Laurent Lassabatere

Numerical evaluation of a set of analytical infiltration equations

In this paper, a set of analytical infiltration equations that are commonly used to evaluate one‐ and three‐dimensional water infiltration from a surface disc source is studied. Both the quasi‐exact analytical formulation and the related approximations for short and long times are assessed. The analytical properties of the quasi‐exact formulation are evaluated using a proposed scaling procedure in order to define the validity domains of related approximations. Both quasi‐exact and approximate analytical equations are then studied with respect to their ability to reproduce numerically generated cumulative infiltrations from a 10 cm radius disc source for four soils (sand, loam, silt, and silty clay) at several initial saturations. The quasi‐exact formulation is suitable for sand, loam, and silt when their soil‐dependent and saturation‐independent shape parameters, γ and β, are properly chosen (between 0.75 and 1 and 0.3 and 1.7, respectively). Approximations derived for the same shape parameters can also be used, provided that their use is restricted to proposed validity intervals. However, none of these equations applies for silty clay, since its hydraulic properties do not fulfill the conditions required for the use of the quasi‐exact formulation.

Turbulent velocity profile in fully-developed open channel flows

The determination of velocity profile in turbulent narrow open channels is a difficult task due to the significant effects of the anisotropic turbulence that involve the Prandtl’s second type of secondary flow occurring in the cross section. With these currents the maximum velocity appears below the free surface that is called dip phenomenon. The well-known logarithmic law describes the velocity distribution in the inner region of the turbulent boundary layer but it is not adapted to define the velocity profile in the outer region of narrow channels. This paper relies on an analysis of the Navier–Stokes equations and yields a new formulation of the vertical velocity profile in the center region of steady, fully developed turbulent flows in open channels. This formulation is able to predict time averaged primary velocity in the outer region of the turbulent boundary layer for both narrow and wide open channels. The proposed law is based on the knowledge of the aspect ratio and involves a parameter CAr depending on the position of the maximum velocity (ξdip). ξdip may be derived, either from measurements or from an empirical equation given in this paper. A wide range of longitudinal velocity profile data for narrow open channels has been used for validating the model. The agreement between the measured and the computed velocities is rather good, despite the simplification used.

Transport of two naphthoic acids and salicylic acid in soil: experimental study and empirical modeling.

In contrast to the parent compounds, the mechanisms responsible for the transport of natural metabolites of polycyclic aromatic hydrocarbons (PAH) in contaminated soils have been scarcely investigated. In this study, the sorption of three aromatic acids (1-naphthoic acid (NA), 1-hydroxy-2-naphthoic acid (HNA) and salicylic acid (SA)) was examined on soil, in a batch equilibrium single-system, with varying pH and acid concentrations. Continuous flow experiments were also carried out under steady-state water flow. The adsorption behavior of naphthoic and benzoic acids was affected by ligand functionality and molecular structure. All modeling options (equilibrium, chemical nonequilibrium, i.e. chemical kinetics, physical nonequilibrium, i.e. surface sites in the immobile water fraction, and both chemical and physical nonequilibrium) were tested in order to describe the breakthrough behavior of organic compounds in homogeneously packed soil columns. Tracer experiments showed a small fractionation of flow into mobile and immobile compartments, and the related hydrodynamic parameters were used for the modeling of reactive transport. In all cases, the isotherm parameters obtained from column tests differed from those derived from the batch experiments. The best accurate modeling was obtained considering nonequilibrium for the three organic compounds. Both chemical and physical nonequilibrium led to appropriate modeling for HNA and NA, while chemical nonequilibrium was the sole option for SA. SA sorption occurs mainly in mobile water and results from the concomitancy of instantaneous and kinetically limited sites. For all organic compounds, retention is contact condition dependent and differs between batch and column experiments. Such results show that preponderant mechanisms are solute dependent and kinetically limited, which has important implications for the fate and transport of carboxylated aromatic compounds in contaminated soils.

