Preliminary Experimental Study On The Formation Of Bars In A Straight Channel Under Scouring Condition

As one of the alluvial river patterns,the characteristics of the straight channel is that the sediment deposited bodies like bars formed in the channel keeps moving and changing.The sediment deposited bodies are often very big,whose development have a great influence on the evolution of the channel.There have been a lot of researches on the movement of small-scale ripples in River Dynamics,but few studies were done on the formation of the large-scale sediment deposited bodies like bars.In recent years,human activities have change the river morphology greatly,the large-and medium-scale water conservancy projects decrease the sediment input of the downstream river,leading to the obvious incision into the riverbed downstream of the projects.In the process of the bed-rebuilding in the downstream river,some researchers observe the formation and emergence of bars.Enough sediment input were often ensured in the previous experimental researches as bars are regarded as the sediment deposited bodies,while the formation of bars under scouring condition are paid little attention to.A series of flume experiments are carried out to study the formation of bars in a straight channel under scouring condition preliminarily.On the basis of bar formation theory and the conditions of natural rivers,the experiment plans are divided into experiments without external disturbances and with external disturbances.Refering to the previous experimental condition of the formation of meandering,the experiments without external disturbances are divided into an experiment with rigid banks with sediment input(Test 1,a reference condition)and an experiment with an initial small straight channel under no sediment input(Test 2).The difference from the previous experiments is the consideration of the fluence ofrigid banks on the formation of bars.The experiments with external disturbances are all under no sediment input and simulating the conditions of natural rivers in which the flow or boundary conditions are changed under the effect of the nattural or human factors,which are divided into an experiment with a fixed plate simulating the disturbance from the an abrupt change of curvature in the bank line(Test 3),studying the effect of the change of inflow condition;an experiment with an initial disturbed bed topography simulating the disturbance from the obvious topographic relief(Test 4),studying the effect of the change of boundary conditions;and an experiment with the waterfall around the tailgate with big aperture simulating the disturbance from a drop in water level(Test 5),studying the effect of the change of water level at the outlet.Under the condition without external disturbances,steady migrating bars form simultaneously along the flume in Test 1 and three migrating bars are maintained in the experimental section.The relative position of bars along the flume keep the same.Small bars form along the channel in the initial stage of Test 2.During the experiment,these small bars keep migrating downstream.During the movement,the length and height of the small bars increase and the incision in the scour pool strengthens along with the deposition on the bed of the lower part and the widening of channel along the flume.Finally the bar near the outlet emerges from the water surface at first.After a certain period of the experiment,most of the sediment scoured from the scour hole deposit near the hole downstream,leading to the formation of a bar around the deposition place.The formation of a bar near the inlet changes the flow direction.Afterwards,a new bar forms in the vicinity of the input zone under the influence of the inlet bar,and the position and size of bars in the lower part change because of the change of inflow.There are some similarities in the results between the experiments without external disturbances and the previous similar experiments,but the observation that formed bars emerged from the water surface in Test 2 has not been observed in the previous experiments,and the detailed analysis on the formation of bars in this paper fills in gaps of the previous studies.Under the effect of the change of flow or boundary conditions aroused by external disturbances,the inflow is disturbed by the plate of the left side,causing the severe erosion in the right side of the cross section.Besides,a recirculation zone forms downstream the plate,and the sand scouring from the right side of the inlet zone deposits in the recirculation zone under the effect of the secondary flow,which contributes to the formation of a bar.The formation of bars disturb the flow and change the flow direction,and then the disturbed flow influences the formation of bars.The communication mechanism between the bars and flow propagates downstream,finally bars form from upstream to downstream in the experimental section.After a large flood in the Test 4,a large bed slope and obvious fluctuations form in the lower part of the experimental section.The large bed slope increases the flow strength of the section.Under the condition of a small discharge afterwards,the flow incises the bed along the low-elevation zone and a new channel forms soon,which makes the area under weaker scouring become the bar.The bed is relatively flat and little fluctuations are observed after a larger flood in Test 5.Soon after the small discharge is applied,due to the water level drop at the tailgate,the drastic conversion from up stream mild flow to the waterfall out of the tail gate makes bed near the outlet experience severe erosion.As the localized incision is intensifying,all the water is gradually collected into the scouring zone and the water level of the zone declines,leading to the outcrop of the area which is not subject to the incision.The formation pattern of bars in the experimental section can be concluded as "unconfined flow and no obvious erosion is seen in the section?scattered bars form due to the water level drop as the headcut passes by?water is gradually collected into main scour path and flow recedes in the secondary scour paths?scattered bars are integrated as a large bar".The above cycle of bar formation is spreading upstream,leading to the formation of large-scale bars in order along the flume.The fomation process of bars under the condition with external disturbances is quite different from that in the previous related experiments and the experiments under the condition without external disturbances.Besides,an experiment aimed to discover the formation of bars under scouring state is introduced as well,in which no bars formed.By the summary of this experiment results without the formation of bars and combination of analysis on the formation of bars without and with external disturbances,the two formation ways of bars are further recognized:the one is under large sediment input and proper width-depth ratio,the other is an extra fixed disturbance to the flow and sediment transport.The formation and development of bars and change of bank line in the experiment are restricted by the fixed walls.When the flow reaches the fixed wall,because the fixed walls are nonerodible,the flow start eroding the bed near the fixed wall,which lowers the local water level and leads to the outcrop of bars.The phenomenon demonstrates that the presence of fixed walls in the straight channel contributes to the development of bars.The experiment result shows that under the effect of the change of flow or boundary conditions aroused by external disturbances,there is a close relationship between the formation of bars and a deep channel.The occurrence of perturbations disturb the previous relatively uniform flow in the experimental section,redirects the main flow,and forces the flow to concentrate into a narrow zone,which increases the flow velocity in the zone and leads to the incision of local bed as a deep channel,making the area under no or weak scouring become bars.The experiment result demonstrates that under some conditions,the erosion aroused by disturbances induces the formation of bars;in this case sediment deposition is no longer a necessary condition for the formation of bars.Comparing the different results of the experiments without and with external disturbances,the follows are found:first the difference in the pattern of bars between the two experiments:migrating bars form in the early stage in the experiment without external disturbances,while only nonmigrating bars form under the influence of disturbances in the experiment with external disturbances;second the difference in the formation process of bars:bars form by the sediment deposition under no external disturbances,while the distinguishing between the bar and the channel is developed by the formation of incising deep channel under external disturbances;third the difference in the characteristics of bars:the formation time,formation order,formation position,length and height of bars are all different between the two kinds of experiments.The above comparisons of experiment results illustrate that continuous disturbances with certain strength have a significant influence on the formation process and size of bars in the straight channel.