Eric Gaume

The Catastrophic Flash-Flood Event of 8–9 September 2002 in the Gard Region, France: A First Case Study for the Cévennes–Vivarais Mediterranean Hydrometeorological Observatory

The Cevennes–Vivarais Mediterranean Hydrometeorological Observatory (OHM-CV) is a research initiative aimed at improving the understanding and modeling of the Mediterranean intense rain events that frequently result in devastating flash floods in southern France. A primary objective is to bring together the skills of meteorologists and hydrologists, modelers and instrumentalists, researchers and practitioners, to cope with these rather unpredictable events. In line with previously published flash-flood monographs, the present paper aims at documenting the 8–9 September 2002 catastrophic event, which resulted in 24 casualties and an economic damage evaluated at 1.2 billion euros (i.e., about 1 billion U.S. dollars) in the Gard region, France. A description of the synoptic meteorological situation is first given and shows that no particular precursor indicated the imminence of such an extreme event. Then, radar and rain gauge analyses are used to assess the magnitude of the rain event, which was particularly remarkable for its spatial extent with rain amounts greater than 200 mm in 24 h over 5500 km2. The maximum values of 600–700 mm observed locally are among the highest daily records in the region. The preliminary results of the postevent hydrological investigation show that the hydrologic response of the upstream watersheds of the Gard and Vidourle Rivers is consistent with the marked space–time structure of the rain event. It is noteworthy that peak specific discharges were very high over most of the affected areas (5–10 m3 s−1 km−2) and reached locally extraordinary values of more than 20 m3 s−1 km−2. A preliminary analysis indicates contrasting hydrological behaviors that seem to be related to geomorphological factors, notably the influence of karst in part of the region. An overview of the ongoing meteorological and hydrological research projects devoted to this case study within the OHM-CV is finally presented.

Uncertainty assessment and analysis of the calibrated parameter values of an urban storm water quality model

Real uncertainties in the parameter values of conceptual models used in hydrology sometimes remain after calibration. While this is unacceptable, uncertainty analysis can provide useful information on the structural properties of the model. This information may then be used to modify the model or establish the limits of usefulness of the measured data. This paper focuses on the estimation and interpretation of the parameter uncertainties in such cases. The standard estimation method, based on the second-order approximation of the shape of the objective function in the vicinity of the optimum parameter set, provides an inaccurate description of these uncertainties. Instead a global approach, consisting of scanning the hypercube of the possible parameter values, is shown to give a good overview of these uncertainties. This gives not only a good general view of the shape of the objective function, but a good basis for interpretation as well. The efficiency of the method is illustrated using a four parameter urban stormwater quality model. The main conclusion is that the lack of measured data and its relative redundancy cause a significant interaction between the parameters, hindering calibration. Another example is given of something being reported by a growing number of authors: that data available in hydrology are only sufficient to support the development of models with limited complexity.

Flood frequency analysis using historical data: accounting for random and systematic errors.

Abstract Flood frequency analysis based on a set of systematic data and a set of historical floods is applied to several Mediterranean catchments. After identification and collection of data on historical floods, several hydraulic models were constructed to account for geomorphological changes. Recent and historical rating curves were constructed and applied to reconstruct flood discharge series, together with their uncertainty. This uncertainty stems from two types of error: (a) random errors related to the water-level readings; and (b) systematic errors related to over- or under-estimation of the rating curve. A Bayesian frequency analysis is performed to take both sources of uncertainty into account. It is shown that the uncertainty affecting discharges should be carefully evaluated and taken into account in the flood frequency analysis, as it can increase the quantiles confidence interval. The quantiles are found to be consistent with those obtained with empirical methods, for two out of four of the catchments. Citation Neppel, L., Renard, B., Lang, M., Ayral, P.-A., Coeur, D., Gaume, E., Jacob, N., Payrastre, O., Pobanz, K. & Vinet, F. (2010) Flood frequency analysis using historical data: accounting for random and systematic errors. Hydrol. Sci. J. 55(2), 192–208.

