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Papers by Mahmoud al-Riffai

In the field of geotechnical engineering, the degree of saturation is a critical parameter since ... more In the field of geotechnical engineering, the degree of saturation is a critical parameter since its effect on pore pressures can significantly influence the slope stability of an earthen dam. In the event of a high intensity storm, quasi-saturated soil conditions in the downstream slope will result in an increase in the local soil coefficient of permeability. In addition, it is likely other activities such as piping may also have further adverse affects. This paper investigates the different methods for achieving unsaturated conditions and maintaining them in earth retaining structures. Furthermore, constitutive relationships that exist between the unsaturated coefficient of permeability and other soil parameters such as soil material, soil layer thickness, poresize and water content are investigated using an integrated approach taking into account hydrologic and geotechnical events. RÉSUMÉ

This paper presents results from an extensive experimental program on dam breaching mechanics and... more This paper presents results from an extensive experimental program on dam breaching mechanics and focuses on the erosion and side-slope instability of the breach channel that occurs on the downstream face of an embankment dam. The experimental work was conducted on homogeneous (non-cohesive) embankment models (30 to 60cm in height). Eight dam models were breached: four embankments with different soil compaction efforts, two embankments with different downstream toe drainage systems (inverted horizontal toe-drain filter vs. no drain) and two embankment dams with different soil compaction using the selected drainage system. It was found that a lower degree of saturation and a higher compaction can delay the breaching process significantly. Results for the different toe-drainage systems reinforce their importance as a model component in laboratory-scale testing for dam breach modeling and must therefore be incorporated in any physical model in order to obtain a realistic simulation of ...

A comprehensive experimental program dealing with three-dimensional overtopping and breach develo... more A comprehensive experimental program dealing with three-dimensional overtopping and breach development as well as two-dimensional overtopping physical tests of noncohesive earth embankments has been conducted on scale models in the Hydraulic Laboratory at the Department of Civil Engineering at the University of Ottawa. The experimental program which consisted of three phases focused on geotechnical and hydraulic aspects of the embankment breach mechanism. The first two phases focused on two test series for the three-dimensional breach overtopping tests: drainage and compaction. The test series were designed to determine the embankment breach characteristics using test parameters which have not been adequately identified or controlled in past noncohesive physical models: initial soil-water state and optimum dry unit weight. Both parameters were controlled in laboratory tests by means of compaction effort and seepage through the embankment body, respectively. The dynamic compaction te...

As part of a larger experimental program on dam breaching mechanics, this paper presents experime... more As part of a larger experimental program on dam breaching mechanics, this paper presents experimental results from breach tests conducted on non-cohesive earth embankment dams under several breach initiation conditions. The initial breach conditions include V-notch breach channels, as well as a more realistic breach initiation over a flat, centrally-tapered crest. A third instantaneous breach initiation was simulated through a crater emulating an explosive effect at the crest of the dam. The relative failure rates of each of the three initial breach mechanisms were compared against each other, as well as against previous experimental studies. Surface velocities through the breach were estimated by large-scale particle image velocimetry using GIS software. Special attention was paid to the geotechnical characteristics, such as moisture content and compaction effort for the embankments in these tests to more accurately scale the laboratory results.

The influence and importance of geotechnical factors such as the unsaturated zone and dry density... more The influence and importance of geotechnical factors such as the unsaturated zone and dry density in overtopped earth dams, as well as other geotechnical processes, has often been neglected or improperly considered in both experimental and numerical dam breaching models. As part of an extensive experimental program on breaching mechanics of homogeneous non-cohesive earth dams and dikes, both the erosion and lateral slope failures within the channel forming on the downstream face of a breached dam was assessed in small-scale homogeneous and non-cohesive dam models. In this particular study, two of the constructed test series are discussed. The first used two different drainage conditions (saturated vs. unsaturated) on the downstream face of the dam models, where two modes of side-slope failures (sliding vs. collapsing) were observed. The same analogy between the failure modes exists when the dry unit weight (second test series) of the dam material was increased (higher vs. lower comp...

