Effect Of Rigid And Emergent Vegetation On Flow Characteristics In A Curved Channel

Flow in channel bends is one of the most common phenomena of water flow in rivers.The characteristics of bend flow affect the dynamics of sediment movement and subsequent river bed evolution.Therefore,investigating the flow patterns in channel bends is one of the major problems in river dynamics.The results from previous studies about the flow characteristics in open and straight channels flow have been mostly concentrated on the changes of water depth,the distribution of flow velocity and the turbulent characteristics of water flow.However,the flow in open and straight channels is mostly found in laboratory flumes,and there are very few open and straight channels in nature.In addition,the flow characteristics along open and straight channels are relatively simple,and the present studies have gained a solid understanding on these features.In contrast,the flow along channel bends is a complex three-dimensional and spiral turbulent flow due to the combined effects of the gravity and centrifugal force.Compared with the flow in straight channels,the main differences in flow features reflect in the redistribution of flow velocity,the deformation of curved flow surface,lateral circulation and so on.The present researches on flow in channel bends mainly focus on the simplified models in laboratory flumes,which also obtain a clear understanding of the flow characteristics in channel bends.However,most of the natural rivers are covered with vegetation where the flow shows a special and complex pattern.Therefore,it is significant and valuable to study the characteristics of water flow along channel bends which are covered with vegetation,and the results obtained can provide practical engineering guide in real field conditions.In this study,a 180° U-shape curved channel was used to simulate the natural curved river.The Acoustic doppler velocimeter(ADV),providing three-dimensional and high resolution flow velocity,was used to investigate the flow characteristics,such as the longitudinal flow velocity profiles,lateral circulation and turbulence kinetic energy(TKE).In this experiment,flow patterns in four different discharge Q(10 L/s,15 L/s,25 L/s,and 30 L/s)and three different vegetation density(0%,2.235%,and 4.47%),totally 12 conditions,were measured.By analyzing and comparing the experimental results under the different conditions,the following results can be obtained:the flow patterns with and without vegetation over a channel bend,the effect of emergent vegetation on flow patterns over a curved channel,the impact of vegetation density with given certain discharge,and the effect of discharge on curved flow features.The experimental results revealed that the longitudinal velocity would increase in the vertical lower regions but decrease in the vertical upper regions,when water flows over a curved channel.As the impact of curved channel increases to a certain extent on the flow,the longitudinal flow velocity along the water depth obviously no longer follow the classic "Logarithmic" distribution.However,the presence of vegetation lessen the effect in curved channels.The longitudinal flow velocity within the rigid and emergent vegetation is approximately constant along the water depth.As the vegetation density increases,the effect of curved channels becomes less significant but the increase of flow discharge will counterbalance the impact of emergent and rigid vegetation in the experimental flume.For different cross sections,the emergent vegetation effectively reduces the lateral and vertical velocity of water flow,and the original circulation in no vegetation conditions disappears.As the vegetation density becomes greater,the velocity vectors at different cross sections becomes smaller,which leads to less obvious in circulation patterns.In the presence of emergent vegetation,even though the flow discharge increases,the horizontal and vertical velocity will accordingly become larger but the secondary circulation will not appear any more.The curved channels would enhance flow mixing,and therefore the turbulent kinetic energy(TKE)would become larger compared with the straight channel flows.The results show that the TKE will increase from the inlet to the 90 degree cross section and then decrease to the outlet of the curved channels.Emergent vegetation can slightly increase the TKE,but the vegetation density and flow discharge are insensitive to the TKE variations.The conclusions drawn in this study will improve the understanding of curved channel.Channel effect on flow characteristics,the interaction between the emergent vegetation and water flow,and the influence of vegetation density and flow discharge on flow patterns over a curved channel.These understanding can be beneficial to the scientific research and engineering applications in sediment dredging,ecological restoration and management practices in rivers and lakes.