Sedimentation assessment and effects in Venda Nova dam reservoir (Portugal)

Sediment quantity and quality in dam reservoirs is often neglected and usually only water quality is monitored in many countries, such as in Portugal. Nevertheless, there are risks associated to sedimentation in dam reservoirs, particularly considering that many dams in the world are ageing into an overextended lifetime. The present study was conducted with the goal to understand how sedimentation monitoring in Portuguese dam reservoirs could be attained, as that information is essential to tackle any sedimentation effects that may have occurred. A dam reservoir in the north of Portugal, the Venda Nova reservoir, was selected for the study. First, historical relevant data was compiled and then new data for sediment and water quality analysis was collected. The results show that since the reservoir was filled, after the dam construction conclusion in 1951, the reservoir morphology has been affected by the sedimentation at different rates and underwent multiple transformations from 1946 (before the dam construction) to 2004 (last topobathymetric survey). An expressive morphology transformation detected in 1984 led to an intervention to remove some of the accumulated material. It was verified, as well, that the asymmetrical contamination of the sediments and water is still strongly influenced by a nearby mine even if now inactive. The methodology applied, if periodically used, will be helpful to understand the sedimentation phenomena in Portuguese and other European dam reservoirs where reservoir sedimentation has been overlooked, and can play a key role to improve river continuity and to comply with the Environmental Quality Standards under the EU Water Framework Directive (2000/60/EC).