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Overview

The Reventazón �鶹��Ƶ project is located on the Reventazón River in the province of Limón in the central part of Costa Rica. It is operated by Instituto Costarricense de Electricidad (ICE), Costa Rica’s national electricity company and is the most downstream of a cascade of hydropower projects on the Reventazón River, located downstream of ICE’s Angostura, Cachí and Río Macho projects. Reventazón HPP came into operation in 2016, becoming the largest hydropower project in Central America with an installed capacity of 305.5 MW. The main project is designed for 292 MW with four turbines that utilise water diverted from the 7 km2 reservoir returning it to the River 4.2 km downstream of the 130 m high dam. An additional small powerhouse (13.5MW) at the foot of the dam provides 15 m3/s environmental flow to the 4.2 km dewatered section of the River. The Reventazón River flows eastwards into the Caribbean Sea 50 km downstream of the project. Figure 1 shows the location of the project site.

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‍Hydrology and sediment

The Reventazón reservoir has a total storage capacity of 293.2 million cubic meters. It is 9.4 km long and has a catchment area of 1,463 km2. The area receives an annual average precipitation of 3500 mm with an average River discharge of 152 m3/s. [1]

A minimum environmental flow of 15m3/s is released from the dam powering a mini plant (13.5 MW) at the foot of the dam. Whereas the principal power station has a design discharge of 240 m3/s, a bottom outlet is designed to discharge up to 600 m3/s for reservoir flushing. The estimated annual sediment load in the reservoir is 4.2 million tonnes taking into account natural suspended load, natural bed load, and Angostura and Cachí Reservoirs flushing loads. Igneous rocks are the predominant soil type in the catchment.

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‍Sediment challenges

Located most downstream on the Reventazón River, Reventazón HPP receives a high sediment load coupled with high sediment fluxes due to flushing operations at the upstream Cachi and Angostura Dams. Anticipated sediment management challenges at Reventazón Dam were accounted for during the design phase of the project. A preliminary assessment estimated that 37% of the total reservoir’s storage volume and 35% of the active storage volume would be lost to sedimentation after 40 years of operation if no sediment management measures were in place. However, with sediment management measures such as drawdown and partial drawdown flushing, the predicted storage loss is significantly less at approximately 8% to 10% of active storage volume.

Management measures

Erosion control measures have been implemented in the upstream catchment, while at Reventazón Dam flushing and adaptive measures are planned to control sedimentation. ICE had been implementing catchment management programs since 2000 prior to the development of the Reventazón Dam jointly with partner agencies, local farmers, businesses and landowners through the Upper Reventazón River Basin Management and Planning Commission (COMCURE). The implemented activities included Payment for Ecosystems Services (PES), sustainable agriculture and reforestation of 16,000 hectares that increased the forest area by 11%. At the time of construction of Reventazón HPP, ICE acquired additional land beyond the area inundated by the reservoir and reforested a buffer zone of 50m around the reservoir perimeter therefore reducing the erosion potential around the reservoir and downstream areas.

A Preliminary assessment of sediment management measures was executed at the design stage using the Reservoir Conservation (RESCON) model, which indicated that flushing is the best alternative among four management options considered, i.e., flushing, hydro-suction sediment removal system, dredging, and dry excavation.

In 2015, ICE prepared an adaptive sediment management plan in August 2015 and monitoring began in 2016. Controlled sediment flushing is planned for the reservoir using bottom outlets with considerations for synchronisation with flushing operations in the upstream Angostura and Cachí Reservoirs.[2] Although the reservoir may not require flushing for 20 years, the reservoir’s operations manual includes a generic procedure for partial dewatering of the reservoir to flush sediment downstream, which may be necessary to minimise downstream erosion.

The ongoing monitoring programme aims to understand the problems related to water quality and sedimentation resulting from the operation and maintenance of the reservoir, before and after its operation in the Reventazón riverbed. Monitoring is done through observation and field measurements to assess river patterns, channel stability, sediment distribution, grain variability and deposition characteristics. Various sections of the river and reservoir are monitored for water quality through physicochemical assessments for suspended solids, Sulphates and ecological indicators such as fish stock and ecological species indicators. The monitoring activities are carried out continuously either bimonthly, monthly or annually, and followed up by mitigation actions. The assessment of channel stability and evolution of the riverbed is made annually whereas assessments for water quality are done on a monthly or bimonthly basis for some indicators[4].

Conclusion

Sediment management measures and plans were incorporated in the design phase of the project encompassing a management plan for the lower Reventazón basin and adaptive management plans for operation of the reservoir and monitoring of downstream areas. Whereas the project has not experienced sediment challenges, an adaptive management plan is in place for future sediment management through drawdown flushing estimated to significantly reduce the sedimentation rate of the reservoir. Continuous monitoring has been ongoing since 2016 to assess and mitigate changes in sediment distribution and water quality that may be resulting from the operation of the reservoir and flow variations.

References

1. Data provided by ICE (Instituto Costarricense de Electricidad). 2017.

2. �鶹��Ƶ Sustainability Assessment Protocol. Reventazón -Official Assessment. 2017.

3. Environmental Impact Study P.H. Reventazón File No. 0331-08-Setena

4. Adaptive Sediment and Water Quality Management Plan: Monitoring Protocol. 2014. ICE.

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