The Cheves project presents an interesting example of an onstream regulating reservoir augmented by a run of river diversion from a second river. Not only does this project incorporate both sediment sluicing and flushing, but it has also been designed to sustain power production using flow from diverted river while the reservoir is emptied for flushing.
Cheves is a run-of-river hydropower plant located in the Huaura River watershed in the highlands above Lima, Peru. The 168 MW pelton plant is owned and operated by Statkraft and produces about 825 GWh annually, enough to power more than 570,000 homes .
The Cheves scheme is illustrated schematically in Figure 1. Cheves uses water from two rivers: Huaura and Checras. The Cheras dam, the 7.2 hectare reservoir, and headrace intake are all located on the Checras river. The reservoir is also feed by a run-of-river (RoR) intake on the Huaura River which can deliver up to 20 m3/s. Alternatively, flow diverted from the Huarua River can also be delivered to the entrance of the desanders, bypassing the Chacres reservoir and intake. During the dry season the Checras reservoir regulates flows from both rivers to provide power peaking services for energy production.
Water is conveyed from the reservoir to three parallel desanders designed to remove 0.25 mm sediment at a design flow of 33 m3/s, before entering the 10 km headrace tunnel leading to the underground powerhouse, where 2 pelton turbines of 84 MW each operate under 602 m of net head. The tailrace tunnel exiting the powerhouse discharges to the Picunche re-regulating reservoir, which evens out flows released back to the river and to downstream irrigation intakes.
The Checras reservoir lost significant volume to sedimentation in its first year of operation. An improved operational strategy was then developed for sustaining the reservoir at the minimum operating level for sediment sluicing during the wet season, together with short periods of empty flushing. These measures significantly controlled sediment trapping in the reservoir. A variety of other sediment-related problems occurred including high rates of damage to both hydro-mechanical equipment and damage to the civil structures that pass the bed load beyond the dam.
Hydrology and sediment
The inflow to the Cheves hydroelectric power plant is a combination of flows from the Checras and Huaura Rivers. The catchment areas for Checras and Huaura rivers are 820 km2 and 890 km2 respectively, with mean flows of 11.65 and 10.2 m3/s. About 60 percent of the inflow comes from the Checras. The daily inflow to Checras reservoir from both rivers is shown in Figure 2.
The Cheves River drains steep mountainous terrain that flows toward the Pacific. These mountains bordering the dry Pacific coast are characterized by poor vegetative cover and high erosion rates. Where there is space to expand laterally, the sediment-laden Checras River develops the multi-channel (braided) form characteristic of rivers with an extremely high bed load. Most of the suspended sediment load is either sand or silt, in approximately equal portions. Only 6 percent of the sediment load consists of clay. In terms of mineralogy, 81 percent of the suspended sediment is comprised of quartz, which is highly abrasive to the steel used in turbine runners. The annual sediment load in each river is summarized in Figure 3.
Within an initial capacity of only 456,000 m3, the Checras reservoir is quite small, yet it acts as an efficient trap for the coarse sediment transported by both the Checras and Huaura Rivers. After three years of operation, the active volume of Checras reservoir had been reduced by 13 percent, to 396,000 m3. The highest deposition occurred within the first year of operation, which prompted operational adjustments to limit sediment accumulation in subsequent years. The biggest sediment challenge is the loss of reservoir volume and associated peaking capacity, and the manner in which to operate the reservoir during high flows to sustain usable capacity. In addition to flushing and sluicing, mechanical excavation during the dry season was performed, starting in 2018. Sediment profiles for pre-impoundment and the first three operational years are compared in Figure 4.
Loss of capacity is not the only sediment challenge. Other sediment-related challenges include: loss of turbine runner efficiency due to deformation by sediment erosion, the cost of frequent change-out and repair of runners, the cost of abrasion-resistant tungsten carbide coatings of turbine runners, plus costly and time-consuming repairs concrete on the spillways due to damage of the civil structures by bed load sediment.
An aerial overview of the headworks is shown in Figure 5, and a photo looking upstream toward the dam is shown in Figure 6. A key sediment management measure incorporated into the design are the three low level outlets (LLO) equipped with radial gates for sediment sluicing and intermittent flushing during the wet season. The environmental releases occur through two bottom outlets, which are also used to fine-tune releases to maintain the target reservoir level.
