TELEMAC 2D Research Articles

Telemac 2D Modeling of Pollutant Transport in the Medjerda River: Impact of Tamarix Vegetation, Bousselem, Tunisia

The aim of this study is to evaluate the impact of pollutant discharges from Bousselem WWTP in the Medjerda River (Tunisia). We chose to model the transport of a passive pollutant from the Bousselem WWTP using Telemac 2D model. The hydrodynamic model calibration was based on identifying the roughness coefficient derived from existing vegetation, mainly from the Tamarix type. Three scenarios of hydrocarbon dispersion representing potential accidents, with concentrations of 0.25, 10, and 50 kg/m3, were simulated. Three successive downstream sections were chosen to track concentration variability in space and over time. The final concentrations recorded (6 km from discharge point) are 500 μg/l, 12 μg/l, and 2.4 μg/l for scenarios of 50 kg/m3, 1 kg/m3, and 0.25 kg/m3, respectively. These values significantly exceed the international norm for drinking water, set at 1 μg/l, and so causing problems for drinking use. The obtained results serve to ensure environmental safety for hydraulic resources.

Physical and numerical modelling of representative tsunami waves propagating and overtopping in converging channels

Tsunami waves pose a threat to coastal communities through overland inundation and overtopping of coastal defence structures and river walls. Tsunami wave overtopping in rivers and in straight or converging channels is yet to be studied in detail. The present study evaluates the performance of current numerical models that are employed in various tsunami warning centres across the world, in terms of their accuracy for the prediction of tsunami wave characteristics in converging channels. The study evaluates each model performance against a benchmark dataset obtained from novel physical model experiments of representative tsunami waves (solitary waves and bores) overtopping a river wall in a converging channel. The complex process of simultaneous steepening and overtopping was observed to be different for solitary waves and bores. The experimental data were then used to compare results from a suite of different numerical models to predict wave heights and overtopping volumes in straight and converging channels. Different numerical models ranging from nonlinear/nondispersive in two-dimensions (2D) to fully nonlinear/dispersive in three-dimensions (3D) were used. The widely used models based on the nonlinear shallow water equations ( ANUGA , BASEMENT , and TELEMAC 2D) fail to capture the wave profiles and heights, and consequently do not accurately predict the overtopping volumes in channels for both breaking and non-breaking waves. A Boussinesq model ( TELEMAC 2D) was found to produce results consistent with experimental measurements for non-breaking waves only, with an error of up to 8% and 20%, for wave heights and overtopping volumes, respectively. On the other hand, the wave heights and overtopping volumes are underpredicted and overpredicted, for solitary waves and bores, respectively, using a three-dimensional nonhydrostatic model ( TELEMAC 3D). When the overtopping occurs in the converging channel, reflection of the incident waves is minimal in both the numerical and the physical models. However, in the absence of overtopping, both the numerical models and the classical analytical long-wave model fail to correctly predict the observed minimal reflection of non-breaking solitary waves in the converging channel. The reasons for this remain to be investigated. • Novel experiments of representative tsunami waves overtopping river wall were conducted. • 2D and 3D numerical modelling results were compared with experimental data. • Nonlinear shallow water models fail to predict solitary wave/bore heights, shapes and overtopping volumes in channels. • Boussinesq model produces satisfactory results for non-breaking solitary waves only. • Numerical models and classical analytical long-wave model fail to predict solitary wave reflection in a converging channel.

Análisis hidro-sedimentológico 2d del comportamiento de un campo de espigones para la protección de márgenes en un río meandriforme

ABSTRACT Alluvial rivers show a highly dynamic morphological patterns, which cause problems in the operation and maintenance of infrastructure located in their environment. In these cases, it is necessary to design structures to control bank morphodynamic processes. This article describes the hydro-sedimentology of the meandering channel called río del Valle (Argentina), for which bank protection was implemented on its outer bank. This study zone was selected for the cutting of an embankment located on the outer bank of the channel due to the excessive lateral migration of the meander. Flow structure and bed morphology patterns are analyzed and discussed using the results of the application of the TELEMAC 2D model coupled to the Sisyphe morphological module. These results allow us to identify vector patterns of velocity and streamlines in the groyne field, showing the development of a recirculation flow between groynes, as well as the influence in the displacement of the core of maximum velocity towards the center of the channel along the bend apex. The bed evolution shows the vertical regrowth of the bed in the groyne field and the reduction of the point bar in the bend entrance.

