Mohamed Elhakeem

Investigating the Role of Clasts on the Movement of Sand in Gravel Bed Rivers

The bed morphology of mountain rivers is characterized primarily by the presence of distinguishable isolated roughness elements, such boulders or clasts. The objective of this experimental study was to provide a unique insight into the role of an array of clasts in regulating sand movement over gravel beds for low relative submergence conditions, H/dc<1, and flow depth, H, to the diameter of the clast, dc, a process that has not been studied thoroughly. To assess the role of clasts in controlling incoming sand movement, detailed flume experiments were conducted by placing 40 equally spaced clasts atop a well-packed glass bead bed for replicating the isolated roughness flow regime. The experiments were performed for moderate (∼2.50τcr* where τcr* is the critical dimensionless bed shear stress) and high (∼5.50τcr*) applied bed shear stress conditions, representative of gravel bed rivers. For comparison purposes, experiments were also repeated for nearly identical flow conditions but without the presence of ...

Calibration and Verification of a 2D Hydrodynamic Model for Simulating Flow around Emergent Bendway Weir Structures

The predictive capability of a two-dimensional (2D)-hydrodynamic model, the finite-element surface water modeling system (FESWMS), to describe adequately the flow characteristics around emergent bendway weir structures was evaluated. To examine FESWMS predictive capability, a sensitivity analysis was performed to identify the flow conditions and locations within the modeled reach, where FESWMS inputs for Manning’s n and eddy viscosity must be spatially distributed for to better represent the river bed flow roughness characteristics and regions where the flow is highly turbulent in nature. The sensitivity analysis showed that high flow conditions masked the impact of Manning’s n and eddy viscosity on the model outputs. Therefore, the model was calibrated under low flow conditions when the structures were emergent and had the largest impact on the flow pattern and model inputs. Detailed field measurements were performed under low flow conditions at the Raccoon River, Iowa for model calibration and verificat...

Evaluation of the reduction in the water storage capacity of Black Lake, AK

Abstract The paper provides a systematic evaluation of the causes that trigger reduction of a lakes water storage volume. This evaluation is demonstrated herein through combined numerical analysis and field work conducted on the south side of the Alaska Peninsula to address the reduction in the water storage volume of Black Lake. The contribution of this research was the development of an integrated approach that adequately quantifies the causes of reduction of water storage volume of lakes. For this purpose, a set of established routing, erosion, riverine, and lake models were integrated to identify quantitatively the causes of the Black Lake water storage reduction. Selection of the models was a compromise between their complexity, compatibility, capability, and available input data. The results of this study suggest that contrary to what has been long thought the lake receives sufficient water influx to maintain its maximum storage capacity. In addition, sedimentation was not found to the cause, because siltation since 1950 has reduced the storage capacity by only 1.0%. The reduction in the lake storage was mainly attributed to severe degradation at the lake outlet. The outlet elevation of the lake has decreased by at least 1.0 m over the past 55 years. This study recommended that a gated weir could be used as a control structure to maintain the elevation at the lakes outlet. In addition, an erosion model predicted that the lake may loose 80% of its current storage capacity within a 100‐year period causing a complete destruction of the ecosystem, if no action is taken.

Density Functions for Entrainment and Deposition Rates of Uniform Sediment

A model has been developed for the prediction of the density functions of bed-elevation and entrainment and deposition rates of sediment in sand bed streams within the lower regime flow condition. The model incorporates both statistical and deterministic parameters in its form. A total of 46 experimental runs have been carried out in a recirculating tilting flume under the equilibrium flow condition using three grain sizes of uniform gradation to validate the model and estimate its parameters. The model parameters are related to the hydraulic conditions of flow and fluid and sediment properties through dimensional and regression analyses. The study has shown that the density functions of bed elevation and entrainment and deposition rates can be approximated quite satisfactorily with the normal distribution curve. Transformation of the density functions into the standardized normal distribution curve provides a unique pattern for all the experimental runs regardless of the sediment grain size, flow condition, and shapes and dimensions of the bed forms. The developed density functions have been utilized to provide a closure for the probabilistic Exner equation for uniform sediment.

