Évolution temporelle de l’envasement des retenues de barrages de Tunisie

La mobilisation et l’exploitation des eaux de surface sont des pratiques anciennes en Tunisie. Les aménagements réalisés au cours du siècle dernier sont exposés à un alluvionnement plus ou moins accéléré. La capacité de stockage des retenues des barrages est sujette à une réduction progressive au cours du temps. Cette perte de capacité, parfois élevée, dépasse les prévisions de l’alluvionnement de la retenue. La quantification des sédiments piégés se base soit sur les bilans de matière solide à l’échelle d’une retenue, soit sur des levés bathymétriques ou topographiques. Les Modèles Numériques de Terrain (MNT) des retenues sont de plus en plus utilisés. Les différentes évaluations sont entachées d’une incertitude. Les retenues des barrages tunisiens perdent annuellement 0,5 % à 1 % de leur capacité par alluvionnement. L’analyse des résultats de mesures montre que l’alluvionnement est lié à l’hydrologie du cours d’eau, à la gestion de la réserve d’eau et aux manoeuvres de dévasement. La comparaison entre les résultats de mesures de l’alluvionnement et les prévisions des projets met en évidence des différences parfois relativement importantes qui sont dues au régime hydrologique des cours d’eau. En effet, une crue exceptionnelle peut provoquer un alluvionnement nettement supérieur à la moyenne annuelle en régime hydrologique normal. Les retenues méandriformes sont comblées rapidement alors que les retenues linéaires offrent la possibilité de soutirage de quantités importantes de sédiments. Les moyennes annuelles des pertes de capacité des barrages en exploitation et des barrages projetés jusqu’à 2010 permettent de quantifier les volumes des sédiments piégés à 500 Mm3. En 2030, la perte de capacité de stockage des barrages en exploitation pourrait atteindre 43 % de leur capacité initiale. Les aménagements amont et les travaux de conservation des eaux et des sols permettent de réduire le taux d’alluvionnement et de prolonger la durée de vie des grands réservoirs.The mobilization and exploitation of surface water are ancient practices in Tunisia. Installations carried out during last century are exposed to a greater or lesser degree of accelerated silting. The storage capacity of reservoirs is progressively reduced over the course of time. This sometimes accelerated loss of capacity of reserves by silting exceeds the forecasts of hydraulic installation projects. The quantification of trapped sediments is based on the assessment of a reservoir’s retained solid matter, either by bathymetric or topographic rising or by Digital Models of Grounds (DMG) corresponding to different dates. These various evaluations of the sediments trapped in reservoirs are, however, sullied with uncertainty. Tunisian reservoirs lose 0.5% to 1% of their storage capacity to silting annually. The analysis of the results presented in this study shows that silting is related to three factors: river hydrology, water reserve management and dredging operations. The comparison between the silting results and the project forecasts highlights relatively significant differences which may be due to river hydrology. Indeed a significant rising in water level can cause an increase in silting above the annual average found during normal hydrological modes. While meandering reservoirs are quickly filled with sediment, linear reservoirs can undergo dredging to remove a significant quantity of sediment. The annual averages in storage capacity losses of the dam under exploitation and its projections up to the year 2010 enable us to quantify the volumes of the sediment trapped as 500 Mm3. In 2030, the loss of storage capacity of the studied reservoirs may reach 43% of their initial storage capacities. Installations upstream and soil and water conservation efforts may reduce the rate of silting and prolong the lifespan of large dams

DEM-Based GIS Algorithms and 3D Geospatial Mapping for Creation of Hydrogeological Models Data in Foussana Basin (Central Tunisia)

Copyright © 2013 Sami Khemiri et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. There are many factors which affect the hydrological, geomorphologic and hydrogeological condition of the area. In order to better comprehend all processes, a Digital Elevation Model (DEM) was developed based on Geographical Information System. This latter appears as an essential tool to facilitate the decision support and can provide very important geological information. In fact, the use of the DEM is growing dramatically with the use of the GIS and the improvement of information extracted from elevation data such as mapping of floods, forest areas, erosion, and lineaments. The spatial distribution of topographic attributes can often be used as an indirect measure of the spatial variability of these processes and allows them to be mapped using relatively simple techniques. The main purpose of this study is to model the natural surface of the earth as the most accurate and the most precise. For this end we have tried in this work to develop various types of Digital Elevation Models DEM of the Foussana rift in Central Tunisia and to demonstrat

Fingerprinting sediment sources in the outlet reservoir of a hilly cultivated catchment in Tunisia

International audiencePurpose: Approximately 74% of Tunisian agricultural soils are affected by water erosion, leading to the siltation of numerous man-made reservoirs and therefore a loss of water storage capacity. The objective of this paper is to propose a methodology for estimating the relative contributions of gully/channel bank erosion and surface topsoil erosion to the sediment accumulated in small reservoirs. Materials and methods: We tested an approach based on the sediment fingerprinting technique for sediments collected in a reservoir installed in 1994 at the outlet of a pilot catchment (Kamech, 2.63 km²). Sampling efforts were concentrated on the soil surface (in both cropland and grassland), gullies and channel banks. A total of 17 sediment cores were collected along a longitudinal transect of the Kamech reservoir to investigate the sediment origin throughout the reservoir. Radionuclides (particularly caesium-137) and nutrients (organic matter, total phosphorous and total nitrogen) were analysed as potential tracers. Results and discussion: The applications of the mixing model with caesium-137 alone or caesium-137 and total organic carbon provided very similar results: the dominant source of sediment was surface erosion, which was responsible for 80% of the total erosion within the Kamech catchment. Additionally, we showed that the analysis of a single composite core sample provided information on the sediment origin that was consistent with the analysis of all successive sediment layers observed in the core. We demonstrated the importance of the core sampling location within the reservoir for obtaining reliable information regarding sediment sources and the dominant erosion processes. Conclusions: The dominance of surface erosion processes indicates that conservation farming practices are required to mitigate erosion in the Kamech agricultural catchment. Based on the results from 17 sediment cores, guidelines regarding the number and location of sampling cores to be collected for fingerprinting purposes are proposed. We showed that the collection of two cores limited the sediment source apportionment uncertainty due to the core sampling scheme to less than 10%

Underground renewal time and mixing of the main mineral waters of Tunisia: A multi-tracer study

International audienceBottled waters are increasingly popular in Tunisia. Most of them come from groundwaters, thus raising the question of the long-term availability of the mineral groundwater resources. Water collected from production springs or wells of eleven mineral waters of Tunisia among the main commercial brands were analyzed for tritium (3H), helium-3, SF6, CFCs and carbon-14. Tracer-tracer plots indicate that all of the groundwaters of our data set are a mixture of modern rainwater with older groundwater flow waters that are beyond the time-scale of the transient tracers. 14C data suggest that the age of these old waters, which constitute the main water reservoir, is of the order of radiocarbon half-life or more. Most of the studied sites have groundwater renewal times in the range 50–150 years. For the sites where tracer concentrations are the most diluted by old waters, this value logically increase to several centuries