Spatial features of hydraulic conductivity in alluvial fans

Abstract:

Alluvial fans are highly heterogeneous due to the complicated depositional processes, which make it difficult to characterize the spatial distributions of the hydraulic conductivity and to evaluate the evolution trends. This study develops a methodology to identify the spatial statistical parameters (mean, variance, correlation range) of the hydraulic conductivity by using the hydraulic and geophysical data. The Chaobai River alluvial fan in the Beijing Plain, China, is used as an example to test the developed methodology for depicting the spatial variations of the alluvial fan. Due to the non-stationary property of the hydraulic conductivity distributions in the alluvial fan, a multi-zone parameterization approach is applied to analyze conductivity statistical properties of different hydrofacies and the composite variance of each zone is computed to describe the evolution of the conductivity along the flow direction of the alluvial fan. Results show that conductivity variances of medium-coarse sand and gravel decrease from Zone 1 to Zone 3 along the flow direction, which is consistent with the scales of the sedimentary transport energy. In the upper zone of the alluvial fan sediments were formed by higher-energy flooding, which induces bad sorting and larger conductivity variances in the sediments. The variance of fine sand also decreases from Zone 1 to Zone 3. The variances of the sub-clay and clay increase from the upper zone of the alluvial fan to the lower zone, where multiple layers of the sub-clay and clay formed by different flooding events have quite different conductivity distributions and statistical properties. With a much larger thickness in the lower zone, the vertical variations of conductivity contribute mainly to the relatively increased variance. The result of this study will provide insight for understanding conductivity heterogeneity and a method for characterizing the conductivity spatial distributions in alluvial fans.