Evaluation of a gravel transport sensor for bed load measurements in natural flows

Abstract A recent acoustic instrument (Gravel Transport Sensor, GTS) was tested for predicting sediment transport rate (bed load rate) in gravel bed streams. The GTS operation is based on the particle collision theory of submerged obstacles in fluids. When particles collide with the GTS cylinder their momentum is recorded in the form of ping rates. The GTS is attractive for further consideration here because of its potential to provide continuous unattended local bed load measurements, especially in areas found in streams that access may be difficult under extreme conditions. Laboratory experiments coupled with numerical simulations for the same flow conditions were performed in order to determine the conditions under which particles of different size will hit the GTS cylinder and be able to register a ping rate. The GTS was able to detect the number of particles with diameter in the range of 15.9 to 25.4 mm, with reasonable accuracy, if the applied Shields effective stress τ*e = τ* − τ*cr was in the range of 0.006 to 0.015. A drawback of the tested prototype GTS, however, was that it exerted increased resistance on the incoming particles. The added drag effects increased the overall resistance that was exerted by the flow on particles and thus increased the likelihood that particles will rest in the ambient region of the cylinder instead of hitting it. Numerical simulation of the flow around the GTS cylinder revealed that changing the prototype geometry from cylindrical to ellipsoid or rhomboid will increase the likelihood of the particles hitting the instrument under the same flow conditions failed by the original tested GTS cylinder.

Implementing streambank erosion control measures in meandering streams: design procedure enhanced with numerical modelling

ABSTRACT Dike structures have been used in many streams to control streambank erosion by redirecting the flow away from the banks towards the centre of the channel. In this study, the 2D finite-element surface water modelling system (FESWMS) was used to enhance the design of dike structures in two meandering stream reaches along the Raccoon River, Iowa, USA. FESWMS was used to find the optimal number and spacing between the dikes and to access their overall performance in controlling river-bank erosion in the study reaches. The study included also field measurements for model calibration and verification. The model results showed that the proposed dike structures, consisting of alternate five bendway weirs and four spurs at each site with a spacing of about 50 m, successfully reduced the flow velocity along the outside bank and increased the conveyance of the flow to the stream centre. The average lengths of the spurs and bendway weirs were about 16 and 32 m, respectively. The estimated velocity and Froude number values along the outside riverbank where the dikes were constructed were less than 1.0 m/s and 0.3, respectively, which are within the recommended values for erodible channel stability design. Site visits one year after the dike construction showed that the dikes were successful in controlling streambank erosion by allowing incoming sediment to deposit between the dikes and along the bank line. Sediment transport measurements at the study sites showed that the maximum deposition was about 0.8 m, which is much higher than a typical averaged deposition value of about 0.3 m/year. Further, recent Google Earth pictures for the sites showed that the bank slope lines almost recovered back in their original profiles and that the dikes became part of the restored banks, a strong indication of the success of the proposed dikes design.

Explicit Solution for the Specific Flow Depths in Partially Filled Pipes

AbstractThis paper presents an explicit solution for the specific flow depths in partially filled pipes of circular cross-sectional area. Four depths encounter in most classical free-surface flow p...

Bedload Model for Nonuniform Sediment

AbstractIn this study, a bedload model for nonuniform sediment is developed considering the density function of sediment entrainment with no active layer. The model is formulated based on physical considerations, dimensional reasoning, regression analysis, and laboratory data. Four parameters are incorporated in the model formulation, namely the Shields stress τ*g′, the critical Shields stress τ*cg, the Kramer coefficient of uniformity cu, and the relative grain size (size of the jth grain fraction to the geometric mean size, dj/dg). The model collectively accounts for the hiding effects and the variation of the transported bedload material fractions from the surface and subsurface material. The findings of the study show that at low values of τ*g′/τ*cg fractions within the range of 0.3<dj/dg<3.0 are present with larger amounts in the transported bedload material than in the substrate, whereas fractions finer or coarser than that range are present with larger amounts in the surface layer than in the subst...

Soil Aquifer Treatment System Design Equation for Organic Micropollutant Removal

Rapid population growth and mass migration from rural to urban centers have contributed to a new era of water sacristy, and a significant drop in per capita freshwater availability, resulting in the reuse of wastewater emerging as a viable alternative. The reuse of wastewater after treatment using the soil aquifer treatment (SAT) has recently gained popularity due to low operating/maintenance cost of the method. However, the presence of organic micropollutants (OMPs) may present a health risk if the SAT is not adequately designed to ensure required attenuation of the OMPs. An important aspect of the design of the SAT system is the large degree of natural variability in the OMP concentrations/loads in the wastewater and the uncertainty associated with the current methods for calculation of the removal efficiency of the SAT for the OMPs. This study presents a novel model for more accurate prediction of the removal efficiency of the SAT system for the OMPs and the fate of the OMPs trapped within the vadose zone. A large data set is compiled covering a broad range of aquifer conditions, and the SAT system parameters, including hydraulic loading rate and dry/wet ratio. This study suggests that removal of OMPs in SAT systems is most affected by biodegradation rate and soil saturated hydraulic conductivity, in addition to dry to wet ratio. This conclusion is reached by the application of the developed prediction model using data sets from the case study SAT systems in Egypt.

Runoff Curve Number and Saturated Hydraulic Conductivity Estimation via Direct Rainfall Simulator Measurements

Surface runoff can be estimated directly from conceptual models such as the runoff curve number (RCN) method or indirectly from physically based infiltration m…