No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Numerical study of breaching at upper parts of homogenous earthen dams
Abstract
In this study, time-dependent finite element analyses of the breaching process in two homogenous earth-fill dams were performed using the finite element method. Breaching was initiated at the middle and corner sections of the upper part of the dam bodies. The numerical results were compared with the findings of the experiments realized on dams 60 cm high, 2 m wide at bottom, 20 cm wide at crest with 1 V:1.5H side slopes at upstream and downstream faces. This numerical study combines time-dependent hydraulic gradient distributions and groundwater flows to assess breach areas, velocities, and flow rates. A Python algorithm was integrated with the Jupyter console, allowing the simulation of the breach mechanism in multiple runs to determine breach parameters. Both numerical and experimental analyses revealed that the dams were exposed to backward erosion, starting at the downstream side of the dam and progressing inward. The compatibility between experimental and numerical results was sought by means of the parameters RMSE, MAE and the statistical performance of the numerical approach was evaluated by using RSR, NSE, and PBIAS. A fairly good agreement was obtained between the experimental and numerical results.
ResearchGate has not been able to resolve any citations for this publication.
From the past to nowadays, earth-fill dams have been built thanks to their
advantages, however, piping is a problem that earth-fill dams can experience and then fail. While there are many studies about the overtopping failures of the dams, there are not too many surveys about dam failures due to piping.
Dams having a height of 0.6 m, a bottom width of 2 m, and a crest width of 0.20
m were built in a channel of 1 m wide, 0.81 m high and 6.14 m long. 3 different
scenarios have been created and the evolution of dam failure resulting from seepage at the dam was recorded by six cameras located at different locations. In the closed system, water was pumped from the lower reservoir to the upper channel. The dam was constructed by using a mixture consisting of 85 % sand and 15 % clay. A circular tunnel with a diameter of 2 cm was created at the middle or corner of the dam according to the scenario and at 6 cm below the dam crest. The breach areas at different time instants at upstream and downstream sides are determined by using the Gauss Area calculation method and by image processing, and then it has been found that methods give close values to each other. Breach discharge and time-varied velocity values were determined by using the continuity equation.
Empirical relations were intended to be derived for the breach flow rate and
empirical relations represented in the literature were trialed by using experimental
findings.
Earth-fill dams have been constructed for decades by compacting natural soil materials near the dam site. Piping is of the most important causes of their failure.
In the scope of this thesis, 2 m in length homogenous earth-fill dams were constructed in a rectangular flume in the laboratory of the Izmir University of Economics. The experimental and numerical investigations on a breach by generating piping were realized with different weak zone scenarios. Three experiments were performed by placing a weak layer cross-section 5x5 cm2 at the dam bottom center. One scenario was performed by locating a weak layer of 2x2 cm2, 28 cm above the bottom.
Temporal breach areas and the breach-wetted areas are evaluated on scaled screenshots by using Gauss’s area formulation. The Temporal breach discharges were calculated from the continuity equation.
Furthermore, finite element analyses on the breaching of homogenous earth-fill dams in different scenarios were performed by comparing the hydraulic gradient with the critical value. In addition to the bottom and middle scenarios, two upper scenarios were also modeled. The water depths were used for each scenario to represent the experimental conditions, and some approaches were made for the weak zones. To simulate the breach mechanism with different loops, a python algorithm was integrated with the Jupyter console. As a result of the simulations, it has been observed that the findings obtained by simulations were in accord with the experimental studies, and the dams were exposed to backward piping starting from downstream towards upstream.
Piping is one of the main problems which threatens stability of earth-fill dams. Realistic approaches are needed for breach mechanism as well as breach geometry and flow. The aim of this study is to realize experiments to provide data needed to perform numerical analyses by making more realistic assumptions. Dam having a height of 0.6 m, a bottom width of 2 m and a crest width of 0.20 m is built in a channel of 1 m wide, 0.81 m high and 6.14 m long. Evolution of dam failure resulting from seepage at upper corner of the dam is recorded by six cameras located at different locations. The time-varied of breach areas at upstream and downstream sides are determined by applying the Gauss Area functions. Discharge of water through the breach and average outflow velocity are determined by using the continuity equation.