Experimental and numerical study of flood dynamics in a river-network-floodplain set-up

Abstract Flood simulations demand mathematical models, which are rigorously calibrated and validated against benchmarking datasets. For this purpose, experiments are conducted in a river-network-floodplain set-up. Hypothetical stepped hydrographs are passed through the channel-network, and fluvial flooding situations are created. Flood depths are recorded at various locations and evolving flood extents are extracted by image processing. TELEMAC 2D is tested against the observed data. The most accurate scheme for flood prediction is identified through sensitivity analysis. Inclusion of the turbulence model is found to improve the accuracy in predicting dynamic flood extents. The model seems to slightly overpredict inundation extents during the rising limb of the hydrographs and underpredict during the falling limb. In addition, certain aspects of a flood such as river–floodplain interaction and junction hydraulics cannot be reproduced with high precision by the 2D model. The experimental datasets can be a valuable resource to mathematical modellers and are freely downloadable.

Comparative study between turbulence models in curved channels

ABSTRACT This paper presents a comparative study between results obtained in two-dimensional computational simulations performed with three different turbulence models: constant viscosity; Elder Model and k˗Ꜫ Model. The simulations were performed using the software Telemac 2D. These results were compared to data obtained from a study in experimental channel with trapezoidal cross-section and composed of straight stretches and curves. The main objective of this comparison is to explore how turbulence models affect the general behavior of the simulated flow. To support these comparisons, statistical analysis were adopted to quantify the differences between the velocity fields obtained in the simulations and that observed in the experimental channel. The results showed that, despite the theoretical limitations, the use of the simpler turbulence closure model, that is the constant turbulent viscosity, can lead to results as good as or better than those obtained with more sophisticated models.

NEW WAVE HINDCAST FOR THE RIO DE LA PLATA ESTUARY

This paper present a wave hindcast for the Río de la Plata Estuary. It was based on the version 5.16 of the numerical wave model WAVEWATCH III ®, forcing with CFSR winds and considering non-stationary water levels and currents obtained from the hydrodynamic model TELEMAC 2D. A multi-mission altimetry database processed by IFREMER was considered as the ground truth. It was taken as reference to calibrate and validate the wave model, and also to do a local validation of CFSR winds. Tuning the Γ coefficient of the JONSWAP parametrization of dissipation by bottom friction (Sbot) and the BETAMAX coefficient of the energy input by wind (Sin), it was possible to correct the large negative BIAS in significant wave height (Hs) obtained with the default configuration. The hindcast cover the period 1985–2016 providing wave parameters with a spatial resolution of 1 km and a time step of 1 hour and full spectra at a 10 km offshore distance along the coast.

Calibration procedure of hydraulic simulations for the microhabitat method

Hydropower plants can profoundly modify the natural morphology and hydrology of rivers and can alter the functionality of habitats for fish living and reproduction. In particular, hydropeaking leads to rapid and frequent changes of the hydrodynamic conditions and it is crucial to ensure aquatic habitat quality is maintained as much as possible during these periods. One present method to determine mitigation measure (minimum and maximum flow, rate of change) is to perform hydraulic simulations in 1D or 2D in the range of flow variation and to evaluate habitat quality for fish with the microhabitat method. The hydraulic model calibration has to be conducted carefully since the model has to reproduce precisely the hydraulic conditions from low to high flow rates (up to several times the mean flow of the rivers). Within this range, the friction coefficient can evolve greatly because at low flows the size of roughness elements become comparable to the water depth.. This phenomenon is observed by performing the modelling and the calibration at 2 stations on a river in French Pyrenees with different cross section shapes, one with progressive overflowing of some banks with large blocks and the other one without such phenomena. Thanks to field measurements of water levels at low and high discharge, the calibration process has shown that the friction coefficient can be multiplied by 2 as a function of the discharge. The paper proposes a methodology to evaluate the most appropriate tool. As water depth is concerned, the 2D simulations (TELEMAC 2D) provide similar results to those obtained with 1D (HEC-RAS) because flow remains unidirectional. Then a sensitivity analysis is carried out to estimate the uncertainty on the fish habitat outputs for a fish species (brown trout in the present study) resulting from several widely used friction laws. These friction laws can lead to different conclusions about habitat suitability depending on the calibrated coefficient. Finally, to perform relevant habitat modelling, it is necessary to measure water levels at several discharges and to describe accurately the spatial variability of roughness height.