Study of the hydrological processes during the Avene river extraordinary flood (south of France): 6–7 October 1997

The Avene flood, with a peak discharge between 600 and 900 m3/s for a drainage area of 57 km2, appears as one of the most important flash floods reported in the French Mediterranean area during the last decades for similar catchment areas. The watershed is ungauged and interviews were conducted to gather valuable information about the flood sequence. Most of the collected data seems reliable. These data indicate that the swelling of the Avene river was sudden and late compared to the measured rainfall sequence. The flood appears to have been mainly produced by surface runoff. Moreover, the late response of the watershed shows that its thin soils, dried out during the preceding summer period, had surprisingly high infiltration rate and capacity: estimated about 100 mm/h and 200 mm respectively.

Influence of the highest values on the choice of log-poisson random cascade model parameters

Abstract Urban drainage systems usually equip small size catchments, with a response time as short as a few tenth of minutes. Hydrological simulation of these urban catchments requires rainfall time series at a very fine temporal resolution. Most of the time, available rainfall data consist of daily or hourly measurements. It is then appealing to design rain simulation models able to disaggregate observed data into rain rate series at shorter time steps. Random cascade models are candidate tools for performing this disaggregation process. The objective of this study is to evaluate the ability of cascade model for performing realistic and robust rain series suited for needs of urban hydrology.

7 – Flood Hydrology Processes and Their Variabilities

Abstract: This chapter presents a brief overview of the basics of flood hydrology. Readers wishing to develop their knowledge of flood hydrology further are invited to refer publications specializing in this subject. The supporting examples featured in this chapter are based on the authors’ works. These examples are often French in origin, but were chosen for their pedagogical and generic nature.

Prévision hydrologique : Cas général

«Praevidere»: voir a l’avance. Autrement dit, anticiper sur l’occurrence d’un evenement et le decrire avec plus ou moins de precision, qu’il s’agisse de la date de sa survenue ou de son ampleur.

Le cycle de l’eau

L’hydrologie s’interesse a un systeme complexe, celui de la phase continentale du cycle de l’eau qu’on presentera en chapitre 2 et qui s’analyse dans des territoires d’extension tres variable, les bassins versants.

Gestion des réserves hydrauliques

L’hydrologie quantitative est un instrument essentiel de la gestion des ressources en eau. Les enjeux d’une telle gestion depassent cependant les limites du present ouvrage. En effet, pour en traiter de facon pertinente il convient d’approfondir prealablement l’ensemble des dispositifs socio-economiques qui contribuent a la formation des arbitrages collectifs concernant la repartition des ressources entre les usages, et tout ceci ne peut etre aborde serieusement sans developper la qualite des eaux, la comprehension fine du fonctionnement des ecosystemes et une connaissance approfondie et detaillee de chaque usage et des instruments institutionnels et economiques qui permettent la regulation economique a l’echelle des bassins versants. Ceci est un domaine en pleine evolution notamment quant a la prise en compt de la biodiversite dans les decisions publiques. Ce manuel n’a pas la pretention d’etre un manuel de gestion des ressources en eau, qui justifie en elle-meme un ouvrage. Il nous a semble cependant important de developper, dans l’ensemble de ce vaste sujet, quelques points de methodes dans lesquels la gestion de la quantite d’eau et des moyens de la reguler sont importants. Nous allons donc dans ce dernier chapitre developper les outils qui ont trait a la gestion des reserves, tant superficielles que souterraines. Cela nous conduira a ouvrir quelques fenetres, tres partielles, sur les questions plus generales de gestion des ressources en eau, sans pretendre pour autant les traiter en tant que telles.

Prédétermination des étiages

L’etiage constitue une partie du processus des debits: il peut etre statistiquement etudie comme le furent les debits de crue, a cette difference pres que le debit minimum instantane est rarement la variable interessante pour l’utilisateur comme l’etait le debit maximum instantane pour l’etude des crues. Ce dernier s’interesse necessairement a des grandeurs couplees debits et durees: debits mensuels, nombre total de jours annuels sous un seuil en debit, nombre de jours consecutifs sous un seuil en debit, etc. Il n’y a donc plus une variable privilegiee a une dimension dont il fault estimer la probabilite d’occurrence ou de depassement comme nous l’avions vu pour les crues, mais une variable a deus dimensions debit-duree.