Hydrology and geomorphology of the Upper White Nile lakes and their relevance for water resources management in the Nile basin

Remote sensing data and digital elevation models were utilized to extract the catchment hydrological parameters and to delineate storage areas for the Ugandan Equatorial Lakes region. Available rainfall/discharge data are integrated with these morphometric data to construct a hydrological model that simulates the water balance of the different interconnected basins and enables the impact of potential management options to be examined. The total annual discharges of the basins are generally very low (less than 7% of the total annual rainfall). The basin of the shallow (5 m deep) Lake Kioga makes only a minor hydrological contribution compared with other Equatorial Lakes, because most of the overflow from Lake Victoria basin into Lake Kioga is lost by evaporation and evapotranspiration. The discharge from Lake Kioga could be significantly increased by draining the swamps through dredging and deepening certain channel reaches. Development of hydropower dams on the Equatorial Lakes will have an adverse impact on the annual water discharge downstream, including the occasional reduction of flow required for filling up to designed storage capacities and permanently increasing the surface areas of water that is exposed to evaporation. On the basis of modelling studies, alternative sites are proposed for hydropower development and water storage schemes. Copyright © 2012 John Wiley & Sons, Ltd.     

Ground penetrating radar and 2D electric resistivity studies for tracing hydrocarbon leakage site,close to Abha City: a case study

This work presents the application of the ground-penetrating radar (GPR) method and electric resistivity tomography (ERT) technique in outlining a zone of contamination due to the light non-aqueous phase liquid (LNAPL) plume underground in the area of an impacted fuel station, close to Abha City. The GPR has been performed using SIR3000 unit with the 100 and 400 MHz antennas. The main objective of the GPR survey was to evaluate the lateral extension of contamination. The complex GPR signature of the plume was well characterized. Low reflectivity zone corresponds to hydrocarbon vapor phase in the vadose zone. Enhanced reflections are associated with free and residual products in the fractured saturated zone directly above the water table. An electric resistivity tomography (ERT) survey was performed on four profiles within the site to investigate the vertical and horizontal extent of the contamination plume and to define the bottom of the landfill. The 2D electric profiles show the presence of low-resistivity (4O to 37 Ω m) anomalies that refers to the presence of accumulated hydrocarbons. From the interpretation of the GPR and ERT profile, it was possible to locate the top and bottom of the contamination plume of the waste disposal site. The radar signal penetrated deep enough and enabled the identification of a second reflector at approximately 10-m deep, interpreted as the hard basement surface which causes the strong amplitude reflection in the GPR profile. The results of GPR and ERT showed good agreement.     

Effect of clayey media parameters on the negative response of a coincident loop1

The aim of this work is to consider the phenomenon of negative response and its detectability in a polarizable half-space formed of Clay-water mixture which commonly exists in water aquifers and which can be used as a simple indicator of the presence of Underground water. We make use of an analytical treatment for the transient voltage induced in a coincident loop lying on a half-space as a basis for our computations. A Cole-Cole model is used to represent the Clay with appropriate parameters. As saturated Clay is characterized by high conductivity and moderate chargeability, it is hardly detectable relative to the practical noise level. However, there are optimum Clay parameters that offer a maximum negative response. It is also observed that for a very short time constant τ in the Cole-Cole model, the amplitude of the negative response decreases with the decrease of the time constant due to the fast decay of the polarization current. Finally, it is found that there is an optimum value for the loop radius which gives the largest value of the negative response and this loop radius depends on the model parameters.     

Multiphase Extraction of Light Non-aqueous Phase Liquid (LNAPL) Using Prefabricated Vertical Wells

Work in this paper describes the results of a field and modeling study conducted to investigate the extraction of LNAPL (jet propellant 4, i.e., JP-4, jet fuel) using Prefabricated Vertical Wells (PVWs) at a former air force base. Field testing consisted of 185 operating hours on 25 rows, each consisting of 7 or 8 PVWs. A total of 133 L of free-phase liquid is removed from the subsurface in addition to 467 kg of organics in the vapor phase as a result of volatilization due to the application of vacuum heads exceeding the JP-4 vapor pressure. Monitored field data are used to calibrate a numerical model for investigating the effect of irreducible water content, LNAPL, and gas contents on system performance. Analysis results show that increasing the irreducible water content from 5 % to 20 % decreases the free-LNAPL specific volume from 0.08 m to 0.067 m, which led to reduction in removal efficiency. Under a continuous saturation condition, more wetting fluid (water), traps the nonwetting fluid (LNAPL) in the soil pores, and reduces its mobility, which eventually impedes the LNAPL recovery. Given the model parameters that simulate field conditions, the PVWs show best removal rates in the gas phase. The model results indicated that 292 L of JP-4 can be recovered in 19.5 days (based on 8 h of operation per day) with an effective rate of 1.87 L/h. At the end of 4.25 years of system operation, the maximum benzene concentration is 0.002 ppm. At a gas extraction rate of 10,273 L/min, a maximum concentration of 0.38 ppm in the vapor phase is achieved after 2.5 years of vapor extraction.     

