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 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 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.
High mountain rivers
Many high-mountain rivers in Asia transport more sediment downstream compared to a few years ago, with such changes in sediment levels having a particularly strong impact on agriculture, water quality, flood management, and hydropower generation.
A study undertaken by the University of Potsdam in Germany has demonstrated the interacting roles of glaciers, vegetation, precipitation, and slope in mobilising sediment and controlling the current sediment transport in rivers.
“The specific sediment yield in catchments with high glacial cover is on average an order of magnitude higher than glacier-free basins, and appears overall higher in Asia’s glacierised catchments than those reported for the European Alps, the Andes, or Norway,” said Bodo Bookhagen, Professor for Geological Remote Sensing at the University of Potsdam. He goes on to explain that the fluvial suspended sediment threatens the water quality downstream and thus the aquatic ecosystems, the river infrastructure such as hydropower plants and bridges as well as agriculture and pastoralism.
The team investigated 151 rivers around the Tibetan plateau and demonstrated that glaciers exert a first-order control on fluvial sediment yield, especially with high precipitation and in high glacier-cover basins.
“Our work highlights the many competing factors in controlling the transported material in river catchments and shows that a more accurate prediction of the sediment volume should consider not only climate change, but also glacier dynamics and vegetation changes and their interactions with slope,” Bodo Bookhagen emphasised.
Vegetation influences sediment transport especially in the Eastern Tibetan Plateau and Tien Shan. Depending on climate zone, vegetation can either promote erosion of material or have a stabilising effect on slopes. These findings call for a systematic basin-wide approach to climate change adaptation in high mountain regions.
Interception of sediment
A joint research pilot project between the US Army Corps of Engineers, the US Army Engineer Research and Development Centre (ERDC), and the Missouri Sedimentation Action Coalition was recently conducted on the Niobrara River in northeast Nebraska. Its aim was to test new technology designed to intercept sediment from the riverbed before it reaches Lewis and Clark Lake.
More than 50% of sediment deposited into the Missouri River reach that makes up the Lewis and Clark Lake delta comes from the Niobrara River. The spring-fed river travels along the northern edge of the Sand Hills of Nebraska and produces a nearly endless supply of sediment through bank, gully, riverbed, and overland erosion.
As a result, the water surface elevation of the Missouri River throughout the Lewis and Clark Lake delta has continued to rise since the lake was formed in 1955 causing increased flood risk, higher ground water levels and water quality and supply issues for nearby communities.
Engineers participating in the project tested a 3.6m bedload sediment collector to determine how much sediment could be captured from the Niobrara River before it reaches the delta.
The speedbump-shaped structure sits on the riverbed and captures sediment as it travels over it and into a trough where it is then pumped out of the river and captured in dewatering bags or large sediment management areas.
The bedload collector method is one of many management methods being considered for Lewis and Clark Lake. The US Army Corps of Engineers and the Missouri Sedimentation Action Coalition hope to publish the Lewis and Clark Lake Sediment Management Plan in the next two years. The plan will include an engineering analysis on the various methods and recommend those that offer the greatest benefits at the lowest costs and with the least environmental impacts.
Overall, engineers consider the equipment and test as being highly successful.
“Under these conditions, the collector was effective, we’re very happy with the results,” said Paul Boyd, hydraulic engineer at the Omaha District of the Army Corps of Engineers. “We’re pulling a lot of sand out of the river.”
Now, Boyd said the challenge is finding a use for the sediment that will offset the cost of removal. He says construction projects could be a potential option.
“Our long-term goal is to try and reduce the cost of harvesting and increase the beneficial use or the value of the sediment – we hope to kind of flip that paradigm and make it cost effective to move sediment” he added.
The ERDC has been researching bedload sediment collection in support of river navigation for more than a decade. Recently, the centre expanded its research to address reservoir sedimentation issues in collaboration with Army Corps districts like Omaha. They are also conducting research with the St. Paul District for reservoir sustainability.
“We were able to fund the work because of the strong partnership with the Missouri Sediment Action Coalition and the association with the Omaha District, which we like,” explained Chuck Theiling, ERDC Environmental Lab research ecologist. “We also have the US Geological Survey scientists helping us determine sedimentation patterns in the river and flow rates and such that will really support our research.”
Theiling says the success of the pilot project could lead to much bigger things.
“If we can take our estimate from this unit and scale it up to a larger unit – conceivably something that might span a whole channel – then we can try to collect most of the sediment going by.”
The research team is currently developing a proposal to return to the Niobrara River in late 2025 to further test the influence of bedform transport of sediment across the collector. They hope to use these tests to improve collection methodology and increase collection efficiency.
Colorado River
The paradigm for sustainable sediment management in the US’ Colorado River relies on two factors:
1. Sand accumulation on the bed of the Colorado River during periods of sediment- rich tributary floods from summer/fall thunderstorms.
2. Dam-released controlled (artificial) floods, referred to as High- Flow Experiments (HFEs), to redistribute the accumulated sand to rebuild eroded bar and floodplain deposits.