The plant is fed by two rivers. The dam and reservoir are located on the Checras and flow is diverted from the Huaura via a run-of-river diversion. Under normal conditions, only water from the Checras is used when its inflow exceeds the 33 m3/s plant capacity. To divert water from the Huaura under these conditions would increase the sediment load entering the reservoir without producing additional power. For this reason, flow from the Huaura intake is normally not used until the Checras inflow drops below the turbine capacity of 33 m3/s.
The Huaura tunnel, with a maximum capacity of 20 m3/s, normally discharges into the Checras reservoir, but there is the option to divert this flow through a conduit in the dam and discharge Huaura water directly to the desanders. This allows the power plant to continue operating whilst the Checras reservoir is empty for flushing or dewatered for repairs. This also allows use of Huaura water alone if there is too much sediment in the Checras.
The desanders were designed to trap particles 0.25 mm or larger for a design flow of 33 m3/s. The desander flushing interval to remove accumulated sediment depends on the sediment inflow, which is correlated to river inflow per the following table.
The project has a comprehensive sediment sampling and monitoring program to better understand sediment behavior and optimize management. The sediment monitoring program at the Cheves hydroelectric power plant consists of regular measurement of sediment concentration in the discharges at the intakes and analysis of mineralogy and particle size distribution of sample sediment. Depth-integrated sediment samples are collected periodically at multiple points in the system to better understand removal efficiency in terms of both total concentration and particle size distribution (Figure 7). The project has an on-site sediment laboratory to facilitate the analysis of these samples. Bathymetric surveys of the reservoir are conducted annually.
Continuous real-time sediment monitoring is being performed with a LISST-ABS acoustic turbidity sensor (Figure 8) which is installed in the forebay upstream of the headrace tunnel entrance (after the desanders). Continuous suspended sediment data from the forebay are shown in Figure 9. If the suspended sediment concentration exiting the desanders exceeds 2500 mg/l, the Checras reservoir can initiate flushing and the plant can operate at a lower rate using Huaura water only. Because exclusive use of Huaura water implies an operation at only 20 m3/s instead of 33 m3/s, the desanders will be operating at a higher level of efficiency due to the reduced hydraulic loading rate, which in turn increases the sedimentation efficiency in the desanders.
Sediment also affects the hydropower plant and civil works. In the plant, the hydro-mechanical equipment is periodically inspected to monitor abrasion damage and abrasion-prone surfaces receive a tungsten carbide coating. The repair cycle for runners is two years, and it is expected that runners will be replaced with a new runner after six or seven repairs, that is, after 12 – 14 years of service. An example of erosion damage to the runners is shown in Figure 10.
Following the analysis of several years of operational and runner erosion data and evaluating the cost of abrasion against the price structure of the PPA, it was determined that the economically optimal strategy for delivering the contracted power, vs the cost of damage to the equipment, would be to deliver contracted power by operating the plant for concentrations up to 2500 mg/l in the forebay.
The civil works also experience significant damage. Bed load sediment is passed through the low-level outlets and across the downstream apron, causing considerable damage to the concrete. This is being combatted with the installation of steel railroad rails and granite blocks, plus regular inspections and repair work.
The cost of sediment and its management includes the following factors:
Reservoir sedimentation: Sediment accumulation reduces the power peaking capacity, which represents lost income, and power production is also lost during flushing.
Concrete repair: The high rate of abrasion of outlet works by bed load sediment requires time-consuming and costly repairs. To minimize abrasion damage, the spillways include both granite blocks and railway rails.
Turbine abrasion: Deformation by erosion of the metal reduces runner efficiency and thus produces less power. This loss is in addition to the cost of applying abrasion-resistant coating and runner repair. Typically a Pelton runner can be repaired six or seven times before it is retired.
Cooling system: Sediment deposition within the generator’s colling system can restrict flow and lead to overheating if it is allowed to accumulate.
Monitoring: As sediment problems become more severe, the monitoring required to optimize operation tends to increase. Although monitoring produces high economic returns in terms of improving operations, it nevertheless represents a cost that may be much smaller for a project with a smaller sediment load.
Sediment monitoring throughout the lifetime of a project is key to developing adaptive measures and economically optimise water resources. Whereas sediment management is important to maintain the reservoir capacity necessary for energy production, the sediment handling approaches at Cheves also demonstrate how a variety of strategies may be required simultaneously to preserve storage capacity while sustaining the continuity of sediment transport along the river.