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Cárcabo reservoir

Territorial and administrative background .

Physical Characteristics of the Basin.

The El Cárcabo wadi basin is practically defined by the Alto del Almorchón, to the West, and the Alto de Higuera and the Herrada to the East. The wadi riverbed starts in Puntal del Ripión (580 masl) and runs 8.5 km until it reaches the site of the dam (10 km until its outlet into the Segura river). The average gradient of the wadi is of 3.8%.

El Cárcabo wadi basin

Environmental information.

There are two protection orders which include El Cárcabo Reservoir. They are the Special Protection Area for Birds (SPA) called Sierra del Molino, Embalse del Quípar y Llanos del Cagitán, and the Site of Community Importance (SIC), called Sierras y Vega Alta del Segura y Río Benamor. .

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Geology and seismology.

Regional geology.

El Cárcabo Reservoir is located in the most northern part of the Baetic Mountains, which in turn is in the north-western segment of the Alpine Perimediterranean Orogenic Belt (alignment of mountain chains with sliding stratum structure originating from the Cretaceous and Tertiary periods which are arranged centrifugally with respect to the sea; Martín Algarra, 1987).

Schematic Division of the Alpine Perimediterranean Orogenic Belt

Schematic Division of the Alpine Perimediterranean Orogenic Belt

Within the Baetic Mountains, the area in question is considered as being in the external areas or South Iberian Domain. The materials in this domain occupy a large extent of the belt and represent a time period which goes from the Triassic to the Miocene epoch. They have a structure characterized by a general detachment between the base (Hercynian Paleozoic) and the deformed covering (Mesozoic and Cenozoic folds, faults and buckled strata), where the clayish-evaporitic Triassic level acts as the detachment material and the buckled strata are generally inclined towards the W and NW. The Palaeozoic base. does not outcrop, remaining at a depth of 5-8 km and made up of materials analogous to the Iberian Massif.

General Geological Map of the Baetic Mountains

General Geological Map of the Baetic Mountains

In addition to this, materials which are autochthonous to the area of our study can be distinguished: those materials which correspond to the pre-mantle formations (Miocene epoch) and those formations made up of post-Orogenic materials which were deposited after the placement of the allochtonous units and which make up the tertiary and quaternary covers.

Geology and Geotechniques of the Storage Area Upstream

This area is, for the most part, made up of Miocene materials, more specifically from the Tortonian period, made up of dark grey loams and loamy limestones (Msb) with gypsum, which can be found in the course of the ravines and below some whitish sandy loams (Ms). The dip is almost horizontal. The quaternary is made up of colluvial materials (pieces of conglomerates, sandstones and limestones mixed with clayey and silty sand), alluvials (formed from limestone and loamy limestone pebbles) and elluvio-colluvials (formed from sandy silt with pebbles). There is a wide strip of allochtonous material which has an average width of 400 m and which crosses the central part of the reservoir from West to East, proceeding from successive Internal SubBaetic sliding strata. These materials are from the Keuper Triassic (motley loams with gypsum and sandstones) and the Eocene (loams, sandstones and nummulite limestones). As far as the geotechnical characteristics of the materials which can be found in the reservoir area the following can be said:

- All of the materials (with the exception of the thick quaternaries), both autochthonous and allochtonous, are practically impermeable.

- The presence of gypsum contributes to the salinity and aggressiveness of the waters.

- The materials are extremely subject to erosion, especially in the whitish sandy loams of the Upper Miocene; as well as to a lesser extent, in the red and green loams of the Upper Miocene and those existing in the Eocene and Triassic

Geology and Geotechniques of the natural downstream boundary

The downstream boundary is located in a very craggy mountain ridge which the El Cárcabo wadi cuts across perpendicularly.

This ridge is 6 km long and 160 m wide as the wadi crosses it and widens out at both ends. There is a 25 m difference in height between one side of the pass and the other formed between the two crags, which the wadi overcomes with a steep gradient and three waterfalls. The first is at the entrance to the pass with a drop of 2.5 m, the second is 25 metres downriver with a drop of 7 m and the third, after a steep stretch enclosed by the last crag has a drop of 10 m as it leaves the last crag. After this last waterfall there is a 4.5 m deep pool.

As a whole the materials of the downstream boundary are conglomerates interspersed with sandstones, sands and clays, which means that there is typical differential erosion, making thick crags from the conglomerates with almost vertical drops, especially on the right bank; and the interspersal of sands and clays produce topographical depressions located between the crags.

On the right bank there are three crags and two depressions; on the left bank five crags and four depressions, the biggest and thickest of these crags being that which is located downriver. Transversally to the riverbed, at the downstream boundary, steep hills appear as soon as we move away from the stream. Most of the sunken areas are covered by a piedmont made up of pieces of conglomerates of different sizes and shapes, which have a clayey sandy matrix and a thickness which varies from several cm to 2-3 metres.

The materials from the downstream boundary are from the Cretaceous era, made up in general from polygenic conglomerates originating from limestone, silica, loamy limestone and sandstone, with a calcareous or sandstone matrix. The conglomerates include lenses of variable length and width of thick grained sandstone, with rich concentrations of carbonates and Lamellibranchia remains, as well as lenses of greenish, white and black coloured, fine grained micaceous sandstones. Green and red clays are also interspersed. The materials at the downstream boundary appear at approximately N-70 with a 75º dip northwards, which makes the crags look vertical. On the wadi riverbed and nearby there are large blocks of conglomerates reaching weights of several tonnes which have fallen off the crags. The alluvial which can be found at the downstream boundary is of little extension and strength.

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The seismic hazard differs depending on the place tested and it is standard practice to classify the land depending on this hazard. Following the criteria set out in the “Reservoir Safety Technical Guide. Geological and Prospecting Studies” published by the Spanish National Committee on Large Dams (SNACOLD) in section 3.3 these are classified in three main groups:

a) Areas of low seismicity ba < 0.04g

b) Areas of medium seismicity 0.04g < ba < 0.13g

c) Areas of high seismicity 0.13g < ba where ba refers to the area’s basic acceleration.

Following this classification, El Cárcabo reservoir is located in an area which we could classify as being of medium seismicity, as the basic acceleration for the Municipality of Calasparra is of 0.07g and for the Municipality of Cieza is of 0.09 g.

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