Client: Inter-American Development Bank
Contract Period: 2016 (Project Duration: 4 months)
GLM performed a bathymetric survey study to determine the storage capacity of Peligre reservoir and concluded that it has lost 58% of its original (year 1952) total capacity. Most of the storage has been being lost as the delta advances toward the dam, as shown. The delta advanced about 1.2 km closer to the dam between 2008 and 2016, for an annual rate of about 150 m/yr.
GLM undertake a sediment sampling campaign using vibecore equipment to document the sediment composition along the reservoir. The majority of the sediment deposited in the reservoir consists of fine sediments. Based on sampling the composition of these deposits is approximately: 49% clay, 50% silt and < 1% sand. All the sediments within approximately 3 km of the dam consist of fine sediment. The only sand found within 5 km of the dam is sand is transported along the bed of the river channel during periods of reservoir drawdown.
Two Sediment removal options were analyzed. Reservoir emptying and flushing was considered not to be feasible due to the high clay content in the Peligre sediments, which makes them very difficult to erode. Thus, due to technical limitations flushing is not considered to be a viable alternative for recovering lost reservoir capacity. Dredging with disposal to upland containment areas was examined but discarded due to high cost and because, given the lack of large volume disposal sites, it is not a sustainable strategy. Dredging with discharge to the river below the dam was considered feasible, and much less costly than upland discharge. Options include dredging the entire volume of sediment, or to focus on dredging only the sands and allowing the fine sediment to pass through the turbines, which would a less costly alternative because it might reduce dredging volume (and cost) by as much as 80%. Under either alternative an electric powered dredge would be used to excavate sediment from the area of the delta face.
GLM performed several simulations of water yield and power generation. These simulations were run using a reservoir operating rule which maximizes the water supply reliability for irrigators. They showed that the reservoir’s operational storage volume, currently at 218 Mm3, could be reduced to as little as 50 Mm3 with very little impact on the total amount of power production. However, there would be a significant reduction in the reliability of irrigation releases.