Majdi Abou Najm

New method for the characterization of three-dimensional preferential flow paths in the field

[1]xa0Preferential flow path development in the field is the result of the complex interaction of multiple processes relating to the soils structure, moisture condition, stress level, and biological activity. Visualizing and characterizing the cracking behavior and preferential paths evolution with soil depth has always been a key challenge and a major barrier against scaling up existing hydrologic concepts and models to account for preferential flows. This paper presents a new methodology to quantify soil preferential paths in the field using liquid latex. The evolution of the preferential flow paths at different soil depths and moisture conditions is assessed. Results from different soil series (Savage clay loam soil versus Chalmers clay loam) and different vegetation covers and soil managements (corn/tilled field versus soybean no-till field in the Chalmers soil series) are presented.

Field method for separating the contribution of surface‐connected preferential flow pathways from flow through the soil matrix

[1] Liquid latex was used as a method to seal visible surface-connected preferential flow pathways (PFPs) in the field in an effort to block large surface-connected preferential flow and force water to move through the soil matrix. The proposed approach allows for the quantification of the contribution of large surface-connected cracks and biological pores to infiltration at various soil moisture states. Experiments were conducted in a silty clay loam soil in a field under a no-till corn-soybean rotation planted to corn. Surface intake rates under ponding were measured using a simplified falling head technique under two scenarios: (1) natural soil conditions with unaltered PFPs and (2) similar soil conditions with latex-sealed large macropores at the surface. Results indicated that the contribution of flow from large surface-connected macropores to overall surface intake rates varied from approximately 34% to 99% depending on the initial moisture content and macroporosity present. However, evidence of preferential flow continued to appear in latex-sealed plots, suggesting significant contributions to preferential flow from smaller structural macropores, particularly in two out of four tests where no significant differences were observed between control and latex-sealed plots. Citation: Sanders, E. C., M. R. Abou Najm, R. H. Mohtar, E. Kladivko, and D. Schulze (2012), Field method for separating the contribution of surface-connected preferential flow pathways from flow through the soil matrix, Water Resour. Res., 48, W04534,

Assessing internal stress evolution in unsaturated soils

[1]xa0Internal stresses in soils evolve as a result of the various interactions among soil particles and the pore fluids in response to natural or human-induced activities. Whether attributed to suction, structure, or the various physicochemical forces that may develop, assessing the internal stresses in soils has been an active area of research in soil science and engineering. This paper presents the restrained ring method as an experimental tool for measuring the stresses that develop in a drying soil when it is restrained from shrinking freely. The internal stress that develops can be related to the water content of the soil. This method is validated using results for both silty clay loam and clay loam soils. Tensile stress at failure (estimated from the restrained ring method) was compared to tensile strength ranges of similar soils, and a good correlation was observed. Moreover, internal stresses measured by the restrained ring method were compared with existing empirical relationships (using suction and degree of saturation). This comparison concluded that suction is not the sole contributor to the soils internal stress.

New Methodology for Density and Water Content by Time Domain Reflectometry

AbstractThis paper provides a new methodology for determining soil water content and density under different compaction energies. A new calibration equation was introduced making use of the voltage drop associated with the first passage of the electromagnetic wave through the soil specimen, the final voltage level after wave propagation ceases, and dry density normalized by the density of water. The new calibration equation allows for the direct calculation of dry density. Values of dry density are then used in the normalized apparent dielectric constant equation to determine the water content. Results of numerous laboratory tests on a wide variety of soils (including coarse-grained soils) showed that the proposed method accommodates different levels of compaction energy. The method also was validated with different field tests at sites commonly used in geotechnical earthwork construction, using soil-specific model coefficients determined from laboratory compaction tests. The results of this study indicat...

AIR QUALITY ASSESSMENT AT A CONGESTED URBAN INTERSECTION

The deficient transportation system of Beirut results in significant economic losses for the city and causes severe traffic congestion in the urban areas. Proposals have been made for grade separations at some of the worst congested intersections. The major concern at these intersections is air quality which greatly depends on the geometric configuration of an intersection. The air quality impact assessment at a typical intersection is presented and potential mitigation strategies for air quality management in urban areas are addressed. Air quality measurements were conducted at representative locations to define existing pollution exposure levels. Also performed were mathematical simulations for several scenarios, both with and without grade separations, changes in vehicle mix, and level of service. By comparing simulated exposure levels with air quality standards, assessment of air quality impact was obtained.

