From the very beginning, the Andévalo dam project has attempted not only to minimise harm to the environment, but to actually improve the conditions of the natural surroundings, says Javier Milans


Andevalo map

ANDÉVALO DAM occupies land belonging to the municipal boundaries of Puebla de Guzmán, Paymogo and El Almendro villages located in the western part of the Spanish province of Huelva, in the region of Andévalo.

The dam simultaneously closes the courses of the river Malagón and the shore of Cobica, water courses that receive other minor ones, including Los Molinos, Agua de Miel and Albahacar on the right bank, and Viguera on the left. All these belong to the Chanza river basin and this, in turn, belongs to that of the Guadiana.

Andévalo dam was constructed to complete the regulation of the Chanza basin. As it is located at the head of the entire distribution system, upstream of the Chanza reservoir, the water resources regulated by the dam can be incorporated immediately in the existing transport infrastructure, meeting any new requirements caused by booming developments in the province of Huelva.

This will ensure urban water supplies, cover part of the demand for irrigation currently served by the coastal aquifers, and supply new areas located in the region of Andévalo itself, in the surroundings of the reservoir.

This regulation work was carried out for the following reasons:

• An increase in the urban consumption of coastal towns (due
to peak tourist development) always marked by strong
seasonal peaks.

• An increase in agricultural activity, brought about by private initiatives, which are enlarging the irrigation areas with modern and profitable technologies.

• The need to promote the agricultural development of the Andévalo region, transforming currently dry but suitable zones into irrigated areas.

• The location of the Andévalo dam, at the tail end of the Chanza reservoir, allows its natural integration on the Chanza-Piedras-Huelva axis, the spinal column for the regulation and distribution of water in the province.

• Water from the Chanza reservoir, on which the coastal towns, the industrial area of Huelva and the majority of the irrigated land in the province depend for supply, is clearly insufficient for the immediate future needs of the region.

• The great irregularity of the hydrological cycle of the rivers that criss-cross the area.

These factors highlighted the urgent need to improve the regulation of water resources of the province, so that part of the water that is released into the sea every year can be used for consumption.

Andévalo dam will, in principle, allow the volume regulated by the Chanza at present to be doubled, making it possible to meet the water needs of the Chanza-Piedras-Huelva system, as well as covering the demands of the new irrigation areas promoted for the economic development of the towns located around the reservoir; Puebla del Guzmán, Paymogo, El Almendro and others.

Environmental Impact

Construction of Andévalo dam means not only the flooding of some 3650ha of its basin, but also the transformation of 15,000ha of holm oak and Mediterranean mountain into citrus fruit or woodland with artificial watering techniques.
However, the work to follow up the environmental problems is centred on the dam and the reservoir itself.

The reservoir extends into the tail of the Chanza reservoir, which constitutes a barrier of around 25km for wildlife, separating the most western part of Andévalo from the rest of the region. Part of the territory is a so-called ‘place of community importance’; although the level of conservation of the present ecosystems may be high, there is room for improvement.

Declaration on Impact

In 1992, the initial project for the construction of Andévalo dam was drawn up. Appendix 22 of the report contained a corresponding environmental impact study.
The first condition of the declaration on environmental impact (DIA), dated 21 July 1994, is of special importance as it limits the height to which the reservoir can be filled and establishes the need to create an ‘environmental follow-up committee’.

It said: ‘the elevation for filling the reservoir is established as 112m above sea level, which corresponds to a reservoir volume of 600hm3 since, with the information currently available, it is considered that under this level and in the case of complying with the conditions of this declaration, no significant environmental impact will be produced’.

The DIA gave a partially favourable declaration as the environmentally viable filling height it specified was less than the 121m asl contemplated in the original base project.
The declaration also called on the Hydrographical Confederation of the Guadiana to carry out: ‘an investigation programme over the next five years to perform an in-depth evaluation of the need for demand to be covered by this reservoir and the potential adverse environmental effects’.

