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Reservoirs

Algeciras Wadi Reservoir

Territorial framework.

Physical Characteristics of the Basin

The catchment area of the Algeciras wadi is located in the south-west of Murcia, on the northern edge of the Guadalentín valley (on its left bank). The reservoir is located on the Rambla de Algeciras, a tributary of the River Guadalentín, in the municipality of Alhama de Murcia.

The surface area of the basin is 44.91 km², and altitudes range between 200 m and 1,320 m above sea level along the length of 15 km running west to east. The basin is surrounded by the Sierras del Cura, de Espuña and de La Muela.

The hydrographic network in the area is composed of numerous gullies, the main one being Valdelaparra, where the source of the Rambla de Algeciras is to be found. The mainly uncemented, detritic nature of the materials of which these gullies are composed, together with the torrential nature of rainfall here in the stormy season, produce a high level of erosion with large amounts of solids being transported, creating a wide-ranging and intensive system of surface drainage.

The dam shoulder and reservoir seen from upstream
The dam shoulder and reservoir seen from upstream
 

The three fundamental variants of the Mediterranean climate (aridity, semi-aridity and sub-humid) are present in the zone, due principally to the presence of the Sierra de Espuña. An arid climate is found in lower-lying areas, especially in the Guadalentín valley and up to 600-700 metres in the Sierras. It is characterised by average annual temperatures of around 18 ºC and rainfall of under 300 mm. Semi-arid conditions are found only above 600-700 metres and are characterised by average annual temperatures of between 14 and 18 ºC and rainfall of between 300 and 500 mm. Above 900-1,000 metres above sea level, we find sub-humid conditions, with average annual temperatures below 14 ºC and rainfall of over 500 mm.

The Algeciras wadi typically has only brief periods of flow but with heavy volume, on account of the torrential local rainfall patterns. This means that most of the water channels in the zone are wadis, i.e. dry rivers with torrential flow patterns, steep gradients and significant solid flow content.

The most important municipalities affected by the reservoir are Alhama de Murcia and Librilla.
 

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Environmental information

The negative economic and social effects which a reservoir can have are those related to the flooding of inhabited areas and agricultural land.

With the modification of flow patterns the possible effects of flooding are reduced, as well as material damage, and the loss of human life and other catastrophes are avoided

The creation of the reservoir in order to abate flooding, the main objective of building the dam, is positive. Among the secondary activities of the reservoir it is possible to mention the following;

  • Domestic supply:the reservoir contributes to solving supply problems in the towns and villages which rely on the right-bank distribution canal of the Tagus-Segura Inter-Basin Canal. Positive.
  • Fishing: the creation of a fishing reserve can have leisure activity benefits for neighbouring towns and villages. Positive.
  • Irrigation: the reservoir guarantees watering every year, and improves the regulation and flexibility of the Inter-Basin Canal. Positive.
  • Health: by guaranteeing the supply to various towns, the reservoir avoids the use of water from privately-owned wells without health controls and guarantees. Positive
  • Aesthetics: A large amount of earth needs to be moved. Unfavourable.
  • Social acceptability: despite the negative aesthetic impact, the reservoir appears to be well received by the community which it is designed to assist. Favourable.

It seems reasonable to assume that although there is significant erosion, this will not cause any special problems due to solids being transported or sedimentation during the useful life of the reservoir.

On the other hand, neither is it probable that the reservoir will produce serious erosion downstream, since in abating the effects of floodwater it makes downriver flow volume much lower than upriver.

Another effect, related to those above, is the erosion produced on the shore of the reservoir due to waves and the variation in water levels, combined with the instability of the banks when wet. However, it is reasonable to conclude that these phenomena should not significantly influence the reservoir’s capacity.

Water and climate.. Important changes in water quality can occur in water which is stored, due to the eutrophication produced by retaining it in temperate zones.
Studies are being made into the annual phosphorous levels in the reservoir, since this substance limits eutrophication, and these studies indicate that the level is between eutrophic and autrophic. Therefore it can be concluded that the impact on water quality is tolerable.In terms of climate, it is thought that the reservoir may have a favourable effect or none at all, but no possible negative impact.

Flora and Fauna. No great changes in the local flora and fauna are predicted, although this is a complex area. Tolerable impact.
 

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Geology and seismology.

