ALBERT

Description: Author(s): Tanguy Le Borgne, Marco Dentz, Philippe Davy, Diogo Bolster, Jesus Carrera, Jean-Raynald de Dreuzy, and Olivier Bour Anomalous dispersion in heterogeneous environments describes the anomalous growth of the macroscopic characteristic sizes of scalar fields. Here we show that this phenomenon is closely related to the persistence of local scale incomplete mixing. We introduce the mixing scale ε as the length for whic... [Phys. Rev. E 84, 015301] Published Wed Jul 20, 2011

Description: Impact of transient groundwater storage on the discharge of Himalayan rivers Nature Geoscience 5, 127 (2012). doi:10.1038/ngeo1356 Authors: Christoff Andermann, Laurent Longuevergne, Stéphane Bonnet, Alain Crave, Philippe Davy & Richard Gloaguen In the course of the transfer of precipitation into rivers, water is temporarily stored in reservoirs with different residence times such as soils, groundwater, snow and glaciers. In the central Himalaya, the water budget is thought to be primarily controlled by monsoon rainfall, snow and glacier melt, and secondarily by evapotranspiration. An additional contribution from deep groundwater has been deduced from the chemistry of Himalayan rivers, but its importance in the annual water budget remains to be evaluated. Here we analyse records of daily precipitation and discharge within twelve catchments in Nepal over about 30 years. We observe annual hysteresis loops—that is, a time lag between precipitation and discharge—in both glaciated and unglaciated catchments and independent of the geological setting. We infer that water is stored temporarily in a reservoir with characteristic response time of about 45 days, suggesting a diffusivity typical of fractured basement aquifers. We estimate this transient storage capacity at about 28 km3 for the three main Nepal catchments; snow and glacier melt contribute around 14 km3 yr−1, about 10% of the annual river discharge. We conclude that groundwater storage in a fractured basement influences significantly the Himalayan river discharge cycle.

Description: Abstract Mountain fronts are the locus of significant variations in river width characterized by narrow bedrock gorges opening to wider unconfined alluvial rivers that are often braided. The contribution of the abrupt change in river valley confinement in modulating the long‐term transport capacity and the resulting equilibrium channel width has never been considered. Here we use a numerical model integrating the full frequency‐magnitude distribution of streamflow to explore the long‐term bedload transport capacity of idealized confined and unconfined channels, both single thread and braided. The model predicts a significant transport capacity loss for a single channel of constant width at the transition between confined gorges and unconfined floodplains that is slightly reduced when floodplain vegetation is present. Because the total transport capacity of a single‐thread channel systematically decreases with channel width in a stochastic framework, only narrower unconfined channels could compensate for the loss of confinement. This prediction contradicts observations of widening ratios ranging from three to eight downstream of gorges in various world rivers. We resolve this inconsistency by demonstrating that a braided river made of narrow channels inset in a wide floodplain can maintain the total bedload transport capacity downstream of gorges by increasing the range of competent discharges. We also show that riparian vegetation may only enhance bedload transport capacity for highly variable discharge regimes and discuss the relevance of various definitions of representative discharges. These results point to the previously unrecognized role of valley confinement in modulating the sediment transport capacity of rivers.

Description: The distribution of groundwater fluxes in aquifers is strongly influenced by topography, and organized between hillslope and regional scales. The objective of this study is to provide new insights regarding the compartmentalization of aquifers at the regional scale, and the partitioning of recharge between shallow/local and deep/regional groundwater transfers. A finite-difference flow model was implemented and the flow structure was analyzed as a function of recharge (from 20 to 500 mm/yr), at the regional-scale (1400 km²), in 3-dimensions, and accounting for variable groundwater discharge zones; aspects which are usually not considered simultaneously in previous studies. The model allows visualizing 3D circulations, as those provided by Tothian models in 2D, and shows local and regional transfers, with 3D effects. The probability density function of transit times clearly shows two different parts, interpreted using a two-compartment model, and related to regional groundwater transfers and local groundwater transfers. The role of recharge on the size and nature of the flow regimes, including groundwater pathways, transit time distributions, and volumes associated to the two compartments have been investigated. Results show that topography control on the water table and groundwater compartmentalization varies with the recharge rate applied. When recharge decreases, the absolute value of flow associated to the regional compartment decreases, whereas its relative value increases. The volume associated to the regional compartment is calculated from the exponential part of the two-compartment model, and is nearly insensitive to the total recharge fluctuations.

