Lake Cachi () is an artificial lake in central Costa Rica created by the Cachí Dam (Represa de Cachí), an arch dam north of Tapantí National Park, to the east-southeast of Cartago in Cartago Province. The main town is Cachí, away from the east bank of the lake. Built in the 1970s, it was one of the first hydroelectric projects in Costa Rica. It has an installed capacity of 102 MW with three units of 34 MW capacity each (Vertical Francis turbines).
The project became operational with the first unit commissioned in 1966, the second unit in 1967, and the third unit in 1978. The Reventazon River provides multiple benefits through the three dams built on it. Out of the three dams, Cachi Dam, not only provides power generation benefits but also controls floods, and recreational facilities in the Lake Cachi. The Rio Macho project on the upstream provides hydroelectric power and the downstream Birris power project also provides drinking water (40% of the metropolitan city’s water supply needs).
The reservoir, in the Rio Reventazón, is an important supplier of electrical power to Costa Rica. It is operated by Instituto Costarricense de Electricidad (ICE).
Geography
Lake Cachi contains the Cachí Dam (Represa de Cach) in the northeastern part of the lake, near the village of Ujarras along the 225 road in the middle reaches of the Reventazon River in the Ujarras valley. The lake is created by the dam on the Reventazon River as it flows in from the northeast winding through the steep-sided valley. The river has a total drainage area of in an elevation range varying from above mean sea level at its highest point to the lowest outflow point into the Atlantic Ocean; out of this, the reservoir created by the Cachi Dam intercepts the upper catchment area of . The annual precipitation in the entire river basin varies from . In 80% of the total catchment area, the relief varies distinctly, with mountains which have slopes between 20 and 85 degrees. The average annual water inflow into the reservoir is at the rate of . The gross storage capacity of the reservoir is . The design flood discharge is /sec. The reservoir has a water spread of which stretches over a length of with maximum water depth of . Sixty percent of the reservoir's catchment is forested, the remainder is agricultural land. Density currents are formed in the reservoir on account of a combination of temperature gradients and high sediment concentration.
Cachi arch dam
The Cachi dam is a thin double-arch concrete structure (stated to be one of the thinnest such dams in the world)) built to a height of above the deepest foundation. It is located in a narrow gorge and has a crest length of , impounding 51 million cubic metres of inflows of the Reventazon River. During the 1991 Limon earthquake, the dam did not suffer any damage even though the epicentre was away from the dam. However, there was a temporary suspension of power generation at the powerhouse located away, due to dislocation of the transformers.
The project was planned, designed and executed by ICE, with support for the design of the arch dam, and for supervision of construction provided by Dr. Laginha Serafim, the Portuguese consultants. The construction of the arch dam was undertaken after extensive explorations of the geological features confirmed the suitability of the site. During construction, two diversion tunnels with gated controls, designed for a discharge of /sec were constructed to divert water away from the work site of the dam foundation.
Power plant
The water from the reservoir is diverted through a long pressure tunnel, a surge tank, a long penstock including a long pressure shaft (steel-lined tunnel) and a power house. The power house has an installation of three units of 34 MW (which would also operate under overload conditions) capacity. They are of vertical type Francis Turbine units which are designed to operate under heads varying between a maximum of and a minimum of . The first two units of the power plant were commissioned in 1966 and 1967, the third in 1978. The project was implemented with soft loan funding provided by the World Bank in two stages after due appraisal.