Evaluation du transport solide et de l’envasement dans le bassin versant de Siliana (Tunisie): cas des barrages Siliana et Lakhmess

Resumé

L’envasement du barrage Siliana est estimé à travers l’étude quantitative des phénomènes érosifs très actifs dans son bassin versant qui couvre une surface de 1036 km². Celui-ci est composé de dépôts alluvionnaires du Quaternaire. Le relief est assez plat et dominé par la présence des montagnes. Le climat dominant est de type semi aride. L’estimation de l’érodibilité des terrains est obtenue à travers l’application de deux modèles; l’Analyse Géomorphologique Quantitative (AGQ) et le modèle Previsioni dell’Interimento nei Serbatoi Artificiali (PISA) utilisant des facteurs physiques et climatiques du bassin versant de Siliana et du sous-bassin Lakhmess. La confrontation de résultats fournis par ces deux modèles avec ceux issus des levés bathymétriques des fonds des barrages étudiés, qui permettent la quantification effective de l’envasement, a permis de valider d’une part l’AGQ comme un outil d’estimation de l’érosion au sein du sous bassin Lakhmess et d’autre part de proposer le modèle PISA comme un modèle d’estimation de l’envasement du barrage Siliana.

Abstract

The evaluation of the sediment transport and the silting in the artificial dam installed on the Siliana river (Tunisia) has been obtained using direct and indirect methods through the qualitative and quantitative study of the erosive phenomena existing in the subtended catchment area and the sedimentation entity accumulated on the dam’s reservoir. The study area, which extends approximately 1036 km², is mainly composed of quaternary alluvial deposits (clay, sandy clay, gravels, limestone). The relief is almost flat (600 meters of altitude), dominated by the presence of mountains of the Tunisian dorsal (the most elevated are: Jebel Bargou, Jebel Sardj). The dominant climate is the semi arid one. The study area is among the zones which are mainly concerned by several erosion phenomena in Tunisia. The indirect evaluation of the lands erodibility of the catchment area has been obtained through the application, with the aid of geo-informatics technologies, of two previsional models: the Quantitative Geomorphological Analysis and the PISA model. The obtained results, for the total Siliana catchment and for Lakhmess sub-catchment, allowed to estimate the entity and the variability of lands erodibility on the basis of some representative factors of the physico-climatic conditions that characterize the study area. The direct measurement of the sediment volumes present in the Siliana dam’s reservoir, has allowed to quantify the effective entity of silting of the same reservoir. The latter, obtained through a bathymetric relief has turned out equal to 14.66 10⁶ m³, to which corresponds a medium erodibility, pertinent to the entire catchment area, of 1322 t/km²/year. The comparison of such value with those obtained from the two indirect applied models has shown, at least for the studied area, the better suitability of the Quantitative Geomorphological Analysis as an instrument of indirect evaluation of the erosion entity in Lakhmess catchment and the PISA model as a good model for estimating the siltation on Siliana dam. The quantitative data obtained from this study, with the qualitative information shown in the numerous thematic charts realized, relative to the various physico-environmental characters of the catchment area, represent an adapted base, to which it will be possible to make reference to the manager responsible for the administration of the dam so as to realize the pertinent and necessary interventions of hydraulic management and soil conservation which are required for the protection of the water resource of the dam.