River Dynamic Characteristics And Pattern Discriminant Method Influenced By Riparian Vegetation

The riparian vegetation affects every basic element of the morphological dynamics system.It changes the flow field and turbulent structure,which affects sediment transport and riverbed morphology.The riparian vegetation also affects the strength of the riverbank,which in turn can change the conditions that may cause the riparian collapse.All of these effects also affect the local morphological dynamics,thus affecting the overall river morphology.Under the concept of eco-environmental protection,it is of great academic and practical significance to study the influence of riparian vegetation on the dynamic characteristics of rivers for scientific research and ecological river regulation projects.In this paper,the effects of vegetation on river evolution,water flow and river pattern will be analyzed from three aspects: micro-scale,small-scale and macro-scale.This paper will make exploratory and pioneering research which is mainly carried out by physical model experiment,theoretical analysis and experienced statistics.Physical model experiment was used to measure the bend-flow structure of channel with shore beach vegetation belt,and the effects of vegetation density on the motion characteristics of the main channel and the secondary flow of the cross section were analyzed.To study the flow structure and turbulence characteristics of vegetation zone and riparian vegetation zone under different riverbed conditions,vegetation density and submerge conditions,the flow field in riparian vegetation zone was measured by physical model experiment.Using self-designed experimental equipment for river evolution with variable slope and seepage separation,natural model experiments were conducted to simulate the river evolution process with or without vegetation planting by changing the riparian vegetation density and the layout of unilateral and bilateral riverbank,analyzing the response of different density vegetation revetment to river dynamic process.To combine the theoretical analysis and empirical statistics,a river pattern discrimination method considering the effect of vegetation feature and based on resistance energy consumption was constructed.The research content and conclusions of this paper are as follows:(1)The hydrodynamic characteristics of curved rivers are not only related to the bending degree and width-depth ratio,but also to bed morphology and riparian vegetation distribution.The vegetation belt of the bend shore has a significant influence on the plane flow field and secondary flow intensity in bends,the response of which in different vegetation densities was analyzed by flume model experiments.The results show that beach vegetation increases the lateral flow velocity gradient of the bend,so that the mainstream of the water flow has a tendency to shift toward the central area of the river.The Under certain vegetation density,the mainstream channel flow appears,and the overall main channel flow velocity is relatively high.The disturbing action of the beach vegetation destroys the large-scale vortex in the river channel effectively restrains the development of the bend circulation.The secondary flow intensity of the curved section varies periodically along the bend,reaching the maximum near the curved section with the minimum curvature of 90 degrees and 270 degrees.The dimensionless secondary flow intensity has a decreasing trend with the increase of the beach vegetation density.(2)Based on the flume model experiment,the distribution law of line velocity under submerged and non-submerged conditions were compared and analyzed.The differences of flow structure and turbulent characteristics of measuring section under emergent condition in different vegetation density and bed conditions were emphatically analyzed.The results show that under the condition of non-submerged flexible leafy vegetation,the flow velocity in most sections with vegetation is higher than that without vegetation,and the vegetation density is not proportional to the increase or decrease of flow velocity.Compared with non-vegetated river flow,the existence of vegetation will increase the turbulence effects.The vertical distribution of velocity and turbulence intensity with different vegetation densities is different under fixed-bed and movable-bed conditions.Generally,the turbulence intensity in the movable-bed is greater than that in the fixed-bed.(3)The riparian vegetation has an important influence on main stream stability characteristics,bend migration characteristics,river bank erosion characteristics and sediment transport intensity,especially in the evolution of the river during the flood period.Given the same flow and sediment boundary conditions,flow movement characteristics,main channel stability characteristics,bend migration characteristics,bank evolution characteristics and sediment transport characteristics are analyzed by controlling the vegetation coverage and single-bank or double-bank layout.The stability of the banks and bed varies considerably due to the different erosion characteristics of riverbanks with or without vegetation.The main stream stability under unilateral vegetation coverage is worse than that under bilateral cover,and river channel evolution under unilateral vegetation cover is often accompanied by alternate development of the main stream and branch.A narrow and deep regime channel more easily forms with a higher vegetation coverage rate.Planting riparian vegetation played a positive role in the erosion resistibility,which effectively reduced the lateral migration rate of the riverbank and at the same time led to a trend in overall scour depth along the riverbank.(4)Rivers always follow resistance laws while processing,and its hydraulic energy consumption is affected by various kinds of factors,including the flow-sediment conditions as well as the feature of river morphology.Therefore,the macro-morphology of the river presents the resistance laws of the flow,and the river channel adapts to different water and sediment boundary conditions by adjusting their morphology.This paper,which is based on the dialectical relationship between the resistance energy consumption and morphology of the river,introduces the river morphological parameter to set up a simple river pattern discrimination method.Through the parametric statistical analysis of the actual several sandy and gravel rivers,and considering the correction o f the influence of riparian vegetation factors on river pattern,the expression representing the river morphological parameter is fitted,and then the standard curves of different river patterns are obtained.Through the demonstration analysis and the classical actual river data verification,the results show that this empirical classification method is simple to calculate and are proved effective in deciding different river patterns.