有植被弯曲分汊河道水流特性研究OA

Objective This study addresses the insufficient understanding of the flow characteristics of such channels in existing research and provides theo-retical support for channel regulation and ecological restoration engineering practices to clarify the flow movement laws of meandering branching channels under the coupled effects of discharge and vegetation density. Methods A generalized physical model test was conducted using the Yujiawan reach of the Xulu section of the Yangtze River as the prototype at the State Key Laboratory of Hydraulics and Mountain River Engineering.The flume was composed of inlet and outlet straight sections and a middle arc section,with the left branch,right branch,and central bar each having a width of 1 m and a longitudinal slope of 3‰.The riverbed was paved with natural sand with particle sizes ranging from 0.5 mm to 10.0 mm.Rigid non-submerged vegetation,with a diameter of 0.8 cm and a length of 30 cm,was arranged in the right branch.Nine groups of clear water steady flow tests were designed using three discharge gradients(55,70,and 90 L/s)and three vegetation density gradients(0,0.02,and 0.08).Each test was subjected to constant scouring for 8 hours until the riverbed reached a stable state.Ultrasonic distance-measuring sensors were utilized to measure the water surface elevation at seven cross-sections.The AiFlow video current measurement system,based on improved Particle Image Velocimetry(PIV)technology,was adopted to obtain the surface flow velocity.The variation characteristics of water surface elevation,longitudinal and transverse water surface gradients,surface flow velocity distribution,and the hydrodynamic axis of the channel were systematically analyzed. Results and Discussions The water level of the meandering branching channel was influenced more significantly by discharge than by vegetation density.A stepwise increase in water level was observed with increasing discharge,whereas only a slight rise in water level occurred with increas-ing vegetation density,and the magnitude of increase in the right branch was slightly greater than that in the left branch.The water level in the di-version area increased due to the backwater effect of the central bar,decreased at the branching mouth with the disappearance of the backwater ef-fect,and then increased along the flow direction in the confluence area due to flow impact and backwater effects.Vegetation exerted a minimal in-fluence on the longitudinal gradient of the channel.The longitudinal gradients in both the diversion and confluence areas increased along the flow direction.In the branching section of the left branch,the longitudinal gradient increased gradually from the branching mouth to the bend apex and then decreased progressively from the bend apex to the confluence mouth.In the branching section of the right branch,the longitudinal gradient first decreased and then increased,reaching a minimum at the bend apex.The transverse gradient of each cross-section increased slightly with in-creasing vegetation density,with a more pronounced increase in the left branch.The transverse gradient was strongly affected by discharge and in-creased sharply with increasing discharge.The transverse gradient in the left branch remained positive,whereas that in the right branch was nega-tive at the branching and confluence mouths and positive within the branching section.Under the same inflow conditions,vegetation produced significantly different effects on flow velocity across different channel sections.The flow velocity in the diversion and confluence areas was less affected by changes in vegetation density,whereas clear differentiation was observed in the branching section.With increasing vegetation density,the diversion ratio and flow velocity in the left branch increased,whereas the discharge through the right branch decreased and the flow velocity was reduced.In addition,vegetation in the right branch caused the flow velocity at the branching mouth to deflect toward the concave bank,and the deflection angle increased with increasing vegetation density.The flow-blocking effect of vegetation in the right branch was identified as the primary cause of hydrodynamic axis deflection.Under vegetation cover conditions,the hydrodynamic axis of the right branch in the branching section deflected toward the concave bank,and higher vegetation density brought it closer to the concave bank.Simultaneously,the hydrody-namic axis of the left branch also deflected toward the concave bank due to the increased diversion ratio.The influence of vegetation on the hy-drodynamic axis depended on discharge,being significant under low discharge conditions and gradually weakening with increasing discharge.Under low discharge,the hydrodynamic axis showed a noticeable deflection when vegetation density increased from 0 to 0.02,but only slight changes were observed when the density increased from 0.02 to 0.08.Under medium and high discharge conditions,the variation trends of the hy-drodynamic axis under different vegetation density gradients remained generally consistent. Conclusions The study revealed the regulatory mechanisms of discharge and vegetation density on the flow characteristics of meandering branch-ing channels.It clarified the variation patterns of water surface morphology,flow velocity distribution,and the hydrodynamic axis of such chan-nels under the water-blocking effect of vegetation.The research results can provide theoretical references for flood control and regulation,the ra-tional configuration of beach vegetation,and river ecological restoration engineering in meandering branching channels.The findings are only ap-plicable to scenarios involving rigid,non-submerged vegetation,with a flow velocity of 0.15～0.35 m/s and a vegetation density of 0.02～0.08.Further experimental studies are required to investigate the effects of other vegetation types,such as flexible and submerged vegetation,on the flow characteristics of meandering branching channels.