Internal erosion, also known as piping, is one of the most important causes of earth-fill dam breaks. Many researchers dealing with numerical analyses in this area make some simplified assumptions about the shape of the breach and the discharge of water flowing through the breach. This study was conducted in the scope of the project supported financially by the Scientific and Technological Research Council of Turkey and it consists of experimental study which aims to provide data needed to perform numerical analyses with more realistic approaches. A dam with a height of 0.6 m, a bottom width of 2 m and a crest width of 0.20 m was built in a flume 1 m wide, 0.81 m high and 6.14 m long. Before the construction of the dam, some common soil mechanics tests were carried out. The dam was constructed by using a mixture consisting of 85 % sand and 15 % clay. A circular tunnel with a diameter of 2 cm was created along the centerline at 6 cm below the dam crest. In the closed system, water was pumped from the lower reservoir to the upper channel. Six cameras located at different locations recorded the evolution of the dam failure. Gauss Area formula was applied to determine the time-varied of the breach areas at upstream and downstream sides. The discharge of water through the breach and average flow velocity were determined by using the continuity equation. The changes in water depth in the channel were also recorded.
Piping is one of the main causes of the earth-fill dam failures. Most of the researchers realizing numerical analyses make some simplified assumptions concerning the shape of the breach and the discharge of water flowing through the breach. The aim of this study is to realize experiments to provide data needed to perform numerical analyses by making more realistic assumptions. The dam having a height of 0.6 m, a bottom width of 2 m and a crest width of 0.20 m is built in a channel 1 m wide, 0.81 m high and 6.14 m long. The evolution of the breach and the discharge through the breach resulting from piping due to seepage at the earth-fill dam bottom was investigated experimentally. The evolution of the dam failure is recorded by six cameras located at different locations. The time-varied of the breach areas at upstream and downstream sides are determined by applying the Gauss Area functions. The discharge of water through the breach and average outflow velocity are determined by using the continuity equation.
Investigating the breach outflow hydrograph is an essential task to conduct mitigation plans and flood warnings. In the present study, the spatial dam breach is simulated by using a three-dimensional computational fluid dynamics model, FLOW-3D. The model parameters were adjusted by making a comparison with a previous experimental model. The different parameters (initial breach shape, dimensions, location, and dam slopes) are studied to investigate their effects on dam breaching. The results indicate that these parameters have a significant impact. The maximum erosion rate and peak outflow for the rectangular shape are higher than those for the V-notch by 8.85% and 5%, respectively. Increasing breach width or decreasing depth by 5% leads to increasing maximum erosion rate by 11% and 15%, respectively. Increasing the downstream slope angle by 4° leads to an increase in both peak outflow and maximum erosion rate by 2.0% and 6.0%, respectively.
An accurate investigation of the landslide dam breach process is crucial for the understanding the breach mechanism and disaster prediction. However, the numerical research on the landslide dam breach process to date is rarely reported, especially regarding the soil-water flow coupling effect incorporated in the erosion process. This paper presents a numerical investigation on the longitudinal breach process of landslide dams via a coupled discrete element method (DEM) and computational fluid dynamics (CFD) with the volume of fluid (VOF). Moreover, a virtual sphere model is proposed to overcome the computational instability caused by the particle size approaching the mesh size. The accuracy and validity of the improved coupled method are verified using a series of single particle sedimentation cases. By employing this method, the longitudinal breach process of landslide dams featuring different materials and hydrodynamic conditions has been simulated. It is found to satisfactorily reproduce the longitudinal breach process of landslide dams including surface flow erosion, backward erosion, head-cut erosion, and water and sediment rebalance or complete breach. The effects of the inflow discharges and dam materials on the erosion process are systematically resolved. The breach flow can cause the rotation trend of particles and lead to the increase of tangential contact force at the initial stage of the dam breaching. During the breach process, both the strength and density of the force chain continue to attenuate. The results obtained from the improved coupled DEM-CFD simulations can reasonably explain the particle-fluid interaction mechanisms, physical and morphological evolution and breach process at both macroscopic and mesoscopic scales.
