Engineering of Koysha HPP with its high RCC gravity dam
Koysha (2,160 MW, under construction) is the fourth hydropower plant of Omo/Gibe river cascade in Ethiopia, comprising Gilgel Gibe (200 MW), Gibe II (420 MW) and Gibe III (1,870 MW), all currently in operation.
Koysha is a step forward in the development of the entire hydropower potential of the Omo river and represents a realization of the large-scale vision started more than 25 years ago with the design of Gilgel Gibe hydroelectric power plant. It testifies our long-term commitment for the progress of Ethiopia’s energy sector.
The project includes several outstanding examples of value engineering and design optimization, carried out by SP throughout the entire design and construction period of the hydroelectric power plant.
The layout includes an RCC gravity dam, approximately 180 m high, that creates a large reservoir of about 6,500 Mm3.
Floods are discharged by means of a gated spillway located on a saddle at the left side of the RCC dam: a large channel with a flip bucket designed for releasing a flow of about 13,100 m3/s in a plunge pool adjacent to the river, downstream of the powerhouse.
The dam is crossed by three steel penstocks feeding an open-air powerhouse. One of the intakes is lowered to allow early generation using the first two units during the first impounding.
The project includes two 6 m diameter middle level outlets embedded in the dam body allowing to control the reservoir level during the first impounding, release artificial/ecological flows and lowering the reservoir level for inspection/maintenance needs.
The river diversion layout includes two concrete culverts on the right side of the river, aimed to divert the river during the dam construction and designed to discharge up to 3,000 m3/s, and an upstream composite RCC/rockfill cofferdam.
Both the river diversion layout and the energy-generation operating rules of Koysha have been optimized taking advantage of the large flow regulation capacity of the upstream Gibe III project.
Tandem operations of the two projects allow to maximize energy generation and provide great flexibility guaranteeing power supply during both dry season and peak-demand period.
The power house is located on the left side of the river, housing eight Francis turbines generating 2,160 MW, together with erection bays and control building. Step-up transformers are located behind the powerhouse. Each unit is connected to three single-phase 400 kV transformers, for a total of 24 transformers that are linked through overhead lines to the 400 kV Switchyard.
Oromia region, Ethiopia
Salini Costruttori S.p.A.
- RCC Gravity Dam (H = 180 m, crest L = 990 m)
- RCC & Rockfill Cofferdam (H = 42 m, L = 330 m)
- Gated Spillway (Q=13,100 m3/s)
- Penstocks (No. 3, L = 0.65 km)
- Power House (open-air, No. 8 Francis turbines, IP = 2,160 MW)
- Switchyard (400kV, No. 14 bays)
Level 1 Design
Level 2 Design
Technical Supervision during Construction for EPC Contractor