Abstract. Occurring at small temporal and spatial scales, flash floods (FF) can cause severe economic damages and human losses. To better anticipate such events and mitigate their impacts, the French Ministry in charge of Ecology has decided to set up a national FF warning system over the French territory. This automated system will be run by the SCHAPI, the French national service in charge of flood forecasting, providing warnings for fast-responding ungauged catchments (area ranging from ~10 tR a NPð ,W ZLOO WKHUHIRUH EH FRPSOHPHQWDU\ WR WKH 6&+$3, ¶V QDWLRQDO ³YLJLODQFH´ V\VWHP ZKLFK FRQFHUQV RQO\ JDXJHG FDWFKPHQWV 7KH )) ZDUQLQJ V\VWHP WR EH LPSOHPHQWHG in 2017 will be based on a discharge-threshold flood warning method called AIGA (Javelle et al. 2014). This method has been experimented in real time in the south of France in the RHYTMME project (http://rhytmme.irstea.fr). It consists in comparing discharges generated by a simple conceptual hourly hydrologic model run at a 1-km² resolution to reference flood quantiles of different (e.g., 2-, 10-and 50-year) return periods. Therefore the system characterizes in real time the severity of ongoing events by the range of the return period estimated by AIGA at any point along the river network. The hydrologic model ingests operational rainfall radar-gauge products from Météo-France and takes into account the baseflow and the initial soil humidity conditions to better estimate the basin response to rainfall inputs. To meet the requirements of the future FF warning system, the AIGA method has been extended to the whole French territory (except Corsica and overseas French territories). The calibration, regionalization and validation procedures of the hydrologic model were carried out using data for ~700 hydrometric stations from the 2002-2015 period. Performance of the warning system was evaluated with various contingency criteria (e.g., probability of detection and success rate). Furthermore, specific flood events were analysed in more details, by comparing warnings issued for exceeding different critical flood quantiles and their associated timing with field observations. The performance results show that the proposed FF warning system is useful, especially for ungauged sites. The analysis also points out the need to account for the uncertainties in the precipitation inputs and the hydrological modelling, as well as include precipitation forecasts to improve the effective warning lead time.
La méthode SHYREG a été développée pour la connaissance régionale des quantiles de débits de crue (débit de pointe et lames d'eau maximales écoulées sur les durées de 1 h à 72 h) pour les périodes de retour de 2 à 100 ans suivant une approche spatialisée. Elle associe un simulateur de pluies horaires et une modélisation simple pluie-débit, mis en oeuvre à une résolution kilométrique. Les quantiles de débits se déduisent directement des distributions de fréquence empiriques des valeurs maximales extraites des très longues chroniques de débit simulées. On obtient alors une base de quantiles de crues que l'on peut agréger à l'échelle de n'importe quel bassin versant, moyennant une règle d'abattement avec la surface. La régionalisation de la méthode a été réalisée sur la France métropolitaine, à l'exclusion de la Corse, en exploitant les données hydrométriques de 1 359 stations de jaugeage et des caractéristiques hydro-climatiques et hydrogéologiques spatialisées permettant de décrire la variabilité du paramètre saisonnier du modèle. Au final, cette régionalisation permet la connaissance des quantiles de débits de crue en tout bassin versant de la France métropolitaine avec une bonne restitution des quantiles de débit de pointe et journalier, pour les périodes de retour comprises entre 2 et 10 ans : un critère de Nash minimum de 80 % est obtenu sur les quantiles de débit de pointe pseudo-spécifique et de débit journalier spécifique des bassins versants non utilisés pour la régionalisation.The SHYREG method was developed for regional flood frequency analysis to estimate peak flow and flood discharges for various durations (1 h to 72 h) and return periods (2 to 100 years), according to a spatialized approach. For each 1-km2 pixel, the method combines an hourly rainfall model with a simple rainfall-runoff model. The discharge flood frequency estimates are deduced directly from the empirical frequency distributions for the maximum values, which are extracted from very long simulated discharge time series. This gives a database of 1-km2 gridded flood quantiles that can be aggregated for any catchment by using an areal averaging method. The method was regionalized for metropolitan France, excluding Corsica, using flow data from 1,359 gauging stations and regional hydroclimatic and hydrogeological characteristics to describe the variability of the rainfall-runoff model parameter. Such regionalization provides flood discharge quantiles for any catchment in metropolitan France for various durations and return periods. Regarding the method performance, accurate estimates of flood quantiles were produced for peak discharge and mean daily discharge for return periods of two to 10 years for gauged basins in dependent validation and cross-validation. A minimum NASH criterion of 80% is obtained for peak flow and mean daily discharge for the catchments not used in the regionalization process
