Recherche d'un modèle de pluie de projet adapté aux précipitations de la zone tropicale africaine : cas d'Adiopodoumé-Abidjan (Côte d'Ivoire)

La structure de 190 épisodes pluvieux enregistrés en 16 ans à la station d'Adiopodoumé est analysée. Les résultats de cette analyse révèlent que les techniques de construction des pluies de projet proposées par DESBORDES d'une part et par HUFF d'autre part s'adaptent bien au cas d'Adiopodoumé. Cette adéquation est mise en évidence par la comparaison entre des hydrogrammes provoqués par des pluies aux caractéristiques connues enregistrés sur des bassins versants voisins, et ceux calculés à l'aide des pluies de projet type DESBORDES et HUFF de mêmes caractéristiques. (Résumé d'auteur

Caractérisation de l'aléa climatique pluvieux en région méditerranéenne : analyse statistique des surfaces pluvieuses

Ces 10 dernières années, certains épisodes pluvieux marquants ont entraîné une prise de conscience du risque encouru par les agglomérations modernes face à des phénomènes hydrologiques particuliers. La gestion du risque pluvial passe par une amélioration de la connaissance de l'aléa pluvieux. Dans cet article, on développe une approche stochastique exploitant le potentiel d'informations contenu dans un échantillon d'épisodes pluvieux extrêmes ayant ou ayant pu engendrer des crues dévastatrices. Une approche spatiale est utilisée pour caractériser l'aléa pluvieux. A partir d'un jeu d'épisodes extrêmes sélectionnés sur une région méditerranéenne entre 1958 et 1993, on estime l'aire des surfaces où les précipitations dépassent un seuil de pluviométrie fixé. L'estimation des aires des surfaces pluvieuses nécessite le recours à un modèle d'interpolation spatiale des hauteurs de pluie. La justification du krigeage climatologique est présentée ainsi que l'estimation des paramètres du modèle retenu. Les distributions des aires des isohyètes, à différents seuils de pluviométrie, sont ensuite analysées. Il apparaît que quelle que soit l'isohyète considérée, une loi gamma peut être ajustée sur l'échantillon de surface. Une relation entre les paramètres des lois permet une généralisation du modèle probabiliste à n'importe quel seuil de pluie compris entre 50 et 300 mm.In the last 10 years many cities in southern Europe have been affected by heavy rainfall events leading to severe runoffs. The assessment of rainfall risk requires a better knowledge of the climate hazards and particularly rainfall hazards. The most usual rainfall risk assessment is based on a stochastic approach and point rainfall frequency analysis remains the most-used method. However, in the Mediterranean region great variations of rainfall depth frequencies can be observed according to the point considered, and according to the period of observation. Moreover the recent hydrological catastrophes which have affected the south of France have been studied on an individual basis and studies based on a global approach, using the whole information contained in a sample of several observations, remain unusual.A rainfall risk assessment has been proposed in the Languedoc-Roussillon, a 28,000 km2 region along the Mediterranean sea. This study has been based on a sample of 93 daily extreme rainfall events, which have occurred in the region. They have been extracted from the Météo-France database for the 1958-1993 period of observation, if a rainfall depth greater or equal to 190 mm in 24 hours or 48 hours (because of the sampling constraints) has been observed at one rain gauge in the region at least. The spatial extension of the rainy surfaces defined at different rainfall thresholds, varying from 50 to 250 mm/24 hours and 50 to 300 mm/48 hours, have been investigated. For a given threshold, the area of the rainy surface corresponding to a given frequency has been estimated.The estimation of the rainy surfaces area has required the choice of a spatial interpolation method: the climatological kriging method has been used. This method is based on the assumption that all the rainfall events came from the same meteorological