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El Canadá �鶹��Ƶ Plant in Guatemala, commissioned in 2003, produces 47 MW. After only two years, the 200,000 m3 off-stream reservoir lost 50 per cent of reservoir storage from severe sedimentation due to high erosion of fine and cohesive sediment. To solve this problem, a tailored hydrosuction dredge powered by gravity was implemented, removing sediment at an average rate of 20,000 m3/month.

Project description

El Canadá �鶹��Ƶ Plant (HPP), commissioned in November of 2003, is a 47 MW peaking plant located in Western Guatemala. It has an annual production of 178 GWh and a load factor of nearly 50 per cent.

Water is taken from an off-stream reservoir through a 1.8-2.15 m diameter penstock with a total length of 2.4 km, which leads to the power house. The net head of 380 m has a variation of 8 m and the discharge is 15 m3/s.

A second power house, Montecristo HPP, uses the same flow to produce an additional 13 MW, which results in a total output of 60 MW.

From the Samalá river, a 1.2 km tunnel leads to El Canadá HPP’s off-stream peaking reservoir. It has a total capacity of 200,000 m3, a live storage of 185,000 m3 and produces a maximum output for approximately 4 hours. A delicate plastic geomembrane covers the bottom and sides of the reservoir.

Hydrology and sediments

The catchment tributary to the reservoir of 807 km2 has an annual average precipitation of 800 mm and the mean annual flow is 476 Mm3. At an elevation of 1,420 m.a.s.l. the intake takes 15 m3/s from the Samalá river to the off-peak reservoir. The catchment is regulated by the upstream Santa María HPP, property of INDE (National power utility), a 7 MW power plant commissioned in 1927 with a 25 m high arch-gravity dam that creates a reservoir in the river with 260,000 m3 of total capacity (see Figure 1).

The sediment load in the Samalá river is highly polluted with organic material from Quetzaltenango city’s sewage, erosion caused by agricultural activity, and solid discharge from unstable slopes affected by heavy tropical rain.

Diversion works were implemented to minimise and remove a significant amount of sediment. However, smaller fractions remain in suspension through the waterway and settle in the reservoir.

Sediment deposited at the reservoir is polluted and has an average size of less than 0.05 mm with a high silt and clay content. As sediments settle, they develop cohesive properties that make it difficult to be removed. Maintenance drawdowns of the reservoir also contribute to consolidate and compact sediments as they are drained.

Sediment problems

After only two years of commissioning, the 200,000 m3 off-stream reservoir lost 50 per cent of reservoir storage from severe sedimentation due to high erosion of fine and cohesive sediment.

An average of 50,000 m3 of sediment is deposited in the reservoir annually.

During heavy rain sedimentation problems grow worse. To mitigate sediment issues in the upstream Santa María reservoir, it incorporated sediment removal operations, which involved flushing sediments once a year during the beginning of the rainy season using bottom outlet gates.

On occasion during big rain events, the use of explosives is required to conduct a controlled blow out of sediment plugs at Santa María reservoir. This results in the release of sediment downstream in which the sediment reaches the headworks of El Canadá reservoir and settles within a short period of time.

Sediment management strategies

Conventional flushing at El Canadá is not possible since the reservoir does not have a bottom gate. The reservoir only has a 600 mm drainage pipe with a low capacity and limited scour effect.

Removal of sediments by excavation is not an alternative because there is no access road, the sediments are highly cohesive and the pond’s delicate plastic geomembrane lining would be destroyed. The option of pouring concrete over the reservoir to cover the plastic lining was briefly considered however conventional dredging was ultimately agreed on.

When about 50 per cent of the volume of the reservoir was lost in 2006, the first measure taken was operating two conventional dredges— one electrical and one diesel. However, the conventional dredges did not perform as expected and were unable to remove the annual sediment inflow.

After several attempts with diesel and electric dredges, an innovative hydrosuction dredging system was implemented in January 2012 (see figures 2 and 3).

A customised 10”-12” hydrosuction reservoir dredge, powered by gravity with efficient water jetting, was especially designed for the reservoir. The system connects to an existing 600 mm drainage pipe from the reservoir (see Figure 4).

Water jetting, in combination with a gravity powered suction, removed the 8 year old and 8 m thick cohesive sediments. Sediment deposits are continuously being removed at an average rate of more than 100 m3 per hour.

Cohesive sediments are disintegrated by a powerful water jetting system. In addition, the unrestricted flow pipeline ensures the removal of large particles up to 240 mm and debris. Furthermore, since the suction of the dredge is powered by gravity, there is no energy consumption for the dredging, and only a small electricity supply is required for the jetting system (see Figure 5).  

From January 2012 to June 2012 it is estimated that the dredge removed more than 120,000 m3 of sediment from the reservoir, which is an average rate of 20,000 m3/month. Figures 6 and 7 show pictures of the reservoir before and after dredging for one month.

The dredge has proved to be cost-effective as it is operated by local labour and the dredging cost is 1.50 $/m3. The sediment concentration at the dredging outlet is of 13.5 per cent with an average capacity of 154 m3 of sediment per hour.

Conclusion

This case study showcases the implementation of a tailored hydrosuction dredge, a cost-effective alternative to conventional dredging technology to remove cohesive sediments. Even though large amounts of sediment continue to flow into the reservoir, hydrosuction dredging is a successful management strategy to maintain the storage capacity of the reservoir and allow the El Canadá HPP to provide its peaking services.

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