Evaluation of the EUROSEM model for predicting the effects of erosion-control blankets on runoff and interrill soil erosion by water

The European Soil Erosion Model (EUROSEM, Morgan et al., 1998) is an event-based soil erosion model which predicts runoff and sediment discharge for different environmental conditions. Applying geotextiles or erosion-control blankets (ECB’s) on the soil surface significantly affects surface seal formation and topsoil properties and therefore controls runoff and soil erosion rates during a rainfall event. Since these within-storm changes of soil surface characteristics and hydrological conditions are not incorporated in EUROSEM, errors in runoff and soil erosion predictions may occur for soil surfaces covered with ECB’s.Therefore, the objective of this paper is to evaluate and improve the performance of a research version of the physically-based erosion model EUROSEM (EUROSEM-2010; Borselli and Torri, 2010) for simulating the effects of ECB’s on runoff and interrill soil erosion by water during intense simulated rainfall events. Results of model simulations are compared with experimental results of interrill erosion using biological (i.e. natural) ECB’s and simulated rainfall. Because ECB’s applied on the soil surface retard seal formation, the differences between observed and predicted runoff rates and sediment discharges are rather high during the first 20-30 min of the simulated rainstorm. Therefore, a simple approach is proposed to cope with the dynamic evolution of some soil characteristics, i.e. saturated hydraulic conductivity, soil erodibility and soil cohesion, during an intense rainfall event. This time-dependent approach improves the predictions of runoff rate and sediment discharge during the first 20-30 min of a rainfall event and increases the model efficiency (i.e. a measure for the goodness of fit) from 0.84 to 0.98 and from 0.48 to 0.68 for the total runoff volume and soil loss, respectively. For most conditions, the predicted final sediment discharge is still considerably larger than the observed values, which can be partly attributed to the deposition of sediment in the bare soil patches (i.e. inter-weave open areas) of the ECB’s, which is not simulated by EUROSEM in this study. This model approach increases our understanding of the effects of ECB’s on within-storm changes in hydrological conditions and soil surface characteristics.     

Investigating the mechanism of downslope bentonite erosion in GCL liners using X-Ray CT

Exposed composite GMB-GCL liners are at risk of downslope bentonite erosion caused by the release of low ionic strength condensed water onto the top surface of the GCL following daily solar heating. This paper investigates the use of X-ray computed tomography (X-ray CT) to quantify the thinning of the bentonite layer and the application of X-ray diffraction techniques (XRD) to investigate the changes in clay chemistry (if any) of the bentonite from the virgin GCL to the eroded bentonite. The effect of specimen size and scanning orientation was investigated resulting in a revised testing procedure in which the CT scanning orientation was changed from horizontal to vertical to permit a longer test specimen which was also sealed at the bottom edge to minimise the edge boundary condition. The X-ray CT results provide highly visual evidence that a) bentonite thinning immediately under the upper cover geotextile is the initial location of erosion, and b) the bentonite core erodes at a significantly higher rate when not covered by a geotextile than when covered by a geotextile. These observations indicate that the upper geotextile of the GCL plays a significant role in controlling the rate of bentonite erosion. Finally, a comparison of the virgin and runoff bentonite properties was conducted to investigate potential changes in swell index, X-ray diffraction results, and concentration of Na and Ca cations. The runoff bentonite was observed to had a significantly higher swell index (40?ml/2?g) than the virgin bentonite (28?ml/2?g) and lower Na and Ca concentrations. This finding is consistent with the observation from XRD analyses of the runoff bentonite which illustrate that the clay fraction of the bentonite is preferentially eroded by the application of DI water.     

Laboratory testing for evaluation of the influence of a small degree of internal erosion on deformation and stiffness

Suffusion is a type of internal erosion, namely, the transportation of soil driven by the seepage force of finer fractions within coarser fractions. The present work uses triaxial confining compression tests to study how a small degree of suffusion affects soil stiffness and deformation. This approach allows the suffusion inside a specimen to impose a downward seepage flow. Although a small degree of suffusion is not detectable, it can lead to the deterioration of earth structures. Linear displacement transducers and clip gages were attached to specimens to accurately measure the localized strain rates. In addition, the turbidity of the discharged water was evaluated. Following the downward seepage, each drained specimen was monotonically compressed. The experimental results show that cohesive soil undergoes suffusion and that most of the clay fractions bonding the sand particles erode. At intermediate strain, the deviator stress decreases in proportion to the degree of suffusion, but this decrease ceases at the critical state. Anisotropic behavior is observed and is tentatively attributed to the disruption of the soil texture, which is qualitatively monitored by scanning electron microscopy. Finally, a simple formula is proposed for evaluating the decrease in stiffness due to suffusion.     

