Estimation des niveaux d'inondation pour une crue éclair en milieu urbain : comparaison de deux modèles hydrodynamiques sur la crue de Nîmes d'octobre 1988

Lors des crues extrêmes en ville, une forte part des écoulements reste en surface. Pour simuler ces inondations, deux modèles sont présentés : le logiciel REM2 U unidimensionnel a pour objectif de simuler la propagation des débits de crue dans l'ensemble d'un réseau de rues alors que le logiciel Rubar 20 bidimensionnel vise à fournir plus d'information sur ces écoulements. Des calculs avec ces deux logiciels ont été menés sur la crue d'octobre 1988 dans un quartier de Nîmes. Lors de cet événement, les hauteurs d'eau maximales ont dépassé deux mètres en certains points et les vitesses 2 m/s ce qui entraînait des passages en régime torrentiel. A partir des données rassemblées sur les sections en travers des rues, des maillages de calcul limités au réseau de rues ont été construits pour les deux logiciels afin de permettre un calcul détaillé. La comparaison des résultats avec les laisses de crue montre des situations très contrastées d'un point à un autre pour une hauteur d'eau maximale moyenne sur l'ensemble de la zone inondée correctement simulée. L'écart sur cette hauteur est, en moyenne, de 1 m ce qui provient des incertitudes sur les observations, sur la topographie et sur les conditions aux limites, des approximations lors de la modélisation et de particularités locales non décrites. Entre les deux logiciels, l'évolution des hauteurs et des vitesses est généralement très proche bien que, comme pour la comparaison avec les laisses de crue, des différences locales importantes sont observées.The hydraulic models that are used to simulate floods in rural areas are not adapted to model floods through urban areas, because of details that may deviate flows and create strong discontinuities in the water levels, and because of the possible water flow running in the sewage network. However, such modelling is strongly required because damage is often concentrated in urban areas. Thus, it is necessary to develop models specifically dedicated to such floods. In the southern part of France, rains may have a high intensity but floods generally last a few hours. During extreme events such as the October 1988 flood in the city of Nîmes, most of the flow remained on the ground with high water depths and high velocities, and the role of sewage network can be neglected. A 1-D model and a 2-D model were used to calculate such flows, which may become supercritical. On the catchments of the streams which cross the city of Nîmes, the rainfall was estimated as 80 mm in one hour and 250 mm in six hours in October 1988, although some uncertainties remain. The return period can be estimated between 150 and 250 years. The zone selected to test the models was an area 1.2 km long and less than 1 km wide in the north-eastern part of the city. It includes a southern part with a high density of houses. The slope from the North (upstream) to the South (downstream) was more than 1 % on average and was decreasing from North to South. Various topographical and hydrological data were obtained from the local Authorities. The basic data were composed of 258 cross sections of 69 streets with 11 to 19 points for each cross section. Observations of the limits of the flooded areas and of the peak water levels at more than 80 points can be used to validate the calculation results. The inputs consisted of two discharge hydrographs, estimated from a rainfall-discharge model from rains with a return period of 100 years, which may result in an underestimate of these inputs. These two hydrographs correspond to the two main structures that cross the railway embankment, which constitutes an impervious upstream boundary of the modelled area. Whereas the western and eastern boundaries are well delimitated by hills above maximum water levels, the downstream southern boundary is somewhat more questionable because of possibilities of backwater and inflows from neighbouring areas.The 1-D software REM2U solved the Saint Venant equations on a meshed network. At crossroads, continuities of discharge and of water heads were set. The hydraulic jump was modelled by a numerical diffusion applied wherever high water levels were found. The Lax Wendroff numerical scheme was implemented. It included a prediction step and a correction step, which implied precise solving of these very unsteady and hyperbolic problems. The software was validated on numerous test cases (Al Mikdad, 2000) which proved the adaptation to problems of calculations in a network of streets.The 2-D software Rubar 20 solves 2-D shallow water equations by an explicit second-order Van Leer type finite volume scheme on a computational grid made from triangles and quadrilaterals (Paquier, 1998). The discontinuities (hydraulic jumps for instance) are treated as ordinary points through the solving of Riemann problems. For the Nîmes case, the grid was built from the cross sections of the streets. Four grids were built with respectively 4, 5, 7 or 11 points for every cross section and these points correspond to the main characteristics of the cross section: the walls of the buildings, the sidewalks, the gutters and the middle point. The simplest crossroads were described from the crossings of the lines corresponding to these points, which provide respectively 16, 25, 49 or 121 computational cells. The space step was about 25 metres along the streets but went as low as 0.1 m in the crossroads; due to the explicit scheme, which implies that the Courant number was limited to 1, the time step was very small and a long computational time was required.The computations were performed with a uniform Strickler coefficient of 40 m1/3/s. Both 1-D and 2-D models provided results that agreed well with observed water levels. The limits of the flooded area were also quite well simulated. However, locally, the differences between calculated and observed maximum water depths were high, resulting in an average deviation of about 1 metre. The reasons for such deviations could come from three main causes. First, the uncertainty of topographical data is relatively high, because of the interpolation between measured cross sections without a detailed complementary DEM (digital elevation model). Second, the observed levels were also uncertain and reveal local situations that are not reconstructed by the hydraulic models which provided maximum water levels averaged on one cell which may not coincide with the exact location of the observations. Finally, modelling means a simplification of the processes, which implies cancelling the level variations due to some obstacles, such as cars, which are not simple to identify.In conclusion, both software packages can model a flood, even a flash flood, in an urbanised area. Research is still necessary to develop methods to fully use urban databases in order to define details more precisely. The improvements to the 1-D software should include a better modelling of storage and of crossroads with an integration of adapted relations for the head losses. 2-D software has a greater potential but the difficulty to build an optimal computational grid means a long computational time, which limits the use of such software to small areas. For both software packages, methods still need to be developed in order to represent exchanges with the sewage network, storage inside buildings and inputs directly coming from rainfall

