Les crues dans le haut bassin de l’Oum Er-Rbia (Amont station Machraa Edahk) entre 1934-2018 : identification, mesure, suivie et extraction (Maroc)

Résumé

Extreme hydrological phenomena (floods, and low water) are the most important natural phenomena in the world. They have intensified in recent years with the effects of current climate change, which has attracted the attention of climatologists and hydrologists to better understand and determine how they work. Morocco is no exception to this general rule since its territory is strongly affected by these extreme events that affect almost all fields, environmental and socio-economic… floods are particularly considered among the most frequent phenomena in Morocco. The Oum Er-Rbia basin, the second largest river in Morocco after the Sebou basin, with an area of 35,000 km2, is considered one of the most vulnerable watersheds to the phenomenon of flooding. Given the importance of the water high escalating levels in the appearance of floods, we studied, i) the different floods in the high basin of Oum Er-Rbia (upstream station Machraa Edahk) between 1937-2018, determining the flow of floods of the main stream of Oum Er-Rbia, the floods of its major tributaries, graduated tributaries, and the measurement and monitoring of flash floods in undeveloped and ungraded tributaries. In order to carry out this study, and to better understand the mechanisms of operation of the flood phenomenon, we are based on two complementary approaches: hydrological and statistical. From a rainfall point of view, the watershed area receives significant rainfall ranging from 1454 mm/year to 263 mm/year, which is marked by great space-time variability. This rainfall variability affects the hydrological behavior of the wadi Oum Er-Rbia and its tributaries. This because the hydrological regime of this wadi is Rain-Snow type with a strong influence of karst and human factor, especially after the hydraulic installations, starting in 2001. This work also showed that the floods were natural between 1934 and 2000 and from 2001, human (hydraulic development, dams) impacted the genesis of the floods of the Oum Er Rbia wadi (anthropogenic floods). In addition, we identified and studied the types of floods recorded in the calibrated and ungraded sub-basins in the upper basin of Oum Er-Rbia. The first phase of the study focused on the analysis and classification of instantaneous floods. This was recorded in six calibrated sub-basins equipped with hydrometric stations between 1975 and 2015. While the second stage focused on the study and monitoring of 11 seasonal sub-basins, not graded, but which in return experienced the appearance with a high frequency of flash floods. To know the hydrological importance of all these basins, whether measured or not, we also studied the evolution and propagation of the waves of the daily floods at the level of the main course of Oum Er-Rbia and the instantaneous floods in the Derna basin. Another phenomenan has been studied and analyzed. It is the invisible (indirect) domestic floods, which greatly affect the urban areas crossed by the Oum Er-Rbia basin. These invisible floods affected the city of Khénifra (upstream of the basin) and Kesba Tadla (downstream of the basin).

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