This study presents a novel experimental technique for measuring the transient response of pore-w... more This study presents a novel experimental technique for measuring the transient response of pore-water-pressures (PWP) through a sand bed on a steep slope, typical to homogeneous non-cohesive earthfill dams or dikes, under supercritical flow using planar overtopping tests. The use of seepage-control elements in 3-D breach overtopping simulations of embankment breach failures carried out in laboratory flumes has been previously investigated by the authors. In this study, the real-time PWPs were recorded using inexpensive tensiometer-transducer probe assemblies (TTPAs), designed, assembled and tested for planar overtopping tests in hydraulic flumes. The TTPAs, comprised of a 2 mm diameter and 2 cm long ceramic probe, were installed at several locations below the downstream embankment slope face to capture the transient PWP response throughout the saturated-unsaturated zone with minimal intrusion in the soil and water mediums. This paper also describes the installation process, calibrat...

As part of a comprehensive experimental program on the failure mechanics of overtopped and subseq... more As part of a comprehensive experimental program on the failure mechanics of overtopped and subsequently breached earth embankments, this study presents the results of a novel experimental technique for measuring pore-water-pressure and erosion rate of a sand bed on a steep slope of an embankment under supercritical flow conditions and a falling head. The influence of seepage induced by the transient drainage conditions occurring on steep slopes on breaching mechanics, such as those of earthfill dams or dikes, is investigated and discussed. The erosion and subsequent slope retreat of the sand bed was captured using high-definition video camcorders and digital photo cameras and analyzed using a photogrammetric technique. Using the record of time-series of bed elevations, z b , the evolution of the bed slope profile was converted into sediment flux, ∂q b /∂s, and an associated erosion rate, ∂z b /∂t. Measurements of real-time matric suction in the unsaturated zone and excess pore-water...

Numerical models are valuable tools for predicting the extent of floods associated with embankmen... more Numerical models are valuable tools for predicting the extent of floods associated with embankment overtopping. The paper presents and compares the results of two widely used dam breaching numerical models: NWS-BREACH and HR-BREACH. The focus of the study is to understand the sensitivity of key geotechnical parameters and their effect on breaching outflow rate, its geometry as well as its temporal and spatial characteristics. Results show that NWS-BREACH produces outflow hydrographs with a generally smaller lag-time where the rise-time was too short, suggesting a rather instantaneous breach. The multiple peaks in the outflow hydrograph using HR-BREACH simulations highlight new concepts that take into account the side-slope failure assessment using the embankment's material tensile and shear strength versus the Culmann method used by NWS-BREACH.

… of the 34th World Congress of the …, 2011

The breach evolution in an overtopped dam constructed of non-cohesive materials can be observed b... more The breach evolution in an overtopped dam constructed of non-cohesive materials can be observed by following the downslope inflection point which migrates upstream as the breach channel slope retreats downwards due to flow-induced erosion. This control section, which defines the locus of Froude-critical points at the breach entrance, has the form of an arc on the upstream face of dam models which were breached in a wide flume at the University of Ottawa's Hydraulic Laboratory. An approach for determining the discharge over the evolving breach crest was developed based on weir equations and compared to the measured breach outflow. It was found that a correction factor for the discharge estimated using the proposed equations must be incorporated to account for constriction effects due to undercutting in the breach channel entrance as well as submergence effects due to side-slope collapses which occur in the downstream section of the breach channel. This study also examines scaling effects in distorted dam breaching models based on the Froude criterion and explores the possibility of using the void ratio as an alternative scaling criterion for mobile bed models simulating dam breach flows.

Natural Hazards, 2010

For over three decades, emergency planners have used numerical models to predict breaching in ear... more For over three decades, emergency planners have used numerical models to predict breaching in earthfill dams due to extreme events such as overtopping. However, current models neglect the role of the unsaturated zone present within the downstream face of an earthfill dam. This leads to an incorrect estimation of the time and space evolution of the breaching process, as such models often oversimplify governing geotechnical aspects such as the presence of the unsaturated soil medium in the vicinity of the breach channel. The stress state in the soil due to matric suction acts as a stabilizing force for the breaching mechanism and influences the erosion of the breach channel, especially during the initial phases of the breaching. The side-slope failure mechanism observed along the breach channel is also influenced by the negative pore-water-pressures in the soil. Based on a comprehensive experimental research program carried out in the Hydraulics Laboratory at the University of Ottawa, Canada, several new concepts are proposed to incorporate geotechnical factors and techniques which must be considered during the construction of earthfill dam models for laboratory testing. Two main findings emerged from this experimental work. First, the installation of a drainage mattress at the downstream toe of the dam depressed the phreatic surface through the earthfill dam body, which caused a lag in the breaching process due to the infiltration and reduced erosion occurring in the breach channel. Second, it is essential to control compaction during the construction of the earthfill dam model, since this significantly influences the erosion, as well as the side-slope failures which occur in the breach channel. Future studies are under way by the authors with the purpose of scaling of parameters such as the matric suction and soil erodibility.

