Tew-Fik Mahdi

Experimental Investigation of the Hydraulic Erosion of Noncohesive Compacted Soils

This paper presents the results of a laboratory investigation whose purpose was to evaluate the effects of compaction on the erodibility of cohesionless soils. By means of a recently developed flume experiment, sediment erosion rates and incipient motion, as a function of shear stress, average velocity, and dry density, have been determined for three compacted sand and gravel mixtures. A preliminary comparison of the incipient motion values shows that granular soils compacted at the Proctor optimum have a higher resistance to free surface flow erosion than those compacted at lower and higher densities. This leads one to infer that the Proctor optimum, generally used as a standard for construction, might also be an optimum for hydraulic resistance and stability. Additional comparison of the experimental data with two commonly used incipient motion criteria also suggests that Yang’s criterion is a better predictor of soil detachment than the Shields-Yalin criterion.

Application of the dynamic bounds method in the safety assessment of flood defences, a case study: 17th Street flood wall, New Orleans

In this paper we provide a computational framework for evaluation of reliability and safety assessment of infrastructures. It is based on the combined application of the dynamic bounds (DB) method and a probabilistic finite element model (FEM). The DB improves the computational efficiency of the FEM when calculating time-dependent failure analyses of coastal and offshore structures, and can speed up the simulation process by several orders of magnitude. Our approach is demonstrated here for an example problem, and shown to be the most efficient method in applications with a limited number of influential variables, which is true for geotechnical and coastal flood defence systems. It is applied to the 17th Street flood wall, a failing component of the flood defence system in New Orleans during Hurricane Katrina. The variation in soil parameters is a critical input in the reliability estimation of this structure, and the calculated probability of failure depends on these assumed values.

Treatment of Checkerboard Pressure in the Collocated Unstructured Finite-Volume Scheme

In this article, a new concept, “local triangle,” is described. A smoother pressure field is then obtained through the utilization of local triangles in a multiblock local triangulation method. The cell face velocity is corrected with a pressure-correction term which is usually underrelaxed. It is demonstrated that this relaxation factor has to be one, and it cannot be the general relaxation factor. It is indicated that the checkerboard oscillations are more significant at higher cell Reynolds numbers and, in turn, in this article, numerical simulations are mainly performed in turbulent flow.

Experimental investigation into rockfill dam failure initiation by overtopping

Abstract Rockfill is the most abundant building material. It is often used for water retention under different contexts, such as dams, embankments or drainage systems. Climate change may cause water levels to rise in reservoirs. As rockfill structures are not able to resist strong overtopping flow, rising water levels will constitute a danger for rockfill dam stability as well as for people living nearby. This work is aimed at the development of an empirical formula that enables calculation of the critical water level of overflow at the crest from the geometrical and physical parameters of a dam. To achieve these objectives, several experimental tests on a rockfill dam model with two different impervious cores, moraine with a sand filter and an empty wooden formwork, were conducted in a hydraulic channel at the hydro-environmental laboratory at École Polytechnique de Montréal. The purpose of these tests was to study the initiation of a riprap failure under the influence of different variables, such as rock size, riprap bank, downstream side slope and bed slope. Results showed linear trends between the critical water level and both the downstream side slope and bed slope. Also, a power trend was observed between the critical level and riprap grain size. A formula that gives the critical overtopping water level was developed from these results.

Comparison of two-dimensional flood propagation models: SRH-2D and Hydro_AS-2D

This article presents a comparison between two two-dimensional finite volume flood propagation models: SRH-2D and Hydro_AS-2D. The models are compared using an experimental dam-break test case provided by Soares-Frazão (J Hydraul Res, 2007. doi:10.1080/00221686.2007.9521829). Four progressively refined meshes are used, and both models react adequately to mesh and time step refinement. Hydro_AS-2D shows some unphysical oscillations with the finest mesh and a certain loss of accuracy. For that test case, Hydro_AS-2D is more accurate for all meshes and generally faster than SRH-2D. Hydro_AS-2D reacts well to automatic calibration with PEST, whereas SRH-2D has some difficulties in retrieving the suggested Manning’s roughness coefficient.

