Study On The Influence Of Flexible Submerged Vegetation Form And Arrangement On Hydraulic Characteristics Of Open Channels

Aquatic vegetation increases the flow resistance and reduces the flood discharge capacity of the river channel.The water flow is easy to form local strong shear flow near the vegetation,resulting in erosion and erosion at the root of the vegetation,thus affecting the stability of the vegetation.However,in order to accurately describe this scouring effect,it is necessary to carry out fine analysis and research on the vegetation flow structure in the river channel.Based on a large number of collected data,this paper makes a systematic summary and analysis of the research status of vegetationcontaining water flow problems at home and abroad.Taking the flexible submerged vegetation myriophyllum plants as the research object,the model plastic grass with aspect ratios of 0.18,0.25 and 0.33 were selected to carry out the open channel flow movement rule test with flexible submerged vegetation in rectangular and quincunx arrangement modes.The flow velocity and water level under different vegetation forms and arrangement modes were measured,and the effects of vegetation forms and arrangement modes on the average longitudinal flow velocity,roughness coefficient,turbulence intensity and Reynolds stress distribution were analyzed.The results show that:(1)The water surface along the route is divided into three areas: upstream backwater area,vegetation zone area and downstream restoration area to study the influence of vegetation morphology and arrangement on the water surface.It is found that the backwater effect is significant when the plum-blossom arrangement and the vegetation width-height ratio are large,and the water surface line decreases greatly under the plum-blossom arrangement in the vegetation zone area,while the water surface line rises rapidly in the restoration zone downstream of the vegetation zone under the rectangular arrangement.When vegetation is planted in open channel,the longitudinal velocity shows an "S" distribution,with a minimum value at 1/2 of the vegetation height.The shape and arrangement of vegetation in the vegetation layer have a great influence on the time-averaged longitudinal velocity.The time-averaged longitudinal velocity in the upper part of the vegetation layer is logarithmic.Under plum blossom arrangement,vegetation has greater influence on head loss,hydraulic gradient and roughness coefficient,while vegetation form has less influence on it.(2)Vegetation form and arrangement have little effect on turbulence intensity in backwater area upstream of vegetation zone,while in vegetation zone area,plumblossom arrangement and increase of vegetation width-height ratio have great effect on turbulence intensity,while in restoration area downstream of vegetation zone,turbulence intensity decreases greatly under rectangular arrangement.Within the vegetation layer,the turbulence intensity increases and reaches the maximum value at the top of the vegetation.Under the rectangular arrangement,the turbulence intensity increases greatly and increases with the increase of the ratio of vegetation width to height.(3)The Reynolds stress of open channel flow with flexible submerged vegetation appears obvious zoning phenomenon.With the increase of vegetation aspect ratio,Reynolds stress increases.The Reynolds stress at the top of the vegetation has a maximum value,which is larger in the quincunx arrangement and decreases toward the bed surface,while decreases greatly in the quincunx arrangement.The results show that vegetation can keep the riverbed smooth and reduce the erosion of riverbed by water flow.Quadrant analysis shows that the "outward interaction" flow pattern is dominant in the upper part of the vegetation layer,the "burst" flow pattern is dominant in the lower part of the vegetation layer,and the "inward interaction" flow pattern is dominant in the lower part of the vegetation layer in the quincunx arrangement mode.