Dynamique et modélisation des crues dans le bassin du Mono à Nangbéto (Togo/Bénin)

Abstract

Reçu le 29 avril 2013; accepté le 28 octobre 2013; la discussion concernant cet article est ouverte jusqu'au 1er mai 2015Editeur Z.W. Kundzewicz; Editeur associé B. Touaibia Résumé Les inondations des dernières années en Afrique de l'Ouest rendent nécessaire la prévision hydrologique pour réduire la vulnérabilité des populations. Nous avons étudié la dynamique des crues dans le bassin du Mono en amont du barrage de Nangbéto (Togo et Bénin) par des méthodes statistiques et déterministes. L'augmentation des précipitations au cours de la décennie 1990 et la mise en eau du barrage de Nangbéto en 1988 ont modifié le régime du Mono, dont les débits d'étiages ont augmenté. Les débits entrants et sortants du barrage sont équivalents, même lors des crues. Les maximums annuels des débits entrants peuvent être modélisés par une distribution des valeurs extrêmes (GEV) de type Fréchet, tandis que ceux des débits sortants, suivent une loi GEV de type Weibull (bornée). Le mécanisme dominant de génération des crues dans le bassin est la montée progressive des eaux associée à l'augmentation de la saturation des sols. L'analyse des débits de crue entre 1988 et 2010 n'a pas montré d'augmentation de la magnitude ou de la fréquence des crues. Le modèle GR4J reproduit bien les débits de crues en période calage, mais ces critères de performance se dégradent lorsque l'on s'en éloigne. Dans ces conditions, l'utilisation de ce modèle pour évaluer les risques futurs de crues serait hasardeuse et il est nécessaire d'envisager d'autres types d'approches.Mots clefs Bénin-Togo ; bassin Mono ; barrage de Nangbéto ; crues ; GR4J ; statistiques ; extrêmes Dynamics and modelling of floods in the river basin of Mono in Nangbeto, Togo/Benin Abstract The devastating floods in recent years in West Africa call for improvement in hydrological forecasts to reduce the vulnerability of local communities. We studied flood dynamics in the Mono basin, in Togo and Benin, upstream of the Nangbeto Dam, using statistical methods. The increase in precipitation in the 23-year period from 1988 (following completion of the Nangbeto Dam) modified the regine of the Mono River, with an increase in stage discharge. There is little difference between the river discharge at the entrance and the exit of the Nangbeto Reservoir, at least at peak flow, which emphasizes that the dam, due to the small capacity of the reservoir, has a limited effect on flow regulation. The annual peak flows entering the reservoir can be modelled using a Frechet-type generalized extreme value (GEV) distribution, while those at the exit follow a Weibulltype GEV distribution. The main mechanism for peak flow generation in the basin is a steady rise of water as a result of increasing soil moisture saturation, rather than isolated rain spells. Peak flows between 1988 and 2010 do not show any change in magnitude and frequency. The GR4J hydrological model is shown to be effective in reproducing peak flows, but its skill decreases as we move away from the calibration period. Therefore, it may be risky t...

Climate and Extreme Rainfall Events in the Mono River Basin (West Africa): Investigating Future Changes with Regional Climate Models

“…Observed hydroclimatic series do not always meet the assumption of data stationarity [40,41]. This is often the case for extreme rainfall and flood discharge.…”

“…To avoid having a value of zero for the scale parameter, αt, we considered the parameterization as φt = log(αt) and thus restricted our analysis to linear dependencies [49]. However, many other types of dependencies can be linearized [41,44,48,50]. The return periods of a GEV model are defined by the cumulative distribution function (CDF), F(x), or the probability density function (PDF), f(x), hat are given by the formula:…”

“…This has also been proved by Ntajal et al [49] downstream of the Mono river basin. However, an in-depth analysis is still needed to prove this assumption because, in principle, very high streamflow is sometimes generated from rapid runoff in response to water excess in soil with a low retention capacity [41,71,76]. This occurs in a very short time period, during which the increase of the evaporative demand is not really a constraint.…”

Future Climate Change Impacts on Streamflows of Two Main West Africa River Basins: Senegal and Gambia

“…In this study, the lumped and conceptual model GR4J [22] that successfully simulates streamflow in a West African context [23][24][25] was used. The structure of the GR4J model is based on a production reservoir, a routing reservoir, and on two unit hydrographs.…”

Comparison of SCS and Green-Ampt Distributed Models for Flood Modelling in a Small Cultivated Catchment in Senegal

“…Consequently, there is a need for a better knowledge of these events and to improve rainfall-runoff models that could be used in the development of prevision systems. However, up to now, only a few studies have applied rainfall-runoff models in West Africa for flood modelling (e.g., [3,4]) or the analysis of the hydrological processes in small catchments less than some tenth or hundreds of squared kilometers [5,6]. In West Africa, flood prediction usually derive from synthetic guidelines [7] or regional studies [8][9][10].…”