Ahmed Kassem

Helical flow couplets in submarine gravity underflows

Active and relic meandering channels are common on the seafloor adjacent to continental margins. These channels and their associated submarine fan deposits are products of the density-driven gravity flows known as turbidity currents. The tie between channel curvature and its effects on these gravity flows has been an enigma. This paper records the results of both large-scale laboratory measurements and a numerical simulation that captures the three-dimensional flow field of a gravity underflow at a channel bend. These findings reveal that channel curvature drives two helical flow cells, one stacked upon the other. The lower cell forms near the channel bed surface and has a circulation pattern similar to that observed in fluvial channels, i.e., with a near-bed flow directed inward. The other circulation cell forms in the upper part of the gravity flow and has a streamwise vorticity with the opposite sense of the lower cell.

Simulation of turbid underflows generated by the plunging of a river

When the density of sediment-laden river water exceeds that of the lake or ocean into which it discharges, the river plunges to the bottom of the receiving water body and continues to flow as a hyperpycnal flow. These particle-laden underflows, also known as turbidity currents, can travel remarkable distances and profoundly influence the seabed morphology from shoreline to abyss by depositing, eroding, and dispersing large quantities of sediment particles. Here we present a new approach to investigating the transformation of a plunging river flow into a turbidity current. Unlike previous workers using experimental and numerical treatments, we consider the evolution of a turbidity current from a river as different stages of a single flow process. From initial commotion to final stabilization, the transformation of a river (open channel flow) into a density-driven current (hyperpycnal flow) is captured in its entirety by a numerical model. Successful implementation of the model in laboratory and field cases has revealed the dynamics of a complex geophysical flow that is extremely difficult to observe in the field or model in the laboratory.

Detection of Partial Blockages in a Branched Piping System by the Frequency Response Method

Steady oscillatory flow in a branched piping system with partial blockages is studied by using the frequency response method. The peak pressure frequency diagrams at the downstream end are developed with the partial blockage at different locations in the system by using the transfer matrix method. A systematic procedure is presented to estimate the size and the location of a single partial blockage in the system. For more than one partial blockage, it is observed that there is a definite relationship between the frequency responses of the individual and combined partial blockages.

Numerical Modeling of Scour in Cohesive Soils Around Artificial Rock Island of Cooper River Bridge

A new laboratory-based methodology for prediction of the maximum scour depth in cohesive soil has recently been developed at the University of South Carolina. Because of the absence of field data, a computational fluid dynamics model, FLUENT, is used to test the scale effects associated with such a methodology. The numerical model was first verified against measurements obtained in the laboratory. The numerical results agreed satisfactorily with the measurements. Then, the numerical model was applied to the rock island protecting the main piers of the Cooper River Bridge, located in Charleston, South Carolina. The scour hole created around the island in the laboratory was scaled up and used in the numerical model. The computed bed shear stresses compared satisfactorily with those scaled up from the measurements and the shear stress at which the field sample begins to erode. It was found that the scour depth of 3.7 m represents the equilibrium state, which is similar to the results scaled up from the laboratory experiments. The numerical results showed that the scour depth of 36 m calculated by the HEC-18 approach is significantly overestimated.

Standard Protocol for Comparing Culvert Hydraulic Modeling Software: HEC-RAS and HY-8 Application

A survey conducted for this study showed that state departments of transportation do not have a common methodology for selecting hydraulic and hydrologic software. Therefore, a standard protocol is developed here to compare hydraulic modeling software, which is the scope of NCHRP Project 20-07(146). The protocol consists of four evaluation components: functionality, detail, user-friendliness, and credibility. A measuring factor is assigned to each evaluation component based on certain criteria, and then the measuring factors of all four components are averaged to determine an overall evaluation factor. A quantitative comparison system for the overall evaluation factor is proposed for comparing and evaluating the modeling software packages on a scale between 0% and 100%. The protocol is universal in nature and can be applied to hydraulic and hydrologic computer models. In this paper the protocol is applied to compare HEC-RAS and HY-8 models as an example for the classification of computer models used for hydraulic analysis of culverts. The model functionality is assessed by comparing the model results with benchmark test data. For that purpose, four test cases are compiled to provide benchmarks for culvert hydraulics. The test cases comprise experimental data and examples from the FHWA Hydraulic Design Series No. 5. Information provided in the documentation of the software packages is used to assess other evaluation components, such as detail, user-friendliness, and credibility. Then, an overall evaluation factor for each model is determined as described above.