FLOODS IN SEMI-ARID ZONE: EXAMPLE OF THE OURIKA (HIGH ATLAS OF MARRAKECH, MOROCCO)
L. Daoudi*, M.E. Saidi**
Université Cadi Ayyad, Faculté des Sciences et Techniques Département de Géologie, B.P. 549, Marrakech, 40 000, Morocco tel: (212) 24 43 34 04 / fax: (212) 24 43 31 70 / *e-mail: daoudi@fstg-marrakech.ac.ma **e-mail: saidi@fstg-marrakech.ac.ma
Received: 21 Sept 2007; accepted: 17 Oct 2007
In the High Atlas of Marrakech, the watershed areas are submitted to important and frequent floods, associated with landslide and rockslide. Morphological and lithological characteristics of the basins watershed have a clear influence on the rising waters strength and the hydrogram's shape. The products of erosion accumulated in the upstream part as gravity accumulation and fans are remobilised during high floods when the discharges increase. This situation make flows muddy and torrential. The consequences of these natural risks can be grave in term of damage and cost. In the Ourika valleys, we are confronted to the amplification and repetition of this process owing to its deepness and narrowness. Furthermore, the degradation of environment speeds up because of the development of tourist activities. All these conclusions suggest the management of these suddenly beating of flows.
Keywords: floods, high atlas, landslide, erosion.
Organization(s): University Cadi Ayyad of Marrakech, Faculty of Sciences and Technology, Department of Earth Sciences.
Position: Responsible of UFR ST03/03 (Formation and Research Unity): Geo environment of arid and semi-arid zone.
Scientific degree: Full Professor. Education: University of Marrakech (1982-1986). University of Paris XI (1986-1987).
University of Sciences and Technology of Lille (France) (1987-1991). University of Marrakech (Faculty of Sciences and Technology) (1991-2007).
Experience: Assistant Professor (1991-1996); Head of Department of Geology (1997-2000), Full professor (2000-2007), Head of UFR (2003-2007).
Main range of scientific interests: geology of clays; environmental geology, coastal environments. Publications: 15 papers in international scientific journals, 10 monographs, 52 participations in international congresses.
Organization(s): University Cadi Ayyad of Marrakech, Faculty of Sciences and Technology, Department of Earth Sciences. Position: Teacher of Hydrology. Scientific degree: Assistant Professor.
Education: University of Agadir (1984-1988), University of Lille (France) (1988-1989), University of Paris VIII (France) (1989-1994), University of Marrakech (Faculty of Sciences and Technology) (1995-2007).
Experience: Assistant Professor (1995-2007).
Main range of scientific interests: hydrology and floods in semi-arid zone.
Publications: 5 papers in international scientific journals, 8 monographs, 22 participations in international congresses.
(
А ЛУВ
N I
Lahcen Daoudi
W Я
Wv
SàT Láff
Mehdi Saidi Mehdi Saidi
Introduction
In arid environments rising waters accompanied by landslides, the consequences of which can be very serious in terms of damage and cost, are quite frequent. In the Ourika basin (High Atlas of Marrakech, Morocco), the rising waters rapidly turn the roads
dangerous and consequently out of use, especially as the valley is linked to the outside world by a unique fragile and vulnerable road.
On August 17th, 1995, the High Atlas of Marrakech, and most particularly the Ourika valley witnessed floods of enormous strength which occurred in an unexpected brutal way [1, 2]. In an unprecedented short time, the
1 id
11
Международный научный журнал «Альтернативная энергетика и экология» № 5 (61) 2008 © Научно-технический центр «TATA», 2008
117
floods caused human casualties (more than 200 death and unaccounted for) and huge material damage. The torrential rain and the mudslides whish followed swept away road infrastructures, agricultural lands, houses, hospitals and schools as well as a great part of the irrigation infrastructures [3]. On the whole, the material damage (vegetable and animal production, hydro-agricultural network and properties) are estimated at 155 million dirhams (about 15 millions US dollars) [4]. This catastrophe caused a great imbalance as far as the production system and the ecological environments is concerned.
This paper is a study of the hydrological, lithological and geomorphological aspects of the Ourika basin. The primordial purpose of this study is: 1 - to understand the dynamics of this natural fragile environments, 2 - to provide indications allowing for a characterization of its hydrological behaviour.
Characterisation of the Ourika basin watershed
The hydrological behaviour of a basin watershed depends mainly on its climatic and geomorphological characteristics. In fact, just like the weather, the physical environment can provide appropriate grounds for brutal pulsations of rising waters. A heavy rain, whish falls on
a sloppy basin with deep watershed and with little permeable substratum, can cause flows on the surface with a very short time response of water concentration.