The DIA conditions the achievement of the original spot height to the development – and later analysis of studies on the natural medium and the demand for water – to evaluate, at a later stage, the environmental viability of the original spot height in the project. For this reason, the declaration distinguishes between two large groups of work: an investigation programme and an environmental supervision programme.

The investigation programme deals again with a study of the natural environment that can be considered for the original spot filling height, and consequently for the volume and original filling surface. These studies focus mainly on four species of fauna: the Iberian lynx, otter, golden eagle and black vulture, together with a study on variation in the water demand. The results will allow the evaluation of the environmental viability of the original spot height by an environmental follow-up committee.

Construction work on the dam was awarded to Dragados, and started on 16 July 1999; the contract contemplating the construction of a dam structurally designed to reach a height of 125m but limiting the height to 116.5m with a spillway located on the right hand side with discharge from 112m asl in accordance with the conditions of the DIA.

The following year, TYPSA was contracted to provide technical assistance and consulting for the follow-up and environmental supervision of the dam works.

Environmental Supervision Programme

The ‘plan for the environmental supervision of the work’ ensures the follow-up and supervision of all the preventative and corrective measures during the construction phase of the dam.

The development of the environmental supervision programme required the preparation of an ‘Environmental Measures Reference Manual’ given the initial breaking up of proposals, conditioners or actions considered in the corrective, preventive or compensatory measures and/or activities with environmental repercussions that need follow-up and supervision. The work is performed through environmental management, with TYPSA providing the team of technical personnel for environmental supervision.

As well as the follow-up and supervision tasks on the work, specific documents were drawn up with corrective measures that in themselves represent a main part of the work. This is the case in the plan for the Deforestation of the Reservoir Basin, which assumes the execution of this work with preservation of forest masses and encouragement of the flora and fauna of the reservoir and its perimeter edge as a basic premise.

Investigation Programme

This programme is intended to provide sufficient environmental information for the Environmental Supervision Committee to determine whether it is environmentally viable to increase the filling height of the reservoir from the approved 112m up to 121m.
As a first step, ‘prospecting’ was carried out in 2002 in a systematic search for the lynx in the area surrounding the reservoir. The method chosen consisted of searching for and collecting excrement for later DNA analysis in the Doñana Biology station laboratory, to determine whether it had come from lynxes. As a result, it was determined, without any doubt, that the Iberian lynx had been extinguished in the western Andévalo.

Two field campaigns were also carried out to study the bird fauna, contracting six people for these teams for a month and a half for each campaign. The data obtained was of good quality and reliability, and provided precise information on the bird communities of the western Andévalo.

Likewise and with respect to the otter and other mammals, photography techniques were perfected and developed to provide information on the location and numbers of otters that would be affected by the flooding of the reservoir.

Environmental Integration

From the very beginning, the Andévalo dam project has given priority to the surrounding environment. In this sense, an attempt has been made not merely to minimise harm to these sites and their fauna, but to actually improve the natural conditions of the surroundings.

With a view to updating the projects for the corrective measures included in the DIA and offering a general review of the need for correction and restoration of the environment in the surroundings of the reservoir, three projects for the restoration and integration of the environment are being developed.

A great number of the actions are centred on terrain of more than 360ha (‘Isla de Quemadillos’) expropriated by the Hydrographical Confederation. A varied group of measures has been brought together including the repopulation of forests; improvement in vegetation; the introduction of the rabbit; roosts for colonies of herons; a building for receiving visitors and work offices; and a passage for the fauna in the Chanza reservoir, (a new action consisting of executing an embankment of earth over the reservoir that connects the two shores and allows the fauna to cross).

Plan of Action

The main environmental actions included in the project can be summarised as follows:

• Creation of artificial ‘floating’ islands for otters and aquatic birds; among these an island of restricted access, 365ha in size, delimited by the Chanza reservoir and the Andévalo dam, in which ecological reserves will be created.

• Repopulation of areas that have been deteriorated by the work with autochthonous vegetation, improving the natural habitats of the local fauna (dunghills for vultures, sleeping places for ardeidas and cormorants, etc).