Zinc and lead transfer in a contaminated roadside soil: Experimental study and modeling

The application of a surface complexation model to simulate the sorption of metals on single sorbents is very well investigated, but very little is known regarding the use of surface complexation modeling to simulate the metal mobility in contaminated roadside soils. The overall objective of this study was to examine whether the use of the surface complexation model (SCM) could correctly describe the migration of zinc and lead in roadside soil under various physicochemical conditions. The release and transport of Zn and Pb was studied by means of batch reactors and saturated chromatography columns. Soil batch experiments were conducted to evaluate the effects of pH variation and ionic strength on the metal mobility from soil. Elution of Pb and Zn was examined in column experiments by using acetic acid at pH5 and EDTA at pH7. The modeling work has focused on the development of a SCM using MINTEQ2 database incorporated in PHREEQC-2 to describe the interactions between trace metals and the main mineral soil components (quartz, iron and aluminum oxides). In this study, it was found that the SCM was able to simulate the mobility of metals from soil by assuming one mononuclear surface reaction between one solution species (Me(2+)) and one type of site on the surface of soil dominant sorbents.

Velocity Distribution in Open Channel Flows: Analytical Approach for the Outer Region

AbstractThis paper presents an integration procedure for the Reynolds-averaged Navier-Stokes equations for the determination of the distribution of the streamwise velocity using the vertical component. This procedure is dedicated to the outer region and central part of channels. The proposed model is applicable to both rough and smooth flow regimes, provided the velocity at the inner-outer boundary has been properly defined. To generate a simplified expansion, a number of hypotheses are proposed, focusing in particular on the analytical modeling of the vertical component by adopting a negligible viscosity. The proposed hypotheses are validated by the experimental data existing in the literature. The proposed simplified expansion is studied through a sensitivity analysis and proved consistent in regards to model experimental data. The proposed model seems capable of demonstrating different kinds of flows, including dip phenomenon flow patterns.

Combined effect of capillary barrier and layered slope on water, solute and nanoparticle transfer in an unsaturated soil at lysimeter scale

It is well recognized that colloidal nanoparticles are highly mobile in soils and can facilitate the transport of contaminants through the vadose zone. This work presents the combined effect of the capillary barrier and soil layer slope on the transport of water, bromide and nanoparticles through an unsaturated soil. Experiments were performed in a lysimeter (1×1×1.6m(3)) called LUGH (Lysimeter for Urban Groundwater Hydrology). The LUGH has 15 outputs that identify the temporal and spatial evolution of water flow, solute flux and nanoparticles in relation to the soil surface conditions and the 3D system configuration. Two different soil structures were set up in the lysimeter. The first structure comprises a layer of sand (0-0.2cm, in diameter) 35cm thick placed horizontally above a layer of bimodal mixture also 35cm thick to create a capillary barrier at the interface between the sand and bimodal material. The bimodal material is composed of a mixture 50% by weight of sand and gravel (0.4-1.1cm, in diameter). The second structure, using the same amount of sand and bimodal mixture as the first structure represents an interface with a 25% slope. A 3D numerical model based on Richards equation for flow and the convection dispersion equations coupled with a mechanical module for nanoparticle trapping was developed. The results showed that under the effect of the capillary barrier, water accumulated at the interface of the two materials. The sloped structure deflects flow in contrast to the structure with zero slope. Approximately 80% of nanoparticles are retained in the lysimeter, with a greater retention at the interface of two materials. Finally, the model makes a good reproduction of physical mechanisms observed and appears to be a useful tool for identifying key processes leading to a better understanding of the effect of capillary barrier on nanoparticle transfer in an unsaturated heterogeneous soil.

Storm water retention and actual evapotranspiration performances of experimental green roofs in French oceanic climate

Green roofs are promising urban management tools from the standpoint of both rainwater management and microclimatology. They are considered as a storm water mitigation technique and may also favour evapotranspiration fluxes, which can be beneficial for urban comfort during summer periods. In France, however, water retention performance of green roofs remains unknown, and published values are often unsuitable. Six experimental roofs, including two thicknesses of growing media, three types of vegetation cover and bare surfaces, were monitored for two years in Nantes and compared to an experimental gravel flat roof. The thickest media combined with the most densely vegetated cover yields the best results in terms of storm water mitigation and actual evapotranspiration. In winter, the rainwater retention performance is clearly dependent on the type of experimental roof vegetation. This kind of experimental set-up is well suited to assisting urban planners design tools for storm water mitigation in buildings.

