Subsurface flow mixing in coarse, braided river deposits

摘要

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: 75?m??×??80?m??×??9?m). 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.