Morphological perspective on the sedimentary characteristics of a coarse, braided reach: Tagliamento River (NE Italy)

- The link between morphodynamics and the subsurface heterogeneity is investigated.
- The high-discharge morphology is overprinted by the low-discharge morphology.
- The high-discharge morphology impacts more the hydraulic subsurface heterogeneity.
- Morphological approaches can underestimate the proportion of cut-and-fill elements.
- Settings leading to different types of heterogeneity are discussed.

In order to understand heterogeneity distribution of hydraulic properties, many studies have proposed models of coarse, braided river deposits. However, they often focus either on the surface/near surface or on ancient deposit analysis. Furthermore, the link between morphological (surface) and sedimentological (outcrops) information has not been fully explored yet. This publication aims to characterize the morphodynamics of a braided, gravel-bed reach of the Tagliamento River (NE Italy) and to assess its relationship with the subsurface heterogeneity. The morphological analysis is based on a LiDAR-derived DEM, aerial and satellite photographs from 1998 to 2011, as well as field observations. A water-stage time series allows the morphological changes to be related to the discharge dynamics. The sedimentological knowledge on coarse, braided deposits was mainly gained from the observation of analog Pleistocene coarse deposits in Switzerland. The main geomorphological elements are identified in terms of their topographic signature and genesis, setting apart the trichotomy water-vegetation-bar. The braidplain is characterized by higher-lying zones with dentritic gully drainage networks and active zones where most of the morphology reworking occurs. Furthermore, two morphologies mark the active zones: a low-discharge morphology (low-discharge incisions and channels, slip-face lobes, etc.) superimposed on a high-discharge morphology (gravel sheets, scours, etc.). Each morphological element is related to a depositional (i.e., sedimentological) element whose preservation potential in the subsurface is assessed as a function of the river-bed aggradation dynamic. The settings that impact the subsurface flow in terms of fast pathway and connectivity are addressed.

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