Modeling multidomain hydraulic properties of shrink‐swell soils

Shrink-swell soils crack and become compacted as they dry, changing properties such as bulk density and hydraulic conductivity. Multidomain models divide soil into independent realms that allow soil cracks to be incorporated into classical flow and transport models. Incongruously, most applications of multidomain models assume that the porosity distributions, bulk density, and effective saturated hydraulic conductivity of the soil are constant. This study builds on a recently derived soil shrinkage model to develop a new multidomain, dual-permeability model that can accurately predict variations in soil hydraulic properties due to dynamic changes in crack size and connectivity. The model only requires estimates of soil gravimetric water content and a minimal set of parameters, all of which can be determined using laboratory and/or field measurements. We apply the model to eight clayey soils, and demonstrate its ability to quantify variations in volumetric water content (as can be determined during measurement of a soil water characteristic curve) and transient saturated hydraulic conductivity, Ks (as can be measured using infiltration tests). The proposed model is able to capture observed variations in Ks of one to more than two orders of magnitude. In contrast, other dual-permeability models assume that Ks is constant, resulting in the potential for large error when predicting water movement through shrink-swell soils. Overall, the multidomain model presented here successfully quantifies fluctuations in the hydraulic properties of shrink-swell soil matrices, and are suitable for use in physical flow and transport models based on Darcys Law, the Richards Equation, and the advection-dispersion equation.

SUSTAINABILITY OF BASIN LEVEL DEVELOPMENT UNDER A CHANGING CLIMATE

Author(s): Alameddine, I; Fayyad, A; Abou Najm, M; El-Fadel, M | Abstract:

Comparative assessment of joint water development initiatives in the Jordan River Basin

ABSTRACT This paper presents a comparative assessment of developmental initiatives to increase regional water supply in the Jordan River Basin using a Multi Criteria Decision Analysis technique with sensitivity analysis on criteria weights. The results indicate that the Red Sea–Dead Sea Conveyance (RSDSC) constitutes an optimal option within the context of alleviating water shortage and promoting regional cooperation. While a surface water pipeline from Turkey emerges as an attractive option when the quantity of delivered water is emphasized, the potential for out of basin water transfers are becoming increasingly less desirable with population growth, political turmoil in the region, development, and climate change challenges.

Temperature Corrections for Time Domain Reflectometry Parameters

AbstractThe waveforms from time domain reflectometry (TDR) tests on soil depend on the temperature of the soil being tested. This study provides a discussion of, and the basis for, temperature corrections for the parameters measured in the TDR test for use in calculating water content and dry density of soil. This paper is a companion to one that provides the basis for an updated ASTM Standard, but does not cover temperature effects. A brief synopsis of the new methodology is provided herein to identify parameters that are sensitive to temperature of the soil being tested.

Improving TDR Measurements through Accounting for Soil Shrinkage Properties

Time domain reflectometry (TDR) is a widely used tool for indirect measurement of soil moisture content. Empirical formulation is used to link the apparent dielectric constant of soil to the volumetric moisture content (Topp’s Equation: Topp et al., 1980) or gravimetric moisture content as a function of soil bulk density (Siddiqui and Drnevich Equation: Siddiqui and Drnevich, 1995). This paper introduces a methodology to account for soil volume change by integrating the true bulk density of the soil into the measurements using the soil Shrinkage Curve (specific volume (cm3/g) versus gravimetric water content). Thus, bulk density becomes a non-constant parameter that can be calculated as a function of the soil water content in the Siddiqui and Drnevich Equation. Experimental evidence demonstrates accounting for soil shrinkage improves the accuracy of TDR measured moisture contents and allows for estimating the shrinkage curve. Direct water content calculation for the Chalmers soil was compared to water contents from TDR readings with and without shrinkage corrections; those with shrinkage corrections showed significantly improved accuracy in TDR-determined soil moisture.