Abstract. Calibration of a conceptual distributed model is challenging due to a number of reasons, which include fundamental (model adequacy and identifiability) and algorithmic (e.g., local search vs. global search) issues. The aim of the presented study is to investigate the potential of the variational approach for calibrating a simple continuous hydrological model (GRD; Génie Rural distributed involved in several flash flood modeling applications. This model is defined on a rectangular 1 km2 resolution grid, with three parameters being associated with each cell. The Gardon d'Anduze watershed (543 km2) is chosen as the study benchmark. For this watershed, the discharge observations at five gauging stations, gridded rainfall and potential-evapotranspiration estimates are continuously available for the 2007–2018 period at an hourly time step. In the variational approach one looks for the optimal solution by minimizing the standard quadratic cost function, which penalizes the misfit between the observed and predicted values, under some additional a priori constraints. The cost function gradient is efficiently computed using the adjoint model. In numerical experiments, the benefits of using the distributed against the uniform calibration are measured in terms of the model predictive performance, in temporal, spatial and spatiotemporal validation, both globally and for particular flood events. Overall, distributed calibration shows encouraging results, providing better model predictions and relevant spatial distribution of some parameters. The numerical stability analysis has been performed to understand the impact of different factors on the calibration quality. This analysis indicates the possible directions for future developments, which may include considering a non-Gaussian likelihood and upgrading the model structure.
Abstract. In Europe, flash floods affect mainly the Mediterranean and mountainous regions, even if other regions also occasionally suffer from them. The catchments involved are usually small and ungauged, with short time of concentration. Forecasting this type of event remains difficult using hydrological models, and assessing the models is even more problematic. Typically, assessment is limited to gauged catchments that have relatively different geomorphological characteristics. The aim of this article is to present a method for assessing the models on real ungauged catchments through the use of damage reports and a multi-threshold approach, with assessment criteria that are based on a contingency table of the Critical Success Index type. The main conclusion, as demonstrated by Irstea's "Adaptation d'Information géographique pour l'Alerte en crue" for "Geographic information adaptation for flood warning" (AIGA) flood forecasting system and by the new version of AIGA for high-altitude catchments, is that while assessing hydrological models on gauged catchments is necessary, it is never sufficient and must be supplemented by assessments on ungauged catchments. This underlines the utility of building flood damage databases that are as exhaustive as possible. Such databases can be a valuable addition to more standard, often limited sources of data, especially for mountainous regions.
Le système national Vigicrues Flash, développé par le SCHAPI, fournit des avertissements aux crues soudaines sur les cours d'eau non jaugés en se basant sur la méthode AIGA-débit. Un modèle hydrologique distribué simplifié intègre, à la résolution de 1 km2, les pluies observées du réseau radar de Météo-France, pour estimer le débit en tout point des cours d'eau. Ces débits sont comparés toutes les 15 minutes aux quantiles de débit préalablement estimés à partir des chroniques de débits simulés (avec le même modèle). Le système affiche les tronçons dépassant les seuils de crue forte et crue très forte, ainsi que les communes impactées, et envoie des messages d'avertissement aux utilisateurs potentiellement concernés. Pour améliorer la pertinence et l'anticipation des avertissements, nous étudions l'intérêt d'intégrer les prévisions immédiates de pluie AROME-PI de Météo-France, sur l'échéance de 6 heures, réactualisées toutes les heures. Afin de prendre en compte les incertitudes des prévisions, les prévisions déterministes successives AROME-PI sont utilisées comme prévisions d'ensemble. Combinées avec différents jeux de paramètres hydrologiques régionalisés, ces prévisions d'ensemble définissent des avertissements probabilisés de risque de crue. L'évaluation sur 13 événements entre octobre 2015 et juin 2017 pour 750 bassins versants montre une amélioration significative en termes de détection et anticipation, en comparaison au système sans prévision de pluie, et l'intérêt des avertissements de crue de type probabiliste.
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