Inondations urbaines : un indicateur géométrique caractéristique du comportement hydraulique du bâti

La prévision des inondations urbaines et de leur impact sur le milieu passe par la modélisation précise et lisible des flux inondants. Leur représentation est cependant rendue difficile par le caractère transitoire et multidirectionnel des écoulements, dans un milieu dont la géométrie est très irrégulière. Cet article traite plus spécifiquement du comportement du bâti africain vis à vis des écoulements, en situation inondante, et des lois de stockage et de vidange que l'on peut définir à différentes échelles représentatives de l'habitat : concession, bloc de concessions. Nous présentons trois propriétés du bâti nécessaires et suffisantes pour décrire le comportement hydraulique du milieu à ces échelles : sa pénétrabilité, sa stockabilité et sa transmissivité. L'étude du comportement hydraulique de l'objet bâti élémentaire, la concession, nous permet de relier ces propriétés à des caractéristiques géométriques de cet objet. Une approche agrégative conduit ensuite à définir un indicateur de la structure géométrique du bâti, l'HistoSeuil, équivalent à une densité d'ouvertures et caractéristique de la pénétrabilité du bâti. L'étude de sa pertinence géométrique, i.e. sa variabilité intra- et inter-quartiers a été réalisée dans le cas particulier de la ville de Ouagadougou (Burkina Faso) ; elle est basée sur le relevé systématique des ouvertures observables sur différentes façades de voiries de trois quartiers de types différents, (habitat individuel et spontané). Sa pertinence hydraulique, i.e. sa capacité à reproduire le comportement hydraulique moyen de l'objet urbain modélisé, est enfin abordée. Développée dans le contexte particulier de Ouagadougou, cette approche est généralisable à des configurations urbaines très diverses.Stormwater runoff generates one of the most critical natural risks in urban environments: impervious surfaces and high drainage network densities lead to frequent urban flooding events, with short process times and within small urban areas. In all parts of the world, urbanisation is growing, and urban flood hazards consequently occur more and more frequently. Examples of important flood damages suffered by urban populations are numerous, especially in tropical regions where the violence and rapidity of tropical storms often lead to an overloading of the drainage system and to the flooding of adjacent built-up areas. Prediction and evaluation of these damages require the determination of some important hydraulic characteristics of the flood, such as maximum water depth or flooding duration. Currently-used models are generally limited to checking the sewer system efficiency. Therefore new models are now expected to represent with accuracy and reliability the stormwater runoff, which can result from sewer system overloading. However, this kind of modelling is hard to carry out because of the geometric complexity of the urban media and because of the rapidity of urban storms and their associated flooding. Moreover, the modelling of the behaviour of the flooded built-up areas should sometimes be integrated into complete models of urban flooding, given their important influence on the hydrodynamics of the flood. However the geometric complexity of these built-up areas prevents us from a complete and accurate description of the different obstacles and water ways encompassed in such areas. Simplified descriptions at a larger scale are consequently to be found.This paper highlights the important physical characteristics that determine the hydraulic behaviour of every hydraulically-independent urban cell, and suggests a way to represent the exchange and storage laws of built-up areas at different scales: individual plots and blocks of plots. The study was performed in the particular case of Ouagadougou's areas. The hydraulic behaviour of every built-up area can be modelled with three important physical characteristics, the two first of which are related to the structure of the surrounding walls:- water perviousness : the ease with which the passing flood can enter or exit the plot. This is dependent upon the aperture density, which can vary according to the façade;- transmissivity: the ease with which water can pass through the plot. It depends on the perviousness of the different external or internal façades of the individual plot. If one façade is waterproof, the transmissivity becomes nil in the perpendicular direction;- storativity : determined with the storage capacity of an individual plot. It is a function of the internal surface area of the plot.These three characteristics are functions of height. Moreover, they are essential and sufficient to describe the behaviour of every basic or global urban object (plot, block of plots...). At the "block of plots" level, the transmissivity and storativity concepts are comparable to the hydraulic roughness and urban porosity concepts that have already been proposed in scientific papers (e.g., Braschi et al. 1991). These two characteristics are sufficient for modelling the hydraulic behaviour of every open urban medium. Nevertheless, some urban media are non-transmissive because of a high connection level between the different obstacles. In these cases the perviousness property is very useful for modelling the different exchanges between the built-up areas and the adjacent flooded roads. The residential urban areas of Ouagadougou, used as an illustration for this study (Figure 1), correspond to this case of partitioned urban areas. The structure of the Ouagadougou's residential districts is standard and is organised around the individual plot, a parcel shielding one or several families: the individual plot is isolated from other plots and from the roads by a surrounding wall that constitutes one of the elementary hydraulic objects of the urban environment. The evolution of the flood water depth in a plot adjacent to a flooded road, determined by equation 1, depends on its floodable surface Sc and on its perviousness. Its perviousness is defined by the geometric characteristics of the apertures present in the wall: the type of aperture, weir or orifice; its height, hs, its length, Ls, and its opening if an orifice, a. Measures of exchanges between roads and plots made during some flood events in Ouagadougou (Hingray 1999) showed that the classical discharge laws for weirs or trough orifices can be used to model these exchanges (equations 2, 3 and 4).We suggest a way to simulate the exchange and storage laws of these built-up areas at a larger scale: the block of plots. An aggregation approach enables us to define a structure indicator: " l'HistoSeuil " (Figure 2). It is based on the description of the lengths of weirs and apertures found in road façades, and is equivalent to an aperture density function. The exchange discharge between the block of plots and the flooded adjacent road can be computed with a simply convolution (equation 6) between this HistoSeuil and the reference discharge laws for broad-crested weirs (equations 7 and 8). The geometric relevance of the indicator is next discussed: it seems to be a relatively stable geometric characteristic of an urban area (Figure 3). This result is given by a systematic survey of the apertures observed in 24 road façades belonging to 3 different districts of Ouagadougou. The two first are traditional residential districts, more and less developed. The "Patte d'Oie" district is fairly old and was established in the 1970's (numerous well developed plots). The second one (Wemtenga 1) is a recent housing estate (1988) (numerous unfinished or empty plots). The final one (Wemtenga 2) is a very recent district of spontaneous development (disorganised built-up area structure). Furthermore, the hydraulic relevance of this indicator, its ability to reproduce the average hydraulic behaviour of a block of plots, is approached. Initial results seem to be positive. If both the hydraulic and geometric relevance of the structure indicator presented in this paper are validated by the additional work that we are carrying out at this present time, this approach may prove to be useful for the hydraulic modelling of built-up areas. Moreover the study of other types of built-up areas could lead to the determination of a hydraulic typology of urban areas. In particular, this study, performed in the case of Ouagadougou, a big city in a developing country, seems to be valid for every city where the built-up areas are highly partitioned

