A limitation of the hydrostatic reconstruction technique for Shallow Water equations

Because of their capability to preserve steady-states, well-balanced schemes for Shallow Water equations are becoming popular. Among them, the hydrostatic reconstruction proposed in Audusse et al. (2004), coupled with a positive numerical flux, allows to verify important mathematical and physical properties like the positivity of the water height and, thus, to avoid unstabilities when dealing with dry zones. In this note, we prove that this method exhibits an abnormal behavior for some combinations of slope, mesh size and water height.Comment: 7 page

FullSWOF: A free software package for the simulation of shallow water flows

Numerical simulations of flows are required for numerous applications, and are usually carried out using shallow water equations. We describe the FullSWOF software which is based on up-to-date finite volume methods and well-balanced schemes to solve this kind of equations. It consists of a set of open source C++ codes, freely available to the community, easy to use, and open for further development. Several features make FullSWOF particularly suitable for applications in hydrology: small water heights and wet-dry transitions are robustly handled, rainfall and infiltration are incorporated, and data from grid-based digital topographies can be used directly. A detailed mathematical description is given here, and the capabilities of FullSWOF are illustrated based on analytic solutions and datasets of real cases. The codes, available in 1D and 2D versions, have been validated on a large set of benchmark cases, which are available together with the download information and documentation at http://www.univ-orleans.fr/mapmo/soft/FullSWOF/.Comment: 38 page

Past Holocene detritism quantification and modeling from lacustrine archives in order to deconvoluate human-climate interactions on natural ecosystem over long time-scale

International audienceWater budget is one of the main challenges to paleoclimate researchers in relation to present-day global warming and its consequences for human societies

FullSWOF: A software for overland flow simulation / FullSWOF : un logiciel pour la simulation du ruissellement

Overland flow on agricultural fields may have some undesirable effects such as soil erosion, flood and pollutant transport. To better understand this phenomenon and limit its consequences, we developed a code using state-of-the-art numerical methods: FullSWOF (Full Shallow Water equations for Overland Flow), an object oriented code written in C++. It has been made open-source and can be downloaded from http://www.univ-orleans.fr/mapmo/soft/FullSWOF/. The model is based on the classical system of Shallow Water (SW) (or Saint-Venant system). Numerical difficulties come from the numerous dry/wet transitions and the highly-variable topography encountered inside a field. It includes runon and rainfall inputs, infiltration (modified Green-Ampt equation), friction (Darcy-Weisbach and Manning formulas). First we present the numerical method for the resolution of the Shallow Water equations integrated in FullSWOF_2D (the two-dimension version). This method is based on hydrostatic reconstruction scheme, coupled with a semi-implicit friction term treatment. FullSWOF_2D has been previously validated using analytical solutions from the SWASHES library (Shallow Water Analytic Solutions for Hydraulic and Environmental Studies). Finally, FullSWOF_2D is run on a real topography measured on a runoff plot located in Thies (Senegal). Simulation results are compared with measured data. This experimental benchmark demonstrate the capabilities of FullSWOF to simulate adequately overland flow. FullSWOF could also be used for other environmental issues, such as river floods and dam-breaks.Comment: 9 page

Connectivity elements and mitigation measures in policy-relevant soil erosion models: A survey across Europe

The current use of soil erosion models in Europe was investigated through an exploratory survey of 46 model applications covering 18 European countries. This revealed novel information on erosion model applications, their parameterisation, incorporation of landscape elements and mitigation measures with implications for connectivity and their use in decision-making in Europe. The model application predictions were applied at national, regional, catchment or field scale. The majority of model applications used the USLE or versions thereof, but a range of semi-empirical, decision-tree and process-based models were also used. The majority of model applications were used for policy relevant purposes such as erosion risk assessment or mitigation measure implementation at a range of spatial scales. The analysis identified an evident prevalence towards the use of national or regional data sets and a highly varying parameterisation of model applications. Landscape elements and mitigation measures with effects on connectivity were implemented in most model applications, but not with a focus on modelling connectivity within the landscape. Altogether, the results demonstrate a need for improving connectivity modelling in diverse agricultural landscapes across multiple scales. Models should be chosen dependent on their ability to reflect erosion risk at different spatial scales. Albeit, harmonisation of data sets, parameterisation procedures and validation approaches is needed for certain modelling scenarios to ensure comparability of soil erosion risk assessment and suitable mitigation practices. Furthermore, we recommend that policy-relevant erosion risk maps should be verified by empirical data and thresholds derived from erosion risk maps should be adapted to regional conditions when used for policy guidelines. Hence, comparability, comprehensibility and regional adaptation are essential qualities of policy-relevant erosion maps

Modeling of connectivity: What for? How do we do it? Where to go?