Influence of vegetation to boundary shear stress in open channel for overbank flow

River hydrodynamicsBed roughness and flow resistanc

On the estimation of the bed-material transport and budget along a river segment: application to the Middle Loire River, France

Sediment load and budgets are a fundamental component of the process-based hydromorphological framework developed by the REFORM project, and are needed to accurately assess the current condition of a river, its sensitivity to change, and its likely future evolutionary trajectory. This paper presents an evaluation of three different methods for estimating both bedload sediment transport and bed-material budget within river channels, using the Middle Loire River as a case study. The first method is based on the stream power concept and does not need any hydraulic calculations. It yields estimates of the sediment transport in the same order of magnitude as measurements but poor results for the bed-material budget in terms of magnitude and tendency. For the second method, hydraulic parameters are computed using the Manning–Strickler equation (or a 1D hydraulic model for steady flow). It provides useful indicators for understanding river dynamics but does not yield significant improvements compared to the first method. The third method uses 1D numerical software for water flow and river bed evolution. It yields the most accurate results for both sediment transport and bed evolution but requires more data and overall more work to construct the model. Guidance is provided on the amount of data required, the competence needed to build the models, and the predictive capability of each of the methods

Attending school after treatment for a brain tumor: Experiences of children and key figures

Reintegration into school is a milestone for childhood brain tumor survivors, as well as for their parents, teachers, and healthcare providers. We explored their experiences following the school re-entry by conducting semi-structured interviews. Thematic analysis resulted in four main themes: "school performance," "psychosocial well-being," "support and approach," and "communication and collaboration." Children were pleased to return to school despite confrontation with adverse outcomes. Parents, teachers, and healthcare providers identified current and future concerns and challenges, as well as opportunities for academic and personal development. Their experiences highlight the importance of coordinated and systematic follow-up in close collaboration with healthcare providers

Short-term perspectives of parents and teachers on school reintegration of childhood brain tumour survivors