A gravity-driven runoff and erosion model for sediment transfers at the catchment scale

The aim of this work is to simulate hydraulic transfers and upstream erosion sources in steep and erodible mountain watersheds with a physicallybased hydraulic model. In such environments, immature debris flows and shallow landslides can be the largest sources of sediments transported at the outlet. To simulate these phenomena, a gravity-driven erosion model and a 1D vertical infiltration model have been developed in the TELEMAC 2D numerical code. In this new erosion model, the motion of the granular flow is described with a fully dynamic system and a Coulomb-like bottom friction treatment, more adapted to the properties of the flow. The new model is first qualitatively evaluated on a theoretical test case: a steep plot with a slope break is used to evaluate the erosion and deposition dynamics of a single immature debris flow. Then, the model is confronted to field data on a real catchment (Draix, in the Southern French Alps). First, the infiltration model is successfully calibrated in order to simulate two different rain events. Then, the new erosion model is applied at the catchment scale. The numerical results show a very realistic behavior compared to the field observation, providing erosion in the upper parts of the hillslopes and deposition in the hydraulic network. This work opens promising perspectives, for example by coupling this new model with a classical and complementary velocity-driven model for the erosion, deposition and transfers in the hydraulic network.

Breach modelling by overflow with TELEMAC 2D: comparison with largescale experiments

An erosion law has been implemented in TELEMAC 2D to represent the surface erosion process to represent the breach formation of a levee. We focus on homogeneous and earth fill levee to simplify this first implementation. The first part of this study reveals the ability of this method to represent simultaneously both formations of the breach and the scour hole at the rear of the levee. The resistance or no of the levee seems consistent with the resistance of the soil material given by a standard classification. But this first part has showed that the lateral development of the breach cannot be represented. In the second part of this study, a simply widening of the breach method based on continuous surface erosion process, has been implemented. The comparison with large scale experiments which has been made, shows that both dynamics of the breach formation and the width are not correctly estimated with the only one surface erosion process. Further developments have to be done to integrate the headcut erosion process and mass failure as sliding of the sides of the breach or undercutting.

Resistance and reconfiguration of natural flexible submerged vegetation in hydrodynamic river modelling

In-stream submerged macrophytes have a complex morphology and several species are not rigid, but are flexible and reconfigure along with the major flow direction to avoid potential damage at high stream velocities. However, in numerical hydrodynamic models, they are often simplified to rigid sticks. In this study hydraulic resistance of vegetation is represented by an adapted bottom friction coefficient and is calculated using an existing two layer formulation for which the input parameters were adjusted to account for (i) the temporary reconfiguration based on an empirical relationship between deflected vegetation height and upstream depth-averaged velocity, and (ii) the complex morphology of natural, flexible, submerged macrophytes. The main advantage of this approach is that it removes the need for calibration of the vegetation resistance coefficient. The calculated hydraulic roughness is an input of the hydrodynamic model Telemac 2D, this model simulates depth-averaged stream velocities in and around individual vegetation patches. Firstly, the model was successfully validated against observed data of a laboratory flume experiment with three macrophyte species at three discharges. Secondly, the effect of reconfiguration was tested by modelling an in situ field flume experiment with, and without, the inclusion of macrophyte reconfiguration. The inclusion of reconfiguration decreased the calculated hydraulic roughness which resulted in smaller spatial variations of simulated stream velocities, as compared to the model scenario without macrophyte reconfiguration. We discuss that including macrophyte reconfiguration in numerical models input, can have significant and extensive effects on the model results of hydrodynamic variables and associated ecological and geomorphological parameters.

A Comparative Analysis of Lajeunesse Model with other Used Bed Load Models - Effects on River Morphological Changes

Among the phenomena that greatly influence the river morphology is the sediment transport, especially the bed load mode causing a significant changes in the river morphology. Indeed, the choice of a model or a methodology that can better quantify sediment transport, remains always poorly understood. In this context, a new approach to studying the morphological evolution of rivers is proposed by Charru in 2004, it is based on a model for the erosion and deposition of the particles under laminar flow. In 2006, Charru proposes an extension of this model to turbulent flow. In more advanced research, Lajeunesse in 2010, realizes an experimental study to support the erosion deposition model of Charru, and proposes a new formula to calculate the bed load transport rate. The current research focuses on the effects of bed load transport on the morphological changes in rivers. In the first part, a comparative analysis of empirical laws of bed load transport with experimental data was conducted, in order to test and validate the new bed load model proposed by Lajeunesse, then to check the grain size effect on the sediment transport capacity. In the second part, we are interested in the study of the morphological evolution in rivers. It was performed through numerical modeling using TELEMAC 2D coupled with SISYPHE. The aim is to understand and analyze the morphological changes in the channel bottom. The analysis of the results presented in this paper showed that through the calculated score, most formulas give satisfactory results. In particular at the grain scale, the new bed load transport relation of Lajeunesse, provides an excellent fit to the experimental data. Finally, we were interested in the study of the morphological changes in the channel bottom, it appers clear that the bed load transport has large impacts on river morphology.