Invited oral presentationabsen

Modélisations numérique et expérimentale du ruissellement. Effet de la rugosité sur les distances de transfert

Mémoire de Géosciences-Rennes n°93, 170 p. ISBN : 2-905532-62-0Soil surface morphology affects overland flow and vice versa. The goal of this study was to gain new insights into control mechanisms and their interactions. Attention is focused on sheetflow in inter-rill areas. A numerical model, based on a conditioned walker technique, is used. Effects of water surface storage on overland flow generation are analysed. Generation of overland flow is described as a development of connections between depressions. Being similar to a percolation process, a formal description of system size on the dynamics of overland flow is performed. Numerically generated surfaces enable one to examine external dynamics (using a runoff coefficient) and internal dynamics (using a transfer distance). The water transfer distance is a relevant measurement to understand system behaviour. This cannot be achieved if only the runoff coefficient is measured. Effects of the different kinds of roughness depend mainly on their amplitudes. Short-range correlation does not affect connection development. Driving particle redistribution, overland flow causes non-obvious modifications of surface geometry. Processes able to modify local morphology have the strongest consequences on overland flow generation. However, usual methods do not characterise such modifications. Applied to water and particle transfers, the redistribution of dissolved matter is also studied.La morphologie de la surface du sol conditionne le ruissellement et réciproquement. Cette étude a été menée dans le but de mieux comprendre les mécanismes de contrôle et leurs interactions. Elle s'est focalisée sur le ruissellement diffus de type inter-rigoles. Ce travail s'appuie sur un modèle numérique basé sur la technique des marcheurs conditionnés. La dynamique du stockage de l'eau en surface est finement analysée en termes de genèse et de propagation du ruissellement. Nous montrons que la propagation du ruissellement, considérée comme un développement de connexions entre dépressions, peut être assimilée à un processus de percolation. Une relation formelle entre la taille du système et la dynamique du ruissellement est proposée. Des surfaces numériquement générées permettent l'examen des dynamiques externes, à partir du calcul du coefficient de ruissellement, et internes, à partir d'un calcul de la distance de transfert. La distance de transfert est une mesure pertinente pour comprendre le comportement du système, le coefficient de ruissellement n'y suffisant pas. Les effets des différentes rugosités dépendent principalement de leurs amplitudes. La corrélation observée à courte distance ne modifie pas le développement des connexions. En étant le moteur des redistributions de particules, l'écoulement d'eau aboutit à des évolutions non triviales de la géométrie des surfaces. Ce sont les processus affectant localement la morphologie de la surface qui ont les plus fortes conséquences sur le déclenchement du ruissellement. Toutefois, les méthodes courantes ne peuvent pas mettre en évidence ce type de modifications. Essentiellement focalisée sur les transferts d'eau et de particules, la redistribution de substances dissoutes est aussi abordée

How soil roughness affects runoff and sediment production ?

Soil surface roughness is a key factor in affecting runoff generation and erosion. Nevertheless, how roughness affects runoff and erosion is not clear, because soil roughness can both converge and diverge surface flow, which can cause opposite effects on sediment production. In this research, we partitioned surface roughness into mounds and depressions and studied their effects on runoff and sediment production separately. The experiments were conducted in a dual soil box system with an up-and-down slope configuration such that erosion from the downslope study box can be quantified with and without receiving runon from the upslope soil box, simulating processes on a slope segment at different hillslope positions. Soil surface in the study box was prepared with and without mounds or depressions and the study was conducted on side by side rough vs. smooth surfaces. We also changed the near-surface hydraulic gradient from a downward drainage to upward seepage to create contrasting soil erodibility for the same soil. Our results showed that runoff was delayed from surfaces with depressions. Once at full runoff, there is no consistent trend in whether smooth or rough (with mounds or depressions) surface produced more or less sediment, even though surface with depressions produced more runoff water. Since increasing soil roughness has been prescribed as an effective soil and water conservation measure, our results indicated that the only benefit from surface roughening is in its runoff delay while rainwater was filling the depressional storage. Therefore, soil roughening may not be effective at all cases and its effectiveness depends on the antecede soil moisture condition and rainstorm pattern. Our results may also lead to changes in current erosion assessment tools, such as the Revised Universal Soil Loss Equation (RUSLE) and Water Erosion Prediction Project (WEPP), in how they handle the surface roughness factor