Gully process analysis in catchments with hill reservoirs on high structural dynamic soils (Tunisia)
Analyse du ravinement de bassins versants à retenues collinaires sur sols à fortes dynamiques structurales (Tunisie)
Résumé
The generation and transfer of sediments in two small catchments that supply hill reservoirs in semi-arid Tunisia have been studied between 1999 and 2003. The risk of reservoirs silting is essentially related to the gullying process in the marl bedrock, through which soil loss is 10 to 15 times more intense than through sheet erosion. Gully erosion was estimated on the basis of diachronic topographic levelling using a laser tacheometer. Digital Elevation Models were subsequently generated in order to compare removed or deposited volumes through time. Gully erosion varies strongly with rainfall intensity and also with the regime of soil management in the cultivated catchment. During the four years of observation, gully erosion yielded 15 to 60 m3 of sediment per catchment hectare. This is contributing to the siltation of the hill reservoirs situated downstream
La production et le transfert de sediments de deux petits bassins versants inscrits dans des marnes et alimentant des retenues collinaires en Tunisie semi-aride ont ete etudies entre 1999 et 2003. Le risque de comblement des retenues est surtout lie au ravinement qui mobilise sur les versants dix a cinquante fois plus de terres que l'erosion areolaire. L'incision des ravins est evaluee par la comparaison diachronique de leves topographiques. Des modeles numeriques de terrain sont ensuite construits pour etablir les bilans volumiques des terres erodees ou deposees dans le ravin. Le ravinement varie fortement selon les intensites des precipitations et le type d'occupation des sols des impluviums. Au cours des quatre annees de l'etude, le ravinement a fourni entre 15 et 65 m3/ha/an de terre aux retenues