LES ECOULEMENTS SUPERFICIELS SUR LE HAUT ATLAS DE MARRAKECH CAS DES DEBITS EXTREMES

Abstract

High Atlas of Marrakech is a geographic area composed by the following watersheds: N'Fis, Gheraya, Ourika, Zat and Ghdat. These watersheds are located on the north side of the High Atlas mountain chain. They are exposed to the atlantic humid air masses witch condenses easily on the upper watersheds. These later are, also, characterized by low permeability, steep slopes and an anastomosed river system. These features lead to a significant rainfall events and violent floods. The flood rise times are quicker and flood peaks are higher compared to the runoff mean. During these floods, the flow velocities and flow rates are very important. Wadis erosion ability is high and the sediment load is always important. Even, with varying intensities, these flash floods are frequent in mountainous semi-arid environment. Peaks of 103 m3/s for example occur every 2 years, and floods of around 485 m3/s have a return period of 10 years. The abruptness and violence of the extreme flows are the main hazards which always threaten the valleys. In order to limit the flood effects, measures have been taken. Furthermore, a flood warning system was installed in the upstream of Ourika valley. Some Wadis banks were reinforced by setting up walls and concrete thresholds. These actions seem to be efficient according to the power and the type of raw.