Response Of Hyporheic Water Exchange To River Micro-topographies At The Meandering Bank And River Confluence

Hyporheic zone is not only a mixing zone or active ecotone in surface water-groundwater system but a key associated node between riverine system and its surroundings,which plays the crucially important roles in controlling the changes of hydrological cycle as well as the fate and transport of contaminants in riverine system.In this study,the meandering bank(along the Weihe River in January and June 2015)as well as river confluence(between Juehe River and Haohe River in January 2016)located in Xi’an,Shaanxi Province,China were two typical river micro-topographies to conduct some experiments for hyporheic water exchange(HWE),hydraulic conductivity(Kv)as well as grain size analysis.One-dimension heat advection-diffusion equation and the thermal method were applied to determine the HWE while permeameter test method was used to calculate the sediment Kv values.The spatio-temporal variability in HWE was evaluated while the correlations of HWE to bank curvatures,riparian vegetation,sediment properties,bedforms,erosional and depositional processes,water depth,and river topography were analyzed to gradually reveal the response of HWE to different river micro-topographies in detail.The conclusions were summerized in this following:1)The significantly seasonal variability in HWE was obtained in January and July 2015(winter and summer).Along the meandering bank in the Weihe River,the HWE patterns of all test points were upward during the two test periods,and the higher vertical fluxes mostly occurred in January 2015.This can be caused by the significant variation of instream flow along with occurrence of dry season in winter and wet season in summer.2)The longitudinal spatial variability of HWE along the flow direction was derived under the condition of the two river topographies.Curvature in riverine system is a controlling attribute to induce the change of patterns and magnitudes of hyporheic water exchange.For the meandering bank at small scales,larger curvatures for either side of convex banks during the two periods easily resulted in higher HWE magnitudes,and the significantly higher vertical fluxes generally occurred near the apex of bend.Meanwhile,in terms of the meandering channel of river confluence at large scales,the change of hyporheic flow pattern from downwelling to upwelling along the right meandering bank of Juehe River was revealed to form the regional and local flow paths,which was inferred to be driven by the local curvature of meander bend morphology and regional groundwater discharge.Additionally,higher HWE magnitudes occurred in the vicinity of bend’s apex.3)Along the meandering bank in summer,riparian vegetation had a large water discharge capacity in HZ,easily influencing the HWE in near-stream system.Significantly higher HWE magnitudes were obtained along the straight bank where more riparian vegetation was adjacent to the bank/water interface,and additionally,a flow pattern from river bank discharging into stream was derived during the campaign in July 2015.In addition,the larger sediment heterogeneity in depositional area,the higher HWE magnitudes.4)The lateral spatial variability of HWE in channel cross-section perpendicular to the flow direction was derived at the river confluence.All hydraulic and sediment properties at the river confluence,including HWE,sediment Kv,sediment grain size and porosity,presented strikingly spatial heterogeneity along the lateral cross-section as well as even different channel segments(M,T and MT).Meanwhile,in the dominant upwelling areas,the change of HWE magnitudes was nearly consistent with that of the Kv values,and both significant higher values mostly appeared in erosional zones near the thalweg paths at the river confluence.It could be potentially induced by erosional and depositional processes subject to the distinctive river morphology.Additionally,the much lower Kv values were obtained in the downwelling areas with mostly silt/clay compared to the downwelling areas,potentially subject to the sediment clogging.5)Furthermore,it was examined that the significant correlations of the VHWE to the Kv,d50,porosity(n),and uniformity coefficient(η)were obtained at the river confluence.Thus,it is suggested that sediment properties and the confluence morphology could be one of driving factors to inducing the variation of HWE magnitudes and patterns in fluvial system,respectively.