Incorporating sedimentological observations, hydrogeophysics and conceptual knowledge to constrain 3D numerical heterogeneity models of alluvial systems

Coarse, braided river deposits are highly heterogeneous in terms of hydraulic properties and make up many groundwater reservoirs worldwide and more than two-thirds of the exploited aquifers in Switzerland. The management of these resources often requires the understanding of the subsurface flow processes and therefore, of the subsurface heterogeneity.
While coarse, braided river deposits were the focus of many studies, the relationship between the braided river dynamics and the resulting sedimentary structures is still poorly known. A better knowledge of this relationship is the key to geologically more realistic modelling of the subsurface heterogeneity by accounting for the former controls on the fluvial system (slope of the floodplain, terraces, lateral confinement, bedrock steps, etc.).
This thesis aims to (i) investigate the link between braided river morphodynamics and subsurface heterogeneity in a hydrogeological context and (ii) develop a numerical model of subsurface heterogeneity that accounts for this link. The sedimentological knowledge on coarse, braided river deposits was mainly gained from the observation of analogue coarse deposits in Switzerland. The Tagliamento River (northeast Italy) was chosen as a field laboratory to thoroughly study the geomorphology (surface) and near-surface sedimentology of coarse, braided rivers. The Tagliamento River is considered to be one of the last large semi-natural rivers of the Alps that has retained much of its natural sediment and discharge dynamics. The observations focused on a single reach sharing similar characteristics with partly confined valleys such as in the alpine foreland.
The sedimentary structures of coarse, braided river deposits can be comprehensively described by a small limited number of sedimentary structures that have specific hydraulic properties. A handful of depositional elements were identified. Among them, the cross bedded trough fills can significantly influence the flow field because their highly-permeable cross beds act as fast flow conduit. Such trough fills most probably form from confluence scour holes.
The geomorphological analysis of the Cimano-Pinzano reach is based on a LiDAR-derived digital elevation model (LiDAR-derived DEM), aerial and satellite photographs, a water-stage time series and regular field observations. Complex aggradation/degradation dynamics resulted in the formation of higher-lying zones incised by a drainage gully network and surrounded by zones that are often reworked by the river. The main geomorphological elements are identified in terms of their topographic signature and genesis, setting apart the trichotomy water–vegetation–bar. 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.). Based on the observations of vertical exposure of coarse, braided river deposits, each morphological element is associated with a depositional element. The preservation potential of the depositional elements is evaluated as a function of the river-bed aggradation dynamics and the resulting subsurface heterogeneity is discussed in terms of its impact on the subsurface flow.
Ground-penetrating radar (GPR) surveys performed on the active zones of the Cimano-Pinzano reach imaged many cross bedded trough fills as observed in ancient deposits. Finding a link between these structures and the evolution of the morphology is challenging. Nevertheless, some hypotheses about the formation of the trough structures are advanced. All the observations suggest the importance of the gravel sheets in the formation of cross bedded trough fills.
An object-based model was developed that mimics the dominant processes of floodplain dynamics. Contrary to existing models, this object-based model possesses the following properties: (i) it is consistent with field observations (outcrops, ground-penetrating radar data, etc.), (ii) it allows different sedimentological dynamics to be modeled that result in different subsurface heterogeneity patterns, and (iii) it is light in memory and computationally fast. To demonstrate its applicability, the object-based model is conditioned to interpreted two-dimensional data and the uncertainty on the three-dimensional subsurface heterogeneity is quantified with Monte Carlo sampling.
The impact of an isolated trough fill complex on subsurface flow mixing is evaluated in terms of advective mixing. The trough fill complex is modelled with the object-based model that is fitted to GPR data. Hydraulic properties are assigned to the model cells and a subsurface flow through the model is simulated. The advective mixing is quantified with 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. These findings depict possible advective mixing found in natural environments and can guide the interpretation of ecological processes such as in the hyporheic zone.
The geomorphological and sedimentological characterisations of the coarse, braided Cimano-Pinzano reach of the Tagliamento River contribute to a better understanding of the morphodynamics of coarse, braided river in relation to the subsurface heterogeneity. The object-based model allows the simulation of various geological settings and the methodology developed for the stereological study can be adapted to other types of data without many changes. Preliminary results of subsurface flow simulations through coarse, braided river deposits show a strong subsurface flow mixing.