Sedimentation |
Above: Erosion on the Río Coca upstream of the former San Rafael waterfall site, Ecuador, showing a newly opened canyon that has formed since 2020 (Molly Wood, USGS)
Above right: Drawing down the reservoir behind Iron Gate dam on the Klamath River in the US Photo credit Swiftwater Films
Below left: View of the Klamath River in the US on 23 December 2023 before drawdown
Below right: View of the drained reservoir beds on the Klamath River. During this drawdown phase the NOAA Fisheries said the river may ‘look a bit messy’ while an intense period of sediment release was underway. (NASA Earth Observatory images by Michala Garrison, using Landsat data from the US Geological Survey 25 February 2024)
The fluctuating backwater area influenced by the
upstream Wudongde Reservoir was slightly eroded, and siltation mainly occurred in the dead storage capacity (below 765m) of the mainstream and tributary estuaries in the perennial backwater area. According to the authors, the differences between these results and those from the reservoir demonstration stage indicate “a lack of understanding about how climate change, human activities, and uncontrolled areas would affect siltation patterns”. They recommend that for future projects, research focusing on climate trend analyses and the comprehensive consideration of human activities should be combined with extensive sediment production monitoring and model parameter calibration.
Sedimentation and fish spawning In their research focusing on the upper reaches
of the Yangtze River in China, which is the largest group of hydropower cascade projects on the world, Yinjun Zhou et al discuss how such hydropower systems can impact flow regimes and sediment transport in rivers, ultimately affecting fish spawning. Previous studies, they add, have examined the effects of fluctuating flow and water temperature on
spawning; however, insufficient attention has been paid to their effects on sediment transport. This study analysed changes in runoff and sediment load in the upper reaches of the Yangtze River over the past 60 years, as well as investigating the effects of cascade hydropower operations from 2009-18 on the spawning abundance of Coreius heterodon in the upper Yangtze River Fish Reserve. The results showed that sediment load and sediment concentration decreased by 56.8% and 56.6%, respectively. This, along with changes in flow regime, led to a reduction in the spawning abundance of this fish species. These findings, the authors say, are critical for evaluating the long-term impacts of cascade hydropower on fish habitats and provide indispensable support for the sustainable operation of such projects.
Mitigating impacts The potential for dam operations to mitigate their
impact on sediment dynamics while preserving hydropower generation targets has not been explored, according to Invernizzi et al. Determined to rectify this, they undertook research looking at the Sekong, Sesan, and Srepok (3S) river basin, an important tributary of the Mekong River which flows through Cambodia, Lao PDR and Vietnam.
32 | July 2024 |
www.waterpowermagazine.com
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