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Crevillente Reservoir

Territorial Framework.

Physical Characteristics of the Basin.

The Crevillente reservoir basin catchment area has a surface area of 11.80 km², its maximum height is 792 masl and it is completely within the municipality of Crevillente. The Barranco del Bosch, in which the reservoir lies, has hardly any flow during the days following significant rainfall. With regard to the latter, is interesting to point out that the rainfall with a return period of 1,000 years generates a peak flow of scarcely 85 m³/s. The Barranco del Bosch basin has no tributaries as such. The surface catchment area has a purely rectangular shape, except for a small fork toward the NW, of about 4 km in a perpendicular direction to the reservoir and about 3 km in a parallel one. This territory receives an average annual rainfall of about 450 mm.

The Normal Maximum Storage Level (MNEN) was established in the original project at the threshold of the overfall spillway and is located on the right embankment of the reservoir. It has a chute and a stilling pool and is situated at a height of 145.00. The total storage volume for the normal maximum level is 12.78 million cubic metres, while the live storage one is slightly reduced to 12.28 million cubic metres. On the other hand, the surface inundated for this level is 90.87 has, the shore is 14 km long and the length of the reservoir 0.9 km upstream.

As for the standard project flood, calculated for a return period of 500 years, the flow is about 75 m³/s, the maximum level reached by the reservoir –the Standard Project Flood Level (NAP) - is 145.52, which corresponds to a storage volume of 13.26 million cubic metres and an inundated surface of 93.03 ha.

Photo view of the reservoir from the right embankment
View of the reservoir from the right embankment

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Geology and Seismology.

Regional Geology.

Crevillente Reservoir is located on posalpídicos sediments, deposited in a predominantly marine atmosphere. These materials lie on the north-eastern edge of the allochthonous Sub-Baetic area and, more specifically, on the eastern end of the Sierra de Crevillente that is formed by an anticline of Triassic, Jurassic and Cretaceous materials, whose WSW-ENE axis dips toward the East below a mighty Miocene series. The materials of the Mesozoic series that appear in the Sierra of Crevillente are gypsiferous loams of Keuper facies, limestones, dolomites and dolomite limestones, loamy limestones and loams.

The Tertiary series begins, discordant, with levels of conglomerate and sandstones (molasses) and continues with loams, clays, poorly-cemented sandstone, limestones and loams, and ending with layers of conglomerate, sandstones and reddish loams from the Pliocene.

The Quaternary series is a recent alluvial one which covers large areas, but outside the zone near the reservoir.

The compact calcareous nature of the Jurassic contrasts with the material loamy- gypsiferous one of the Triassic, which morphologically gives rise to a marked differential erosion that results in high mountain peaks, bordered by deep ravines with vertical walls. In the Aspe valley, this aspect is attenuated by the filling of alluvial materials in the depressions. On both sides of this core there are Tertiary mountainous regions whose differences in level are more moderate, but also quite broken up by differential erosion, with valleys parallel to the structure and consequent collectors, frequently inside faults. These Tertiary materials (Middle Miocene to Upper Pliocene) present frequent lateral variations of related litofacies, both with the immediate local flows and with the various depths of sedimentation.

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

All along the bed there is a rapidly decreasing thickness of alluvial material downstream, as it is natural due to the morphology of the bed, because the bed sediment is at its maximum at the narrowing. This thickness ranges from between a maximum of 4 m, to about 2 m and less than one metre. During the phase of the primitive construction project in the year 1972, seven (7) mechanical probings were carried out, with a total length of 600 m drilled and distributed in 3 different profiles; two of them transverse to the ravine and the other one longitudinal. The percentage of core was quite variable in each probing, because some stretches were found in which it was almost 100% and others in which recovery was practically nil. The results of the profiles indicated that the right embankment, in its first metres, was in worse conditions than the left.

In order to evaluate the permeability of the area, water admission tests were carried out, taking as an acceptable limit approximately 4 l/m/minute for an injection pressure of 10 atm.

Total losses were not very frequent, except in one probing, in which, apart from the percentage of core recovery was very irregular, admissions were noticeably greater than in the rest of the probings.

The reservoir basin is completely built on Miocene terrain. It is impervious, since it is in areas downcut by erosion in loams and clays. There are levels of sandstones and limestones that are not very permeable and which are filled with impervious material.

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

The terrain of the downstream boundary in which the dam is built corresponds to a series of levels of concordant loams, limestones and sandstones that dip 30º and 40º toward the South. All these materials form the south slope of a gentle anticline. During the phase of the primitive construction project in the year 1972, seven (7) mechanical probings were carried out, with a total length of 600 m drilled and distributed in 3 different profiles; two of them transverse to the ravine and the other one longitudinal. In all the probings natural radioactivity tests were carried out. In the final project, when the typology of the dam was changed to that of loose materials with an impervious clay core, the boundary was brought slightly forward upstream and so the geological-geotechnical characterization of the land, on dipping downstream, was critically assumed and confirmed during the execution of the works.

The lithological column that was deduced from the probings is quite similar in all, since it begins with detritic limestones that gradually give way to detritic sandstones and, later on, to progressively less detritic loams. There are some levels of detritic limestones, sandstones or clays at various depths that mean the changes are not abrupt. These levels begin with a series of detritic limestones that gradually give way to detritic loams, appearing in some parts a level of sandstones. Then there is a level of yellowish sandstones, in which a level of light grey clays is inserted. Next can be observed a great layer of loams and detritic loams of about 25 m, and in which some correlations of the levels of sandstones and limestones appear, although, in general, there does not seem to be any continuity of them. Further down, there is a level of detritic yellowish limestones of variable thickness that gives way to a level of clays and dark grey loams. All the probings end with a level of loams of diverse tonalities that start off being detritic and become more clayey as they go down.

Geologically-speaking, no problems were found for the construction of the dam in the natural downstream boundary for the storage area chosen in Amended Project 02/79 of the Crevillente Canal Regulating Reservoir. Finally, the dam was built further upstream in order to adapt the loose-material dam, with a central clay core, to the morphology of the area. The core rests, at least in the central two thirds, on impervious materials formed by more or less clayey loams and, to a lesser extent, on calcareous and sandy loams in, at least, 250 m that the probings went through. They are more than 4 m thick, they dip downstream (concordant with the structure of the area) and their permeability is more than sufficient. On the abutments, the core rests on levels of sandstones and detritic sandy limestones that, since their imperviousness is doubtful, made it necessary for them to be cleaned and their joints sealed.

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