Measurement and conceptualization of maternal PTSD following childbirth: Psychometric properties of the City Birth Trauma Scale-French Version (City BiTS-F).

The City Birth Trauma Scale (City BiTS) assesses posttraumatic stress disorder symptoms following childbirth (PTSD-FC). Recent studies investigating the latent factor structure of PTSD-FC in women reported mixed results. No validated French questionnaire exists to measure PTSD-FC symptoms. Therefore, this study first aimed to validate the French version of the City BiTS (City BiTS-F). Second, it aimed to establish the latent factor structure of PTSD-FC. French-speaking women with infants aged 1 to 12 months old (n = 541) completed an online cross-sectional survey. Questionnaires included the City BiTS-F, the PTSD Checklist, the Edinburgh Postnatal Depression Scale, and the anxiety subscale of the Hospital Anxiety and Depression Scale. Additionally, sociodemographic and medical data were collected. The two-factor model with birth-related symptoms (BRS) and general symptoms (GS) fit the data well, whereas the four-factor model was not confirmed. The bifactor model with a general factor and the BRS and GS gave the best fit to the data, suggesting that use of the total score in addition to the BRS and GS subscales scores is justified. High reliability (α = .88 to .90) and good convergent and divergent validity were obtained. Discriminant validity was calculated with weeks of gestation, gravidity, history of traumatic childbirth and event, and mode of delivery. The City BiTS-F is a reliable and valid measure of PTSD-FC symptoms in French-speaking women. Both total score and BRS or GS subscale scores can be calculated. This psychometric tool is of importance for clinical and research purposes. (PsycInfo Database Record (c) 2021 APA, all rights reserved)

Le partage de la ressource en eau sur la Durance en 2050 : vers une évolution du mode de gestion des grands ouvrages duranciens ?