situation, but some studies have shown that there may be different meteorological situations (TOURASSE, 1981; RIVERAIN, 1997). Thus the sensitivity of the interpolation model according to this assumption has been tested. A different interpolation model has been estimated for each season because the information about the meteorological situations which have generated the selected events is not available. Only the variogram over June to August differs significantly from the "annual" variogram. The differences between the rainy surfaces area estimated with the "seasonal" variogram and the "annual" one did not exceed 10% in proportion of the areas estimated with the "annual" variogram. The rainy surface areas are less sensitive to the climatological assumption. For each time step and each rainfall threshold considered, it has been observed that the two parameter Gamma law could best fit the frequencies of the rainy surface areas. The relation between each of the Gamma law parameters and the rain threshold has been estimated (relations R1 and R2). The quantiles of the rainy surface areas have been estimated with two methods :- directly from the fitting of a Gamma function to the sample of rainy surface areas; - using the previous relation to estimate the Gamma function parameters. It has been observed that the quantiles estimated with the second method were close to those estimated with the first method, even if the fitting errors of the R1 and R2 relations were considered. Such a result allows one to estimate the regional frequency of a rainy surface areas defined at each threshold between 50 and 300 mm/48 hours or 50 and 250 mm/24 hours. However extrapolations beyond the studied threshold intervals should not be done because the R1 and R2 relations are empirical.The isohyets area quantiles have been defined: they represent the isohyet area corresponding to a given rainfall threshold and a given return period. The isohyet area quantiles may be very large; for example at the 200 mm / 48 hours threshold the isohyets area represents 15% of the region (4500 km2). This can be explained by the time step dt. The isohyets area represents the dynamics of the convective cells integrated over dt, which remains unknown but is greater than 48 hours. Moreover for a given rainfall threshold and a given event, several separate isohyets could be observed. However in this study only the all areas corresponding to the different isohyets have been estimated. Thus it could give a very large area when the event affects the all region.The ratio between the isohyet area quantiles at the 48-hour and 24-hour time steps evolved from 1.3 to 20: it increased with the rainfall threshold for a given return period. This can be explained by the strong dynamics of the convective cells which generate the highest rainfall depths, compared to the rain cells at a larger spatial scale, which generate lower rainfall depths. Thus the isohyet areas defined at a high rainfall threshold are sensitive to the time steps than isohyet areas defined at a smaller rainfall threshold.The frequencies estimated in this study have been regional frequencies, but it appears that the isohyet areas are not independent of the event's location. However, at this stage the sample is too small to allow a study of conditional frequencies. In order to perform this study the sample has already been enlarged by considering all the French Mediterranean region which have been affected by heavy rainfall depths. It has been based on all the information included in the Météo-France data base over this region (since 1870). The rainfall threshold used to select the rainfall events has been diminished to 90 mm/ 24 hours to include the high intensity events over short time steps which could generated severe floods, especially over small catchments.Combined with the information about the meteorological situations, the development of this work should allow improved studies of the relations between the rainy surfaces and the meteorological situations at the origin of the rainfall events

