Abdul A. Khan

Numerical Simulation of Flood Inundation due to Dam and Levee Breach

A numerical model has been developed to simulate flood inundation due to dam and levee breach. The model solves the conservative form of the two-dimensional shallow water equations using a finite volume method. The intercell flux is computed by upwind method and the water-level-gradient is evaluated by weighted average of both upwind and downwind gradient. The newly developed model is tested with various types of examples, including a partial dam-break problem, oblique hydraulic jump due to contraction of a side boundary, and a real life case of flood wave propagation in the Toce River Valley. It is found that the scheme is inherently robust and stable, and is able to predict complex flow phenomena that involve subcritical flows, supercritical flows, transcritical flows, overland flows, and overtopping flows.

Modeling flow over an initially dry bed

The one-dimensional groundwater flow equations are coupled with the St. Venant equations to simulate the flow resulting from a sudden removal of a dam over an initially dry downstream bed. The St. Venant equations are solved using a Petrov-Galerkin finite element scheme, while the groundwater flow equations are solved using the Bubnov-Galerkin finite element scheme. The comparison of the computed water surface and discharge per unit width profiles with the corresponding analytical solutions, for a dambreak over frictionless horizontal bed, show that the model possesses excellent phase accuracy, for both positive and negative waves, and can predict the discharge distribution accurately. Also, the computed water surface profiles are compared with the available measured data for the dambreak flows over smooth and rough surfaces of horizontal and sloping channels. In addition, the results obtained from only the St. Venant equations with minimum depth criteria are presented for comparison with the above model.

Discontinuous Galerkin Method for 1D Shallow Water Flows in Natural Rivers

Abstract A numerical model is proposed for the solution of one-dimensional shallow water flow equations for natural rivers. This model is based on the total variation diminishing Runge-Kutta discontinuous Galerkin finite element method. In natural rivers, the cross-section shape and bed slope can be quite irregular, which requires a compatible discretization scheme for the bed slope term and net pressure force term. Therefore, in this model, the hydrostatic pressure force term and the wall pressure force term are combined and a new discretization for the resulting term is introduced. This formulation is shown to prevent unphysical flow due to improper treatment of bottom slope term. The mass and momentum flux term are calculated by HLL Riemann solver. A scheme is presented to model flow over dry bed. To evaluate the numerical scheme, tests are conducted for idealized dambreak problems in parabolic (with wet and dry beds), and rectangular channels, hydraulic jump in a rectangular channel, dambreak in the Teton River (Idaho, USA) and the Toce River (Northern Alps, Italy), and flooding event in the East Fork River (Wyoming, USA). The comparison of the computational results with analytical and laboratory results of dam break flows shows that the model is capable of handling flow over dry areas. The simulation results for hydraulic jump show the discharge conservation property and shock prediction capability of the model. The dambreak and flood simulations in natural channels show that the model is capable of handling flows in highly varying bed topography and channel geometry.

Investigation of Flow through Orifices in Riser Pipes

In this study the discharge coefficient for circular orifices of different size in two different sizes of riser pipe is investigated experimentally. This type of outlet structure is common in detention ponds to achieve runoff volume control from developed areas in order to meet outflow discharge and water quality requirements. The discharge coefficient is determined by recording the drop in pond water level with time as water flows out of the orifice. The discharge coefficient is found to be a function of head over the orifice, location of the orifice above the floor of the tank, and the ratio of the orifice diameter to riser pipe diameter. The discharge coefficient increases as the head over the orifice decreases and height of the orifice above the floor of the tank decreases. The discharge coefficient reduces as the ratio of the orifice diameter to pipe diameter increases and eventually reaches an asymptotic value. Also, with all other variables being the same, the discharge coefficient is lower for larger size riser pipe. An equation that is a function of head over the orifice and height of the orifice above the floor of the tank is fitted for each ratio of orifice diameter to piper diameter. The coefficients of the equation are found to be a function of the ratio of orifice diameter to piper diameter. A minimum coefficient of determination of 0.78 for the fitted equation suggests that the fit can be used to determine the discharge coefficient for orifices in circular riser pipes.

Scour upstream of a circular orifice under constant head

The transport mechanism as well as the extent and the shape of the scour upstream of a circular orifice is investigated under steady flow conditions for different sediment sizes and heads on the orifice. The main mechanism of sediment transport is found to be vortex-related, appearing randomly within the scour hole. The scour length, maximum scour width, and maximum scour depth are evaluated for various sediment sizes and under different heads on the orifice. The non-dimensional scour depth profiles along the orifice centreline are found to be similar. The non-dimensional scour depth profiles across the width of the scour hole at various locations upstream of the orifice are found to be similar as well. The flow characteristics related to the scaled centreline longitudinal velocity and the scaled position of the maximum longitudinal velocity are found to approach those for the unbounded orifice at the equilibrium scour condition.