In an earth-fill dam, the effect of seepage has been studied by applying a finite element method using the SEEP2D program. This is in order to determine the quantity of seepage through the dam. The total head measurements, core permeability, and anisotropy ratio (kx/ky) (Case study: Khassa Chai Dam, Iraq) are taken as the main parameters. The effect of the different water heads of the reservoir were tested on the seepage. The results showed that any increase in the water heads caused an increase in seepage quantity. Also, it was found that the seepage rate decreases by about 8.7%, 13.2%, and 15.3% at levels of water 454, 471, and 485 m.a.s.l, respectively by changing the core permeability from 10-6 m/s to 10-7 m/s. It has been concluded that the clay core plays a significant role in decreasing the seepage quantity and existing gradient. The results of testing the effect of anisotropy ratio on seepage showed that an increase in (kx/ky) ratio leads to an increase in seepage quantity. Output variables and input variables have been linked by the ANN model that governs seepage quantity through zoned earth dams and existed gradients. The results showed that both models present a good estimation for the determination of coefficient R2 (0.9003, 0.933).
Overtopping erosion is one of the main factors responsible for the destruction of earthen structures due to floods. Considering the shortage of existing studies and the need for further research to properly understand the processes leading to breaching failure, this study investigates the effect of compaction level (dry density) on the overtopping erosion response of model compacted silty sand dams using a laboratory flume apparatus. From side-view video recordings of the homogeneous earthen dam cross-section, digital image-processing techniques are employed to track and forensically analyse and interpret the initiation and progression of the erosion edge. The results indicate that the overall pattern of initiation and development of overtopping erosion depends on the compaction level, with greater compactive effort (higher shear strength of soil) reducing its progression speed, thereby increasing the time period for the erosion edge to reach the dam’s upstream crest (tB) and breaching failure. Artificial neural network and response surface methodology (RSM) approaches are investigated for estimating the experimental tB values, with the RSM-derived third-order polynomial found to produce good predictions for the three compaction levels investigated. Finally, recommendations are given for further research, including employing the experimental set-up presented for investigating other types of dam failure.
Earth-fill dams are the most common types of dam and the most economical choice. However, they are more vulnerable to internal erosion and piping due to seepage problems that are the main causes of dam failure. In this study, the seepage through earth-fill dams was investigated using physical, mathematical, and numerical models. Results from the three methods revealed that both mathematical calculations using L. Casagrande solutions and the SEEP/W numerical model have a plotted seepage line compatible with the observed seepage line in the physical model. However, when the seepage flow intersected the downstream slope and when piping took place, the use of SEEP/W to calculate the flow rate became useless as it was unable to calculate the volume of water flow in pipes. This was revealed by the big difference in results between physical and numerical models in the first physical model, while the results were compatible in the second physical model when the seepage line stayed within the body of the dam and low compacted soil was adopted. Seepage analysis for seven different configurations of an earth-fill dam was conducted using the SEEP/W model at normal and maximum water levels to find the most appropriate configuration among them. The seven dam configurations consisted of four homogenous dams and three zoned dams. Seepage analysis revealed that if sufficient quantity of silty sand soil is available around the proposed dam location, a homogenous earth-fill dam with a medium drain length of 0.5 m thickness is the best design configuration. Otherwise, a zoned earth-fill dam with a central core and 1:0.5 Horizontal to Vertical ratio (H:V) is preferred.