Influence de l'évolution dans l'espace et le temps d'un réseau de pluviomètres sur l'observation des surfaces de pluie en fonction de leur aire

La caractérisation précise de l'aléa climatique nécessite l'exploitation de mesures reposant sur la période d'observation la plus longue possible. Souvent cette information est constituée de mesures au sol à partir de postes pluviométriques. L'évolution dans l'espace et dans le temps des réseaux de pluviomètres introduit un biais dans toute étude stochastique spatiale ou ponctuelle reposant sur des séries de valeurs échantillonnées à partir d'un tel réseau. On se propose dans cet article de quantifier la potentialité d'un réseau de pluviomètres à intercepter des surfaces de pluie, en fonction de leur aire et des caractéristiques de ce réseau à une date donnée. On procède par simulation à partir du réseau de pluviomètres géré par Météo-France sur la région Languedoc-Roussillon, étudié sur une période de 123 ans. On définit la notion de pourcentage d'observation, qui représente la proportion de surface pluvieuse affectant la région et qui ont été interceptées par le réseau de mesure. Toutes études statistique reposant sur des séries de mesure échantillonnées à partir du réseau seront biaisées, étant donné qu'entre 1958 et 1993, on observe qu'une proportion des surfaces pluvieuses de moins de 2000 km2 qui ont touché la région étudiée. Ce pourcentage d'observation est ensuite utilisé pour débiaiser les estimations de l'aléa pluvieux régional reposant sur le réseau de pluviomètres.The most usual rainfall risk assessment, based on a stochastic approach, or an accurate quantiles estimation, requires long series of observations. Most of the time when long periods of observation are considered, the available information consists of data from daily rain gauge networks which are evolving in space and time during these periods. As the rainy surfaces which generate the highest intensities are localised in space, the intergauge distances may be too large to "observe" all the rainfall events occurring over a given network. Thus it could bias the stochastic results based on values sampled from such a network, especially when extreme rainfall events are considered. The aim of this paper is to estimate the capacity of a daily rain gauge network to intercept rainy surfaces according to their area and the network density. The results have been used to estimate the bias introduced in rainfall risk assessment using the regional frequencies of isohyets areas observed in the studied region.The network studied is the Languedoc-Roussillon daily rain gauge network, in a French region along the Mediterranean sea. The network has been developed by Météo-France since 1870. The number of gauges put into service has varied during the 1870-1993 period of observation: from 3 gauges in 1870, the maximum reached was 353 gauges in 1969 and 1972, which represented a spatial mean density of 12.6 gauges/ 1000 km2. Since 1972 the number of gauges has decreased; in 1993 the gauge density was the same as in 1963, with 10.6 gauges/ 1000 km2. Nevertheless the clustered gauges have been reduced, as have the maximum intergauge distances, and the network has become more homogeneous over the region.Using simulation, the percentage of rainy surfaces which have affected the region, and which have been observed by the rain gauge network, has been estimated, as a function of the rainy surfaces area and the rain gauge density. It could be interpreted as the empirical expression of the probability to observe a given rainy surface with a given network configuration. Two periods have been considered, 1870-1957 and 1958-1993. Two simulation methods have been used: in the first the rainy surfaces have been considered to be static and in the second their motion has been taken into account. It has appeared that considering the motion of rainy surfaces yields the same results as the static method but with a different rainy surface geometry. The small differences between the percentage of rainy surfaces observed by the network in both cases can be explained by the simulation methods. It has been shown that the average probability over the period from 1870 to 1957 of observing a given rainy surface is 2 to 4 times less than the average probability over the 1958-1993 observation period, during which the gauge density has increased and the network has become more homogeneous over the region: over the 1870-1957 period the rain gauge network intercepted 50% at least of the rainy surfaces equal to or larger than 700 km2 but in the 1958-1993 period 50% at least of the rainy surfaces were observed if their area exceeded 80 km2. If the rainfall event which affected the N"mes hydrological system on 2-3 October 1988 is considered, these results have shown that the average probability over the 1870-1957 observation period to observe such an event is 2 times less than over the 1958-1993 observation period.In a recent study, a rainfall risk assessment has been made over the Languedoc-Roussillon region, using the frequencies of the isohyets areas defined for different rain thresholds, for 24-hour and 48-hour durations. These isohyets areas have been estimated on the basis of a sample of 93 rainfall events selected over the Languedoc-Roussillon region from 1958 to 1993 (Neppel et al., 1998). A method to estimate the bias introduced by the network in the estimation of the isohyets area return periods has been carried out, using this empirical probability estimated with the static simulation method. It has been shown that the bias only affects the more frequent isohyets area quantiles, corresponding to return period of 1 year for 48-hour duration and 1 to 3 years for 24-hour duration. Moreover, for this sample and this network, it has been shown that the bias would be negligible compared to the quantiles 5% confidence limits, whatever the return period and the time step. It must be noted that with this sample the 5% confidence limits of the quantiles sometimes reach 100% of the quantiles. The results are related to the sample and the network configuration, and they should not be extended to other areas or other samples: a larger sample over the same region could lead to narrower confidence limits, in which case the bias might no longer be negligible. In particular, the use of historical data needs to consider the longest observation period. Usually the rain gauge density decreases over such observation periods, which leads to a lower empirical probability of observing rainy surfaces according to their area. Thus the bias influence may increase, especially compared with the quantiles 5% confidence limits which are reduced when the sample is enlarged. Nevertheless the method described here is general and may be transposed to other geographical zones, provided that the isohyets area frequencies and the empirical probability of observing a rainy surface according to its area, corresponding to the network under consideration, are known.The current tendency in France is to reduce the number of daily rain gauges, managed by volunteers, and to replace them by automatic rain gauges. However in such a case the density would decrease and reach that observed in 1900. When rainfall risk assessment is considered, this study has shown the drawbacks of such a policy

