Lake Cachi (Spanish: Lago de Cachí) 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
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
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).
2 Cachi arch dam
3 Power plant
4 Sedimentation studies
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 3,000 square
kilometres (1,200 sq mi) in an elevation range varying from
3,432 metres (11,260 ft) 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 919 square kilometres (355 sq mi). The
annual precipitation in the entire river basin varies from 1,200 to
8,000 millimetres (47 to 315 in). 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 104 cubic metres per second
(3,700 cu ft/s). The gross storage capacity of the reservoir
is 51 million m3 (1.8 billion cu ft). The
design flood discharge is 3,500 cubic metres
(120,000 cu ft)/sec. The reservoir has a water spread
of 324 hectares (800 acres) which stretches over a length of 6
kilometres (3.7 mi) with maximum water depth of 69 metres
(226 ft). 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
80 metres (260 ft) above the deepest foundation. It is located in
a narrow gorge and has a crest length of 70 metres (230 ft),
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 86 kilometres (53 mi) away from the
dam. However, there was a temporary suspension of power generation at
the powerhouse located 12 kilometres (7.5 mi) 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 600 cubic metres
(21,000 cu ft)/sec were constructed to divert water away
from the work site of the dam foundation.
The water from the reservoir is diverted through a 5,942-metre
(19,495 ft) long pressure tunnel, a surge tank, a 566-metre
(1,857 ft) long penstock including a 116-metre (381 ft) 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 246 metres (807 ft) and a minimum of 221
metres (725 ft). 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.
The output of the hydroelectric power plant at Cachí will increase
from 100 megawatts to 160 MW. The expansion of the power plant will
include a new 40 MW generator. An existing generator will also be
upgraded. The work is scheduled to begin in 2012. The project should
be finished by 2015, according to the Costa Rican Electricity
Institute (ICE). Once expanded, the Cachí power plant will be able to
supply the electricity needs of 330,000 people. The project is being
funded by a $140 million loan from the Central American Bank for
Economic Integration (CABEI).
Power evacuation from the switch yard of the power station was planned
with step-up substation equipped with three-phase step-up transformers
and evacuated through double circuit 132 kV transmission lines
connecting Cachi with the Rio Macho power plant.
The annual sediment load into the Cachi reservoir estimated at 0.81
million tons, which amounts to 1% storage volume of the reservoir, had
been assessed at the planning stage to be distributed at 54% as
thalweg deposits (flushed through low level sluice provided in the
body of the dam), 21% getting deposited on the terraces, 18% flowing
out through the spillway by gate operation and power intake through
turbines and 7% trapped as bed load in the reservoir. This process is
witnessed in two distinct parts of the reservoir – the upper part
and the lower part – the upper part gets filled with sand and coarse
sediments and the lower part, which is the deep river channel, gets
deposited with fine sediment on its terraces which needs to be flushed
by reservoir operation (a high concentration of sediments was noted
near the power intake). However, for seven years after the project was
commissioned, the reservoir was not desilted, which resulted in
trapping of 82% of the sediment inflow into the reservoir and which
moved towards the body of the dam and interfered with diversion of
flow through the power intake in the body of the dam for power
generation. The dam is provided with one scouring sluice at the bottom
in the main river channel adjacent to the intake screen of the power
intake that draws water for power generation. To keep the reservoir in
a serviceable condition to derive planned power benefits, it became
obligatory to drain the reservoir to its lowest level (though it
caused an inevitable economic loss in power generation during the
filling stage every year) and flush the deposited sediments downstream
so that the intake does not get choked and allows sediment flow into
the turbines. The first flushing operation was carried out in three
well defined stages in 1973 and thereafter repeated every year till
1990. The downstream impact on the river regime as a result of the
silt flushed from the reservoir was also studied when a peak
concentration of 400 grams/litre were noted in the form of turbid
water not only on the flood plains but also where the river merged
with the Atlantic Sea. It was expected that these deposits would be
flushed during the flood season. However, as a result of the silt
flushing, some detrimental effect on the biota was noted by local
people of the area in the downstream reaches of the river below the
The effectiveness of the flushing operations has been studied over the
years by hydrographic surveys of the reservoir using turbidimeters,
side scan sonar, sub-bottom profiler, repeated echo sounding, sediment
coring and X-ray techniques. The studies have indicated that the
sluices provided in the dam to flush the sediment deposits in the
reservoir are effective given the "duration and degree to which the
reservoir is drawn down and on the discharge capacity of the sluices"
and also the shape of the reservoir, which in the case of Cachi is in
a narrow gorge. Studies indicated that the average diameter of the
sediments deposited in the reservoir was of the order of 0.04 mm.
It was also noted that on the terraces of the reservoir flushing was
not very effective since they were covered with water hyacinths which
had trapped the sediments. The old river channel also had indicated a
deposit rate of about 2 metres (6 ft 7 in), which, however,
is now regularly flushed out by opening the scouring sluice.
The Cachí reservoir is now flushed of sediment deposits on an almost
yearly basis. The field studies on the flushing operation carried out
in 1996 indicated that about 250,000 tonnes were deposited within the
reach between 10 kilometres (6.2 mi) and 30 kilometres
(19 mi) downstream from the dam. Of these, 82% were channel-bed
deposits while 18% were deposited on the river banks.
The dam takes advantage of the head available in the river, which for
some 65 kilometres (40 mi) creates white water suitable for
rafting. Surrounded by mountains, the lake was created when the
Instituto Costariccense de Electricidad built the dam across the river
to supply San José with hydroelectric power.
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