Geographic and climatic situation The Ourika basin at Aghbalou, about forty kilometers south of Marrakech (Fig. 1), is situated between 31° and 31°20' North and between 7°30' and 8° West. Several aridity indexes place the sector in a semi-arid zone with a sub humid tendency where oceanic (west perturbations), continental and mountainous influences interfere. The average annual temperature is 17.6 °C at Aghbalou, but the difference in temperatures between the hottest mounts (July) and the coldest (January) can reach 15 °C. The region is characterized by precipitations of a spatio-temporal variability and by relative irregularity in superficial flows. The average annual rainfall is 584 mm per year at the Aghbalou station with a 34 % variation rate. The monthly and seasonal variability is even more marked respectively by 50 and 55 % variation rate. According to the Marrakech hydraulic regional office, the average annual discharge at the basin vary from 0.59 m3/s to 29.6 m3/s. However, the marked trace of the Ourika flows corresponds to the highly occasional flow discharge which can reach several hundred of cubic meters per second.
Fig. 1. Ourika basin watershed: location map and hydrographie network
International Scientific Journal for Alternative Energy and Ecology № 5 (61) 2008
© Scientific Technical Centre «TATA», 2008
Morphological characteristics The basin watershed shape can have important hydrological consequences, mainly the rain-discharge relationship and the evolution of the flows in periods of rising waters. In other words, and besides the nature of the rainfall, it is basin's morphological characteristics that condition the shape of hydrograms observed downstream the basin. Several formulae and indexes illustrate these characteristics (Table 1). The Gravelius compactness index (Kc = 0.28 P/VS); where P is the perimeter and S the surface) offers and idea of the basin's geometrical shape; it is the range of 1.3. This relatively mediocre compactness grants the basin with an elongated shape. The main flow forms a linear valley, fed on the two banks, by a succession of tributary ravines (Fig. 1). This situation allows for the waves of rising waters to swell downstream while being fed by the tributaries.
The analysis of the distribution of the altitude parts is made of the basis of a topographical map at 1/100 000 m Oukaimeden-Toubkal. The altimetric distribution at the Ourika basins shows the predominance of sites situated between 1600 and 3200 m (75 %); the average altitude reaching 2500 m, Fig. 2.
1200 m
Table l
Morphological characteristics of the Ourika basin watershed
Perimeter (km) 104
Surface (km2) 503
Compactness index 1.3
Length of the main (km) 45.5
Length of the equivalent rectangle (km) 39.2
Width of the equivalent rectangle (km) 12.S
Maximum altitude (m) 4001
Minimum altitude (m) 1070
Average altitude (m) 2500
Average slope of the main flow (%) 2.15
Average slope of the main tributaries (%) 9.35
Average slope of the basin watershed (%) 35
Moins de 1200 1200-1600 m Jj 1600-2000 m 2000 - 2400 m 2400-2800 m 2800 - 3200 m 3200 - 3600 m plus de 3600 m
Fig. 2. Hypsometry of Ourika basin
Международный научный журнал «Альтернативная энергетика и экология» № 5 (61) 2008 " ^ © Научно-технический центр «TATA», 2008
The calculation of the Ourika slopes shows that the main flow slopes are not particularly high (0 to 5 %). However, the speed and the violence of the flows are mainly governed by the most important slopes of the tributaries. All the tributaries feed the main flow with very important slopes. The Tarzaza that drains the Oukaimeden massif follows an average slope of 11 %, but the most sloppy little valleys are situated upstream the basin with slopes reaching, in some places 30 to 40 %, Fig. 1 and 2.
Geological context of the basin
On the geological side, the basin slope offers two types of facies (Fig. 3):
- The upstream part, situated at heights superior to 2000 m, is composed of igneous and metamorphic rocks wish form the Atlas chain platform. We found especially plutonic, mainly granite and granodiorite rocks, volcanic (andesites, rhyolites, ...) rocks and metamorphic rocks (such as gneiss and migmatites).
This crystalline mosaic is appropriate for a rapid flow of the rain waters.
- The septentrional part, situated at heights inferior to 2000 m, is composed of permo-triasic and softer quaternary deposits. These facies are formed by conglomerates, sandstones, siltites and clays [5]. The lithological observations deduced from the geological map at 1/500 000 m and from land prospections show that soft to averagely soft rock represent an outstretch inferior to 40 % while the hard substract represents about 65 % of the basin outstretch. Thus, the source of the blocks and the drifting pebbles carried by Ourika came essentially from the platform which constitutes the axial part of t
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.