Shallow Water Flow Analysis with Moving Boundary Technique Using Least-squares Bubble Function

This paper presents a computational simulation method for a river problem. For the actual flow problem, it is necessary to compute flow velocity, water elevation and water region at the same time. For the basic formulation, the unsteady shallow water equations are used. As the numerical approach, implicit FEM is proposed by bubble function. To control numerical stability and accuracy, LSBF (Least-Squares Bubble Function) is used to solve the finite element equations. Also, the fixed boundary technique is combined to deal with wet and dry areas in the moving finite element mesh. Some numerical tests are shown to check this method.

Discontinuous Galerkin Method for 1D Shallow Water Flow in Nonrectangular and Nonprismatic Channels

A total variation diminishing Runge-Kutta discontinuous Galerkin finite element method for the solution of one-dimensional (1D) shallow water flow equations for natural channels is presented. The hydrostatic pressure force and the wall pressure force terms are combined to simplify the calculations and prevent unphysical flow attributable to improper treatment of the bottom slope term. The treatment of the combined term that appropriately accounts for the momentum flux is given. HLL and Roe Riemann solvers are assessed for the mass and momentum flux terms. Numerical tests are conducted using prismatic rectangular and nonrectangular channels as well as non prismatic channels and natural channel for dam break, supercritical flow, transcritical flow, and dry-bed problems. Slope limiters based on flow cross section area, water surface, and water depth are evaluated. The tests show that HLL and Roe solvers provide similar accuracy. However, the slope limiter based on flow area provides more accurate solutions f...

A novel vibration-based monitoring technique for bridge pier and abutment scour

Scouring degrades the overall health of a bridge by removing the bed material surrounding the piers and abutments. If undetected, scour may lead to the catastrophic failure of a bridge resulting in hundreds of millions in repair costs. The loss of a bridge due to undetected scour formations can also hinder emergency evacuations since riverbed scouring typically occurs in peak flow periods such as hurricane or flood events. To take requisite precautions against such catastrophic events, a monitoring system that can reliably detect scour formation, without being adversely affected by the environmental conditions, is essential. This article presents a novel scour monitoring technique that exploits the differences between the low-frequency ambient excitations exerted on a thin, flexible plate located in the flow versus the same device located in the sediment. The underlying principle is that a flexible plate excited by the turbulent flow vibrates at significantly higher amplitudes compared to an identical plate surrounded by sediment. To validate this principle, a simplified numerical model is developed to guide the design of the scaled laboratory device; next, a prototype model is built in the laboratory and tested in an indoor flume. The energy content of the sensor in the flow is measured to be one to two orders of magnitude greater than the sensor in sediment. The findings obtained at various flow conditions indicate that this technique can supply reliable information on the water/sediment interface, and thus scour and refill processes. Experimental results also demonstrate that the presence of a scour hole further improves the ability to detect the interface location. Additionally, the results show that maximum slope of the sensor energy content as a function of the sensor depth can be used as a feature to estimate the water/sediment interface.

Novel vibration-based technique for detecting water pipeline leakage

Abstract Exacerbating the imbalance between demand for freshwater and available water resources is the sub-optimal performance of water distribution systems, which are plagued with leaks that cause significant losses of treated freshwater. This paper presents an approach for leak detection that involves continuous monitoring of the changes in the correlation between surface acceleration measured at discrete locations along the pipeline length. A metric called leak detection index is formulated based on cross-spectral density of measured pipe surface accelerations for detecting the onset and assessing the severity of leaks. The proposed non-invasive approach requires minimal human intervention and works under normal operating conditions of the pipeline system without causing any operational disturbances. The approach is demonstrated on a 76 mm diameter polyvinyl chloride pipeline test system considering varying leak severities. The preliminary results presented in this paper seem promising and lead to several interesting questions that will require further research.

Leaching of DOC, DN, and inorganic constituents from scrap tires.

One concern for recycle and reuse of scrap tires is the leaching of tire constituents (organic and inorganic) with time, and their subsequent potential harmful impacts in environment. The main objective of this study was to examine the leaching of dissolved organic carbon (DOC), dissolved nitrogen (DN), and selected inorganic constituents from scrap tires. Different sizes of tire chips and crumb rubber were exposed to leaching solutions with pHs ranging from 3.0 to 10.0 for 28days. The leaching of DOC and DN were found to be higher for smaller size tire chips; however, the leaching of inorganic constituents was independent of the size. In general, basic pH conditions increased the leaching of DOC and DN, whereas acidic pH conditions led to elevated concentrations of metals. Leaching was minimal around the neutral pH values for all the monitored parameters. Analysis of the leaching rates showed that components associated with the rubbery portion of the tires (DOC, DN, zinc, calcium, magnesium, etc.) exhibited an initial rapid followed by a slow release. On the other hand, a constant rate of leaching was observed for iron and manganese, which are attributed to the metal wires present inside the tires. Although the total amounts that leached varied, the observed leaching rates were similar for all tire chip sizes and leaching solutions. Operation under neutral pH conditions, use of larger size tire chips, prewashing of tires, and removal of metal wires prior to application will reduce the impact of tire recycle and reuse.