Integrated tool for risk assessment in agricultural management of soil erosion and losses of phosphorus and nitrogen

In recent years, increased attention has been focused on models for risk assessment of source areas in agricultural landscapes. Among the simplest of such models are index tools, which have been developed particularly for phosphorus (P) and to some extent nitrogen (N). However, only a few studies have considered the development of an integrated management strategy that includes erosion and losses of both P and N. Accordingly, the major objective of this study was to initiate the development of an integrated risk assessment tool, consisting of indices for erosion, P and N. The strategy used to create the integrated tool was based on the assumption that all input data at field scale should be readily available either from ordinary agricultural statistics or from the farmer. The results from using the indices in a pilot case study catchment illustrated that losses of P and N had often different critical source areas. The P index was highest for fields with manure application and/or high soil P status or with autumn ploughing, and the N index was highest for fields with excessive N application. The integrated risk was greatest for areas with manure application and some areas with a high erosion risk in combination with high nutrient application rate. Additionally, four different management options were assessed: (1) reduced fertilisation, (2) catch crops, (3) autumn ploughing, and (4) no autumn ploughing. The results verified that reduced nutrient application and stubble during autumn and winter led to the largest decrease in index values, and it was also apparent that management changes in high-risk areas had the greatest impact on the indices. Overall, our findings indicate that the present integrated risk assessment tool with readily available input data can be used to rank farm fields according to risk of soil erosion and losses of P and N.     

Inverse analysis of laboratory data and observations for evaluation of backward erosion piping process

An inverse analysis procedure has been developed to interpret collected pore pressure data and observations during backward erosion piping (BEP) initiation and progression in sandy soils. The procedure has been applied to laboratory models designed to mimic the initiation and progression of BEP through a constricted vertical outlet. The inverse analysis uses three-dimensional (3D) finite element method (FEM) to successively produce models of the hydraulic head regime surrounding progressive stages of BEP based on observations at the sample surface and pore pressure measurements obtained from the laboratory models. The inverse analysis results in a series of 3D contour plots that represent the hydraulic-head regime at each stage of the BEP development, allowing for assessing the development of BEP mechanism as well as calculating the critical hydraulic conditions required for various BEP stages to initiate and progress. Interpretation of the results identified four significant stages of the piping process: (1) loosened zone initiation, (2) channel initiation and progression, (3) riser sand fluidization, and (4) loosened zone progression. Interpretation of the hydraulic head contour plots allows assessment of the critical hydraulic gradients needed to initiate and progress various components of the BEP development.     

Experimental mapping of elastoplastic surfaces for sand using undrained perturbations

Elastoplastic models are commonly used in modern geotechnical practice to numerically predict displacements, stresses, and pore pressures in large construction projects. These elastoplastic models use presumed functional forms for yield and plastic potential functions that are rarely obtained from experimental measurements. This research describes a simple experimental technique that can be used to obtain the slopes of the plastic potential and yield functions during shear based on the deformation theory of plasticity. The method imposes small perturbations in the direction of the stress increment by closing the drainage valve, thereby abruptly switching from drained to undrained loading conditions during plastic loading. Elastoplastic moduli are obtained immediately before and after the perturbations from the measured deviatoric stress, mean effective stress, deviatoric strains, and volumetric strains for the stress paths immediately before and immediately after closing the drain valve. During drained shear, samples were sheared while the mean effective stress was maintained constant. Combining tests performed at several confining stresses, the proposed method was able to map conventional isotropic yield and plastic potential surfaces and predict their evolution for a wide range of stresses. The proposed technique can also be used for kinematic yield surfaces and to develop new and more accurate elastoplastic constitutive models.     

Experimental study on mitigating wind erosion of calcareous desert sand using spray method for microbially induced calcium carbonate precipitation

Wind erosion is one of the significant natural calamities worldwide, which degrades around one-third of global land. The eroded and suspended soil particles in the environment may cause health hazards, i.e.allergies and respiratory diseases, due to the presence of harmful contaminants, bacteria, and pollens.The present study evaluates the feasibility of microbially induced calcium carbonate precipitation(MICP)technique to mitigate wind-induced erosion of calcareous desert sand(Thar desert of Raj...     

A pilot study of constructed wetlands using duckweed (Lemna gibba L.) for treatment of domestic primary effluent in Israel

Constructed wetlands are well known as highly efficient system to treat wastewater from different sources. This treatment system is cost-effective for reuse in desert areas. A continuous flow, free water surface (FWS) pilot wetland using the duckweed plant Lemna gibba L. was constructed at the Blaustein Institute for Desert Research in Kiryat Sde Boker of the Negev, Israel, and operated on domestic primary effluents. Water quality and system efficiency were observed during the experiment for reuse purposes. Results indicated that, hydraulic residence time averaged 4.26+/-0.61d, average influent flow rate was 0.234+/-0.027m(3)/d and hydraulic load 0.22+/-0.03m/d. Hydraulic efficiency in the system was high and allowed good settling conditions. Suspended solids and organic matter removals were the highest and effluent concentrations were 13.1+/-9.7 and 40.3+/-11.9mg/l for TSS and total BOD(5), respectively. Nitrogen removal was lower (10-20%) but slightly increased with higher nitrogen loads. Therefore, nitrogen content in the plants was high (4.3+/-0.5%/kg dry plant). Phosphorus removal was negligible. High removal for fecal coliform (approximately 95%) and effluent turbidity (> 50%) were also observed.