Journal of Hydraulic Engineering, 2011

Canadian Journal of Civil Engineering, 2013

As part of a larger experimental program on dam breaching, this paper presents experimental resul... more As part of a larger experimental program on dam breaching, this paper presents experimental results for a series of breach tests conducted on homogeneous non-cohesive embankments under several initial breach conditions. The initial conditions utilized different geometries including a V-notch cut into the crest of the embankment, which is typical of a number of previous experimental studies, as well as the more realistic mechanisms of overtopping over a flat crest, and the novel breach overtopping mechanism of simulating a crater caused by an explosive blast. The relative failure rates of each of the three initial breach geometries were compared against each other, as well as against previous experimental studies. It was found that the breach initiation mechanism has a strong influence on the time required for the breach to progress to failure. Based on the experimental results, a four-stage breach process is proposed to describe breach morphology and outflow.

Predicting the breach outflow hydrograph due to overtopped embankment dams is important for the d... more Predicting the breach outflow hydrograph due to overtopped embankment dams is important for the development of flood inundation maps. Scaling the breach outflow characteristics from overtopped non-cohesive geometrically distorted dam models in the Hydraulic Laboratory at the University of Ottawa is investigated using the Froude criterion and substantiated by scale series tests. Another objective of this study is to examine the use of void ratio as an alternative scaling criterion for mobile bed models simulating dam breach flows, using a second scale series test. Photographic images of the breach channel captured during the test revealed an upstream-control section forming on the upstream face of the dam model in the shape of an arc, which migrates upstream as the breach channel slope retreats downwards due to erosion. The arc resembles an ogee-crest, defining the locus of Froude-critical points upstream the breach channel entrance. The discharge over the arched crest was approximated using weir equations and compared to the measured breach outflow. It was found that a correction factor for the estimated discharge must be accounted for constriction effects due to undercutting in the breach channel entrance and submergence effects due to side-slope collapses occurring in the downstream section of the breach channel. While the Froude criterion is substantiated for the distorted scale series for horizontal dimensions, scale effects did exist laterally. With respect to the compaction scale-series, scale effects in both the horizontal and lateral dimensions were minor.

In the field of geotechnical engineering, the degree of saturation is a critical parameter since ... more In the field of geotechnical engineering, the degree of saturation is a critical parameter since its effect on pore pressures can significantly influence the slope stability of an earthen dam. In the event of a high intensity storm, quasi-saturated soil conditions in the downstream slope will result in an increase in the local soil coefficient of permeability. In addition, it is likely other activities such as piping may also have further adverse affects. This paper investigates the different methods for achieving unsaturated conditions and maintaining them in earth retaining structures. Furthermore, constitutive relationships that exist between the unsaturated coefficient of permeability and other soil parameters such as soil material, soil layer thickness, poresize and water content are investigated using an integrated approach taking into account hydrologic and geotechnical events. RÉSUMÉ

This paper presents results from an extensive experimental program on dam breaching mechanics and... more This paper presents results from an extensive experimental program on dam breaching mechanics and focuses on the erosion and side-slope instability of the breach channel that occurs on the downstream face of an embankment dam. The experimental work was conducted on homogeneous (non-cohesive) embankment models (30 to 60cm in height). Eight dam models were breached: four embankments with different soil compaction efforts, two embankments with different downstream toe drainage systems (inverted horizontal toe-drain filter vs. no drain) and two embankment dams with different soil compaction using the selected drainage system. It was found that a lower degree of saturation and a higher compaction can delay the breaching process significantly. Results for the different toe-drainage systems reinforce their importance as a model component in laboratory-scale testing for dam breach modeling and must therefore be incorporated in any physical model in order to obtain a realistic simulation of ...

A comprehensive experimental program dealing with three-dimensional overtopping and breach develo... more A comprehensive experimental program dealing with three-dimensional overtopping and breach development as well as two-dimensional overtopping physical tests of noncohesive earth embankments has been conducted on scale models in the Hydraulic Laboratory at the Department of Civil Engineering at the University of Ottawa. The experimental program which consisted of three phases focused on geotechnical and hydraulic aspects of the embankment breach mechanism. The first two phases focused on two test series for the three-dimensional breach overtopping tests: drainage and compaction. The test series were designed to determine the embankment breach characteristics using test parameters which have not been adequately identified or controlled in past noncohesive physical models: initial soil-water state and optimum dry unit weight. Both parameters were controlled in laboratory tests by means of compaction effort and seepage through the embankment body, respectively. The dynamic compaction te...

