Effects Of Bedform Geometry And Permeability On Hyporheic Exchange Characteristics

Hyporheic exchange driven by bedforms is an important advective process in river water environment system,which affects the migration,transformation and flux process of water,solutes,contaminants,colloids and fine particles between overlying water and pore water,and it plays a key role in the health and function of river ecosystem.The geometry and permeability of the bedform constituent units are two important influencing factors for the hyporheic exchange flux,and their comprehensive impacts on the hyporheic exchange characteristics need to be further studied.The impacts of the geometry(the ratio of the spacing of adjacent bedform constituent units to the height)and the permeability variation(relative to the bed)of the bedform constituent units on the hyporheic exchange flux under the sandy bed conditions have been investigated by laboratory annular flume experiments and combining with numerical simulations,and the relationship between the effective diffusion coefficient and its main influencing parameters and the influencing mechanisms have been analyzed by a parameterization method.The main results are as follows:1.The experimental results indicate that the interaction between the adjacent bedform constituent units has an important influence on the hyporheic exchange characteristics.With the change of the ratio of the spacing between the adjacent constituent units to the height(w/h),the effective diffusion coefficient shows a nonmonotonic variation.Within the range of the experimental parameters(3≤w/h≤11),under the combined influence of turbulence penetration and pumping exchange,with the increase of w/h,the hyporheic exchange flux initially increases and then decreases,and reaches the maximum at w/h≈6～8,that means that the combined effect of turbulence penetration and pumping exchange is the strongest.2.The relative variation of the permeability of the bedform constituent units has a significant effect on the hyporheic exchange characteristics.Compared to that in the case of the homogeneous coarse sand wave bed,the dimensionless effective diffusion coefficient increases monotonically with the increase of w/h for the case of less permeable and impermeable discrete bedforms.Compared to that in the case of the homogeneous coarse sand flat bed,the interfacial mass exchange rate decreases obviously when w/h≤7 and increases when w/h≥8 for the case of the less permeable discrete bedform;within the range of 3≤w/h≤11,the interfacial mass exchange rate always decreases for the case of the impermeable discrete bedform.3.The numerical simulation results show that the interaction between adjacent constituent units has a significant effect on the local flow structure under the condition of two-dimensional discrete bedform.With the increase of w/h,the recirculation zone and interfacial pressure distribution between adjacent constituent units change significantly.When 3≤w/h≤4,a large recirculation zone occurs between the adjacent bedform components,and the influence of the recirculation zone on the external flow field is weak.The pressure difference between the upstream high pressure zone and the downstream low pressure zone increases;When 5≤w/h≤6,the influence of the recirculation zone on the external flow field is enhanced,and there are two recirculation zones,including large and small recirculation zones,and the inhomogeneity of the interface pressure distribution increases;When 7≤w/h≤11,the interaction between the adjacent elements turns from strong to weak,and the pressure difference between the high pressure area on the upstream side and the low pressure area on the downstream side decreases and tends to be steady.4.The mechanism analysis results show that the hyporheic exchange flux is influenced by the coupling between the geometry and the permeability of the bedform constituent units,which is closely related to the relative contribution of turbulence penetration and pumping exchange.When the bedform permeability is the same as that of the sediment bed,the relative contribution of pumping exchange and turbulence penetration changes with the increase of w/h.Affected by the inhomogeneity of the interfacial pressure distribution,the pumping exchange initially increases and then decreases with the increase of w/h,which makes the hyporheic exchange flux to show a similar trend;when the bedform permeability is less than the sediment bed permeability,the overall interfacial permeability decreases,and the turbulence penetration weakens.The mass exchange inside the low permeability bedform constituent unit weakens,and the pumping exchange tends to be attenuated.