23. Sediment Assessment of Medjerda High Valley Dams (Tunisia) Using PISA Model

The High Valley of Medjerda characterized by a semi-arid climate and a strong relief, suffer from an intense erosion contributing to the sediment deposit in Dams. The percentage of sedimentation in Mellegue and Sidi Salem Dams are 81 and 29%. In this paper we analyzed the factors contributing to the siltation of Sidi Salem dam, assessed the dam siltation by PISA (Previsioni Interimento Serbatoi Artificiali) model. To study the spatio-temporal variability of precipitation, time series of rain were collected from 16 rainfall stations, ranging from 1980/81 to 2014/15. The missing data were reconstructed using the weighting method. The breaks in the pluviometry series detected by homogeneity tests (Pettitt, Buishand and SNHT) which make it possible to study the distribution of the pluviometric regime on the monthly and annual scales of the watershed with 5 stations that have fewer breaks compared to the others. The average annual rainfall determined by spatial interpolation with the method of Theissen is 477 mm. The sensitivity study of PISA model brought out that the eroded area (which depends on the area of the watershed and land cover) has a significant influence on the rate of sedimentation compared to the other parameters. Then we proceeded to assess the soil losses in the sub-basins (numbered 7) after determining the specifics parameters. By adding up the siltation indices of each sub-basin, we obtain a cumulative sedimentation rate equal to 415 Mm³. This result is overestimated by 2.45 times compared to the bathymetric survey (169 Mm³).