Regional geology.

Lithology.Geologically the area is composed of both Baetic and sub-Baetic units and overlying Tertiary sediments deposited on the Baetic erosion surfaces. Large stretches of sedimentary accumulations from the Quaternary are found on the floors of the valleys which separate the various mountainous parallel ranges formed by Baetic materials, and with a morphology of glacis and alluvial deposits.

The Algeciras Wadi Reservoir is situated among various Baetic structural units (Sierra España to the south and south-east). To the north are formations belonging to the Sub-bético Interno (internal sub-Baetic) (the Sierras of Ponce, Ricote and Oro

The stratigraphic sequence of the complex Baetic units form the backdrop on which the post-mantle sediments have been deposited, formed by Neogene and Quaternary deposits. In this zone these formations constitute the Alhama de Murcia-Alcantarilla basin..

In the Baetic geo-tectonic area, there are three stratigraphic units which are from the Tortonian period:

  • The Lower Tortonian appears in Sierra Carrascoy and presents a lithological facies which is predominantly detritic, with metamorphous rock conglomerates (phyllites, slates and quartzites), with dolomites and dolomitic limestones.
  • The Middle Tortonian appears on the slopes of the Sierra de la Muela, with a marine facies, and consists of a sequence of ochre calcarenite, violet-coloured conglomerates of mixed origins and yellowish sandy loams.
  • The Upper Tortonian – Andaluciense appears in the basin of the Algeciras wadi and is the most widespread unit in the area. It is formed by a sequence of blue loams interspersed with whitish calcareous loams and gypsiferous loams. Castellar peak is crowned with these formations of gypsiferous loams interspersed with thick banks of crystallised gypsum.

Although it is not seen in the immediate surroundings of the reservoir, there is a volcanic complex in the town of Barqueros, made up of volcanic ash with dikes and lava flows

Tectonics. The features of the area correspond to a tectonic group of the Alpine Orogeny, whose effects have been felt since the Jurassic.

It is thought that the Baetic units were laid down in at least two separate tectonic phases, involving strong horizontal movements. During the first, the units were separated. During the second, overthrusting caused a considerable tectonic compression and discontinuity in the degree of metamorphic re-crystallisation in the contacts between units. Later, large-scale folding affected the units.

In the last phase of the orogenic history, the Baetic lands around the site of the reservoir were actively affected by faulting, especially normal faults.

Tectonically speaking, the lithological units found in the area of the dam are not compatible with normal Baetic formations, and the terrain shows various phases of deformation. These movements can be seen by the occurrence of polygenic conglomerates produced by the intense erosion of the emergent Baetic mountain-ranges. During the Tortonian sedimentation slight deformations can be seen which bring about intra-formational brecciations and gentle folding which herald the arrival of the post-Miocene marine transgression. There is intense deformation in landscapes of the Upper Tortonian and Andaluciense.

The movements are evident in all the lithological units around the site of the reservoir. The layers dip at various angles of intrusion with gentle folding on the internal part of the Algeciras wadi basin, whilst at the edge, near the Guadalentín, there are various fractures in a SW-NE direction.

The most recent movements have given rise to the rejuvenation of structures which already existed, causing diverse tectonic features, of which the most spectacular in terms of morphology is the valley of the River Guadalentín.

At the NW edge of the valley of the Guadalentín there are indications of geological discontinuities, with flexural slip folding.

Geology and Geotechniques of the Storage Area Upstream of Dam.

The Tertiary materials which underlie the reservoir and its surrounds are as follows:

  • Sandy-loamy polygenic conglomerates, sometimes loose and sometimes strongly cemented, with phyllite, quartzite, dolomite, limestone and sandstone pebbles, form the hill on the right bank downstream of the reservoir. There is a variety of geo-technical outcrops, from rockfill material to a violet impermeable granular substance similar to rañas, or ochre or calcarenite sand.
  • The loams are abundant on both banks of the wadi throughout the reservoir. For morphological reasons a distinction is made between the underlying blue loams and the gypsiferous loams in the upper area (stratified and with a high gypsum level).
  • The calcarenites and molasses of the Cerro del Castro de la Atalaya, ochre in colour, are similar to those in the Sierra de la Muela, and in tectonic terms lie discordantly tectonic on the loams of the storage area upstream of the dam.

