Subsurface flow mixing in coarse, braided river deposits

Abstract

Coarse, braided river deposits show a large hydraulic heterogeneity on the metre scale. One of the main depositional elements found in such deposits is a trough structure filled with layers of bimodal gravel and open-framework gravel, the latter being highly permeable. However, the impact of such trough fills on subsurface flow and advective mixing has not drawn much attention. A geologically realistic model of trough fills is proposed and fitted to a limited number of ground-penetrating radar records surveyed on the river bed of the Tagliamento River (northeast Italy). A steady-state, saturated subsurface flow simulation is performed on the small-scale, high-resolution, synthetic model (size: 75m × 80m × 9m). Advective mixing (i.e. streamline intertwining) is visualised and quantified based on particle tracking. The results indicate strong advective mixing as well as a large flow deviation induced by the asymmetry of the trough fills with regard to the main flow direction. The flow deviation induces a partial, large-scale rotational effect. These findings depict possible advective mixing found in natural environments and can guide the interpretation of ecological processes such as in the hyporheic zone.