Page - 607 - in Book of Full Papers - Symposium Hydro Engineering

Several possible solutions can be taken into consideration:  sediment by-passing into the downstream area;  drawdown routing involves discharging high flows through the dam dur- ing periods of high inflows to allow sediment to be transported through the reservoir minimizing sedimentation;  drawdown flushing, opposite to sluicing, focuses on scouring and re- suspending deposited sediment and transporting it downstream. It in- volves the complete emptying of the reservoir through low-level gates;  turbidity current venting can be done when inflowing water with high sed- iment concentrations forms a higher density current that flows along the bottom of the reservoir without mixing with the lower density clear wa- ters;  dredging with specialized equipment to remove the sediments from the dam. In the last years, sedimentation in artificial reservoirs became more and more important, especially in reservoirs where their catchment basin is af- fected by important surface erosion rates due to poor vegetation coverage (degradation of the vegetation by human action), or to the presence of heavy rains in very short periods (flash floods). Climate change has increased the frequency of flash floods, therefore sedimentation in artificial reservoirs is one of the most important challenges of Public/Private Authorities responsible for artificial reservoir management. Sedimentation is really important because there are situations, all around the world, where original reservoir capacity has been dramatically reduced by 30/50%, generating important losses in terms of electricity production, reduc- tion of water for irrigation and human use. This paper deals with the positive experience happened in an artificial reservoir located in the eastern part of the Italian Alps (Ambiesta reservoir) managed by A2A, that is one of the most important energy supplier in Italy. Few years ago, A2A launched a tender to select a Contractor able to propose a cost-effective technology to remove about 23,000 m3 of sediments settled near the water intake structure located on the right flank of the valley close to the dam, affecting dam bottom outlet, and water intake for hydroelectric purposes. The tender has been awarded to THETIS COSTRUZIONI & DRAG- FLOW which designed, supplied, installed, tested and managed a dredging system which allowed to complete the activity required by A2A fulfilling con- tractual time and very stringent environmental constraints established by River Tagliamento Authority. In the following pages the reader will find in detail reference data used to dredging system design, characteristics of the dredging system itself, on site activity and monitoring plan during dredging, and finally the conclusions. 607