Etude de la variabilité des apports hydrologiques des oueds Rhumel-Endja au barrage Béni Haroun (Algérie Orientale). Enjeux du climat et de l’environnement.

Abstract

This research is concerned with the contribution of deep knowledge of Algerian hydrology, throughout the example of a large, physically contrasted basin in eastern Algeria, formed by the Rhumel and Endja wadis. These two wadis drain an area of 5315 km² and 2160 km², respectively, and converge into the El Kebir oued where the largest dam in the country is erected, named ""Béni Haroun"". The study of the spatio-temporal variability of the input (rainfall) and output (flow) signals in the basin are is based on time series of flow and rainfall data. The statistical approach of the ""Bertin matrix"" method applied for rainfall over 42 years made it possible to detect the different wet and dry periods. The severe drought character appears between 1987/88 to 2001/02 and a wet phase occurs from the hydrological year 2002/03 with a higher frequency of wet and very wet years. In addition, the use of spectral wavelet methods wavelet coherence demonstrated an evolution of rainfall and flows, structured according to several modes of variability which are expressed for different time scales. The variability of the flows in the sub-basins is strongly influenced by rainfall variability origin (the coherence percentages remain at a level > 50%) and only a relatively small part of this variability can be explained by other physiographic factors. In the context of daily scale, the variability is more important, mainly, during extreme events, especially floods. The analysis of floods and low flows made use of numerous statistical tools to characterize and model these extreme events. The analysis of the data of the water balance (2003/04-2017/18) of the Beni Haroun dam confirms the return to a greater frequency of wet years from 2003, year of impoundment of the dam. The average observed inflow (1013.9 hm³/year) is much higher than the theoretical inflow (435 hm³/year) defined by the project studies. The outflows are considerable, in particular those linked to the flood spills, analyzed on flood chronicles which marked the life of the dam. In addition, the data on solid transport measured at the hydrometric stations show a much greater erosive dynamic in the Wadi Endja sub-basin. At the Beni Haroun dam site, the cumulative solid input is underestimated compared to the result of the bathymetric campaign carried out in 2013, which resulted in an annual siltation of 8.3 hm³/year (a specific degradation of the basin of the order of 1728 t / year / km²). Finally, the results of physico-chemical analyses from 2005 to 2014, available on the site of the Beni Haroun reservoir, show that a large percentage of the contents linked to agricultural, industrial and domestic pollution is higher than the standards, which indicates that the water of the reservoir of the dam cannot be used for drinking water supply without prior treatment. However, the quality of the water in the dam has improved significantly during 2018 compared to the data of 2014.