SAIDI, Mohamed El Mehdi et al. Les crues de l’oued Ourika (Haut Atlas, Maroc): Événements extrêmes en contexte montagnard semi-aride. []. , 97, pp.113-128. ISSN 1647-581X.

^lfr^aLe bassin versant de l’Ourika est un hydrosystème du Haut Atlas de Marrakech. Le substratum y est très peu perméable, le couvert végétal faible et clairsemé, les pentes élevées et les vallées encaissées. Le bassin, orienté vers le Nord et le Nord-Ouest, est exposé aux perturbations atlantiques qui peuvent générer des pluies importantes et intenses. La conjonction de ces facteurs est à l’origine de pulsations brutales et violentes du cours d’eau. L’aptitude de l’oued à l’incision et à l’érosion est forte et la charge solide est toujours importante. Bien qu’avec des intensités variables, ces crues rapides en contexte semi-aride montagnard se répètent. Un certain nombre d’aménagements ont été entrepris pour atténuer les effets des crues et un système d’alerte et d’annonce de crues est installé en amont du bassin. Certaines de ces infrastructures ont prouvé leur efficacité, mais d’autres n’ont pas résisté aux fortes crues postérieures.^len^aThe 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.^lpt^aA bacia do rio Ourika é um hidrossistema do Alto Atlas de Marrakech. O substrato local é muito pouco permeável, a cobertura vegetal, escassa ou rara, as vertentes elevadas e os vales encaixados. A bacia, orientada a Norte et a Noroeste, esta exposta às perturbações atlânticas que podem dar origem a chuvas importantes e intensas. A conjugação destes factores está na origem de pulsações brutais e violentas do curso de água. A apetência do rio à incisão e à erosão é forte e a carga sólida é sempre importante. Embora com intensidades variáveis, as cheias rápidas, neste contexto semi-árido montanhoso, repetem-se. Neste trabalho propõem-se algumas medidas para minimizar os efeitos das cheias e um sistema de alerta e aviso das cheias foi instalado a montante da bacia. Certas destas infra-estruturas têm provado a sua eficácia mas outras não resistiram a fortes cheias posteriores.

: .

        · | | |     ·     · ( pdf )