The San José hydropower project on the eastern slopes of the Andes features an off-stream reservoir. This is due to the heavy sediment bed load of the main river, which consists of cobbles and boulders. This strategy was not included in the project’s original design.
The 124 MW San José hydropower project, located on the eastern slopes of the Andes in the Cochabamba department in Bolivia, harnesses the hydropower potential of the upper catchment of the Chapare River, utilising the outflow waters of the Corani and Santa Isabel hydropower plants. Owned by the state-owned company ENDE Corani, the San José project is expected to be commissioned in 2018. The project features two cascade power plants: San José I and San José II, with 55 MW and 69 MW respectively of installed capacity.
As shown in figure 1, the hydropower project has multiple intakes feeding into a single off-stream reservoir. In addition, another tunnel along the right bank of the Paracti river will collect the inflow from tributary rivers.
From the regulating reservoir, Aguas Claras, water is conveyed to the surge shaft and then through the penstocks to the first power plant. From San José I, water is discharged into Miguelito, a small reservoir with 35,000 m3 of storage capacity, before being conveyed to San José II. The characteristics of both plants are shown in figure 2.
In the original project design, it was intended to build a conventional reservoir, but the heavy sediment load in the main tributary, Mágala River, which has a bed load of cobbles and boulders (see figure 3), meant this was unlikely to be a suitable approach. Even though the annual mechanical cleaning of the reservoir was originally considered as an option, the combination of high cost, the large size of the bed load, and operational considerations demonstrated the desirability of using an off-stream reservoir.
Sediment management strategies
The off-stream reservoir is best suited for providing short-term storage (pondage) for the San José run-of-river project, providing daily flow regulation for power peaking. The off-stream reservoir will exclude the sediment-laden floods from the power plants, therefore protecting the turbines and acting as a very efficient sedimentation basin.
The off-stream regulating reservoir was made as small as possible due to the limited size of the site and the required rock excavation. The project is designed to allow hydraulic cleanout of the reservoir by empty flushing to the Santa Isabel River, and also provides for the entrance of mechanical equipment for the dry excavation of sediment not removed by flushing.