Local Ignition in Carbon/Oxygen White Dwarfs -- I: One-zone Ignition and Spherical Shock Ignition of Detonations

The details of ignition of Type Ia supernovae remain fuzzy, despite the importance of this input for any large-scale model of the final explosion. Here, we begin a process of understanding the ignition of these hotspots by examining the burning of one zone of material, and then investigate the ignition of a detonation due to rapid heating at single point. We numerically measure the ignition delay time for onset of burning in mixtures of degenerate material and provide fitting formula for conditions of relevance in the Type Ia problem. Using the neon abundance as a proxy for the white dwarf metallicity, we then find that ignition times can decrease by ~20% with addition of even 5% of neon by mass. When temperature fluctuations that successfully kindle a region are very rare, such a reduction in ignition time can increase the probability of ignition by orders of magnitude. If the neon comes largely at the expense of carbon, a similar increase in the ignition time can occur. We then consider the ignition of a detonation by an explosive energy input in one localized zone, eg a Sedov blast wave leading to a shock-ignited detonation. Building on previous work on curved detonations, we find that surprisingly large inputs of energy are required to successfully launch a detonation, leading to required matchheads of ~4500 detonation thicknesses - tens of centimeters to hundreds of meters - which is orders of magnitude larger than naive considerations might suggest. This is a very difficult constraint to meet for some pictures of a deflagration-to-detonation transition, such as a Zel'dovich gradient mechanism ignition in the distributed burning regime.Comment: 29 pages; accepted to ApJ. Comments welcome at http://www.cita.utoronto.ca/~ljdursi/thisweek/ . Updated version addressing referee comment

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

Sur l'origine de l'augmentation apparente des inondations en région méditerranéenne

