Enciso Dam

Description of the Environmental Measure

Problem detected:

The quarry proposed for the extraction of the aggregate necessary for the production of the concrete for the dam is located on a hill in the Sierra de la Hez (Arnedillo – La Rioja), at an altitude of 1200 m. This is located next to a SPA (Special Protection Area, a designation under the European Union Directive 79/409/EEC of 2 April 1979 on the Conservation of Wild Birds), which is inhabited by a couple of Bonelli’s Eagles (Hieraaetus fasciatus), a species almost disappeared from the Spanish Northern Plateau and threatened with extinction.

The extraction of the limestone aggregate must be carried out through blasting with explosives, a method which may have an impact on these animals because of the noise and vibrations. The Environmental Impact Statement (D.I.A.) limits blasting activity out of the breading season of this bird of prey (established between the 1st of February and the 15th of July). The fact that blasting could only be performed between August and December signifi- cantly compromised the completion deadline for the works, as it was necessary to have a great amount of the aggregate produced and collected (around 70%) prior to producing the concrete and placing it onsite.

Solutions adopted:

In order to minimise the impact on the couple of Bonelli’s Eagles, from the outset contact was maintained with the Environmental Department of the La Rioja (Regional) Government. Firstly, the operation of the quarry was planned in such a way that the excavation face would act as a screen to reduce the noise and dust caused by the blasts, the earthmoving machinery used during the operation and the aggregate crushing and sorting plant. This would help to decrease the noise levels transmitted to the SPA located in the vicinity of the works.

At the same time, a biologist was hired to track the pair of eagles and see to what extent the works affected the conditions of their habitat and their reproductive capacity. An initial series of visits were performed prior to blasting in order to check the condition of the couple of eagles and confirm if they used all of the nests, mountain ledges, and feeding areas, whose locations were provided by the staff of the Environmental Department.

Together with the staff of La Rioja Government, it was decided that it was important to start blasting in December, one of the months at the beginning of the breeding season, so that either the individuals would get used to the noise and vibrations or they would have timeenough to look for a nest further away from the area, where they would not be affected by the execution of the works.

The timetable scheduled for blasting was between 13:00 and 16:00 so that, should the eagle leave the nest due to the noise of the explosion, it would have time enough to return to it before the eggs or chicks got cold.

Furthermore, measures were taken to reduce the levels of air blast waves, such as not using an outdoor detonating cord, minimising the explosive load per millisecond delay and using suitable lengths of tamping.


  • Measurements taken in the area of nests indicated that there were no substantial vibrations or increases in noise during blasts. It was also confirmed that the noises and vibrations caused by the crushing and sorting plant, and by the rest of the machinery used in the operation of the quarry, had no impact on the normal activity of the couple of eagles.
  • All of the above demonstrates that, through the preliminary planning of the operational methods for the actions to be carried out (surveying, types of detonators, days and hours, etc.) and by incorporating various teams into the process (production, earthmoving company, quality assurance unit and the La Rioja Government), it was possible to make the continual operation of the quarry feasible and compatible with the presence of the couple of Bonelli’s Eagles, and it was even discovered during the first season that a new, young specimen has arrived from elsewhere.