Vertical Subsurface Flow Mixing and Horizontal Anisotropy in Coarse Fluvial Aquifers: Flow Field

Abstract:

A stochastic object-based model for hydrogeological aquifer characterization of coarse braided-river deposits has been developed based (1) on sedimentological observations of ancient coarse braided-river deposits and ground penetrating radar surveys on the floodplain of the Tagliamento River (Northeast Italy) and (2) on observations of the morphodynamics of the Tagliamento River.

We assume that at each large flow event, flow-confluence scours are formed and filled during their migration, and subsequently covered by downstream moving gravel sheets of poorly sorted sediments. The object-based model mimics this behaviour: at each iteration truncated ellipsoids (scour-fill deposits) are distributed, as a marked point process, at the previous simulated floodplain surface and are followed on top by a layer with a specified thickness (gravel sheets) that define the next floodplain topography. The truncated ellipsoids have erosional and depositional properties. Compared with the scour-fills, the gravel sheets show much less variability and a much smaller hydraulic conductivities with only a vertical anisotropy. Accordingly, specific three-dimensional hydraulic conductivity tensors and porosity values are stochastically attributed to the truncated ellipsoids and the horizontal layers.

This object-based model is able to simulate a large range of the morphodynamics of braided-river systems (scenarios) that result in different sediment sorting processes and, in turn, in specific hydraulic heterogeneity and connectivity patterns of high permeable units.

In the context of surface water-groundwater interaction the model is used to assess the impact of some scenarios on the groundwater flow field as well as on solute transport. We evaluate the vertical mixing of the groundwater flows, the horizontal flow deviation caused by the anisotropy of the scour-fill deposits as well as the solute transport behaviour. We discuss the specific hydraulic responses in relation with the heterogeneity and connectivity variability.