Within the covering Quaternary materials on the wadi bed, it is possible to discern some debris deposits formed by limestones, sands, gravels and limestone pebbles, very irregularly distributed. On the sloped banks there are also conglomerate glacis, scree, fragmentary rubble, and alluvial fans.

The series of clayey loams is fairly uniform until we reach the area of the natural downstream boundary for storage area, where, on its downstream face, on both embankments, there is an unusual contact between clayey loams and the polygenic conglomerate. This contact is produced by a sudden change of angle in a very short distance, and a flexurally faulted structure can be seen. This formation can be seen on both banks just on the downriver side of the dam.

The following should be noted in respect of the geo-technical features of the land under the reservoir:

  • The general imperviousness of the reservoir bed is very good. In general terms, the clayey loams are, by their very nature, totally impervious. The formation which appears here is composed of a sequence of clayey loams and Quaternary cover. The drainage network is intense and very well-defined, no changes due to lithological variation being observed. Therefore, the complete watertightness of the reservoir storage area can be ensured. However, it is important to point out that the natural downstream boundary of the dam, especially Castro Hill, where the right-hand abutment is situated, there is a high level of permeability in the pockets of conglomerate molassic limestones. This made it necessary to move the axis of the dam upstream so that the metal grating of the core rested on the abundant underlying blue loams appearing in the ravine on the hillside. In this way the whole area was sealed and covered. This, together with the side key dam of the narrow part of the ravine, avoided seepage into the pockets of milonitised material which appear in the side mound of this hill, where a fringe of intense fracturing has been found. On the left bank of the natural downstream boundary, this problem does not exist, as long as the maximum normal level of the reservoir is maintained, so that the porous formations of the upper pockets (sandy limestones and gypsums) are not reached.
  • In terms of embankment stability, the morphology of the reservoir storage area shows a high degree of changeability in the sequence of clayey mares, with high, steep banks. The stability of the embankments when they are dry is very favourable, although it is possible to detect a progressive withdrawal, resulting in a certain degree of verticality of the embankments. The crumbly and highly degradable nature of the sequence of clayey loams and the typical features of this formation are properties which combine to create favourable conditions that could lead to certain instabilities during the saturation process (filling and emptying of the reservoir).

Geology and Geotechniques of the Natural Downstream Boundary.

The Algeciras Wadi Dam is located some 550 metres from the Tagus-Segura Transfer Main Right-Bank Canal siphon. The axis of the dam is about 100 metres downstream of the orographic closure (gorge).

From the topographic point of view the natural downstream boundary corresponds to the wadi level, which is about 200 metres above sea level. The general direction of the wadi is WNE-ESE, with a series of meanders controlled by tectonic factors such as faults and diaclases, as well as lines of folds and the direction of the regional fault angle

The shape of the natural downstream boundary is asymmetrical, with steeper banks on the left embankment (30 or 40 degrees above the horizontal one) than on the right embankment (20 to 25 degrees). Furthermore, there is a small platform or terrace on the right embankment at 210 to 215 metres above sea level. At the level of the wadi, between 200 and 205 metres above sea level, significant narrowing occurs, the slopes being rather a lot steeper than what is average on both embankments, with an alluvial terrace width of between 40 and 70 metres.

Both embankments are cut by numerous small ravines and gullies, partly related to lithoclases and/or evolutionary erosive phenomena. The right embankment is mostly parallel in its structural surface to the strata in the slopes, whereas on the left there are numerous rock deposits, debris deposits, and upturned strata.

Seismology.

In general terms, the province of Murcia is located between the most important seismic activity zones in the Iberian Peninsula. It is considered by the Spanish National Committee on Large Dams as a zone of medium seismic activity. However, in the Seismoresistant Construction Regulations PDS-1 of 1974 it is classified as a Level 3 seismic activity zone, denoting a high level of activity..

The nearest epicentres to the dam site correspond to specific geological structures such as the dip of the River Guadalentín and the bordering Paleozoic Baetic basements.

The area where the Algeciras Wadi Reservoir is located is within the tectonic zone of the Baetic Cordilleras, and there are a large number of epicentres whose tremors are due to structural readjustments, which still continue today, along the most important fault-lines, possibly on account of North-South compression at the union of the African and European tectonic plates.

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