Congrès SHF: Water Tensions in Europe and in the Mediterranean: water crisis by 2050?, Paris, FRA, 08-/10/2015 - 09/10/2015International audienceUne vision prospective de la gestion de l'eau du bassin de la Durance et des territoires alimentés par ses eaux à l'horizon 2050 a été élaborée, appuyée par une chaine de modèles incluant des représentations du climat, de la ressource naturelle, des demandes en eau et du fonctionnement des grands ouvrages hydrauliques (Serre-Ponçon, Castillon et Sainte-Croix), sous contraintes de respect des débits réservés, de cotes touristiques dans les retenues et de restitution d'eau stockée pour des usages en aval. Cet ensemble, validé en temps présent, a été alimenté par des projections climatiques et paramétré pour intégrer les évolutions du territoire décrites par des scénarios de développement socio-économique avec une hypothèse de conservation des règles de gestion actuelles. Les résultats suggèrent à l'horizon 2050 : une hausse de la température moyenne de l'air impactant l'hydrologie de montagne ; une évolution incertaine des précipitations ; une réduction des stocks de neige et une fonte avancée dans l'année qui induisent une réduction des débits au printemps ; une diminution de la ressource en eau en période estivale ; une diminution de la demande globale en eau à l'échelle du territoire, cette demande étant fortement conditionnée par les scénarios territoriaux élaborés ici ; la satisfaction des demandes en eau en aval des ouvrages considérées comme prioritaires, au détriment de la production d'énergie en hiver (flexibilité moindre en période de pointe) et du maintien de cotes touristiques en été ;une diminution de la production d'énergie due notamment à la réduction des apports en amont des ouvrages hydroélectriques

Heterogeneity of patients with functional/dissociative seizures: three multidimensional profiles

Objective Current concepts highlight the neurological and psychological heterogeneity of functional/dissociative seizures (FDS). However, it remains uncertain whether it is possible to distinguish between a limited number of subtypes of FDS disorders. We aimed to identify profiles of distinct FDS subtypes by cluster analysis of a multidimensional dataset without any a priori hypothesis. Methods We conducted an exploratory, prospective multicenter study of 169 patients with FDS. We collected biographical, trauma (childhood and adulthood traumatic experiences), semiological (seizure characteristics), and psychopathological data (psychiatric comorbidities, dissociation, and alexithymia) through psychiatric interviews and standardized scales. Clusters were identified by the Partitioning Around Medoids method. The similarity of patients was computed using Gower distance. The clusters were compared using analysis of variance, chi-squared, or Fisher exact tests. Results Three patient clusters were identified in this exploratory, hypothesis-generating study and named on the basis of their most prominent characteristics: 1. A “No/Single Trauma” group (31.4%), with more male patients, intellectual disabilities, and nonhyperkinetic seizures, and a low level of psychopathology; 2. A “Cumulative Lifetime Traumas” group (42.6%), with clear female predominance, hyperkinetic seizures, relatively common comorbid epilepsy, and a high level of psychopathology; and 3. A “Childhood Traumas” group (26%), commonly with comorbid epilepsy, history of childhood sexual abuse (75%), and posttraumatic stress disorder, but also with a high level of anxiety and dissociation. Significance Although our cluster analysis was undertaken without any a priori hypothesis, the nature of the trauma history emerged as the most important differentiator between three common FDS disorder subtypes. This subdifferentiation of FDS disorders may facilitate the development of more specific therapeutic programs for each patient profile