Geophysical surveys to investigate the relation between the Quaternary Nile channels and the Messinian Nile canyon at East Nile Delta, Egypt

This paper compares the pattern of historical Eastern Nile Delta branches, delineated by geoelectric resistivity investigation, with the path of the Nile canyon of Messinian time revealed in seismic reflection. A close correlation is evident between the Late Quaternary Nile River channels and the Messinian canyons (wadies) in the eastern Nile Delta. This indicates that the Messinian canyons were a persistent topographic (or geomorphic) feature until historical branches. The results may also have implications for long-term patterns of subsidence in the delta area.     

Stress-strain analysis of geogrid-supported liners over subsurface cavities

Summary Finite element analyses were conducted to investigate the magnitude of tensile strains imposed on landfill liners due to the formation of subsurface cavities. The study incorporated the significance of using geogrids to reduce the magnitude of strains and possibly the potential for collapse of landfill liners. Variations of key parameters included depth of overburden (D) and diameter of the cavity (B). Estimated stress distributions were compared to theoretical values obtained from a model reported in the literature. Results indicated that, contrary to conventional wisdom, the critical area based on the mechanics of arching was above the edge of the cavity where stress concentration occurred. Incorporation of geogrid reinforcement reduced the magnitude of tensile strains. The tensile force in the geogrid was dependent upon the size of the cavity, the depth of the overburden, and the applied pressure.     

Repeated Load Triaxial Testing of Recycled Concrete Aggregate for Pavement Base Construction

This paper presents the results and analysis of Repeated Load Triaxial Testing (RLTT) conducted on three basecourse products; two crushed concrete, or Recycled Concrete Aggregate (RCA) materials, and a local Virgin Aggregate (VA). The objective of this testing program was to study the impact of applied stress states on resilient modulus and permanent deformation of the materials using the AUSTROADS RLTT approach (Vuong and Brimble 2000). A series of RLTT was performed under drained conditions on cylindrical specimens statically compacted at different levels of initial moisture content. The RLTT specimens were tested at 60, 80 and 90 % of Optimum Moisture Content (OMC) and to a single target dry density ratio of 98 % of Maximum Dry Density from Modified Proctor compaction testing. Duplicate specimens were tested for each moisture level. On sample measurements were made of sample deformation. It was found that the resilient moduli of the two RCA products ranged between 340 and 715 MPa, while corresponding moduli from tests on VA varied from 270 to 450 MPa. Resilient modulus was found to be dependent on both moisture content and applied stress. The two specimens of VA prepared at 90 % OMC failed during the permanent strain testing stages of the AUSTROADS test protocol. Moreover the recycled materials could be used as basecourses according to interpretation of the AUSTROADS RLTT data by Vuong and Arnold (2006).     

Experimental Evaluation of Recycled Aggregate Porous Concrete Piles for Soft Ground Improvement

The use of sand compaction pile or gravel compaction pile is nowadays a common approach for soft ground improvement. In this article, a recycled aggregate porous concrete pile has been developed by replacing natural aggregates with recycled aggregates to overcome issues related to bulging failure or reduced section geometries. Such issues may arise during installation and during the early stages of operation. In addition, the proposed approach utilizes recycled aggregates instead of natural materials. To investigate the applicability of the recycled aggregate porous concrete pile method as a ground improvement technique, a series of laboratory model consolidation tests was performed on soft clay soil reinforced with sand compaction pile, gravel compaction pile, and recycled aggregate porous concrete pile, respectively. The results indicated that the settlement reduction effect of recycled aggregate porous concrete pile was significantly higher than the sand compaction pile and gravel compaction pile methods. The stress sharing ratio from the experimental program showed good agreement with those calculated by elasticity theory. Comparative analyses of the recycled aggregate porous concrete pile versus sand compaction pile and gravel compaction pile approaches, under the same replacement area ratio and surcharge pressure, showed significantly improved consolidation time, settlement reduction, and stress sharing effect.     

Granitoid evolution in Sinai,Egypt, based on precise SHRIMP U–Pb zircon geochronology

Late-stage Pan-African granitoids, including monzogranite, syenogranite and alkali granite, were collected from four separate localities in Sinai. They were selected to represent both the calc-alkaline and alkaline suites that have been viewed as forming separate magmatic episodes in the Eastern Desert of Egypt, with the transition to alkali granite at ~ 610 Ma taken to mark the onset of crustal extension. Although intrusive relations were observed in the field, the emplacement ages of the granitoids cannot be distinguished within analytical uncertainty and they all formed within a restricted time span from 579 to 594 Ma. This indicates that the two suites are coeval and that some calc-alkaline rocks were also likely generated during the late extensional phase. These ages are identical to those recently obtained from similar rocks in the North-Eastern Desert, confirming that Sinai is the northern extension of the Eastern Desert Pan-African terrane of Egypt. Rare inherited zircons with ages of ~ 1790 and ~ 740 Ma are present in syenogranite from northeastern Sinai and indicate that older material is present within the basement. A few zircons record younger ages and, although some may reflect later disturbance of the main zircon population, those with ages of ~ 570 and 535 Ma probably reflect thermal events associated with the extensive emplacement of mafic and felsic dykes in both northeastern and southern Sinai.