Based on model tests of earthen dam breach due to piping failure, a numerical model was developed. A key difference from previous research is the assumption that the cross-section of the pipe channel is an arch, with a rectangle at the bottom and a semicircle at the top before the collapse of the pipe roof, rather than a rectangular or circular cross-section. A shear stress-based erosion rate formula was utilized, and the arched pipe tunnel was assumed to enlarge along its length and width until the overlying soil could no longer maintain stability. Orifice flow and open channel flow were adopted to calculate the breach flow discharge for pressure and free surface flows, respectively. The collapse of the pipe roof was determined by comparing the weight of the overlying soil and the cohesion of the soil on the two sidewalls of the pipe. After the collapse, overtopping failure dominated, and the limit equilibrium method was adopted to estimate the stability of the breach slope when the water flow overtopped. In addition, incomplete and base erosion, as well as one- and two-sided breaches were taken into account. The USDA-ARS-HERU model test P1, with detailed measured data, was used as a case study, and two artificially filled earthen dam failure cases were studied to verify the model. Feedback analysis demonstrates that the proposed model can provide satisfactory results for modeling the breach flow discharge and breach development process. Sensitivity analysis shows that the soil erodibility and initial piping position significantly affect the prediction of the breach flow discharge. Furthermore, a comparison with a well-known numerical model shows that the proposed model performs better than the NWS BREACH model. Keywords: Earthen dam, Piping failure, Overtopping failure, Breach flow, Numerical modeling, Sensitivity analysis
Sparmos embankment dam was built in 1990, forming an off-stream reservoir. It is built on rock foundation using weathered gneiss classified as SC with 25% fines. It has a maximum height (crest to downstream toe) of 15.8m. The dam developed wet patches and leakages at the downstream slope soon after the first filling of the reservoir. The leakages that were observed for many years, but left unattended, intensified in March 2016 and led to the breach of the dam and the rapid emptying of the reservoir. Immediately after the reservoir rapid draw-down, a sliding failure was observed on the upstream slope, near the right bank. The paper presents the failure and investigates the mechanism of internal erosion following Bulletin B164. It is most probably a case where contact erosion started along segregated construction layer interfaces and 27 years later developed into piping that caused the breach of the dam. RÉSUMÉ : Le barrage en terre de Sparmos a été construit en 1990 et forme un réservoir hors cours d'eau de la rivière. Il a été construit sur un fond rocheux, ayant des recharges (amont et aval) de gneiss altéré classé dans la catégorie SC avec 25% de grains fins. Sa hauteur maximale (de la crête à la pied aval) est 15.8m. Le barrage a développé des zones humides et des fuites sur la pente en aval peu après le premier remplissage du réservoir. Les fuites observées depuis de nombreuses années, mais laissées sans surveillance, se sont intensifiées en mars 2016 et ont conduit à la rupture du barrage et au vidange rapide du réservoir. Immédiatement après la vi-dange rapide du réservoir, un glissement a été observée sur la pente en amont, près de la rive droite. Cet article se réfère a l'échec et étudie le mécanisme de l'érosion interne d'après le Bulletin B164. C'est probablement un cas où l'érosion de contact a commencé au long des interfaces des couches de construction séparées et 27 ans plus tard, s'est transformée en renard qui a été l'origine de la rupture du barrage 1 INTRODUCTION
This paper discusses a piping model that fundamentally differs from Sellmeijer’s model in relation to the structure of the formula used to calculate the critical hydraulic gradient at which backward erosion leads to dike failure. In this model, the laminar groundwater flow in the sand layer is schematized by characteristic discharges, which are estimated by Darcy’s Law. The laminar pipe flow and the incipient motion of the particles are described using equations of Hagen–Poiseuille, Darcy–Weisbach and Shields. The shear-stress concept of Grass is used to include the effects of the non-uniformity of the sand mixture on pipe erosion. Sellmeijer’s piping equations and the proposed piping model are compared using over 100 laboratory experiments and some field observations.
Backward erosion piping is an important failure mechanism for cohesive water retaining structures which are founded on a sandy aquifer. At present, the prediction models for safety assessment are often based on 2D assumptions. In this work, a 3D numerical approach of the groundwater flow leading to the erosion mechanism of backward erosion piping is presented and discussed. Comparison of the 2D and 3D numerical results explicitly demonstrates the inherent 3D nature of the piping phenomenon. In addition, the influence of the seepage length is investigated and discussed for both piping initiation and piping progression. The results clearly indicate the superiority of the presented 3D numerical model compared to the established 2D approach. Moreover, the 3D numerical results enable a better understanding of the complex physical mechanism involved in backward erosion piping and thus can lead to a significant improvement in the safety assessment of water retaining structures.
A new and relatively simple equation for the soil-water content-pressure head curve is described. The particular form of the equation enables one to derive closed-form analytical expressions for the relative hydraulic conductivity, when substituted in the predictive conductivity models of N. T. Burdine or Y. Mualem. The resulting expressions contain three independent parameters which may be obtained by fitting the proposed soil-water retention model to experimental data. Results obtained with the closed-form analytical expressions based on the Mualem theory are compared with observed hydraulic conductivity data for five soils with a wide range of hydraulic properties.
