Study On The Spatial Variation Of Hydraulic Conductivities Of Streambed Sediments In Semi-arid Areas

The interactions between groundwater and surface water are a common naturalphenomenon. Hydraulic conductivities of hyporheic zone play a direct role in controlling thehydraulic connection between groundwater and surface water. Characterization of spatialvariations of streambed hydraulic conductivity is a necessary step for developing integratedmanagement plans of surface-groundwater resources and in the design of restoration systemsof the rivers in semi-arid areas. This study chooses the Wei River in Shaanxi Province ofChina and the Platte River in Nebraska of USA to analyze the spatial variations of streambedhydraulic conductivity of hyporheic zone. In this study, in-situ permeameter test and grainanalysis method were used to determine the hydraulic conductivity of streambed sediments.From the following aspects, we studied the spatial variation of hydraulic conductivities ofstreambed sediments in the Wei River. First, the in-situ permeameter test was used todetermine the vertical hydraulic conductivities (K_v) of the upper and lower layers of thestreambed sediment in the Wei River. Second, the in-situ permeameter test method wasextended to determine the horizontal hydraulic conductivities (Kh), vertical hydraulicconductivities (K_v), hydraulic conductivities parallel to bedding direction, and hydraulicconductivities perpendicular to bedding direction in exposed riverbed sediments. Third, thehorizontal hydraulic conductivities (Kh) in a floodplain in the Wei River were also measured.The results indicate that (1) the upper K_vis greater than the lower K_vin submergedstreambeds,(2) the maximum K value is the hydraulic conductivities parallel to beddingdirection, and the minimum K value is the hydraulic conductivities perpendicular to beddingdirection,(3) in the floodplain, the sediment grain sizes become smaller from the bottom up,accordingly the hydraulic conductivities decrease upward, as a result of that thehydrodynamic forces became weaker during the deposition of the upper part of the floodplain.Empirical formulas based on grain analysis were used to calculate the hydraulic conductivitiesof the streambeds. The K results from the grain size distribution were used to compare withthose from field tests. It was found that the results from the method of standpipe can betterreflect the actual sedimentary structure. Moreover, we used Ordinary Kriging to analyze the spatial variation of the vertical hydraulic conductivities (K_v) in the Platte River. Firstly, thevariogram models were derived from K_vwhich were measured from the in-situ permeametertests. Then, Ordinary Kriging was used to interpolate K_vfor "unknown" locations. Wecompared interpolation results with measured values, and compared interpolation results byOrdinary Kriging with results by the methods of inverse distance to a power and movingaverage. The results demonstrate that Ordinary Kriging interpolation can result in loss ofvariation. Finally, the Hunt (1999) river-groundwater theory model was used to simulatestream-aquifer interactions caused by groundwater pumping. The hydraulic conductivities ofthe Wei River were used to represent several hypothetical streambeds. The simulation resultssuggest that the hydraulic conductivity (K) is an important hydrogeology parameter to controlthe exchange rate between surface water and groundwater.