Flooding of the Saguenay region in 1996: Part 1—modeling River Ha! Ha! flooding

This paper presents an application of the model UMHYSER-1D (Unsteady Model for the HYdraulics of SEdiments in Rivers One-Dimensional) for the representation of morphological changes along the Ha! Ha! River during the 1996 flooding of the Saguenay region. UMHYSER-1D is a one-dimensional hydromorphodynamic model capable of representing water surface profiles in a single-river or a multiriver network, with different flow regimes considering cohesive or non-cohesive sediment transport. This model uses fractional sediment transport, bed sorting, and armoring along with three minimization theories to achieve riverbed and width adjustments. UMHYSER-1D is applied to the Ha! Ha! River (Quebec, Canada), a tributary of the Saguenay River, for the 1996 downpour. The results permit forcing data verification and prove that some cross sections are not the right ones. UMHYSER-1D captures the trends of erosion and deposition well although the results do not fully agree with the collected data. This application shows the capabilities of this model and predicts its promising role in solving complex, real engineering cases.

Flooding of the Saguenay region in 1996. Part 2: Aux Sables River flood mitigation and environmental impact assessment

AbstractAfter the flooding of the Saguenay region in July 1996, several rivers, including the Aux Sables River, experienced unusual water discharge, causing flooding and morphological damage. This paper addresses flood mitigation and environmental impact assessment of the Aux Sables River following the July 1996 flooding. The consequences of the flood are summarized followed by a review of various proposed solutions for a similar flood. The option of dredging the Aux Sables River to increase discharge without causing flooding raises the issue of suspended sediment concentration, since the intake water at Jonquiere would be at risk. Utilizing newly developed software, UMHYSER-1D, the suspended sediment impact assessment for the Aux Sables River enables the maximum permissible sediment discharge to be released into the river while avoiding any risk of pollution for the population of Jonquiere city. Using UMHYSER-1D to mitigate this risk confirms the important role of numerical modeling in solving complex engineering problems.

Flood modelling improvement using automatic calibration of two dimensional river software SRH-2D

Abstract River model calibration is essential for reliable model prediction. The manual calibration method is laborious and time-consuming and requires expert knowledge. River engineering software is now equipped with more complex tools that require a high number of parameters as input, rendering the task of model calibration even more difficult. This paper presents the calibration tool O.P.P.S. (Optimisation Program for PEST and SRH-2D) and then uses it in multiple calibration scenarios. O.P.P.S. combines PEST, a calibration software and SRH-2D, a bi-dimensional hydraulic and sediment model for river systems, into an easy-to-use set of forms. O.P.P.S is designed to minimise the user’s interaction with the involved program to carry out rapid and functional calibration processes. PEST uses the Gauss–Marquardt–Levenberg algorithm to adjust the model’s parameters by minimising an objective function containing the differences between field observation and model-generated values. The tool is used to conduct multiple calibration series of the modelled Ha! Ha! river in Québec, with varying information content in the observation fields. A sensitivity study is also conducted to assess the behaviour of the calibration process in the presence of erroneous or imprecise measurements.

Evaluation of the overflow failure scenario and hydrograph of an embankment dam with a concrete upstream slope protection

The standard procedure in Quebec, Canada, for evaluating the failure of an embankment dam, per the Loi sur la sécurité des barrages, specifies a 30-min-long failure scenario with a breach width equal to four times the maximal height of the dam. We demonstrate a new method for evaluating the flood overtopping failure scenario for embankment dams with concrete upstream slope protection, using Toulnustouc dam for example computations. Our new methodology computes safety factors for a range of potential failure mechanisms taking into account geotechnical, hydraulic, and structural factors. We compile the results of our investigations of the various dam failure mechanisms and compare the corresponding dam failure hydrographs to the current hydrograph specified in the standard analysis procedures. Our investigations tend to invalidate the current standard procedures for evaluating the failure of rock-fill dams with concrete upstream faces, by indicating that the current standard procedures underestimate the peak failure discharge and overestimate the time to the peak discharge.

Manhole Storage Capacity Influence on Transient Flow Modeling during Storm Sewer Flooding Event

Drainage systems are designed to quickly convey rainwater from urban areas to areas of natural flow (rivers, streams, lakes, etc). Hence, they are sized with the assumption of steady free surface flow. With the frequent flooding of storm sewers and corresponding damage to public and private infrastructure (Schmitt et al. 2004), numerical modeling of transient flow has now become unavoidable. Several models of transient flow are available, each with its advantages and limitations. This chapter examines the impact of the inclusion of manhole size on the filling celerity of urban drainage systems. It is assumed that the pressurization of the drainage network starts with the rapid filling of a manhole, i.e. as soon as the water level in the manhole reaches the crown of the pipe being filled. Based on this assumption, a filling model is built to analyze the influence of manhole capacity on the propagation of the surge wave. This model uses the method of characteristics within the conduit and an innovative method based on gravity waves. To evaluate the impact of manhole size on the filling speed of the Storm Water System (SWS), the proposed methodology is applied to a theoretical case study. Results show that manhole size can accelerate or reduce the filling speed of connecting conduits.