The bedform-driven hyporheic exchange plays a crucial role in mass transport within natural aquatic ecosystems like streams and rivers. This study aimed to unveil the impact of geometric features of impermeable discre...The bedform-driven hyporheic exchange plays a crucial role in mass transport within natural aquatic ecosystems like streams and rivers. This study aimed to unveil the impact of geometric features of impermeable discrete bedforms on hyporheic exchange by experimentally measuring quantitative hyporheic exchange flux data and variation characteristics in an annual flume. The experiments encompassed diverse conditions involving the ratio of bedform wavelength (λ) to wave height (h) and relative submergence. The study also analyzed the dependence of the effective diffusion coefficient on the geometric characteristics of bedform composition elements. The experimental results showed that, in comparison to a permeable flat bed, the presence of an impermeable discrete bedform tended to either attenuate or enhance hyporheic exchange, contingent on the geometric characteristics of bedform composition elements. The hyporheic exchange flux exhibited an initially increase followed by a decrease with increasing λ/h, with turbulence penetration emerging as the dominant mechanism governing hyporheic exchange for cases with relatively denser bedform composition elements (e.g., λ/h = 4.0). The effective diffusion coefficient peaked at λ/h around 6.0–8.0, owing to a significant augmentation in the relative contribution of pumping exchange to gross hyporheic exchange. Furthermore, the hyporheic exchange intensity generally increased with decreasing relative submergence, primarily attributed to the augmented relative contribution of pumping exchange to gross hyporheic exchange.展开更多
Metal contaminants from surface water pollution events often enter hyporheic zones,under certain conditions,they may be released back into streams,causing secondary pollution to the water quality.The present study inv...Metal contaminants from surface water pollution events often enter hyporheic zones,under certain conditions,they may be released back into streams,causing secondary pollution to the water quality.The present study investigated the effects of adsorption,permeability,and anisotropy of sediment beds on the release of zinc ions(Zn^(2+))from the hyporheic zone into overlying turbulent flows using large-eddy simulations(LES).The volume-averaged Navier-Stokes equations and advection-diffusion equation with adsorption term were used to describe the sediment in-flow,adsorption,and convective diffusion of Zn^(2+) within the sediment layer.The effects of sediment permeability on the Zn^(2+)concentration distribution and mass transfer processes were investigated by time-averaged statistics of flow and concentration fields.The results show that adsorption becomes stronger as the pH value increases,leading to a slow increase in Zn^(2+)concentration in the overlying water layer and reaching a lower steady-state concentration.Higher overall permeability of the sediment layer can enhance mass and momentum exchange near the sediment-water interface(SWI),and intensify the release of Zn^(2+)from the sediment layer into the overlying water.As the wall-normal permeability of the sediment layer increases,the normal turbulent intensity strengthens,momentum transport enhances,the wall-normal Zn^(2+)concentration flux increases,the effective diffusion coefficient increases,and the concentration in the overlying water increases.展开更多
In the Elkhorn River,burrows,tubes,and sediment mounds created by invertebrate bioturbation were observed in the exposed streambed and commonly concentrated on the fine-sediment patches,which consist of silt,clay,and ...In the Elkhorn River,burrows,tubes,and sediment mounds created by invertebrate bioturbation were observed in the exposed streambed and commonly concentrated on the fine-sediment patches,which consist of silt,clay,and organic matter.These invertebrate activities could loosen the thin layer of clogging sediments and result in an increase of pore size in the sediments,leading to greater vertical hydraulic conductivity of the streambed(Kv).The measurements of the vertical hydraulic gradient across the submerged streambed show that vertical flux in the hyporheic zone can alter directions(upward versus downward)for two locations only a few meters apart.In situ permeameter tests show that streambed Kv in the upper sediment layer is much higher than that in the lower sediment layer,and the calculated Kv in the submerged streambed is consistently greater than that in the clogged sediments around the shorelines of the sand bars.Moreover,a phenomenon of gas bubble release at the water-sediment interface from the subsurface sediments was observed in the groundwater seepage zone where flow velocity is extremely small.The bursting of gas bubbles can potentially break the thin clogging layer of sediments and enhance the vertical hydraulic conductivity of the streambed.展开更多
基金supported by the National Natural Science Foundation of China(Grant No.11472168).
