Experimental Study On Water Flow Characteristics Of Open Channel Under The Action Of Flexible Floating Vegetation

Aquatic vegetation causes increased resistance to water flow and reduces the flooding capacity of the river.Vegetation roots can cause scour erosion and affect its stability,while strong localised shear flows tend to form in the vicinity of the vegetation.In order to accurately describe this scouring effect,a refined analysis of the vegetation flow structure in the river channel is required.In this study,we used flexible floating vegetation of the genus Tennantaceae as the object of study,and selected a flexible model plastic grass with a root length of about 5.5 cm in a rectangular arrangement to test the water movement in an open channel with flexible floating vegetation.and Reynolds stress distribution.The results of the study show that:(1)Under the cover of floating vegetation,the surface line along the course of the water appears to be congested,declining and rebounding,with three zones:the upstream congestion zone of the vegetation belt,the vegetation belt zone and the downstream recovery zone of the vegetation belt.In the vegetated zone,the larger the vegetated area,the higher the longitudinal flow velocity in the bed dominated zone,the higher the roughness near the edge wall of the vegetated zone,and the lower the roughness near the midline of the flow;the decreasing trend of longitudinal flow velocity from bottom to top at each measurement point under different flow rates.The higher the vegetation cover density,the better the flow restoration conditions downstream of the vegetation zone;the higher the overflow,the larger the "C" shape opening characterising the flow pattern.(2)Under floating vegetation cover,the rise in roughness of the flow along the open channel relative to the unvegetated flow is inversely proportional to the larger the vegetation gap and proportional to the overland flow.Different cover densities,different arrangements and different overflow rates of floating vegetation cover will have approximately the same effect on the roughness along the open channel,but the flow characteristics in the study area are very different.(3)Under the cover of floating vegetation,the longitudinal turbulence intensity shows a maximum near the vegetation root system and gradually decreases towards the free water surface and bed surface.The trend of longitudinal turbulence intensity in the upstream congested zone is similar to that in the unvegetated zone,while the longitudinal turbulence intensity in the vegetated zone has an inverse "S" shape,and the maximum value of turbulence intensity in the downstream recovery zone of the vegetated zone is shifted towards the bottom of the riverbed.(4)Under the cover of floating vegetation,the vertical turbulence intensity increases and then decreases with the increase of water depth,and the peak value is shown near the vegetation root system,the higher the overwater flow,the peak value in the downstream recovery area is closer to the free water surface.The higher the overflow,the closer the peak in the downstream recovery zone is to the free water surface.After the water passes through the vegetation zone,the peak is shifted towards the bottom of the river bed,and the trend of turbulence intensity on both sides of the vertical direction slows down.The higher the density of vegetation cover,the more significant the trend of increasing turbulence intensity in the vertical direction from the side wall to the midline of the flow.(5)The vortex volume is mainly distributed at the junction of the vegetation-dominated zone and the bed-dominated zone.The vortex makes the pressure in this zone low,producing a large pressure gradient in front of and behind the vegetation root system.As the cover density of floating vegetation increases,the vortex and impeded areas on the surface of the floating vegetation become the main distribution area for vorticity stresses,and either too low or too high flow rates may destabilise the flow,leading to changes in the Reynolds stress distribution.Below the critical point of transition between jet and downsweep flow regimes,the dominance of the downsweep flow regime is high or low in proportion to the vegetation cover density,and the downsweep of fluids near the vegetation root system is enhanced,facilitating the deposition of material in the channel.