A large number of embankment structures, including dams, levees,
dikes, and barriers, have been built by humans or formed naturally
along rivers, lakes, and coastal lines around the world. Most of
these structures play very important roles in flood defense,
although many are also used for water supply, power generation,
transportation, and sediment retention. Because these structures can
sustain only limited safety levels and are subject to decay, they may
fail owing to various triggering mechanisms (Costa 1985; Foster
et al. 2000; Allsop et al. 2007), particularly with a high probability
of failure under extreme conditions. These failures pose significant
flood risks to people and property in the inundation area and cause
an interruption of services provided by these structures. Examples
of such events include the failure of the Teton Dam in 1976 (Ponce
1982) and the New Orleans levee failures during Hurricane Katrina
in 2005 (Sills et al. 2008), as shown in Figs. 1 and 2. Clearly, understanding
and prediction of embankment failure processes are crucial
for water infrastructure management.
Watershed models are powerful tools for simulating the effect of watershed processes and management on soil and water resources. However, no comprehensive guidance is available to facilitate model evaluation in terms of the accuracy of simulated data compared to measured flow and constituent values. Thus, the objectives of this research were to: (1) determine recommended model evaluation techniques (statistical and graphical), (2) review reported ranges of values and corresponding performance ratings for the recommended statistics, and (3) establish guidelines for model evaluation based on the review results and project-specific considerations; all of these objectives focus on simulation of streamflow and transport of sediment and nutrients. These objectives were achieved with a thorough review of relevant literature on model application and recommended model evaluation methods. Based on this analysis, we recommend that three quantitative statistics, Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error to the standard deviation of measured data (RSR), in addition to the graphical techniques, be used in model evaluation. The following model evaluation performance ratings were established for each recommended statistic. In general, model simulation can be judged as satisfactory if NSE > 0.50 and RSR < 0.70, and if PBIAS + 25% for streamflow, PBIAS + 55% for sediment, and PBIAS + 70% for N and P. For PBIAS, constituent-specific performance ratings were determined based on uncertainty of measured data. Additional considerations related to model evaluation guidelines are also discussed. These considerations include: single-event simulation, quality and quantity of measured data, model calibration procedure, evaluation time step, and project scope and magnitude. A case study illustrating the application of the model evaluation guidelines is also provided.
Knowledge of pore water pressure in an earth dam is crucial for analysing its mechanical stability. In classical calculations of these pressures, great uncertainty exists regarding the permeability of the materials and the representation of their spatial variability. In this article, a probabilistic analysis of pore water pressures based on field data is performed to represent the permeability with a 2D random field established from statistical and geostatistical analyses. This random field is introduced in a model based on the Finite Element Method (FEM) and the influence of the spatial variability of permeability on pore water pressure is then studied using Monte-Carlo simulations (MCS).
This paper combines seepage analysis with limit equilibrium analysis to investigate the safety of an earth dam over time and during drawdown. The numerical investigation is accomplished by directly coupling the de-terministic software packages Seepage/W and Slope/W with the StRAnD reliability software. The first-order reliability method is employed in reliability analysis. Sensitivity analyses reveal that saturated hydraulic conductivity (k s), friction angle (ϕ′) and cohesion (c′) are the random parameters with the greatest contribution to failure probabilities. The cumulative effect of random saturated hydraulic conductivity (mainly) makes critical times and critical slip surfaces significantly different in the probabilistic and deterministic analyses.
Earth dams are widespread throughout the world and their safety has gained increasing concern from geotechnical engineering societies. Although probabilistic stability analysis approach has been widely applied to the safety assessment of geotechnical structures, few studies have been performed to investigate the effects of water level fluctuations on earth dam slope stability considering uncertainties of soil parameters. This study proposes an efficient probabilistic stability analysis approach by integrating a soft computing algorithm of multivariate adaptive regression splines (MARS). The calibration of a MARS model generally requires a large number of training samples, which are obtained from repeated runs of deterministic seepage and slope stability analyses using the GeoStudio software. Based on the established MARS model, the earth dam slope failure probability can be conveniently evaluated. As an illustration, the proposed approach is applied to the probabilistic stability analysis of Ashigong earth dam under transient seepage. The effects of the uncertainties of soil parameters and water level fluctuation velocity on the earth dam slope failure probability are explored systematically. Results show that the MARS-based probabilistic stability analysis approach evaluates the earth slope failure probability with satisfactory accuracy and efficiency. The earth dam slope failure probability is significantly affected by the water level fluctuation velocity and the coefficient of variation of the effective friction angle.