Development of a life safety model to estimate the risk posed to people by dam failures and floods

Dam owners face important decisions about the ways in which finite resources should be allocated to ensure the continuing safe operation of ageing dams. Dam safety risk assessments depend on credible estimates of loss of life for hypothetical failure events to aid in quantifying risk and making decisions concerning how structures are maintained. This paper briefly describes the development of a model, called the ‘life safety model’, to provide a physics-based, dynamic model to estimate loss of life and evacuation times that can result from extreme flood events. The life safety model has an agent-based simulator that enables the model to represent a myriad of probable scenarios, which could result from a flood event. Unknown variables such as the effectiveness of warning, road capacity and time-varying population density can be tested in a range of scenarios. The life safety model uses results of flood water depth and velocity from two-dimensional hydraulic models such as Telemac 2D over the course of the event, to represent the flood hazard. Unlike many other models and methods of this kind the life safety model includes a dynamic interaction between the receptors and the flood hazard. The paper describes the application of the model to the Malapasset dam disaster that occurred in 1959 in the South of France. The application of the life safety model to this disaster demonstrated that the model was capable of making accurate estimates of loss of life from an actual flood event.

Présentation du projet SIMBAR sur la modélisation des barrages anti-hydrocarbure

EIGSI, CEDRE, La Rochelle University, CETMEF and LNHE are partners of the project Simbar. An oil spill boom model is constructed with a global point of view. Structural computation and fluid mechanics are used together. New boom conception and design can be evaluated by using the model. Sea current velocity is the main problem for boom efficiency. The actual limit is 0.35 to 0.5 m/s. We propose the study of 3 kinds of oil (light, heavy, and emulsification). A reference for the boom geometry is chosen. It is based on a standard product (buoyancy diameter 55 cm, skirt height 75 cm). Design, material, and anchorage systems are the main investigations done. The physical limit of a boom comes from different effects: droplet carrying, overflow, and submersion. The visualisation of the flow around a 2D boom section is achieved with an experimental channel. The Froude number is used to construct a mock-up at scale 1/15. The “Smoothed Particles Hydrodynamics” SPH numerical method is used to compute the water/oil flow. A monochromatic wave simulates the sea motion on the shore. A membrane finite-element model computes the mechanical stress. The concentration of the mechanical stress is located on the chain in the bottom of the skirt. The computation of the oil boom of the river Elorn (Brest Bay) is achieved at the different stages of a tide. The boom is 1030 m long. Tide and river flows are superposed (TELEMAC 2D computation). The boom leakage comes mainly from the vertical skirt angulation.

Evidence for spatially variable friction from tidal amplification and asymmetry in the Pertuis Breton (France)

The semi-diurnal tides are amplified and distorted as they propagate into the Pertuis Breton, a semi-enclosed shallow basin in the north-eastern part of Bay of Biscay, in France. This paper investigates the influence of bottom friction on amplification and phase lag of the tidal constituent M2 and its overtide M4 in the Pertuis Breton. A fine resolution two-dimensional (2D) numerical model is implemented to simulate tidal propagation. The model solves the depth-averaged shallow-water equations on a finite element grid using the TELEMAC 2D software. A two-zone parameterisation of friction coefficient is introduced to evaluate the impact of smooth mud flats on the tidal asymmetry and amplification in the Pertuis. Fitting the model to observed amplitudes and phases of M2 and M4 evaluates the decrease of Chezy friction coefficient from the mud flats to the rest of the Pertuis as 100:60. This conclusion is supported by the direct estimation based on morphology and composition of sea bed in the Pertuis Breton.

Assessment of TELEMAC system performances, a hydrodynamic case study of Anglet, France

The wave propagation and flow modules of the TELEMAC system have been applied to the Adour River mouth and the adjacent beaches of Anglet (France). The wave propagation has been simulated using the phase-averaged model TOMAWAC. In addition, the phase-resolving REFDIF S model has been also used, as the diffraction of wind waves, which is not taken into account in TOMAWAC, can occur in the vicinity of the River mouth. Next, the hydrodynamics has been simulated using the two-dimensional depth-averaged flow TELEMAC 2D model. The numerical results are compared with field measurements collected in March 2003. Model performance statistics are calculated in order to assess the accuracy of the predictions at six locations. Regarding the computation of the wave propagation, a root mean square error (RMSE) is evaluated for the significant wave height, the mean period and the peak direction of propagation. For the flow computation, a mean absolute error (MAE) is used as it is applicable to vector quantities. A classification is also adopted that categorises the quality of the results. Regarding the significant wave height, the average relative RMSE is of 18.2% for the TOMAWAC computations whereas the error is of 30.8% for the REFDIF S simulations. Concerning the peak wave direction, the RMSE is about 10°. Both models predict the wave direction normal to the shore whereas the experimental parameters are more variable. However, the areas, where energy concentrates, are well reproduced. The classification of errors shows that TELEMAC 2D flow predictions are in a range from excellent to reasonable.