Comorbidités psychiatriques et épilepsie

National audienceLes comorbidités psychiatriques des épilepsies sont nombreuses et fréquentes. Un patient épileptique sur trois présente au cours de sa vie une pathologie psychiatrique (contre une personne sur cinq en population générale). Les études retrouvent des prévalences augmentées chez les patients épileptiques, en particulier pour les troubles de l’humeur, les troubles anxieux et les troubles psychotiques. Les troubles psychiatriques précèdent, accompagnent ou compliquent les différentes formes d’épilepsie. On sait, de plus, aujourd’hui que l’existence d’un trouble de l’humeur ou d’un trouble psychotique chez un sujet non épileptique augmente significativement le risque ultérieur de développer une épilepsie. Ces comorbidités ont un impact considérable, non seulement en termes de souffrance psychique et de qualité de vie, mais également sur le contrôle des crises épileptiques et sur l’efficacité et la tolérance des traitements antiépileptiques. De toute évidence, le lien qui unit épilepsie et troubles mentaux n’est pas celui d’une causalité unidirectionnelle où les troubles mentaux se réduiraient aux conséquences de l’épilepsie sur la santé mentale. En réalité, il s’agit moins d’une causalité que d’une association ; la relation entre pathologies épileptiques et psychiatriques est bidirectionnelle, voire triangulaire – certains facteurs physiopathologiques exposant les sujets à la fois à la survenue de troubles épileptiques et psychiatriques. L’usage des critères diagnostiques issus du DSM s’avère souvent problématique dans le cas des comorbidités psychiatriques de l’épilepsie. En effet, bon nombre de patients épileptiques présentent des symptômes psychiatriques sévères et invalidants mais atypiques, spécifiques qui ne réunissent pas l’ensemble des critères nécessaires au diagnostic d’un trouble particulier. Une attention particulière doit être notamment portée aux rapports temporels entre les crises d’épilepsie et l’apparition des symptômes psychiatriques. On distingue ainsi les troubles psychiatriques péri-ictaux (pré-ictaux, ictaux, postictaux) des troubles psychiatriques interictaux. Nous détaillerons, en outre, le syndrome dysphorique interictal, la psychose postictale et les crises non épileptiques psychogène

Modélisation conceptuelle globale du régime du transport particulaire sur les fleuves tropicaux d'Afrique : application aux bassins du Niger supérieur et du Bani, Mali