As part of a larger experimental program on dam breaching mechanics, this paper presents experime... more As part of a larger experimental program on dam breaching mechanics, this paper presents experimental results from breach tests conducted on non-cohesive earth embankment dams under several breach initiation conditions. The initial breach conditions include V-notch breach channels, as well as a more realistic breach initiation over a flat, centrally-tapered crest. A third instantaneous breach initiation was simulated through a crater emulating an explosive effect at the crest of the dam. The relative failure rates of each of the three initial breach mechanisms were compared against each other, as well as against previous experimental studies. Surface velocities through the breach were estimated by large-scale particle image velocimetry using GIS software. Special attention was paid to the geotechnical characteristics, such as moisture content and compaction effort for the embankments in these tests to more accurately scale the laboratory results.

The influence and importance of geotechnical factors such as the unsaturated zone and dry density... more The influence and importance of geotechnical factors such as the unsaturated zone and dry density in overtopped earth dams, as well as other geotechnical processes, has often been neglected or improperly considered in both experimental and numerical dam breaching models. As part of an extensive experimental program on breaching mechanics of homogeneous non-cohesive earth dams and dikes, both the erosion and lateral slope failures within the channel forming on the downstream face of a breached dam was assessed in small-scale homogeneous and non-cohesive dam models. In this particular study, two of the constructed test series are discussed. The first used two different drainage conditions (saturated vs. unsaturated) on the downstream face of the dam models, where two modes of side-slope failures (sliding vs. collapsing) were observed. The same analogy between the failure modes exists when the dry unit weight (second test series) of the dam material was increased (higher vs. lower comp...

This study presents a novel experimental technique for measuring the transient response of pore-w... more This study presents a novel experimental technique for measuring the transient response of pore-water-pressures (PWP) through a sand bed on a steep slope, typical to homogeneous non-cohesive earthfill dams or dikes, under supercritical flow using planar overtopping tests. The use of seepage-control elements in 3-D breach overtopping simulations of embankment breach failures carried out in laboratory flumes has been previously investigated by the authors. In this study, the real-time PWPs were recorded using inexpensive tensiometer-transducer probe assemblies (TTPAs), designed, assembled and tested for planar overtopping tests in hydraulic flumes. The TTPAs, comprised of a 2 mm diameter and 2 cm long ceramic probe, were installed at several locations below the downstream embankment slope face to capture the transient PWP response throughout the saturated-unsaturated zone with minimal intrusion in the soil and water mediums. This paper also describes the installation process, calibrat...

As part of a comprehensive experimental program on the failure mechanics of overtopped and subseq... more As part of a comprehensive experimental program on the failure mechanics of overtopped and subsequently breached earth embankments, this study presents the results of a novel experimental technique for measuring pore-water-pressure and erosion rate of a sand bed on a steep slope of an embankment under supercritical flow conditions and a falling head. The influence of seepage induced by the transient drainage conditions occurring on steep slopes on breaching mechanics, such as those of earthfill dams or dikes, is investigated and discussed. The erosion and subsequent slope retreat of the sand bed was captured using high-definition video camcorders and digital photo cameras and analyzed using a photogrammetric technique. Using the record of time-series of bed elevations, z b , the evolution of the bed slope profile was converted into sediment flux, ∂q b /∂s, and an associated erosion rate, ∂z b /∂t. Measurements of real-time matric suction in the unsaturated zone and excess pore-water...

Numerical models are valuable tools for predicting the extent of floods associated with embankmen... more Numerical models are valuable tools for predicting the extent of floods associated with embankment overtopping. The paper presents and compares the results of two widely used dam breaching numerical models: NWS-BREACH and HR-BREACH. The focus of the study is to understand the sensitivity of key geotechnical parameters and their effect on breaching outflow rate, its geometry as well as its temporal and spatial characteristics. Results show that NWS-BREACH produces outflow hydrographs with a generally smaller lag-time where the rise-time was too short, suggesting a rather instantaneous breach. The multiple peaks in the outflow hydrograph using HR-BREACH simulations highlight new concepts that take into account the side-slope failure assessment using the embankment's material tensile and shear strength versus the Culmann method used by NWS-BREACH.