Extraordinary Flood Response of a Small Urban Watershed to Short-Duration Convective Rainfall

Abstract The 9.1 km2 Moores Run watershed in Baltimore, Maryland, experiences floods with unit discharge peaks exceeding 1 m3 s−1 km−2 12 times yr−1, on average. Few, if any, drainage basins in the continental United States have a higher frequency. A thunderstorm system on 13 June 2003 produced the record flood peak (13.2 m3 s−1 km−2) during the 6-yr stream gauging record of Moores Run. In this paper, the hydrometeorology, hydrology, and hydraulics of extreme floods in Moores Run are examined through analyses of the 13 June 2003 storm and flood, as well as other major storm and flood events during the 2000–03 time period. The 13 June 2003 flood, like most floods in Moores Run, was produced by an organized system of thunderstorms. Analyses of the 13 June 2003 storm, which are based on volume scan reflectivity observations from the Sterling, Virginia, WSR-88D radar, are used to characterize the spatial and temporal variability of flash flood producing rainfall. Hydrology of flood response in Moores Run is characterized by highly efficient concentration of runoff through the storm drain network and relatively low runoff ratios. A detailed survey of high-water marks for the 13 June 2003 flood is used, in combination with analyses based on a 2D, depth-averaged open channel flow model (TELEMAC 2D) to examine hydraulics of the 13 June 2003 flood. Hydraulic analyses are used to examine peak discharge estimates for the 13 June flood peak, propagation of flood waves in the Moores Run channel, and 2D flow features associated with channel and floodplain geometry.

Airborne LiDAR in support of geomorphological and hydraulic modelling

AbstractThis paper considers the application of airborne laser altimetry (LiDAR) to the provision of elevation data at accuracies and spatial densities commensurate with the current generation of high‐resolution hydraulic models. Three sets of issues are addressed with reference to a Telemac 2D model of a morphologically complex estuary in eastern England. First, the quality of airborne LiDAR data is assessed via multiscale calibration against surveyed sections and supplementary control points. Second, image processing techniques are used for (i) identification of multiple regions of interest within large LiDAR mosaics; (ii) subregional infilling of voids left by data ‘drop‐outs’; (iii) filtering and subsampling to match topographic information content with model resolution and the density of the computational mesh. Third, the implications of improved terrain data are considered with reference to the estimation of elevations and potential tidal volumes for a number of discrete flood compartments and the modelling of hypothetical inundation scenarios. After minor offset correction to ensure registration with local benchmarks, the quality of LiDAR elevation data within a 12 km × 4 km coverage is found to be consistent with the published specification (±0·10 to ±0·15 m). Image‐processing tools provide an efficient means of managing information content and interfacing this with the GIS‐based functionality of the pre‐processing software used in conjunction with the hydraulic model. The accuracy and spatial resolution of the LiDAR data allow the identification of subtle but important topographic variations between adjacent flood compartments. In many cases, differences in model results obtained using these data relative to previously estimated average flood compartment elevations are small. Importantly, however, LiDAR provides topographic information at an accuracy and resolution close to the present limits of model representation. Reliable representation of form allows the modeller to concentrate on the physical aspects of model parameterization, whilst minimizing the conflation of parameter effects with those of poorly constrained geometric representation. Copyright © 2003 John Wiley & Sons, Ltd.

Fuzzy rule-based model for contaminant transport in a natural river channel

Fuzzy rules are used to model solute dispersion in a river dead zone, such that the turbulent diffusion is determined by a fuzzy inference system which relates the local mean velocity shear to the longitudinal velocity fluctuations. A finite-volume hybrid scheme is applied to a non-orthogonal grid for which a mean velocity field is produced using the computational fluid dynamics (CFD) package Telemac 2D. At each cell face fuzzy rules predict a fuzzy number, and these numbers reflect the possible magnitudes of turbulent velocity fluctuations. These are input to the finite-volume model using a single-value simulation method. Multiple model runs produce a fuzzy number for the solute concentration in each cell. The results of the fuzzy model are then compared with data collected in a field experiment with rhodamine dye in the River Severn.