Description: Author(s): Pietro de Anna, Tanguy Le Borgne, Marco Dentz, Alexandre M. Tartakovsky, Diogo Bolster, and Philippe Davy We study the intermittency of fluid velocities in porous media and its relation to anomalous dispersion. Lagrangian velocities measured at equidistant points along streamlines are shown to form a spatial Markov process. As a consequence of this remarkable property, the dispersion of fluid particles ... [Phys. Rev. Lett. 110, 184502] Published Wed May 01, 2013

Description: The “precipiton” method is one of the particle-based approaches, which consists of routing elementary water volumes on top of topography with erosive and depositional actions. Here we present an original way to calculate both river depth and velocity from a method that remains embedded in the precipiton framework. The method solves the governing equations for water depth, where the water depth is increased by a constant quantity at each precipiton passage, and decreased by a value based on a flow resistance equation. The precipitons are then routed downstream on top of the resulting water surface. The method is applicable even if the precipitons are routed one by one (i.e. independently of each other), which makes it simple to implement and computationally fast. Compared to grid-based methods, this particle method is not subject to the classical drying-wetting issue, and allows for a straightforward implementation of sediment transfer functions between the river bed and running water. We have applied the method to different cases (channel flow, flow over topographic bumps, or flood prediction on high-resolution LIDAR topography). In all cases, the method is capable of solving the shallow water equations, neglecting inertia. When coupled with erosion and sediment transport equations, the model is able to reproduce both straight and braided patterns with geometries independent of grid size. Application of the model in the context of multi-thread rivers gives new insight into the development of braiding instability.

Description: Mountain ranges are frequently subjected to mass wasting events triggered by storms or earthquakes and supplying large volumes of sediment into river networks. Besides altering river dynamics, large sediment deliveries to alluvial fans are known to cause hydro-sedimentary hazards such as flooding and river avulsion. Here we explore how the sediment supply history affects hydro-sedimentary river and fan hazards, and how well can it be predicted given the uncertainties on boundary conditions. We use the 2D morphodynamic model Eros with a new 2D hydrodynamic model driven by a sequence of flood, a sediment entrainment/transport/deposition model and a bank erosion law. We first evaluate the model against a natural case: the 1999 Mount Adams rock avalanche and subsequent avulsion on the Poerua river fan (West Coast, New Zealand). By adjusting for the unknown sediment supply history, Eros predicts the evolution of the alluvial river bed during the first post-landslide stages within 30 cm. The model is subsequently used to infer how the sediment supply volume and rate control the fan aggradation patterns and associated hazards. Our results show that the total injected volume control the overall levels of aggradation, but supply rates have a major control on the location of preferential deposition, avulsion and increased flooding risk.. Fan re-incision following exhaustion of the landslide-derived sediment supply leads to sediment transfer and deposition downstream and pose similar, but delayed, hydro-sedimentary hazards. Our results demonstrate that 2D morphodynamics models are able to capture the full range of hazards occurring in alluvial fans including river avulsion aggradation and floods. However, only ensemble simulations accounting for uncertainties in boundary conditions (e.g., discharge history, initial topography, grain size) as well as model realization (e.g., non-linearities in hydro-sedimentary processes) can be used to produce probabilistic hazards maps relevant for decision making.

Description: Quantification of the recharge in fractured aquifers is particularly challenging because of the multi-scale heterogeneity and the range of temporal scales involved. Here, we investigate the hydraulic response to recharge of a fractured aquifer, using a frequency domain approach. Transfer functions are calculated in a range of temporal scales from 1 day up to a few years, for a fractured crystalline-rock aquifer located in Ploemeur (S Brittany, France), using recharge and groundwater level fluctuations as input and output respectively. The spatial variability of the response to recharge (characteristic response time, amplitude, temporal scaling) is analyzed for ten wells sampling the different compartments of the aquifer. Some of the transfer functions follow the linear reservoir model behavior. On the contrary, others display a temporal scaling at high-frequency that cannot be represented by classic models. Large scale hydraulic parameters, estimated from the low-frequency response, are compared with those estimated from hydraulic tests at different scales. The variability of transmissivity and storage coefficient tends to decrease with scale, and the average estimates converge towards the highest values at large scale. The small scale variability of diffusivities, which implies the existence of a range of characteristic temporal scales associated with different pathways, is suggested to be at the origin of the unconventional temporal scaling of the hydraulic response to recharge at high-frequency.

Description: . [1]  In order to improve discrete fracture network (DFN) models, which are increasingly required into groundwater and rock mechanics applications, we propose a new DFN modeling based on the evolution of fracture network formation – nucleation, growth and arrest – with simplified mechanical rules. The central idea of the model relies on the mechanical role played by large fractures in stopping the growth of smaller ones. The modeling framework combines, in a time-wise approach, fracture nucleation, growth and arrest. It yields two main regimes. Below a certain critical scale the density distribution of fracture sizes is a power law with a scaling exponent directly derived from the growth law and nuclei properties; above the critical scale, a quasi-universal self-similar regime (UFM, [ Davy et al ., 2010]) establishes with a self-similar scaling. The density term of the dense regime is related to the details of arrest rule and to the orientation distribution of the fractures. The DFN model so defined is fully consistent with field cases former studied. Unlike more usual stochastic DFN models, ours is based on a simplified description of fracture interactions, which eventually reproduces the multiscale self-similar fracture size distribution often observed and reported in the literature. The model is a potential significant step forward for further applications to groundwater flow and rock mechanical issues.