| The Three Gorges Reservoir area is located in the canyon and mountains,with deep rivers,broken surfaces and severe soil erosion,and the incoming sediment brought by soil erosion threatens the safe operation of the Three Gorges Reservoir.Vegetation is the key to restoring the Three Gorges floodplain habitat and controlling the sediment.As a threatened vegetation endemic to the Three Gorges floodplain,the reintroduction of the Myricaria laxiflora to the Three Gorges floodplain is not only necessary for the recovery of the Myricaria laxiflora population,but also an important measure to restore the fragmented ecological environment of the Three Gorges floodplain.Current research on flow and sediment retention is limited in reintroduction.In this paper,a space-time strategy was used to select 1-2 year old(small seedlings),3-4 year old(medium seedlings)and 5-7 year old(large seedlings)Myricaria laxiflora as test material and to analyse the changes in morphological and biomechanical characteristics with intra-and inter-annual growth and the corresponding changes in flow characteristics in the wake area during late August(dormant period),late September(early growth period)and late October(vigorous growth period)respectively.The bending and tensile properties of Myricaria laxiflora were measured using an electronic universal materials machine,and the flow velocity array in the wake zone of sparse water cedar branches was measured using an ADV and a recirculation channel.This is the first time that the response of the biomechanical properties of Myricaria laxiflora to seasonal changes has been considered,and the first time that the flow characteristics of the wake area of Myricaria laxiflora have been analysed.The main results of this paper are summarized as follows:(1)The dynamic growth of Myricaria laxiflora during intra-annual(from dormancy period to vigorous growth period)results in an increase in the frontal area for interaction with flow,an increase in the bending resistance and a decrease in the tensile resistance of the biomechanical properties.The frontal area of small seedlings at early and vigorous growth period is 1.2 and 2.4 times that of the dormancy period,respectively,the flexural stiffness at vigorous growth period is 22.3 times that of the dormancy period,and the tensile Young’s modulus at dormancy period is 1.8 times that of the vigorous growth period.The frontal area of medium seedlings at early and vigorous growth period is 2.8and 5.6 times that of the dormancy period respectively,the flexural stiffness at vigorous growth period is 10.7 times that of the dormancy period,and the tensile Young’s modulus at dormancy period is 2.0 times that of the vigorous growth period.The frontal area of medium seedlings at early and vigorous growth period is 1.3 and 2.6 times that of the dormancy period respectively,the flexural stiffness at vigorous growth period is 3.5 times that of the dormancy period,and the tensile Young’s modulus at dormancy period is 1.5times that of the vigorous growth period.(2)The intra-annual growth of Myricaria laxiflora increases the inhibition of mean velocity,the velocity gradient with the surrounding flow increases and the turbulent exchange of flow is enhanced.The area of dormant vegetation inhibition is smaller and the reduction in mean velocity in the wake area is smaller.After the restoration of growth,the blocking area increases and the cutting effect on the flow velocity in the wake area is enhanced.During the dormant period,the generation of turbulent kinetic energy in the wake is controlled by a combination of bed friction and vegetation.The velocity gradient behind the vegetation increases after entering the growth period,and the turbulent energy production in the wake is dominated by the vegetation.As vegetation resumes growth,the turbulent kinetic energy in the wake numerically increases.The influence of intra-annual growth on the dominant flow regime of water flow is relatively small.For the submerged Myricaria laxiflora,the mixed layer sweeping flow regime dominates and sediment is easily deposited on the downstream bed of the vegetation.The energy spectrum analysis revealed that with intra-annual growth,the energy increase corresponding to the dominant frequency of the wake,i.e.,enhanced current fluctuations,increased vortex motion,and increased water turbulence.(3)The number of main stems,stem diameter and frontal area increase with interannual growth of Myricaria laxiflora.Bending and tensile capacity also increase with inter-annual growth.Small seedlings sprout only one main stem,while medium and large seedlings tiller sprout into the basic form of a multi-branched shrub.Stem diameter also increases markedly with inter-annual growth,averaging about 1.36 cm for small seedlings,2.26 cm for medium-sized seedlings and 4.60 cm for large seedlings.The frontal area of large seedlings was 2.6 and 14.3 times larger than that of medium and small seedlings respectively.The inter-annual biomechanical properties of the vegetation are mainly controlled by the variation in diameter,with large seedlings being able to withstand 3.0and 3.7 times the maximum bending force of medium and small seedlings,and large seedlings being able to withstand 5.0 and 8.9 times the breaking tensile force of medium and small seedlings respectively.(4)The interannual growth of Myricaria laxiflora further inhibits mean velocity and increases turbulent exchange of flow.Smaller seedlings have the least reduction effect on mean velocity.A comparison of the effect of medium and large seedlings on mean velocity shows that medium seedlings can reduce mean velocity to a greater extent over a limited area,while large seedlings can affect mean velocity over a larger area.When the seedlings are small,the vegetation alone has an effect on the velocity.As the age of the seedlings increases,the lateral deflected flows caused by the two vegetation integrate with each other and may form an erosion zone between the two vegetation.Smaller seedlings have the least effect on the turbulent kinetic energy generation in the wake area.The greater velocity gradient created in the wake areas of the medium seedlings,combined with the slight oscillation of the medium seedlings under the experimental conditions,resulted in the turbulent kinetic energy of the medium seedlings in a limited area having a numerical value greater than that of the large and small seedlings.However,large seedlings influence the generation of turbulent kinetic energy on a larger scale due to their larger frontal area.All three seedling Myricaria laxiflora show a dominant ejection flow pattern in the lower water column,which facilitates the transport of sediment and other material in the lower layer to the downstream bed along with the flow.The energy spectrum analysis shows that the dominant frequency corresponds to the energy peak of medium seedlings > large seedlings > small seedlings,i.e.the vortex motion is most intense in the medium seedling wake.(5)The intra-annual dynamically increasing bending stiffness and frontal area combine to promote the retain flow and sediment effect of the Myricaria laxiflora.From dormancy period to vigorous growth period,the frontal area of Myricaria laxiflora increases exponentially.The bending stiffness of the vegetation increases as it enters its vigorous growth period.This causes the vegetation are subjected to greater drag force during the vigorous growth period than the dormancy period,the degree of bending is still consistent with the dormancy period.The dynamic increase in bending stiffness allows the vegetation to retain flow and sediment over a wider range of water depths.At the same time the sediment particles and the nutrients deposited can further contribute to the growth and development of vegetation. |
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