Ecological Engineering Approaches to Improve Hydraulic Properties of Infiltration Basins Designed for Groundwater Recharge

Infiltration systems are increasingly used in urban areas for groundwater recharge. The reduction of sediment permeability by physical and/or biological processes is a major problem in management of infiltration systems often requiring expensive engineering operations for hydraulic performance maintenance. To reduce these costs and for the sake of sustainable development, we proposed to evaluate the ability of ecological engineering approaches to reduce the biological clogging of infiltration basins. A 36-day field-scale experiment using enclosures was performed to test the influences of abiotic (light reduction by shading) and biotic (introduction of the macrophyte Vallisneria spiralis (L.) or the gastropod Viviparus viviparus (Linnaeus, 1758)) treatments to limit benthic biofilm biomass and to maintain or even increase hydraulic performances. We coupled biological characterization of sediment (algal biomass, bacterial abundance, total organic carbon, total nitrogen, microbial enzymatic activity, photosynthetic activity, and photosystem II efficiency) with hydraulic conductivity measurements to assess the effects of treatments on sediment permeability. The grazer Viviparus viviparus significantly reduced benthic biofilm biomass and enhanced hydraulic conductivity. The other treatments did not produce significant changes in hydraulic conductivity although Vallisneria spiralis affected photosynthetic activity of biofilm. Finally, our results obtained with Viviparus viviparus are promising for the development of ecological engineering solutions to prevent biological fouling in infiltration systems.

Reduction of feasible parameter space of the inverted soil hydraulic parameter sets for Kosugi model.

Abstract Effective soil hydraulic parameters of soil vegetation atmosphere transfer (SVAT) models can be derived in a cost-efficient way by inverse modeling. Nevertheless, a serious drawback of SVAT models based on Richards’ equation is that they require as many as five unexploited correlated hydraulic parameters. To reduce the feasible parameter space, we propose a method to prevent nonphysical combinations of soil hydraulic parameter sets obtained by optimization. We adopt the soil hydraulic analytical model by Kosugi because it enables the feasible parameter space to be reduced by predicting parameter &sgr; from Rm, which are the variance and mean of the log-transformed soil pore radius, respectively. To further decrease the parameter space, we derive two models to predict saturated hydraulic conductivity, Ks, from three or four Kosugi soil water retention parameters, respectively. These two models are based on the combination of the Hagen-Poiseuille and Darcy equations that use three semiempirical parameters (&tgr;1, &tgr;2, and &tgr;3) calibrated on large UNSODA and HYPRES databases. Our derived models are compared with a version of the Mishra and Parker (1990. Ground Water. 28:775–777) Ks model being modified to account for the parameters of Kosugi’s relationships. The results show that the uncertainties of the developed Ks model are comparable to the uncertainties of Ks measurements. Moreover, the developed Ks model outperforms the Mishra and Parker model. Therefore, the developed method will enable one to substantially reduce the feasible range of the inverted Kosugi’s hydraulic parameters.

Impacts of thinning of a Mediterranean oak forest on soil properties influencing water infiltration

Abstract In Mediterranean ecosystems, special attention needs to be paid to forest–water relationships due to water scarcity. In this context, Adaptive Forest Management (AFM) has the objective to establish how forest resources have to be managed with regards to the efficient use of water, which needs maintaining healthy soil properties even after disturbance. The main objective of this investigation was to understand the effect of one of the AFM methods, namely forest thinning, on soil hydraulic properties. At this aim, soil hydraulic characterization was performed on two contiguous Mediterranean oak forest plots, one of them thinned to reduce the forest density from 861 to 414 tree per ha. Three years after the intervention, thinning had not affected soil water permeability of the studied plots. Both ponding and tension infiltration runs yielded not significantly different saturated, Ks, and unsaturated, K−20, hydraulic conductivity values at the thinned and control plots. Therefore, thinning had no an adverse effect on vertical water fluxes at the soil surface. Mean Ks values estimated with the ponded ring infiltrometer were two orders of magnitude higher than K−20 values estimated with the minidisk infiltrometer, revealing probably soil structure with macropores and fractures. The input of hydrophobic organic matter, as a consequence of the addition of plant residues after the thinning treatment, resulted in slight differences in terms of both water drop penetration time, WDPT, and the index of water repellency, R, between thinned and control plots. Soil water repellency only affected unsaturated soil hydraulic conductivity measurements. Moreover, K−20 values showed a negative correlation with both WDPT and R, whereas Ks values did not, revealing that the soil hydrophobic behavior has no impact on saturated hydraulic conductivity.