Subsurface flow mixing in coarse, braided river deposits

Abstract

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: 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.

Imaging scours in straightened and braided gravel‐bed rivers with ground‐penetrating radar

“…Subsurface models based on the stacking of individual riverbed morphologies (e.g., Webb ; Pirot, Straubhaar and Renard ) underestimate scour fill deposits that strongly impact subsurface flow and transport because of their generally highly permeable lithologies with regard to the other coarse, braided river deposits (Jussel et al . ; Huber and Huggenberger ).…”

“…). The following fundamental rules guide the interpretation of GPR data: (i) continuity of the dominant reflections, (ii) differences in reflection patterns and (iii) angular unconformity between reflection patterns (Huber and Huggenberger ).…”

“…The open‐framework gravel is much more permeable than the other deposits (up to three orders of magnitude). Scour fills consisting in open‐framework–bimodal couplets strongly impact subsurface flow and transport (Huber and Huggenberger ) and therefore river water–groundwater exchange (Huggenberger et al . ).…”

A review of methods for measuring groundwater–surface water exchange in braided rivers

“…This allowed, for the first time, the visualisation and analysis of the morphology of large braided rivers (e.g., Hicks et al, 2006;Huggenberger, 1993;Lane, 2006). A number of studies have looked at the surface water features of braided rivers (e.g., Davies et al, 1996;Meunier et al, 2006;Young and Warburton, 1996), as well as aquifers created by braided river deposits (e.g., Huber and Huggenberger, 2016;Pirot et al, 2015;Vienken et al, 2017). However, the connections between the two have been less explored.…”

A pseudo genetic model of coarse braided‐river deposits

“…Length-scale characterization studies [Sapozhnikov and Foufoula-Georgiou, 1996;Foufoula-Georgiou and Sapozhnikov, 2001; Hundey and Ashmore, 2009] have revealed self-affinity and scale invariance in the geomorphology of braided rivers. Detailed outcrop analyses [Klingbeil et al, 1999;Labourdette and Jones, 2007;Bayer et al, 2011] and ground-penetrating radar measurements and interpretations Bridge et al, 1995;Lunt and Bridge, 2004;Huber and Huggenberger, 2015b] have reinforced the knowledge of sedimentary structures and heterogeneity in braided-river aquifers. According to Huber and Huggenberger [2015b], essential features of braided-river aquifers may be attributed to a layering of successive gravel sheets eroded by scours and then filled with cross-stratified deposits.…”