root system,and the effect of reducing the size and frequence of slump block and the limitation of channel migration etc.By these studies,scientific evidence for the river ecological protection and vegetation restoration in the source region of the Yellow River can be provided.The main achievements are as follows:(1)Along the horizontal axis of the convex bank,from the riverbank to the middle of the beach,the soil type presents gradient change from silty gravel to silty sand and then to silty soil.Soil moisture content decrease first,and then increase.The minimum water content is 40.27%;the average soil density is 1.47 g·cm-3.At the depth 0?45cm,the average soil density and average moisture content of the river banks take the trend of increasing and decreasing respectively.Alpine meadows are the main vegetation types in this study area.Under the effect of scouring and silting of bend flow,convex bank vegetation can be divided into four zoning generally:hydrophyte,hygromesophytes,mesophytes,and subshrub.The composition,coverage degree and plant species of the zoning increase gradually from the river bank to the outside of the convex bank.The vegetation of concave bank is relatively uniform and there is no obvious succession change.(2)Through the quadrat survey,the dominant plants can be determined by the important values of plants in each vegetation zone.The importance value of Blysmus sinocompressus in I,II zone is the highest and plays an important role in protecting and armoring the river bank.The dominant plants in III,IV zone are Oxytropis ochrocephala,Kobresia capillifolia,Carex moorcroftii and Potentilla fruticose.(3)The in-situ pullout test of the four dominant plant roots is carried out in the riverbank collapse profile of concave bank in the study area.The following is the order of the average pulling strength:Potentilla fruticosa>Blysmus sinocompressus>Carex moorcroftii>Kobresia capillifolia;here is the order of the average tensile strength of four dominant plants by single root tensile test:Potentilla fruticosa>Carex moorcroftii>Blysmus sinocompressus>Kobresia capillifolia.The relationship between tensile strength,anti-drawing strength and root diameter is negative correlation of power function.The roots of cyperaceae plant Kobresia capillifolia and shrub plant Potentilla fruticosa possess the highest pulling strength and tensile strength,72.44MPa and 31.19MPa respectively.The results show that the plants with strong tensile strength also have strong pulling strength in soil,and the friction between roots and soil is high.During the process of bank failure,at the top of the concave bank,the anti deformation capacity of root system that pass through the collapse profile is weakened with the crack width increasing.The smaller the tension speed is,the more abundantly the plant plays its tensile characteristics.(4)Along horizontal direction of the river bend,shear strength of soil-root composite increases first,then decreases.And with the depth increasing,the shear strength of soil-root composite take the tendency of decrease.The results obtained by indoor remolding direct shear tests show that the optimal root content is 13%with the root length of the soil-root composite 2cm,and the cohesion reaches a peak of23.13k Pa.The correlation between root area ratio,soil-root composite moisture content and cohesive force is the highest,with the correlation values 0.754 and 0.75respectively.The freeze-thaw cycle has a weakening effect on the shear strength of the soil-root composite in the study area.During the pocess of 20 times freezing and thawing cycle,the cohesion of soil samples and soil-root composite samples are decreased gradually,with the number of freezing-thawing cycles increased.The cohesion of soil-root composite samples with different root content decrease less than that of non-root soil samples,which show a certain inhibitory effect on the change of temperature frost heave.(5)The riverbank in the study area is composed of soil-root composite layer with high cohesive force in the upper layer and sandy gravel in the lower layer,the typical binary structure riverbank.Beam failure is the main type of bank collapse,which refers to a block rotates forward about a horizontal axis somewhere in the block.The average length,width and thickness of the failure blocks in the study area are1.96±1.17m,0.70±0.22 m,and 0.56±0.25m respectively.The vertical root length of slump blocks is in good agreement with the thickness of the failed blocks after natural logarithmic transformation,and the volume of the slump blocks is correlated power function with root length.According to the mechanical balance principle of cantilever beam,the critical equilibrium equation of cantilever block in the study area is established,and the critical width formula of the collapse block was derived.Meanwhile,the root enhancement coefficient of the WWM model was modified,the critical width formula of the collapse block was verified by real cases,with a margin of error of 0.41%between the theoretical and measured values,and the width of river bank collapse is 0.725m.According to the calculation during dry period,freeze-thaw period,flood period and falling period of two selected riverbanks in 2016,the stability coefficients were 11.8,6.51,1.07 and 0.65 respectively,which indicates that the riverbank is in the critical stable state during flood period,and the falling period is in the intense stage of the bank failure in the study area.(6)Through the spatial positioning surveying,the data of position change of5.3km curved channel are analyzed,and the interannual collapse width of riverbank cross section is 0.905m.Landsat remote sensing images in 1995 and 2014 were analyzed and processed,the river range with the length of 12km and width of 2km,the meandering river take regular left-right migration,the curved river keeps good shape,and the average annual migration is 1.590m.According to the comparison analysis of remote sensing during the 20 years and GPS-RTK,the width of river bank collapse is 0.905m;the average slump blocks width of 145 failure blocks is 0.703m,which is in good agreement with the width of 0.725m derived from the riparian balance formula.The results can define and verify the lateral fluvial processes and inter-annual variation of the meadow meandering river in the study area. |
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