• Special interest is given to actions aimed at maintaining the routes for the lynx to access the area. This is a mammal in danger of extinction, which could be affected by the barrier effect of the reservoir. With the objective of minimising this danger, the execution of various embankments between the two sides of the river has been anticipated.

• Restoration of tree and bush strata on the sites that will be converted into islands in the interior of the reservoir, to encourage the nesting of aquatic birds and, at the same time, establish areas for rest during their migration.

• Regeneration of the vegetation on the banks in the areas of copses located at the tail end of the reservoir, improving the habitat of the otters and other aquatic species.

• Holm oaks in the basin are being recovered by means of transplanting.

• The adaptation of more than 20km of animal tracks to green ways.

• Eradication of the eucalyptus and substitution with Medite-rranean vegetation.

Author Info:

Javier Milans is with TYPSA. For more information, email him at:

Main dam structures

Dam body
The dam is made of earth fill: rockfill dyke with an impermeable central core of clay. It has a total height of 76m above the river Malagon. The dam is 12m wide and 1830m long. The present freeboard is set to 13m allowing for the NMN to be established at an elevation of 121m at a later stage (final situation of the project).
The shoulders have a 2 / 1 slope in the lower part (from the shoulder located at the elevation of 70/80 upstream and 95 downstream to the riverbed), and 1.8 / 1 from these shoulders to the top of the dam.
The standard section is made up of the following enclosures, described in order and starting upstream: 3m thick rip-rap for upstream shoulder protection; upstream shoulder, formed by an all-in-one slate and whin rockfill; central vertical clay core with a symmetrical layout with regard to the axis of the dam and 0.25/1 slopes on both sides. It is protected with a transition layer 5m wide upstream; and downstream by a filter 4m wide, plus a 5m transition layer; downstream shoulder, formed by the same all-in-one material used for the upstream shoulder. The face is covered with a thin layer of topsoil.

The spillway, located in the hill on the right side, has a stationary lip. It only has one opening with a 13m useful length, and a threshold at a spot height of 112m (extendable in the future to a spot height of 121m).
The excess elevation of the reservoir for evacuating the flood plain in the project is of 1.60m with the NMN at a spot height of 112m and of 1.15m when the NMN is extended to 121m. A depth of 2.10 m with the NMN at the spot height of 112 and of 1.50 m with the NMN at the spot height of 121 are needed to evacuate the far distant flood plain. Therefore, it will have a very large clearance (13m) in the initial situation. When the lip is extended to the spot height of 121m there will be a normal clearance of 4m (over NMN) and a minimum clearance of 285m (over NAP). Excess elevation due to waves on the flood plains is 1.85m.

River Diversion
The Andevalo dam has a series of peculiarities that making the river diversion solution different from those usually used. The dyke closes two riverbeds (the rivers Cobica and Malagon) that flow very near each other and there is a low, gentle hill that almost interconnects them in the contiguous area upstream of the dam. This has notably simplified the construction problem, since it has allowed the flows from the river Malagon to be diverted via this hill to the Cobica, while the Dam for closing the Malagon was being built.

First phase – With the diversion in the first phase, the bottom outlet and intake gallery were built, located on the left bank of the Malagon, with its entrance at an elevation of 71m. Through this entrance, the discharges from the floods were evacuated in the second phase of the diversion. This phase started when the Malagon dyke exceeded the elevation of 85m, building the cofferdam and counter-cofferdam of the Cobica valley at that time, forcing the water from this river to pass into the Malagon via the interconnection trench (in the opposite direction to the flow in the 1st Phase).

Second phase – The diversion gallery is a reinforced concrete structure with a stilted arch of 3 centres and a flat edge, a total width of 8m and height of 6.21m. Its length is 230m, with a 2% inclination. Its drainage capacity, during the construction of the dam, was 350m3/sec in free water surface, which allowed the 100 year return period flood to be laminated and evacuated. It was built in the open air on the left bank of the MalagÃ3n, with a curved form that runs along and is built into the rock, its upper limit is situated at the same elevation as the base of the rastrillo of the dam core.


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