Study On Hydraulic Characteristics Of Open Channel Flow In Flexible Submerged Vegetation Zone

Natural aquatic vegetation is an important part of the river and plays a key role in its treatment and remediation.But natural aquatic vegetation also complicates the structure of the water flow.At present,scholars in related directions have studied more on the structural changes of water flow around vegetation,but less on the characteristics of water flow within the flexible submerged vegetation zone.This paper is based on summarizing the research results at domestic and international.Taking the submerged flexible vegetation of Myriophyllum as the research object.The hydraulic characteristics of water flow in flexible submerged vegetation zone under different vegetation arrangement,submerged degree and flow were studied by indoor flume experiment.The resistance characteristics,flow velocity characteristics and turbulence characteristics of the open channel flow in the flexible submerged vegetation zone were obtained.The main findings are as follows.(1)Due to the blocking effect of flexible vegetation on water flow,the water surface line along the open channel containing flexible inundated vegetation belt shows a congestion phenomenon in the inlet area of the vegetation belt and a decreasing trend into the vegetation belt,and the water depth increases in the outlet area of the vegetation belt.As the vegetation inundation increases,the viscous force of water flow at high Reynolds number decreases,and the degree of inversion of flexible vegetation and the hydraulic slope increase.The coefficient of roughness increases significantly in the inlet area of the vegetation zone,follows an increasing trend along the flow,reaches a maximum value and then gradually decreases,levelling off in the middle section of the vegetation zone.At low flows,the integrated roughness coefficient is larger in the vegetated section.The resistance effect of the flexible submerged vegetation is greater in the plum-shaped arrangement than in the rectangular arrangement.(2)Vegetation obstructs the passage of some water,and the average vertical velocity in the front area between the flexible submerged vegetation is small,gradually increasing along the flow,and then leveling off;the average vertical velocity in the area on both sides of the vegetation is small and the velocity fluctuates more along the lateral area near the middle line of the vegetation.As the vegetation inundation increases,the flow area increases and the mean vertical velocity tends to decrease;the variation of the velocity distribution along the flow decreases and increases along the lateral flow distribution.The flow velocity is greater in the rectangular arrangement than in the plummeting arrangement;the velocity increases faster in the plummeting arrangement than in the rectangular arrangement.The variation in flow velocity is small at high flows.The velocity of the water in the flexible submerged vegetation zone is distributed in an"S"pattern along the water depth,with the minimum value occurring at 0.47～0.52 h_vheight and an exponential distribution in the free water zone.At low inundation levels,the variability of the time-averaged longitudinal velocity distribution tends to increase at different water depths,with significant fluctuations at 0.4h_v.(3)The turbulence intensity increases in the front and rear areas between the flexible inundated vegetation and near the vegetation median,and decreases in the areas on both sides of the vegetation;the turbulence intensity within the vegetation layer tends to increase along the water depth,and continues to decrease in the free water surface area.Inundation level and arrangement have a strong influence on the distribution of turbulence intensity in the vegetation zone.The rate of increase and decay of turbulence intensity increases with increasing inundation.The turbulence intensity is greater in the plum-shaped arrangement than in the rectangular arrangement.The longitudinal turbulence intensity is greater than the vertical turbulence intensity.The Reynolds stress is linearly distributed,with the maximum value occurring at0.52～0.65 h_v height,where the flexible vegetation canopy zone is located.