State of the art of reservoir sedimentation management in Spain

Part of the total reservoir storage capacity in Spain (56 km3) is lost due to sedimentation processes taking place. Surveys carried out in 121 reservoirs indicate that 6% of them have undergone a capacity reduction of over 50%. However, most of them (81%) are characterised by a reservoir capacity loss below 20%. The most frequent methods used to control reservoir sedimentation in Spain fall into one of the following groups: reduction of sediment yield through basin management and removal of the sediment deposits that have built up. Reduction of sediment yields includes soil and water conservation programmes, upstream sediment trapping debris dams, and bypassing of sediment. In Spain, soil and water conservation programmes, mainly those involving afforestation practices and engineering measures, have been carried out since 1877. Flushing and excavation practices are being used to remove part of the sediment build-up from some reservoirs. With regard to this, the empty flushing of the Joaquin Costa (Barasona) reservoir is highlighted, since it was the first controlled flushing undertaken in our country. Prior to the flushing, all sectors concerned in the Barasona Reservoir activities were consulted. Sediment evolution and environmental factors, such as water quality, fisheries, riparian vegetation, fauna, etc., were studied before, during and after the flushing. In general terms, the experience was found to be so satisfactory that it is being taken into consideration when planning other reservoir flushings. In Spain, all reservoirs are provided with bottom outlet(s), so that they can be subjected to flushing. This has been a deciding factor when considering regular flushing as one of the most suitable method for sedimentation control. Once the specific characteristics of each individual reservoir have been taken into consideration, regulations and standards are drawn up and applied to ensure minimum impact and the maximum accuracy of flushing operations. It is expected that this silting mitigation measure could soon be regularly applied to all reservoirs.