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Pliego Reservoir

Territorial Framework.

Physical Characteristics of the River Basin.

The River Pliego, tributary of the Mula on the right bank, arises from the confluence of Rambla del Madroño, Rambla del Huérfano, Rambla de Malvariche and Barranco de la Hoz which collect the runoff from Sierra Espuña. Rambla de Malvariche is the longest course in this fluvial network, approximately 30 km, going as far as Pliego Reservoir.

The northern area of Pliego Reservoir basin is characterised by plains furrowed by Rambla del Madroño and Rambla de la Herteña. On the other hand, the southern area covers part of Sierra Espuña. The highest point in the basin is Espuña, with 1,583 m, in the Barranco de la Hoz basin.

The Pliego reservoir basin is crossed by Taibilla Canal, but not by any railway line.

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Environmental information.

The land through which the Rambla del Madroño, the Rambla de la Herteña and the Rambla del Huérfano run is a plain with almond tree plantations, sometimes combined with apricot trees. There are also some scattered vineyards, the largest one being found at Llano de Aparicio. Intensive farming, barley and wheat, can also be found scattered throughout this flat territory.

The most important extensions of esparto grass (Stipa tenacissima) can be found in the Rambla de Malvariche basin. Nowadays esparto grass is hardly grown and the existing clumps are natural ones, due to the aridity and dryness of the land. Those lands on hillsides and on not-too-uneven ground are rotated to be used for dry farming crops such as almond trees.

Clumps of esparto grass can appear associated with the Aleppo pine (Pinus halepensis) and scrubland such as rosemary (Rosmarinus officinalis) and thyme (Thymus sp.).

In quantitative terms, the Pliego Reservoir river basin is mainly composed of woodland. The mountain landscape of Sierra Espuña is dominated by three species of pine reforestation: the Aleppo pine (Pinus halepensis), to a greater extent, the Maritime pine (Pinus pinaster) and the Austrian pine (Pinus nigra). The highest areas in these mountains are colonized by thorny scrubland of scarce height that adapts to the inclemency of the wind. Among these woodland species we can highlight the Phoenician juniper (Juniperus phoenicea), the West Prickly or Cade juniper (Juniperus oxycedrus), broom (Genista sp.), buckthorn (Rhamnus lycicoides) and Kermes oak (Quercus coccifera).

The pine grove is the environment most used by the Espuña red squirrel (Sciurus vulgaris hoffmanni), an endemic species of this area, the wild boar (Sus scrofa), the goshawk (Accipiter gentilis), the eagle owl (Bubo bubo), the jay (Garrulus glandarius) and the crossbill (Loxia curvirostra).

The heights of the basin are dominated by the golden eagle (Aquila chrysaetos) and the chough (Pyrrhocorax pyrrhocorax). In these high areas it is easy to spot the Barbary sheep (Ammotragus lervia), a hunting species introduced in the Parque Natural de Sierra Espuña from Morocco.

Farmlands are inhabited by the swallow (Hirundo rustica), the white wagtail (Motacilla alba), the kestrel (Falco tinnunculus), besides mammals such as the field mouse (Apodemus sylvaticus) and amphibians such as the common salamander (Salamander salamander), the common toad (Bufo bufo), the Iberian water frog (Rana perezi) and the European tree frog (Hyla arborea).

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Geology and Seismology.

Regional Geology.

The Pliego river basin is mostly within the so-called Espuña-Pliego Tertiary structural unit associated with the Pre-Baetic one of the Mula Unit to the North. It is made up of materials deposited after the first tectonic event but affected by the movements of the following ones.

The Tertiary materials of Sierra de Espuña range from the Paleozoic era until the Eocene and are hardly affected by Alpidic Metamorphism. The Sub-Baetic materials go from the Triassic period to the Oligocene and do not present metamorphic features. The Neogene and Quaternary materials occupy a series of corridors that separate the various mountain ridges made up of Baetic materials.

The Mula unit belongs to Sub-Baetic - Pre-Baetic geotectonical area which is characterized by tectonics with surface principal structures and numerous low-angle thrust faults, this area having an important seismic activity.

The Mula unit is mainly composed of a succession of materials ranging from the Oligocene period to the Upper Miocene and Quaternary.

To the east of Sierra de Ponce there are Oligocene formations consisting of a thick series of loamy limestones characteristic of a lacustrine sedimentation, followed by compact limestones with Lepidocyclina, which are followed, discordant, by some molassic limestones, possibly from the Aquitanian period.

To the north of the Sierra de Espuña or South-southeast of the Mula unit, appear the formations of the Pliego river basin, featuring on the Oligocene base some reef limestone and conglomerates, followed by the Lower Oligocene formed by ochre loams with alternating detritic levels and then the Upper Oligocene, basically coastal with strong continental influence, composed of shales, limonites and sandstones, also appearing, clearly discordant, some molassic limestones from the Aquitanian period.

Above these materials, to the west, there are deposits from the Middle Miocene, formed by greenish-grey loams and conglomerate sandstones of calcareous cement; to the east and in the vicinity of the town of the Pliego, there are elements from the Upper Miocene, being discordant on the Lower Miocene; in this case, they are formed by loose clays, conglomerate sands and some levels of molasses.

All the above-mentioned is covered by a silt deposit plain that fossilizes a previous paleo-relief and which is formed by distance deposits of loamy and slightly gypsiferous silts with a certain percentage of sands in diffuse lenses.

The existing terrace deposits, fluvial alluvium and loose hillside soils are from the Quaternary, Holocene, period.

Geology and Geotechniques of the Storage Area Upstream of the Dam.

