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Reservoirs

La pedrera Reservoir

Territorial Framework and Management.



Physical Characteristics of the Basin.

La Pedrera Reservoir basin catchment area has a surface of 36.34 km² and its maximum height is 300 masl; it is completely within the province of Alicante, in an orographically uneven area, and whose natural flows are practically non-existent. The reservoir is located in the wadi called Rambla de Alcoriza and in order to build it, it was necessary to construct, apart from the main dam, three saddle dams at the sides in order to close off two water-courses located on the right bank -Dikes I and II - and a third on the left bank -Dike III -. The surface slope is of a markedly rectangular shape of about 8 km perpendicular to the dam and about 4.5 km parallel. This territory receives an average annual rainfall of about 275 mm.

The Normal Maximum Level of the reservoir that, due to the characteristics of the catchment area, does not have a spillway, is limited by the dam crest -height 110.50 -, and it is located at a height of 105.00. The total reservoir volume for the normal maximum level is 246.09 million cubic metres. On the other hand, the surface inundated for this level is 1,272 ha, the length of the shore, 31 km, and the length of the reservoir goes about 6 km up the wadi.
 

Photo view of the reservoir, dam and intake tower from the right bank
View of the reservoir, dam and intake tower from the right bank

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Geology and Seismology.

Geology and Geotechniques of the storage area upstream of the dam.

La Pedrera dam and reservoir are located at the most eastern end of the Baetic Mountains. This area is formed by materials from the Tertiary period, from the Vindobonian age that were folded towards the end of the Helvetian age and during the Tortonian stage, in the middle of the Miocene period. The Miocene materials have been folded into steep dips, which in some cases are over 70º, amongst which we must highlight El Hurchillo anticline, whose north flank slopes down into the Segura trench.Here we find, in an almost vertical position, layers of sandstones and limestones.

From this anticline stretches a fold of a large radius that affects the Helvetian materials, which includes the materials of the storage area upstream the dam and the downstream boundary. Once the folding had taken place, a process of intense erosion began that broke up the calcareous roof –not very thick- of the anticline and produced the inversion of the relief by digging out a great hollow in the bluish-grey loams of the anticline core. In the next layer, corresponding to the Tortonian stage, starts the continental

Morphologically, the area of the reservoir can be described as a wide erosive depression surrounded by gentle reliefs that are somewhat higher on its northern side, due to the pronounced contrasts of its lithology. The great anticline, whose core is formed by loamy material from the Miocene protected with weak upper strata that are harder (sandstones, conglomerates and limestones), was eroded producing a wide valley or hollow that marks a great contrast in the relief. All the substratum of the area is formed by grey and blue loams from the Miocene that are very thick; in the reservoir basin these loams appear covered in greyish and brownish-grey clays with a high proportion of silt from the Quaternary period; it is about 12 m thick. To the sides of storage area upstream of the dam, the summit edges are made up of a mixture of loam-limestone and sandstones that, as they are more resistant to erosion, form the divide. These ridges are the remains of the broken cover of the anticline that produces gentle outward slopes, which are steeper toward the interior.

The strong erosion that took place in the bluish-grey loams led, in some cases, to the slipping of small scales of limestones that, on the one hand, ended up covering more recent layers of the Tortonian stage, as occurred on the south-eastern hillside of the Cabezo Gordo and, on the other, produced some gravity faults. Some slip faults were also produced between the limestones alternating with the bluish-grey loams, to the East and the South-east of the Cabezo del Moro. In short, the dam is located in the monocline series that closes the anticline on the North, while the storage area upstream the dam lies on the anticline core of bluish-grey loams that are impervious and which, only in the area of the natural downstream boundary for the storage area, the presence of Tortonian conglomerates gave rise to problems of imperviousness.
 

Geology and Geotechniques of the downstream boundary.

In order to determine the geological composition of the natural downstream boundaryfor the storage area, both of the main dam and the key dams, in 1971 the Public Works Geological Service (SGOPU) made twenty one (21) mechanical probings of lengths of between 15.5 m and 102.2 m and a total length of 1,128 m, distributed in the two axes which at that time were being considered as the possible location for the dam, and in the two hills of the right riverbank. These probings were supplemented with six electric probing profiles, with a total of 72 units. In each of the probings a pile, a surface nozzle and a stand pipe piezometer were placed, to measure the water levels. Below there is a description layers and the materials that appear in the studied environment, as contained in the SGOPU report in August 1971.

As a whole, a thickness of about 700 m was calculated for the Miocene period, of which 300 m corresponded to the Vindobonian age made up of loams, limestones, sandstones and pudding stone. The natural downstream boundary and the storage area ustream of the dam are entirely located in this formation and the following layers were defined:
 

  1. Bluish-grey loams. The base of the Vindobonian age is made up of detritic levels that do not appear in the studied area, followed by a series of highly impervious clayey loams of a bluish-grey colour that occupy practically the whole of the reservoir basin. They are quite thick, since, in the deepest probings, a depth of more than 100 m was registered and they did not go through the layer. On the surface, they are strongly eroded and weathered.
  2. Light grey limestones.They are about 5 or 10 m thick and dip 20º toward the North. Erosion has swept the loamy overlying materials and dug a hill in the right riverbank (Hill 2). The imperviousness of this stretch is guaranteed because it is followed by another one of impervious bluish-grey loams.
  3. 3. Bluish-grey clayey loams. They are very similar to those described above, the calcareous layer being packed tight in between both, which ensures its imperviousness. This soft stretch has been quite affected by erosion and another Hill has been formed on the right riverbank in the direction of the stratification (East-West).
  4. Light grey limestones. Crowning the Helvetian series is a stretch of loamy limestones 40 to 50 m thick, with alternating levels of loamy materials and sandstone, which has resisted erosion only in certain areas and that is well represented in Cabezo Gordo. Beneath the broken limestones appear the bluish-grey loams described above, and, on top of these, in some cases, pieces of the calcareous stretch, more rigid, have slipped.
  5. Conglomerates and sandstone. Discordant with the Helvetian age, a thick conglomerate of heterometric stones of a diverse nature appears. They are moderately rounded and agglutinated with calcareous cement. There are important changes in the series with lateral variations and insertions of sandstone. In the upper part, sandy levels prevail. The thickness in the probings carried out varies from 3.40 m to 27.00 m. This detritic formation presents problems of permeability and occupies quite an extensive strip that stretches along the left bank of Arroyo Grande and the upper part of Cerro de la Pedrera, ending at the confluence of Arroyo Grande and Rambla de Alcoriza.
  6. Sandstones and loams. A thick series of medium-grain sandstones and loams, with some levels of conglomerates are concordant over the previous stretch. It appears downstream from the ravine, has a great extension and are partly eroded.
  7. Alluvial materials.Finally, according to the surface drainage network, there are areas covered by alluvial materials mainly composed of stones, silts and clays that, due to the reduced slope of the river, are several metres thick in the area of the natural downstream boundary for the storage area.

As for the quarries where the various materials for the dam made of loose materials were to be obtained, it was advised that the rockfill should be from Helvetian light grey limestones, and the quarry chosen be in Cerro Gordo, also for the purpose of ruling out any later move. The report also proposed the use of conglomerates of the right bank that, in this way, would increase the stability of the right abutment, and clayey loams as the impervious nucleus.

Seismology.

The seismic activity of the area is very high and has intensity X on the MKS scale. Therefore, a horizontal seismic coefficient of 0.138 was adopted and a vertical one of 0.069 in contrast to the ones officially proposed of 0.1 and 0, respectively, for an earthquake with a 500 year return period.
 

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