Chile - La Confluencia
Key project features
The Confluencia hydropower project, located in the Chilean Andes, is fed by water from seven intakes. The Tinguiririca intake features an off-stream reservoir to provide peaking services. It also serves as a very efficient sedimentation basin, which helps to protect the turbines.
As shown in the scheme of figure 1, the project uses the water from the Tinguiririca, Portillo and Azufre Rivers, as well as another four small tributaries. The main intakes, Tinguiririca and Portillo, feature weirs and desanders to divert water to the headrace tunnels, of 9.2 km and 12 km, with design discharges of 26.5 m3/s and 25 m3/s respectively. The secondary intakes can divert between 1.5 and 2.3 m3/s. Tinguiririca intake also includes an off-stream regulating reservoir with 1.2 Mm3 of storage to provide peaking services. It can supply water for six hours of generation at full capacity.
The annual observed sediment load through turbines is 71,000 t/yr. The sediment concentration at Confluencia is between 80-1,000 ppm, with about 25 per cent hard minerals. The river bed load consists of cobbles and boulders.
The expected generation is 656 GWh, but due to the sediment problems the production loss is almost half. The current average annual generation is 374 GWh.
The rainy season in 2016 was characterised by increasing runoff, which led to higher sediment transportation. The heavy sediment load of the Tinguiririca River caused major damage to the Tinguiririca facilities and access roads. Figure 2 shows an aerial view, where the sediment load has turned the river brown.
The sediment-related costs at the Confluencia project are estimated to be 8 per cent of the project’s annual revenue. The main sediment-related costs are the maintenance and replacement of components damaged by the extreme sediment concentration, and the sediment mechanical dredging at the intakes.
The civil works to repair the Tinguiririca facilities and access road caused by the high precipitation and heavy sediment load in 2016 include mechanical dredging, the expansion of two gutters to 500 m and 450 m, and other works to improve the access roads.
The expected generation is 656 GWh, but due to the sediment problems the production loss is almost half."
The off-stream reservoir is best suited to providing short-term storage for the Confluencia run-of-river plant, providing daily flow regulation for power peaking.
This off-stream reservoir only uses the Tinguiririca intake and includes a desander prior to the 1.2 Mm3 regulating pond (see figure 3). The single regulating pond has been sized with respect to the powerhouse discharge rather than the intake capacity, as no regulation volume is provided for the other intakes. Nevertheless, the other intakes discharge directly to the powerhouse following desanding. Desanders are typically provided to reduce sediment load and also to prevent the deposition of coarse sediment in the conveyance facilities. However, the off-stream reservoir for power peaking will be much larger than a desander, and will act as a very efficient sedimentation basin for the material not trapped in the desander. This is beneficial from the standpoint of protecting turbines.
The off-stream reservoir entered into service in May 2011 but by 2017 had not yet experienced a sedimentation problem in the reservoir that required dredging. The sediment consists of silt, with negligible amounts of clay, and after an initial period of accumulation the rate of sediment accumulation slowed substantially or halted because the sediment was being carried out of the pond by normal operational drawdowns in the off-stream reservoir.
Graphs and figures
This is part of a series of sediment management case studies collated by the International Hydropower Association with support from the South Asia Water Initiative (SAWI), trust funds to the World Bank. For more case studies, visit www.hydropower.org/sediment-management.