Sur le fleuve Niger, les relations entre concentrations en Matières En Suspension (MES) et débits liquides montrent, à l'échelle d'une crue annuelle, des cycles d'hystérésis orthogrades. Le modèle présenté dans cet article reproduit les variations saisonnières de ces MES à partir du seul débit liquide. Il suppose que les MES proviennent de deux sources distinctes : le système " versants + réseau hydrographique secondaire ", siège d'une érosion saisonnière temporaire et le réseau hydrographique principal, siège d'une érosion permanente. Le modèle représente schématiquement la production de MES provenant de ces deux sources par le biais de deux réservoirs de MES. Le premier contient un stock en MES temporaire et limité. Ce stock, maximum au début de la crue annuelle (stock initial), est mobilisé et entraîné au cours de la saison pluvieuse en produisant un flux journalier, supposé être, à un instant donné, proportionnel au stock restant et à une fonction de puissance du débit. Le second, contient un stock de MES illimité et disponible en permanence. La mobilisation de ce stock produit un flux journalier, supposé être aussi une fonction de puissance du débit, et dont l'importance sera limité par la capacité du cours d'eau. Les cinq paramètres du modèle sont calibrés à l'aide des données acquises durant huit années hydrologiques (1991/92 à 1998/99) sur deux stations du Niger amont (Banankoro et Douna). Malgré les limites d'utilisation actuelles liées à la détermination du stock initial, le modèle présenté reconstitue de façon satisfaisante les variations annuelles des concentrations en MES et offre des perspectives intéressantes pour modéliser l'évolution temporelle des MES observées tant pour les fleuves tropicaux unimodaux que pour les petits bassins versants africains. En termes de flux annuels, le modèle n'apporte pas d'amélioration sensible par rapport à un ajustement statistique simple entre les volumes écoulés et les flux de MES. Cependant, il permet aussi de déterminer les variations de flux au cours de l'année, information qui ne peut être obtenue avec un modèle de régression statistique.Estimating temporal variability of suspended sediment concentrations in a watershed is important for a number of reasons (e.g., sediment yield estimation, provision of input data for reservoir sediment-deposition models and water quality models). Three different approaches have been adopted for modelling erosion and sediment transport: physical erosion models (Wicks and Bathurst, 1996); conceptual models (Negev, 1967; Pinheiro and Caussade, 1996); and empirical models (Walling, 1977; Asselman, 1977). Physical and conceptual models usually require rainfall intensity data. In African tropical river catchments, however, temporal and spatial variability of rainfall are not well known; in case of the Niger, water discharge is the only reliable hydrological parameter. This study proposes a model of temporal changes in suspended sediment concentrations using only water discharge data, thereby eliminating the need for rainfall parameters.Daily discharge and weekly suspended sediment concentration data (from 1991/92 to 1998/99) gathered at two monitoring stations of the Upper Niger (Banankoro and Douna) (Figure 1) were used to study relationships between suspended sediment concentration and river discharge.At each gauging station on the Niger, the relationship between water discharge and suspended sediment concentration during the annual flood is characterized by clockwise hysteresis (Figure 3). Moreover, several other African single-annual-flood rivers-unimodal rivers-also exhibit this type of relationship (Kattan et al., 1987; Olivry et al., 1988; Orange, 1992). This cyclicity suggests a three-stage description of sediment transport dynamics: (1) At the beginning of the rainfall season, sediments are imported by hill-slope surface runoff, re-entrainment of deposits in the channel network, and riverbed erosion. The first two sources consist of easily mobilizable material available throughout the catchment at the beginning of the hydrological year. (2) Sediment availability decreases with time as the soil becomes stabilised by vegetation during the rainy season. Erosion is consequently reduced, despite increased discharge. (3) During the period represented by the falling limb of the flood hydrograph, mobilizable material has been depleted or cannot be entrained; what suspended sediment there is originates upstream and from bank and bed erosion-permanently available sources. Non-seasonal sediment sources are grouped under the label of "continuous erosion."We propose a lumped conceptual model of suspended-sediment concentration variations over the hydrological year. The model divides the erosion, transport and deposition processes into those acting on hill-slopes and those acting in the channel network, and assumes that both are explainable by water discharge Q(t) alone. The hill-slope/channel distinction is based on the fact that suspended sediment transport in a river depends not only on transport, bank and bed-erosion capacity, but also on the amount of available material in the drainage catchment.Sediment transport (in tons per day) for the hydrological year is thus computed as the sum of two independent daily contributions of sediment discharge. The first, Fmob(t), originates from a limited reservoir which is full at the beginning of the flood and available only temporarily, during the rainy season. At a given time t, the sediment input Fmob(t) is proportional to the amount remaining in the limited reservoir and to a power function of water discharge (Eq. (1), Eq. (2)). The second reservoir, temporally and quantitatively unlimited, injects a daily sediment discharge Fec(t) which is a power function of water discharge (Eq. (3)); Fec(t) is limited only by river capacity. The final concentration is obtained from Equation (4).The five model parameters were calibrated with concentration and discharge data from hydrological years 1991/92 to 1995/96. The unlimited reservoir parameters were estimated using Equation (6) with data taken during the decreasing stage. Initial sediment in the limited reservoir was estimated with Equation (7), using observed concentrations and concentrations derived from continuous erosion (Eq. (3)). Two parameters related to the decrease in initial sediment loads were obtained by optimization of the mean Nash criterion between observed and calculated concentrations (Figure 6). Some physical interpretations were ascribed to coefficients related to the limited reservoir.Despite the limitations of assuming a single initial mobilizable reservoir, predictions of temporal sediment-concentration patterns during the annual flood were satisfactory (Figure 7, Table 2). The model also simulated some observed sediment concentration peaks associated with sudden water discharge variations during the rising limb of the annual flood. Best results were obtained by varying initial sediment reservoir estimates for different hydrological years (Table 3)-the model could therefore be improved by highlighting parameters that determine sediment loads at the beginning of the hydrological year.The model does not give better estimates of annual sediment yield than simple regression of annual water volume (Table 4). However, it is able to reproduce the temporal variability of the sediment flux during the annual flood. The small size of the data set makes evaluation of the performance of this model difficult; for better assessment, it should applied to other data gathered on the same catchments or on other large tropical rivers. Model parameter values could also be explained by drainage basin characteristics

Stochastic generation of multi-site daily precipitation focusing on extreme events

