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José Bautista Reservoir

Territorial Framework and Management.

Physical characteristics of the basin.

The River Guadalentín drainage area is located in the south-western part of the Region of Murcia, between the Sierras of Lavia, El Cambrón, Espuña and El Cura, to the North and those of Las Estancias, La Carrasquilla, Almenara and Carrascoy to the South and South-east. The basin’s watershed intercepted by the José Bautista Martín Reservoir, meets the watershed for the general basin of the Guadalentín in the Sierra del Cura to the North and the Sierra de Carrascoy to the South.
The reservoir’s drainage area covers an area of 3,142 km² and is at altitudes of between 111 metres above sea level at the dam and 2,045 metres above sea level at Cerro Poyo, in Sierra de María, at the headwaters of the Caramel or Alcaide River.
The reservoir has a capacity at maximum normal level (131.50 metres above sea level) of 5.55 million cubic metres and covers an area of 80 ha.
Librilla and Alhama in Murcia are the two Municipalities covered by the Reservoir’s surface.
In the basin we can find the three different subgroups of the Mediterranean climate- arid, semi-arid and subhumid-, due in particular to the presence of the Sierra de Espuña. The arid climate can be found in the lowest areas, particularly in the Guadalentín valley and up to altitudes of 600-700 metres in the mountains. It is characterized by mean annual temperatures of around 18 ºC and a rainfall of below 300 mm. The semiarid subgroup is only found over altitudes of 600-700 metres and is characterized by mean temperatures which range between 14 and 18 ºC and rainfall of between 300 and 500 mm. Above altitudes of 900-1000 metres we can find the subhumid subgroup with mean annual temperatures below 14 ºC and rainfall of more than 500 mm.

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Geology and geotechniques of the dam.

The area inundated by the reservoir essentially affects materials from the Quaternary epoch.

To a lesser extent, in the area near the reservoir, Miocene materials have also been inundated, made up of the same facies which form the ravines.

The Quaternary materials which make up the storage area are:

  • Greyish-white and yellowish-white clays and loams; they correspond to the letters Qm in the geological map.
  • These materials cover practically the entire surface affected by the storage area, as well as a large part of the Guadalentín valley floor.
  • Present alluviums and terraces of the River Guadalentín (Qal and Qt) occupy the sunken areas formed by the river course and the areas adjacent to it.

Geometrically, they are always found below the Quaternary clays and loams (Qm) which fossilize them. When they appear, they tend to be small outcrops close to the natural downstream boundary, where the clays and loams (Qm), the main constituents of the reservoir area, have been eroded. There is also a small outcrop in the riverbed close to the bridge on the secondary road from Librilla to Casas Nuevas which has been affected by the reservoir.

From a tectonic point of view, no accidents, such as faults or folds, which affect the reservoir area, need to be reported. If anything, there is an accumulation of clays and loams (Qm) with horizontal stratigraphy.

From a geotechnical point of view there is one important factor to be considered which is how easily the loams (Qm) which form the reservoir area erode. In fact, the water courses formed during more or less torrential periods of rainfall, create deep furrows cut into these loams producing their immediate erosion. Similarly, landslides are produced in the areas where the river course touches one or other embankment of the hillside. These landslides are not significant, due mainly to the fact that the hills in the area upriver of the natural boundaries, where the reservoir storage area widens leading to a low altitude plain, (Guadalentín valley) are not particularly high, though superficially there are some substantial landslides.

Geology and Geotechniques of the Natural Downstream Boundary.

The area of the natural downstream boundary of José Bautista Martín Reservoir, on the River Guadalentín in the Municipality of Librilla (Murcia), has been studied in detail on several occasions.

The River Guadalentín enters the Miocene substratum in this area. From the aerial photograph and the geological map we cannot guarantee the existence or absence of more or less important faults dividing the Miocene substrate in the area where the reservoir is located. There are signs of some vertical faults following the river’s alignment, though this extreme has, until now, been difficult to confirm.

The last explorations carried out included three probings: one in the bed and another two in the hillsides, in order to provide certain information on supposed faults appearing in previous reports.

Test drilling of the river bed.

The materials which appear in the test drilling of the river bed are Miocene deposits (they have been dated in accordance with their microfossils). The lithology varies throughout the test drilling. The minerals range from clayey carbonated limes (which could be denominated loams) to poorly cemented conglomerates.

In the lithological column, only three different types of soils were distinguished:

  • Conglomerates with a clayey cement
  • Brown or reddish-brown clayey loams (or loamish clays)
  • Microconglomerates and/or sandstones with calcareous cement

From the test drilling of the river bed, a particularly thick area was detected formed by conglomerates whose deformability is the lowest of those found in this test.

At greater depths, gravel appears less frequently and below the altitude of 70 metres above sea level they do not appear again in practically the next twenty metres examined in the drilling.

Test drilling of the left abutment.

The test drilling of the left abutment revealed a similar lithological formation, though, in the upper part, different layers were detected (whitish carbonated limestones, gravels and a layer of grey loamish clay) which are probably Quaternary deposits.

Conglomerates with a clayey cement are less frequent in this test drilling. The ground appears to be formed from finer materials.

At the end of the drilling the recovery of the loams is clearly better than in the shallower areas.

Test drilling of the right embankment.

This test drilling perforated the Miocene from its mouth. In the area located at the level of the riverbed (the first fifty metres) there are cemented conglomerates. Some complete samples have been recovered which have later been highly resistant in the laboratory. These conglomerates alternate with clayey loams similar to those found in the riverbed and the left abutment.

The deepest area of the test drilling indicates the predominance of poorly cemented sandstones which are recovered as sand from the test drilling.

The presence of sandstones in the left abutment and their absence in the other test drillings leads us to thinking that there is a discontinuity close to the foot of the right embankment.

The difference in the thickness of the clayey cement conglomerates found in the different drillings suggests that the presence of conglomerates is erratic.

In any case, the decrease in the abundance of the conglomerates from the riverbed to the left embankment, and, in particular within the area of the riverbed, at depth, is a fact already which other examinations have already highlighted and which is also hinted at in these studies.

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