OBJECTIVE: To discover short-term changes in perspectives of parents and teachers of childhood brain tumour survivors on school reintegration, in order to reveal similarities and differences between them over time. METHODS: Semi-structured interviews were conducted with parents and teachers of five children at the start and the end of a 1-year period following the child\u27s school re-entry. RESULTS: Thematic analysis of data resulted in three main themes: \u27the child\u27s performance and wellbeing\u27, \u27the school\u27s attitude and approach\u27 and \u27communication and working together\u27. Parental concerns about child-specific changes and the school\u27s approach to the child could either decrease or increase over time. Teachers remained focused on assessing their pupil\u27s learning potential and finding ways of appropriate support. Their different perspectives on communication and working together became more pronounced. CONCLUSIONS: This study emphasizes the importance of clear communication and collaboration, coordinated follow-up and availability of healthcare professionals during the child\u27s school reintegration

Modélisation hydrosédimentaire de la basse plaine de l'Agly

National audienceLa basse plaine de l'Agly est constituée par les alluvions apportés par le fleuve au fil des siècles. L'endiguement du fleuve dans sa partie la plus aval dans la seconde moitié du vingtième siècle a réduit l'apport massif de sédiments au lit majeur. Malgré la présence d'un barrage à l'amont, des volumes importants de sédiments sont encore apportés et doivent transiter jusqu'à la mer. Pour comprendre l'importance relative des différents processus en oeuvre dans la basse plaine et la répartition des sédiments entre les différentes zones de dépôt potentiels, une modélisation numérique du transport de sédiments a été effectuée. Deux événements de crue sont examinés : la crue de 1992 et la crue de 1999 pendant laquelle les endiguements se sont rompus. En alternative à l'utilisation d'un modèle hydrosédimentaire bidimensionnel adapté à la simulation de dépôts sur la plaine alluviale, un modèle couplé 1D/2D est proposé : la modélisation unidimensionnelle du lit endigué peut être une alternative efficace (elle évite un maillage fin du lit mineur) si elle réussit à transcrire l'évolution dissymétrique de la géométrie du lit dans les courbes, la modélisation bidimensionnelle restant indispensable pour simuler l'étalement des sédiments déposés sur le lit majeur. La comparaison entre les deux types de modélisation pour différentes valeurs des paramètres sédimentaires permet à la fois d'évaluer l'intérêt de cette modélisation couplée et d'identifier l'importance des crues rares dans l'évolution du lit prévisible à long terme

Errors in 2-D modelling using a 0th order turbulence closure for compound channel flows

peer reviewedWhen dealing with flood issues, compound channels flows are often encountered in the field. This paper investigates the errors that can be expected when using 2-D modelling for compound channel flows, comparing the simulations with experiments. Three flow configurations are analyzed: uniform, gradually varied and rapidly varied flows. The last configuration is obtained by setting a transverse embankment on the flood plain. Errors are estimated on the sub-section mean velocity, discharge andwater depth, on the mixing layer width and on the depth-averaged stream-wise velocity and lateral shear stress. Depending on the flow configuration and on the studied parameter, relative errors significantly vary from nearly zero to 50%. The influence of the 0th order turbulence closure on the mean flow and the dimensions of the recirculation zone behind the embankment is also investigated, using either constant eddy viscosity or Elder's model. © 2012 Taylor & Francis Group, London

Débordement d'égout dans le modèle urbain MURI

River Flow 2018, Villeurbanne, FRA, 05-/09/2018 - 08/09/2018International audienceSewer overflow is one of the main causes of urban floods. Using an urban model called MURI recently built by Irstea, a series of experiments are carried out in order to simulate overflowing cases. The first configuration that was studied is a single street of 5.4 m in length, 0.15 m in width with a slope of 0.1% and two street inlets (of which the orifice diameter is 14 mm) that ensure the connection with the sewer network. Inflows of 2 and 3 L/s are used for the street and an inflow of 0.5 L/s for the sewer network that is partially closed in order to obtain overflow at the street inlets. The rising of the weir downstream the street permitted to have enough water depth to measure the vertical profiles of velocities. This study allowed us to investigate how the overflow process influence the water depth and the velocity field around the street inlet. The differences of results between the two street inflows reveals the complexity and explains the difficulty to calculate exchange flow.Ppour comprendre les inondations par débordement de réseau, des expériences ont été effectuéees dans le modèle urbain MURi au laboratoire d'hydraulique d'Irstea à Villeurbanne. la complexité des écoulements rend difficile le calcul des débits d'échange