文摘The bedform-driven hyporheic exchange plays a crucial role in mass transport within natural aquatic ecosystems like streams and rivers. This study aimed to unveil the impact of geometric features of impermeable discrete bedforms on hyporheic exchange by experimentally measuring quantitative hyporheic exchange flux data and variation characteristics in an annual flume. The experiments encompassed diverse conditions involving the ratio of bedform wavelength (λ) to wave height (h) and relative submergence. The study also analyzed the dependence of the effective diffusion coefficient on the geometric characteristics of bedform composition elements. The experimental results showed that, in comparison to a permeable flat bed, the presence of an impermeable discrete bedform tended to either attenuate or enhance hyporheic exchange, contingent on the geometric characteristics of bedform composition elements. The hyporheic exchange flux exhibited an initially increase followed by a decrease with increasing λ/h, with turbulence penetration emerging as the dominant mechanism governing hyporheic exchange for cases with relatively denser bedform composition elements (e.g., λ/h = 4.0). The effective diffusion coefficient peaked at λ/h around 6.0–8.0, owing to a significant augmentation in the relative contribution of pumping exchange to gross hyporheic exchange. Furthermore, the hyporheic exchange intensity generally increased with decreasing relative submergence, primarily attributed to the augmented relative contribution of pumping exchange to gross hyporheic exchange.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12172207,92052201).
文摘Metal contaminants from surface water pollution events often enter hyporheic zones,under certain conditions,they may be released back into streams,causing secondary pollution to the water quality.The present study investigated the effects of adsorption,permeability,and anisotropy of sediment beds on the release of zinc ions(Zn^(2+))from the hyporheic zone into overlying turbulent flows using large-eddy simulations(LES).The volume-averaged Navier-Stokes equations and advection-diffusion equation with adsorption term were used to describe the sediment in-flow,adsorption,and convective diffusion of Zn^(2+) within the sediment layer.The effects of sediment permeability on the Zn^(2+)concentration distribution and mass transfer processes were investigated by time-averaged statistics of flow and concentration fields.The results show that adsorption becomes stronger as the pH value increases,leading to a slow increase in Zn^(2+)concentration in the overlying water layer and reaching a lower steady-state concentration.Higher overall permeability of the sediment layer can enhance mass and momentum exchange near the sediment-water interface(SWI),and intensify the release of Zn^(2+)from the sediment layer into the overlying water.As the wall-normal permeability of the sediment layer increases,the normal turbulent intensity strengthens,momentum transport enhances,the wall-normal Zn^(2+)concentration flux increases,the effective diffusion coefficient increases,and the concentration in the overlying water increases.
基金the Program for Changjiang Scholars and Innovative Research Team of China Ministry of Education(No.IRT0811).
文摘In the Elkhorn River,burrows,tubes,and sediment mounds created by invertebrate bioturbation were observed in the exposed streambed and commonly concentrated on the fine-sediment patches,which consist of silt,clay,and organic matter.These invertebrate activities could loosen the thin layer of clogging sediments and result in an increase of pore size in the sediments,leading to greater vertical hydraulic conductivity of the streambed(Kv).The measurements of the vertical hydraulic gradient across the submerged streambed show that vertical flux in the hyporheic zone can alter directions(upward versus downward)for two locations only a few meters apart.In situ permeameter tests show that streambed Kv in the upper sediment layer is much higher than that in the lower sediment layer,and the calculated Kv in the submerged streambed is consistently greater than that in the clogged sediments around the shorelines of the sand bars.Moreover,a phenomenon of gas bubble release at the water-sediment interface from the subsurface sediments was observed in the groundwater seepage zone where flow velocity is extremely small.The bursting of gas bubbles can potentially break the thin clogging layer of sediments and enhance the vertical hydraulic conductivity of the streambed.
