Erosion & sedimention control |
An economic haemorrhage
Sedimentation can pose a significant threat to hydropower development and reservoir sustainability. Here IWP&DC looks at how recent global experiences are trying to address this problem which has been described as ‘an economic haemorrhage’
DESPITE BEING BLESSED WITH abundant hydropower resources and a theoretical potential of 83,000MW, Nepalese experience is demonstrating how sedimentation can pose a significant threat to hydropower development and reservoir sustainability. According to Dahal et al in Nature’s Scientific Reports, monsoon storms contribute to a substantial portion of runoff in such Himalayan regions, with most of the country’s rivers carrying significant sediment loads that can catalyse reservoir sedimentation. Some of the country’s existing reservoirs have been
Above: Abrasion of hydropower plant turbines due to increased sediment transport. Example from Nepal Photo: Bodo Bookhagen
Below: A sediment-covered glacier in the northwest Himalayas in a region where a lot of sediment is produced and transported away in the rivers Photo: Bodo Bookhagen
experiencing significant capacity loss, such as the Kulekhani Reservoir which was commissioned in 1982, and there is now a ‘pressing need for effective sediment management strategies’ country wide’. As the authors warn, sedimentation is the primary factor in reducing a reservoir’s useful life, making it ineffective for both flood control and hydroelectricity generation and, ultimately, having an adverse impact on a project’s economic feasibility and long-term sustainability. Anticipating how much sediment a reservoir can trap is a critical component of economic forecasts for a planned dam. However, as gathering sediment flow information is much more expensive and difficult in comparison with streamflow data, estimating sedimentation rates is a difficult task. And this may eventually lead to a scarcity of reliable sediment data. In their research, Dahal et al focused on the proposed 218MW Naumure Multipurpose Project (NMP) which will feature a 169m high CFRD in the West Rapti River. Situated in the mid-western region of Nepal, this river is recognised as being ‘dynamic’ due its extremely erosive characteristics that increase its capacity to carry large amounts of sediment load into the river. The study modelled sedimentation in the Naumure Reservoir during a 50-year operating period utilising the Hydrological Engineering Centre’s River Analysis System (HEC-RAS). A versatile hydraulic modelling tool, HEC-RAS can simulate flow profiles, sedimentation processes, and water quality dynamics. The results revealed that about 6.22%, 11.61%, 15.94%, 22.96% and 25.65% of the storage capacity of NMP reservoir will be depleted in 10, 20, 30, 40 and 50 years respectively Admitting their study was limited by the
unavailability of flow and sediment data, plus the omission of the influence of climate change, the authors acknowledge giving consideration to climate change during the project implementation phase will replicate the trend of sediment deposition in the NMP reservoir, and help with best sediment management practices during the project operational phase. They also recommend setting up flow and sediment
32 | April 2025 |
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measurement stations in the river and its tributaries, while regular measurement and monitoring by trained personnel will enable the required data to be readily accessed with high accuracy.
EU soil study Soil erosion is both a major driver and consequence
of land degradation with significant costs which are critical to understand and quantify. Indeed, the global costs of removing accumulated sediment in reservoirs is estimated at more than US$21 billion. Despite significant developments in improving
soil erosion assessments over large spatial scales in the last 20 years, the availability of data concerning siltation and sediment management in European reservoirs is rather limited. And as Panagos et al state in the Journal of Cleaner Production, ‘an economic haemorrhage of this magnitude makes it imperative to address the problem’. Working as part of the EU Soil Observatory working
group on soil erosion, the authors say the objective of their study was to provide a comprehensive and regionalised understanding of the requirements and costs from sediment management through capacity restoration in European reservoirs. To do so, they combined information on rates of sedimentation in reservoirs with cost estimations from available mechanical techniques used for sediment removal. Modern technology now means that sediments
extracted from reservoirs can also be reused as a secondary raw material in multiple applications and contribute to a circular economy.
Sediments can be used for soil stabilisation, land filling, and multifunctional soils. In recent years, the dredged material from sediments has also been used for cement in concrete production, as alternative material in road building, and as a raw material in the production of bricks and ceramics. In this study, the authors provide an estimation of the off-site costs (sediment removal) of soil erosion in the EU, by combining local cost estimations and Pan- European soil erosion assessments. There’s an estimated 135Mm3
of accumulated
sediments produced by water erosion in the entire EU and UK, with the removal of sediments from EU dams costing at least €5–8 billion annually. Such cost estimations have substantial associated uncertainties and the authors say these predictions could be improved with more detailed data on costs and sediment yields. In addition, the costs do not consider possible mitigation measures to reduce reservoir sedimentation.
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