… of the 34th World Congress of the …, 2011

The breach evolution in an overtopped dam constructed of non-cohesive materials can be observed b... more The breach evolution in an overtopped dam constructed of non-cohesive materials can be observed by following the downslope inflection point which migrates upstream as the breach channel slope retreats downwards due to flow-induced erosion. This control section, which defines the locus of Froude-critical points at the breach entrance, has the form of an arc on the upstream face of dam models which were breached in a wide flume at the University of Ottawa's Hydraulic Laboratory. An approach for determining the discharge over the evolving breach crest was developed based on weir equations and compared to the measured breach outflow. It was found that a correction factor for the discharge estimated using the proposed equations must be incorporated to account for constriction effects due to undercutting in the breach channel entrance as well as submergence effects due to side-slope collapses which occur in the downstream section of the breach channel. This study also examines scaling effects in distorted dam breaching models based on the Froude criterion and explores the possibility of using the void ratio as an alternative scaling criterion for mobile bed models simulating dam breach flows.

Natural Hazards, 2010

For over three decades, emergency planners have used numerical models to predict breaching in ear... more For over three decades, emergency planners have used numerical models to predict breaching in earthfill dams due to extreme events such as overtopping. However, current models neglect the role of the unsaturated zone present within the downstream face of an earthfill dam. This leads to an incorrect estimation of the time and space evolution of the breaching process, as such models often oversimplify governing geotechnical aspects such as the presence of the unsaturated soil medium in the vicinity of the breach channel. The stress state in the soil due to matric suction acts as a stabilizing force for the breaching mechanism and influences the erosion of the breach channel, especially during the initial phases of the breaching. The side-slope failure mechanism observed along the breach channel is also influenced by the negative pore-water-pressures in the soil. Based on a comprehensive experimental research program carried out in the Hydraulics Laboratory at the University of Ottawa, Canada, several new concepts are proposed to incorporate geotechnical factors and techniques which must be considered during the construction of earthfill dam models for laboratory testing. Two main findings emerged from this experimental work. First, the installation of a drainage mattress at the downstream toe of the dam depressed the phreatic surface through the earthfill dam body, which caused a lag in the breaching process due to the infiltration and reduced erosion occurring in the breach channel. Second, it is essential to control compaction during the construction of the earthfill dam model, since this significantly influences the erosion, as well as the side-slope failures which occur in the breach channel. Future studies are under way by the authors with the purpose of scaling of parameters such as the matric suction and soil erodibility.

Journal of Hydraulic Engineering, 2011

Canadian Journal of Civil Engineering, 2013

As part of a larger experimental program on dam breaching, this paper presents experimental resul... more As part of a larger experimental program on dam breaching, this paper presents experimental results for a series of breach tests conducted on homogeneous non-cohesive embankments under several initial breach conditions. The initial conditions utilized different geometries including a V-notch cut into the crest of the embankment, which is typical of a number of previous experimental studies, as well as the more realistic mechanisms of overtopping over a flat crest, and the novel breach overtopping mechanism of simulating a crater caused by an explosive blast. The relative failure rates of each of the three initial breach geometries were compared against each other, as well as against previous experimental studies. It was found that the breach initiation mechanism has a strong influence on the time required for the breach to progress to failure. Based on the experimental results, a four-stage breach process is proposed to describe breach morphology and outflow.

Predicting the breach outflow hydrograph due to overtopped embankment dams is important for the d... more Predicting the breach outflow hydrograph due to overtopped embankment dams is important for the development of flood inundation maps. Scaling the breach outflow characteristics from overtopped non-cohesive geometrically distorted dam models in the Hydraulic Laboratory at the University of Ottawa is investigated using the Froude criterion and substantiated by scale series tests. Another objective of this study is to examine the use of void ratio as an alternative scaling criterion for mobile bed models simulating dam breach flows, using a second scale series test. Photographic images of the breach channel captured during the test revealed an upstream-control section forming on the upstream face of the dam model in the shape of an arc, which migrates upstream as the breach channel slope retreats downwards due to erosion. The arc resembles an ogee-crest, defining the locus of Froude-critical points upstream the breach channel entrance. The discharge over the arched crest was approximated using weir equations and compared to the measured breach outflow. It was found that a correction factor for the estimated discharge must be accounted for constriction effects due to undercutting in the breach channel entrance and submergence effects due to side-slope collapses occurring in the downstream section of the breach channel. While the Froude criterion is substantiated for the distorted scale series for horizontal dimensions, scale effects did exist laterally. With respect to the compaction scale-series, scale effects in both the horizontal and lateral dimensions were minor.