The paper deals with the analysis of triggering conditions and evolution processes of piping phenomena, in relation to both mechanical and hydraulic aspects. In particular, the aim of the study is to predict slope instabilities triggered by piping, analysing the conditions necessary for a flow failure to occur. Really, the mechanical effect involved in the loads redistribution around the pipe is coupled to the drainage process arising from higher permeability of the pipe. If after the pipe formation, the drainage goes prevented for pipe clogging, the porewater pressure increase can lead to the failure or even the liquefaction, with a subsequent flow slide. To simulate the piping evolution and to verify relevant stability conditions, a iterative coupled modelling approach has been pointed out. As example, the proposed tool has been applied to the Stava Valley disaster (July, 1985), demonstrating that piping might be one of triggering phenomena of the tailings dams collapse.
Internal erosion processes due to piping in an earthen levee built in a laboratory flume are reported in this paper. Different sand, silt and clay mixtures are used for constructing the levee. An image processing technique using an edge-detection algorithm is successfully applied to track the erosion process from the recordings of both side and bottom views. The change in the depth of erosion during the piping phenomenon for different soil composition is studied. The results show that small changes in clay content in the soil mixtures significantly affect the erosion rate. The average depth is approximately equal to the average bottom width of erosion in the piping zone. An exponential equation to estimate the depth of erosion as a function of time and the coefficient of soil erodibility is presented. Results for the critical shear stress and the coefficient of soil erodibility agree well with those obtained from the hole erosion test but not with those obtained from the jet erosion test.
The Teton Dam failure flood is modeled with an explicit scheme in a network of channels and reservoirs taking into consideration the progressive inundation and drying of the flood plain. Previous studies on the same flood have been made using one dimensional models producing averaged stage and discharge hydrographs across the flood plain. Results of the models were compared with data from field surveys made after the flood. It is found that the network analysis allows for more accurate representation of flood propagation in a complex topography.
The paper describes the results of a statistical analysis of failures and accidents of embankment dams, specifically concentrating on those incidents involving piping and slope instability. The compilation of dam incidents includes details on the characteristics of the dams, including dam zoning, filters, core soil types, compaction, foundation cutoff, and foundation geology. An assessment of the characteristics of the world population of dams was also carried out. By comparing the characteristics of the dams which have experienced failures and accidents to those of the population of dams, it was possible to assess the relative influence of particular factors on the likelihood of piping and slope instability.Key words: dams, failures, piping, instability database.
The performance of two popular watershed scale simulation models — HSPF and SWAT — were evaluated for simulating the hydrology of the 5,568 km2 Iroquois River watershed in Illinois and Indiana. This large, tile drained agricultural watershed provides distinctly different conditions for model comparison in contrast to previous studies. Both models were calibrated for a nine-year period (1987 through 1995) and verified using an independent 15-year period (1972 through 1986) by comparing simulated and observed daily, monthly, and annual streamflow. The characteristics of simulated flows from both models are mostly similar to each other and to observed flows, particularly for the calibration results. SWAT predicts flows slightly better than HSPF for the verification period, with the primary advantage being better simulation of low flows. A noticeable difference in the models' hydrologic simulation relates to the estimation of potential evapotranspiration (PET). Comparatively low PET values provided as input to HSPF from the BASINS 3.0 database may be a factor in HSPF's overestimation of low flows. Another factor affecting baseflow simulation is the presence of tile drains in the watershed. HSPF parameters can be adjusted to indirectly account for the faster subsurface flow associated with tile drains, but there is no specific tile drainage component in HSPF as there is in SWAT. Continued comparative studies such as this, under a variety of hydrologic conditions and watershed scales, provide needed guidance to potential users in model selection and application.
Failure of horse creek earth dam
- M C Hinderlider
Experimental Study on the Piping Erosion Process in Earthen Embankments
- Y A Sharif
- M Elkholy
- Hanif Chaudhry
- M Imran
Internal Erosion Risks
- Usbr Usace
Progress report on methods for estimating the probability of failure of embankment dams by internal erosion and piping
- R Fell
- C H Wan
- M Foster
Documented cases of earth dam breaches
- V M Ponce
Internal erosion and piping evolution in earth dams using an iterative approach
- F Saliba
- R B Nassar
- N Khoury
- Y Maalouf