Many multi-site stochastic models have been proposed for the generation of daily precipitation, but they generally focus on the reproduction of low to high precipitation amounts at the stations concerned. This paper proposes significant extensions to the multi-site daily precipitation model introduced by Wilks, with the aim of reproducing the statistical features of extremely rare events (in terms of frequency and magnitude) at different temporal and spatial scales. In particular, the first extended version integrates heavy-tailed distributions, spatial tail dependence, and temporal dependence in order to obtain a robust and appropriate representation of the most extreme precipitation fields. A second version enhances the first version using a disaggregation method. The performance of these models is compared at different temporal and spatial scales on a large region covering approximately half of Switzerland. While daily extremes are adequately reproduced at the stations by all models, including the benchmark Wilks version, extreme precipitation amounts at larger temporal scales (e.g., 3-day amounts) are clearly underestimated when temporal dependence is ignored

Influence of spatial discretization, underground water storage and glacier melt on a physically-based hydrological model of the Upper Durance River basin

International audienceThe SAFRAN-ISBA-MODCOU hydrological model (Habets et al., 2008) presents severe limitations for alpine catchments. Here we propose possible model adaptations. For the catchment discretization, Relatively Homogeneous Hydrological Units (RHHUs) are used instead of the classical 8 km square grid. They are defined from the dilineation of hydrological subbasins, elevation bands, and aspect classes. Glacierized and non-glacierized areas are also treated separately. In addition, new modules are included in the model for the simulation of glacier melt, and retention of underground water. The improvement resulting from each model modification is analysed for the Upper Durance basin. RHHUs allow the model to better account for the high spatial variability of the hydrological processes (e.g. snow cover). The timing and the intensity of the spring snowmelt floods are significantly improved owing to the representation of water retention by aquifers. Despite the relatively small area covered by glaciers, accounting for glacier melt is necessary for simulating the late summer low flows. The modified model is robust over a long simulation period and it produces a good reproduction of the intra and interannual variability of discharge, which is a necessary condition for its application in a modified climate context

Hiérarchie des incertitudes sur les étiages futurs liées au climat et à l'hydrologie

International audienceThis works aims at providing a understanding of the different types of modelling uncertainties associated with future low-flow projections. Hydrological projections come indeed at the bottom of the well-known cascade of uncertainty and therefore potentially integrate all types of modeling uncertainties related to both climate and hydrological. The specificity of low flow characteristics is their tight dependency on catchment processes, suggesting a non negligible contribution of the hydrological modelling step to the overall uncertainty. This study builds on an extensive top-down climate impact approach set up within the R2D2-2050 project. This integrated assessment project aims at informing water resource adaptation strategies for the Durance river basin located in the Southern French Alps, by confronting future hydroclimate projections to prospective scenarios of water demand and water use. Future water resource estimates have been based on 11 runs from 4 GCMs from the ENSEMBLES project under the A1B emissions scenario. These large-scale projections have been further statistically downscaled with 3 probabilistic methods based on the k-nearest neighbours resampling approach, leading to 330 spatially distributed climate projections. Downscaled projections have then been used as forcings to 6 diverse hydrological models, from global conceptual to physically-based fully distributed, in order to derive transient hydrological projections from 1961 to 2065 for up to 25 stations over the basin. Most of the Durance subcatchments are under the influence of both the Alpine and Mediterranean climates, which leads to two distinct periods of low flows, one in winter when precipitation is stored as snowpack and one in late summer after the snowmelt. Analyses have therefore been applied separately to the two periods to infer the future evolution of low flows characteristics and its consistency among the range of available projections. Several analysis of variance (ANOVA) frameworks have further been applied to quantify the respective contributions of climate and hydrological modeling uncertainties on the results and their evolution over time. Results first show contrasted evolutions according to the season and the catchment, with little evolution in winter low flows for high-elevation catchments, and a dramatic decrease in summer low flows over the whole basin. The contribution of the hydrological modelling step to the overall uncertainty is moreover limited in winter, but high in summer with a pronounced increase over time. This last result critically suggests a divergence of climate change responses between the hydrological models in terms of summer low flows, potentially due to difference in the implementation of snowmelt processes