There are 3 main units that have been distinguished in the Pliego Storage Area. The two base ones correspond to the Oligocene period, and the third, belonging to the Plio-Quaternary, is composed of alluvial formations, colluvial soils, terrace deposits and recent alluvial soils.

Among the Tertiary deposits, there is, both in the downstream boundary and in its surroundings, along the Rambla del Campillo and the River Pliego itself, a detritic facies formed by alternating shales, limolites and sandstones generally with calcareous cementation.

In Rambla del Partidor de Anguilas, there is a discordant formation over the previous one, made up of some molassic limestones and conglomerates that possibly belong to the Aquitanian period.

The above-mentioned formations lie, in general, below deposits from the Plio-Quaternary age, in the whole area of influence of the storage area upstream of the dam. These deposits are composed of alternating sands, silts and clays with occasional gravels and round stones, of a whitish-greenish colour with ochre and yellowish tones.

They are paleo-relief silt formations and can reach significant thicknesses of about 10 metres or more. Above, there are deposits of Quaternary formations, generally not very thick, which are composed of a mainly clayey soil with a variable content of pebbles and gravels.

The alluvial deposits that appear in the courses of the river and wadis are presented, generally, with a reduced thickness of about one metre and made up of clayey silts, with pockets of gravels and pebbles, basically calcareous and, occasionally, sandy.

Some relatively young geomorphological features can be found within the storage area, through which the river and wadi courses run. Fluvial dynamics present an active channel with some occasional side branches. This system shows a combination of full channels and occasional layers of gravels, and ledges that vary depending on if they are downstream or upstream. It is also necessary to point out the existence of some chains of gravels and round stones that surround the outfall of the wadis into the main course.

Due to the limited sedimentary control of this type of fluvial basin, these geomorphological and morphological characteristics determine short times of concentration, large pressures caused by the transport of sediment load, difficulties in natural flood abatement and the existence of critical points in the bed, in particular, narrowings.

Geology and Geotechniques of the natural downstream boundary for the storage area..

The downstream boundary is very near the place known as Molino de Charrancha, in an area of the river course between two meanders, downstream from the confluence of Rambla del Campillo, a tributary of the River Pliego on the left riverbank.

The hillsides present different morphological features, the left one being more spread out than the right. The left hillside, from 362 m from the flat surface of the crest to 340 m, presents an average gradient of about 25º with not very steep torrential courses.

The right hillside presents a bigger escarpment of approximately 25 m, at the foot of which there is a lower terrace about 20 m wide.

Pliego Reservoir is located on materials from the Upper Oligocene-Lower Miocene period covered by the Pleistocene. On the hillsides there are discontinuous colluvial soils, on the lower part of the same terraces of a limited thickness. The alluviums are scarce and discontinuous, not very thick and fundamentally mobile during flood events.

The above-mentioned deposits from the Upper Oligocene-Lower Miocene make up the substratum of the whole area. This formation includes four types of materials, exclusively distinguishable by the percentage of sands and fine materials existing in each layer.

The thick end includes sands more or less calcareous (calcarenites) barely cemented by carbonates; they have siliceous and micaceous grey sandy grains and other clasts of carbonates (smooth grains) with a percentage of very variable fine silts (30-60%) forming the matrix; they are also indistinctly found in banks of 0.80 m or in layers of 5-6 cm with all the intermediate graduations, their stratigraphic limits usually very marked, however, inside the same stratum, it is very frequent to find lateral changes, slumping structures and lenses.

The other end of the series is made up of slightly-calcareous aleurites.

These layers are predominantly thin without any visibly marked structure, except for their general folding.

As intermediate elements we can consider the alterations of the two previous elements in centimetric layers ranging between 4 to 15 cm, or milimetric ones of between 2 and 8 mm. The greater or lesser content in sandstone banks in the series enables us to distinguish more or less competent levels in the terrain.

The Pleistocene materials are fundamentally silty and quite loose. The lower ones, which fossilize the paleo-relief cut out during the Oligocene-Lower Miocene, are slightly more clayey and contain a small percentage of carbonates; they appear as more compact and their density is high. The structures, in this group, are not easy to define, only the differentiation of a surface level of 4-5 m, much thinner with abundant signs of internal erosion, and another lower one that is denser and with a variable thickness being possible. The latter is discontinuous, filling a paleo-relief in which the highest points were only covered at the end of the deposition, already in the upper package.

On the base of the lower silts there are deposits of subrounded gravels of a calcareous nature with a diameter of 2 to 5 cm, filled with silty clays and their cement. These form the old alluviums of the fossilized drainage network, and are between 1.5 and 3.0 m thick.

The existing Tertiary deposits form a N 80º E± 10º monocline series with a northern dip of about 28º-40º. Downstream, about 100 m from the dam, the dip increases to about 50º-52º.

In the area of the ravine, potential diaclases with closed fissures can be found, which make the ledges visible, hollowed out as mechanically weak levels.

A series of campaigns for additional tests regarding the primitive project were carried out on the right embankment of Pliego Reservoir, in order to be able to classify its right abutment geologically and geotechnically, and so define its excavation as well as the treatment to be made of the so-called Plio-Quaternary lands. The intention of this study was also to establish the characteristics of the paleo-relief of the substratum soil, formed by Tertiary packages of sandstone, shales and limonites.


Pliego Reservoir is located between the municipal districts of Pliego and Mula, for which Annex 1 of Seismoresistant Construction Regulations NCSR-02 establishes a basic seismic acceleration of 0.09 g, the aforementioned Regulation therefore being applicable.

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