Off stream reservoir
The 135 MW San Francisco hydropower plant and offstream reservoir is the largest on the Caldas cascade, located in the municipality of Chinchiná, 25 km west of the City of Manizales, Colombia. The cascade is owned and operated by Central Hidroeléctrica de Caldas (CHEC) and consists of three hydropower plants and multiple intakes feeding two off-stream reservoirs: San Francisco and Cameguadua. The project scheme is illustrated in Figure 1.
The San Francisco off-stream reservoir captures and stores water from several tributaries to the Cauca River, providing capacity for power peaking services and also reducing the sediment load reaching and abrading turbines. Intermittent empty flushing was applied during the first decades of service to sustain reservoir capacity, but when a flash flood entered the reservoir during a flushing event, it suddenly scoured a large volume of sediment, overwhelming the assimilative capacity of the receiving body and producing a fish kill in the Cauca River, one of Colombia’s principal rivers.
San Francisco reservoir receives flows from Cameguadua reservoir via tailwater from the Insula and Esmeralda (22 m3/sec) hydropower plants, plus the San Francisco River (7 m3/sec). The peaking power plant was designed to operate at 50 m3/s and discharge to the Cauca River. The two reservoirs in series are fed by intakes with desanders that divert flows from the Chinchiná, Campoalegre, San Francisco and San Eugenia rivers and Estrella Creek, plus a small unregulated inflow from Cameguadua Creek. Catchment management activities are being undertaken in more sensitive watershed areas to reduce sediment yield.
A majority of the sediment entering the system comes from the Chinchiná River, which is both the largest single source and which also has an active volcano (Nevada Ruíz) in its upper watershed, which constitutes an important source of erosion.
The system’s run-of-river intakes include instream gates to pass floods, while water for power is diverted through coarse screens followed by desanders for removal of sand larger than about 0.15 – 0.20 mm dia. The intakes do not pass large sediment-laden floods into the reservoirs, allowing large floods and their associated high sediment loads to bypass the intake rather than passing through the reservoir. As a result, these off-stream reservoirs experience a much lower sedimentation rate than a conventional onstream reservoir. Simulations at Cameguadua reservoir indicate that, by virtue of being offstream, ~ 90 percent of the annual sediment load is excluded from the reservoir, with desanders also helping to exclude much of the coarse sediment load.
Nevertheless, the off-stream reservoirs do act as very efficient sedimentation traps; they provide a much longer detention time than desanders and trap much of the fine sediment load not removed by the desanders. This makes it necessary to periodically remove sediment by either empty flushing or dredging to maintain the reservoir’s storage capacity.
Because the 20 MW Insula hydropower plant operates as a run-of-river facility without power peaking, the storage volume in Cameguadua reservoir is not critical, but it does capture sediment and thus protects turbines, while also acting as a headpond. However, it is essential to sustain storage capacity at the San Francisco reservoir for power peaking. There, sediment accumulation has decreased reservoir capacity at the rate of ~300,000 m3/year, an annual loss of about 4 percent (Figure 2). After 25 years of operation, and with the cessation of empty flushing activities to scour sediment, San Francisco reservoir had lost most of its storage capacity to sediment accumulation.
After empty flushing was stopped, dredging was performed in both reservoirs from 2008 to 2018 to recover capacity, discharging the sediment to both Cameguadua Creek and Cauca River, thereby restoring sustain sediment transport capacity along the fluvial system. However, the permit for this discharge was not renewed due to environmental concerns, including concern over impacts from an unplanned operation of the low-level outlet which produced a high sediment discharge to the Cauca River. Consequently, the reservoir has continued to accumulate sediment and lost useful storage capacity, putting at risk the ability to regulate flows to focus power production during hours of peak demand.
Sediment management is undertaken in the watershed as well as along waterways. To reduce sediment yield from steep soils, CHEC has been conserving land in the upper basins of these tributaries since 1960. By 2014 around 6,000 hectares had been dedicated to forest protection in the Chinchiná, Campo Alegre and San Francisco river basins supplying the CHEC generation system. 
Empty flushing was initially used to sustain reservoir capacity but was halted due to downstream environmental impacts. Thereafter dredging of both reservoirs was undertaken. A photo of dredging underway in 2011 is presented in Figure 3 showing the point where dredged material is discharged to the bottom outlet of Cameguadua reservoir. During the dredging processes, the sediments extracted from each reservoir were also characterised and bathymetric studies were carried out to quantify the benefits of dredging to achieve volume recovery.
However, when dredging permits were not renewed by the environmental authority ANLA (Autoridad Nacional de Licencias Ambientales) due to environmental concerns, CHEC began working closely with environmental authorities, using field data from the initial dredging experience to demonstrate that the dredging operations typically had low environmental impact, and to identify and modify the specific circumstances that produced adverse downstream consequences. In 2020 a modified sustainable sediment management plan was submitted following ANLA guidelines, incorporating results and lessons from the past dredging operations and data collection activities. 
CHEC is currently awaiting approval from the environmental authority to implement phase I of the new sustainable sediment management programme, which would facilitate the recovery of the reservoir’s storage while minimizing environmental impacts. To provide better control over the rate of sediment release, while still restoring the sediment flow into Cauca River, the sustainable sediment management plan will focus on the gradual release of sediment by dredging during periods when the natural flow in the receiving body provides very high dilution ratios (~1000:1), allowing the power peaking capacity to be sustained while maintaining ecological systems in the receiving river.
The San Francisco reservoir initially operated successfully due to the low sedimentation rate typical of an off-stream reservoir as compared to on-stream reservoirs. Nevertheless, sustainable sediment planning is necessary to manage the eventual sedimentation of off-stream reservoirs to maintain hydropower operations. Because of the smaller sediment load that enters these off-stream reservoirs, dredging represents a sediment management alternative that is economically feasible and has minimal environmental impacts. The discharge of dredged sediment to the large Cuca River below the dam will sustain the continuity of sediment flow along the fluvial system, and by ensuring adequate dilution the impact of this sediment discharge is minimized
1. CHEC website. Accessed September 2021. https://www.chec.com.co/Portals/0/documentos/informes%20sosteniblidad/Informe%20de%20Sostenibilidad%20CHEC%202014_compressed.pdf
2. Johanna, C.H.S. 2020. Thesis: Sediment discharge to surface water bodies. Analysis of legal criteria applicable to the activity of dumping applicable to the generation of energy in the reservoirs of the generation in the reservoirs of the Caldas S.A E.S.P. hydroelectric power plant. https://bdigital.uexternado.edu.co/bitstream/handle/001/3983/GTAAA-spa-2021-Descarga_de_sedimentos_a_cuerpos_de_agua_superficiales_Analisis_de_criterios_juridicos_condicionantes_de_la_actividad?sequence=1&isAllowed=y
3. Morris, G. 14th International Symposium on River Sedimentation, 2019. Management Strategies to Combat Reservoir Sedimentation. Workshop: Numerical Modeling. In: Sediment Management Of Reservoirs And Run-of-river Hydropower Plants. Chengdu, China – September 17, 2019. http://www.waser.cn/waser/uploadfile/2019/10/14/20191014175054548003.pdf