Impacts Of Aquatic Vegetation On Hydrodynamics Of Gravity Currents

Gravity currents,driven by the density difference between the flow and ambient environments,frequently occur in natural and engineering environments.From generation to disappearance,gravity currents usually propagate a long distant.Aquatic vegetation poses an important impact on the propagation of gravity currents.Vegetation is commonly found in wetland and estuary environments,and the presence of vegetation will change the characteristics of gravity current motions,which will affect the stability and species diversity in wetland ecosystems and the living quality for local residents.Therefore,it is of great importance to study the propagation of gravity currents within vegetation for the purpose of protecting wetland and estuaries and improving residents' life quality.In this paper,two high-speed digital cameras(one is from the top-view and the other one is from the side view)were firstly employed to obtain the three-dimensional motions of lock-exchange gravity current through a large number of repetitive experiments,i.e.these experiments were carried out under the same conditions.The front location,speed,height,and angles of gravity current and their uncertainties were analyzed,and the effects of three-dimensional current motions on these uncertainties were investigated.The images from the top-view side show that the head position of density currents across the flume are different,indicating the presence of the three-dimensional motion patterns.The results show that the gravity current is composed of lobes and clefs,and the three-dimensional patterns of current motions due to lateral diffusion and the wall friction may be the cause of the uncertainty for current motions.The front location,speed,height,and angles of gravity current follow normal distributions at the selected time.The uncertainties of these parameters were measured by the coefficient of variation(COV,the ratio of the standard deviation to the mean).For the front locations,the COV values decrease rapidly at the initial stage,and subsequently linearly and slowly decrease below 6%.The COV values for the front speed also rapidly reduce and then maintain a near constant value,approximately equal to 5%.Based upon the experimental results,the uncertainty for the front location or speed of gravity currents due to the three-dimensional gravity current motions is approximately 5-6%of the averaged front location or current speed.Secondly a high-speed camera was applied to acquire the evolution process of the gravity currents,and a PIV system was used to investigate the microstructures of gravity currents;the effects of rigid vegetation,herb vegetation,and bottom roughness on characteristics of gravity current motions.Experimental results shows that for gravity currents in different saline concentrations,the gravity current motions could be divided into slumping phase and self-similar phase.The presence of vegetation and bottom bed roughness can significantly reduce the transition point of gravity currents from slumping phase changed into self-similar phase.However,the current speeds during the slumping phase would not be significantly affected.As gravity currents flowed through vegetation,the gravity currents profile evolved from the classic elliptic shape into a well-defined triangular shape.The results also revealed that when the vegetation was sparse,the front speed of current would be slightly affected by the vegetation height,but when the vegetation was dense the current speed would be significantly impacted by the vegetation height.As the non-dimensional vegetation height(the ratio of vegetation height to water depth)was 0.21 with the vegetation density being 18.0%,the gravity current would flow along the upper edge of vegetation and within vegetation at the same time.Since the current density on the top of vegetation was larger than that within vegetation,the Rayleigh-Taylor instability would occur.In addition,the entrainment coefficient of gravity current within vegetation decreased as the currents moved further,and the entrainment coefficient was smaller compared to the case without vegetation.When the density current moved into the submerged vegetation,some current climbed on the top of the vegetation where a new bottom boundary for gravity current was formed and negative vorticity was produced.In the upper boundary of the current,the current kept mixing with the ambient fluid and induced the positive vorticity.The results also performed that the vegetation would reduce the current speed as well as the strength of positive vorticity,and the vegetation density was proportional to the reduction degree of the positive vorticity.When gravity currents through the short vegetation(30 cm in length,3 cm in height),it rode on the top of vegetation and moved forward,and then contain the typical contour outflow the vegetation;presented a movement process that accelerated first,followed by uniform forward,then decreased,and finally slowed down.When gravity currents propagated into a long vegetation patch(80 cm in length,6 cm in height),part of the currents were blocked by the vegetation and only part of gravity currents could flow through vegetation.The current speed firstly accelerated,then kept constant,rapidly decelerated and finally flowed slowly.When gravity currents propagated through vegetation on one side,the lateral entrainment velocity was negatively correlated with front position and current velocity,but the decreasing rate of the lateral entrainment velocity was gradually reduced.When gravity currents flowed through the one-side submerged vegetation patch,the streamlines in the ambient environments were scattered.On the other hand,for gravity currents flowing through the one-side emergent vegetation patch,the streamlines were more regular and uniform.The cross-section energy of the gravity currents increased firstly and then decreased with the time,but the energy peak was negatively related to the vegetation height.When gravity currents propagated on the rough bed,the Froude number of gravity currents did not change significantly at the slumping phase but decreased with the front position at the self-similar phase.The entrainment coefficient showed a decreasing trend with the increasing front position and the Richardson number.The bed roughness could strengthen entrainment and increase the entrainment coefficient.When gravity currents moved over the rough bed,the upper interface of gravity currents kept mixing with the environment and induced the positive vorticity,the lower interface formed the positive and negative vorticity due to the no-slip bottom boundary condition and the complexity of the flow structure at the bed.The results from the study provide scientific basis for ecological environment protection and nearshore sedimentation issues in vegetated areas of wetlands and estuaries.