Resumo

SAIDI, Mohamed El Mehdi et al. The Ourika floods (High Atlas, Morocco), Extreme events in semi-arid mountain context: Eventos extremos num contexto semi-árido montanhoso. Comunicações Geológicas [online]. 2010, n.97, pp.113-128. ISSN 1647-581X.

The watershed of the Ourika River is a part of the Marrakech hydrosystem of the High Atlas. In the montaneous part of the watershed, the bedrock has a low permeability, the vegetation is sparse on steep slopes and rare on the upper areas, the main valleys are narrow and deep. The N to NW exposure favours rain falls which are usually strong, short in duration, and sometimes very intense. The conjunction of these physical and climatological factors is at the origin of violent pulses of the Ourika River, which are characterized by high velocities and rates of flow, active erosion and strong sediment transport. These extreme events in a semi-arid mountain setting are a repetitive phenomenon of variable intensity. The rising time of the floods is very short and the maximum rate of flow is much higher than the mean rate. Within the 34 years of data, floods occurred in any month of the year, excepted December. 44% of the floods occurred in spring, and 25% in summer which is the dry season in Morocco, except in high mountain regions. These summer floods are the result of intense stormy rains, related to a hot and stormy weather at the front of Trade winds. The hydrograms are steep and narrow, mostly simple, monogenic and clearly distinct. They are dissymmetric: the rising level phase is short, and the drop in level is slower. Because of their suddenness and violence, these flash floods represent a major natural and recurrent risk for the touristic valley. For the last decades, several damaging floods in Morocco lead to develop a policy of management of the water resources in the country. Locally, after the flash flood of the Ourika Valley of 1995, several structural and non stuctural actions were undertaken. To reduce the high vulnerability of the Valley new equipments have been set up. A warning monitoring system is installed in the upper reaches of the valley, it can display an alert to the population along the valley. Supporting walls and gabions reinforce the banks of the river. Works to widen some narrows of the river bed have regulated the flow of the river. Concrete sills and gabions break the flow of the main tributaries. The efficiency of these engineering structures is presently evaluated.

Palavras-chave : Ourika River; High Atlas; morocco; flash flood; management.

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