En septembre 2002, les régions méditerranéennes françaises et notamment le département du Gard ont été affectées par des précipitations d'une extrême intensité. On estime que 80% de ce département a été inondé, on dénombre 23 victimes et les dégâts ont été évalués à 1.2 milliards d'euros. Cette catastrophe hydrologique soulève à nouveau les problèmes de la fréquence de ces événements et de l'augmentation des forts cumuls de pluie ces dernières années. L'objet de cet article est d'apporter quelques éléments de réponse, notamment à travers l'analyse régionale des pluies extrêmes journalières ayant affecté la région Languedoc-Roussillon de 1958 à 2002.La fréquence régionale des pluies extrêmes est estimée en prenant en compte la superficie couverte par ces événements en fonction des hauteurs pluviométriques. A l'échelle régionale la période de retour de l'événement varie entre 80 ans pour la superficie touchée par au moins 200 mm à 140 ans pour celle couverte par 300 mm.La stationnarité des fréquences des pluies extrêmes est analysée à partir des chroniques du nombre annuel d'événements pluvieux dépassant 200 mm, 250 mm et 300 mm en 24h maximum, entre 1958 et 2002 sur la région. Les tests de stationnarité ne révèlent pas de tendance significative à l'augmentation de ces fréquences. Les données historiques aboutissent aux mêmes conclusions. L'augmentation réelle des inondations est en fait principalement liée à l'augmentation de la vulnérabilité des bassins.In September 2002, the Gard department in the South of France was affected by heavy precipitation that covered a broad geographical area. It was estimated that 80% of the department was flooded; there were 23 victims and the damage was evaluated to be 1.2 billion euros. This hydrological catastrophe raised questions about a possible increase in the frequency of these events during recent years, since several other severe flooding events have been observed in the region over the last 15 years. The aim of this article is to explore these questions through a regional analysis of the extreme daily rainfall that affected the Languedoc-Roussillon region between 1958 and 2002. The daily rain data were used because they are the most available type of information over the observation period. Usually, the rainfall hazard description is based on statistical analysis of the maximum rainfall depth observed at a given rain gauge. However, because the spatial variability of rainfall in the Mediterranean region, such results are only representative of local rainfall conditions. Moreover, this type of analysis does not take into account the spatial coverage of the precipitation, which is another factor influencing the resulting floods. Thus, the regional frequency of extreme rainfall was estimated by taking into account the area covered according to a given rainfall depth. For each rainfall event, a rain field was built using a kriging interpolation (NEPPEL et al., 1997). The isohyet area defined a rainfall threshold from 10 to 300 mm with a step of 10 mm calculated for each rainfall event. For each rainfall depth from 10 to 300 mm with a step of 10 mm, the probability distribution of the isohyet area was estimated. The regional rainfall hazards were described with the Depth-Area-Frequency curves (DAF) for 24-h periods. It was shown that at on regional scale, the return period of the last event varied between 80 years for the surface affected by at least 200 mm and 140 years for the surface covered by 300 mm. Compared with other major events that have occurred in the region, it appears that the September 2002 event one was characterized by :1. the spatial extension of the heavy rainfall, for example more than 1800 km² were affected by at least 400 mm in less than 24 h;2. the spatial localisation of the heaviest rainfall depths, which were measured over the highest relief (1000 m to 1500 m) as usual in the 'cévenols' meteorological situation, but rather in the plain where the altitude lies between 200 m and 300 m.The stationnarity analysis of the extreme rainfall frequency was based on the annual number of events exceeding 200 mm, 250 mm and 300 mm over a 24 h maximum duration, between 1958 and 2002. The hypothesis of random events against the hypothesis of a trend or a sudden break in the mean was examined through several statistical tests. The procedures used were the rank correlation test, PETTITT's test, BUISHAND's test, HUBERT's segmentation procedure, a linear regression procedure, and the turning points procedure. Detailed descriptions of these tests can be found in KENDALL and STUART (1977), LUBES-NIEL et al. (1998) and WMO (2000). Except for the rank correlation test, all the procedures led to the conclusion that the three series are randomly distributed at the level of significance 1%, 5% and 10% respectively. Thus no significant increase in extreme rainfall frequency seems to appear. Although the study period was short, 45 years, compared with climatological variability, LUBES-NIEL et al. (1998) show that the procedures used were adapted in detecting trends in 50-yr time series. In considering historical rainfall data before 1958 in the same region, at least two extreme rainfall events could be compared with the event on 8-9 September 2002: in October 1940, 840 mm of rainfall were measured during 24 h in the Pyrénées-Orientales district and in September 1900, 940 mm were observed over 24 h in Valleraugue, upstream in the Herault catchment. Furthermore, if the evolution of the rain gauge network density is taken into account, one can argue that such an event could have occurred more frequently. Indeed, the number of rain gauges has varied from 162 gauges in 1900 to 330 today. It has been shown that the number of observed rainfall events varied according to the area of the events and the network density (NEPPEL et al., 1998b). For example, an event of 150 km2 (corresponding to the area covered by more than 600 mm in September 2002) had a probability of 70% to be observed by the network between 1958 and 1993. If one considers the period 1920-1939, this probability decreases to 30%.In addition, the basin vulnerability has increased. The regional population has grown from 1,460,000 inhabitants in 1949 to 2,300,000 in 2000. At the same time, urbanization has expanded widely. Moreover, this new population came from other districts, and they are not familiar with the Mediterranean rainfall regime and the resulting flash floods. Buildings have often been constructed near rivers, which are attractive building sites, and sometimes even in the river's main channel, increasing the flooding risk and the flood damages. Thus, rather than climate change, for which the effect on extreme rainfalls cannot be proved, the development of basin urbanisation and vulnerability could explain the apparent increase in floods. As the regional population is expected to reach more than 3,000,000 by 2030, it is necessary to take into account the flood risk in future urban planning