As Grams et al explains in River Research and Applications, the management protocol which specifies narrowly defined sand accumulation periods and HFE implementation windows, is based on implementing HFEs in late autumn during the period of greatest sediment enrichment, before higher winter releases for hydropower erode the accumulated sand from the riverbed.
However, low dam releases associated with drought in recent years have changed the pattern of sand accumulation and low reservoir elevations have prevented HFE implementation in the defined window.
The authors give details about an alternative strategy for HFE planning and implementation that was tested opportunistically in April 2023 following lower than normal winter dam releases.
The findings showed that sand enrichment and sandbar building equalled or exceeded that of HFEs conducted under the established management protocol. Furthermore, they show that ‘management goals for sediment under conditions of prolonged drought may be achievable but will likely require substantial changes in dam management strategies’.
Malaysian challenges
Kenyir Lake Basin in Malaysia is the largest man-made lake in Southeast Asia, and crucial for the region’s hydrology and socio-economic activities, including water supply, flood control, hydropower generation, fisheries, and tourism However, the management of reservoir sedimentation here is proving to be a complexly critical challenge for sustainable development across the region.
Multiple factors are associated with sedimentation in Kenyir Lake and include land use changes, deforestation, agricultural activities, and natural erosion processes. All of which are being exacerbated by climate change, with extreme weather events.
New research carried out by Wahab et al in the Journal of the Malaysian Institute of Planners, proposes a multi-faceted management strategy which involves implementing sediment control measures to reduce sediment influx, establishing regular monitoring programmes to track sedimentation trends, and engaging the local community in sediment management efforts.
“Community involvement is paramount in ensuring the success of sedimentation management initiatives,” the authors explain. “Engaging local stakeholders in decision-making processes and promoting awareness about sustainable practices can foster a sense of responsibility towards the lake’s health. Moreover, interdisciplinary collaboration among government agencies, research institutions, and non-governmental organisations is crucial to developing and implementing comprehensive sediment management plans.”
Wahab et al add that such a comprehensive approach aims to provide a framework for managing sedimentation challenges in tropical reservoir systems and will contribute to a broader understanding of reservoir sedimentation issues.
Ethiopian experience
Although Ethiopia is facing challenges in safeguarding its land and water resources, there has been limited research to assess sediment accumulation in reservoirs. Research suggests many dams constructed across the region in recent years have become silted up with sediment, impacting their intended purposes and calling out for the establishment of effective sedimentation monitoring practices.
A recent study was undertaken to estimate the sedimentation and service life of the Angereb reservoir in Ethiopia’s Upper Blue Nile Basin. Results showed a total of 2.18 Mm3 of sediment was deposited on the reservoir bed between 2007 to 2022, with storage capacity decreasing by 62.28% in 16 years of operation, with an average annual volume reduction of 3.9%. Recommendations include addressing soil erosion issues throughout the whole watershed with tailored conservation practices and investment.
References
Analysing sedimentation patterns in the Naumure Multipurpose Project (NMP) reservoir using 1D HEC-RAS modelling by Vishan Dahal, Subash Kunwar, Shishir Bhandari, Subash Chaudhary, Saurab Gautam, Nabin Bhatt & Ram Krishna Regmi. www.nature.com/Scientific Reports (2024) 14:22134. https://doi.org/10.1038/s41598-024-73883-x
Understanding the cost of soil erosion: An assessment of the sediment removal costs from the reservoirs of the European Union by Panos Panagosa, Francis Matthewsa, Edouard Patault, Carlo De Michele, Emanuele Quaranta, Nejc Bezak, Konstantinos Kaffas, Epari Ritesh Patro, Christian Auel, Anton J. Schleiss, Arthur Fendricha, Leonidas Liakos, Elise Van Eynde, Diana Vieira, Pasquale Borrelli. Journal of Cleaner Production 434(2024)140183. https://doi.org/10.1016/j.jclepro.2023.140183
Implementation of Controlled Floods for Sediment Management on the Colorado River in Grand Canyon Under Aridification by Paul E. Grams, David J. Topping, Gerard Salter, Katherine A Chapman, Robert B Tusso, Erich R Mueller. River Research and Applications, 2025; 41:334–348 https://doi.org/10.1002/rra.4374
Reservoir Sedimentation Management: A Sustainable Development Challenge in The Kenyir Lake Basin, Malaysia by Noorjima Abd Wahab, Mohd Khairul Amri Kamarudin, Mohd Ekhwan Toriman, Siti Nor Fazillah Abdullah, Sianturi Novdin Manoktong. Journal of the Malaysian Institute of Planners Volume 23 Issue 1 (2025), Page 97 – 111
Understanding sedimentation trends to enhance sustainable reservoir management in the Angereb reservoir, Upper Blue Nile Basin, Ethiopia by Tessema YM, Zimale FA and Kebedew MG (2024.) Front. Water 6:1387915. doi: 10.3389/frwa.2024.1387915
