Sudesh Dahal


Last Name

Dahal

First Name

Sudesh

ORCID

0000-0003-3526-1051

Organisational unit

03820 - Boes, Robert / Boes, Robert

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2022 - 20259
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Publications1 - 9 of 9
  • Alpine Reservoir Sedimentation: Challenges in Modelling Rapidly Varying Flow Conditions at Inflow Section
    Item type: Other Conference Item
    Dahal, Sudesh; Evers, Frederic M.; Boes, Robert; et al. (2025)
    Book of Extended Abstracts of the 41st IAHR World Congress – Innovative Water Eng. for Sustainable Development
    Reservoir sedimentation has a negative impact on dam-reservoir systems causing progressive reduction of the storage volume and impaired functionality of dam safety outlets. The global situation is alarming because the rate of loss of storage capacity is not balanced by the construction of new dams. With increasing water/energy demand amidst climate change impacts, the sedimentation issue is more amplified and management actions need to be prioritized to ensure sustainable storage. An in-depth investigation of governing processes is fundamental to plan for management strategies. In this study, the sedimentation dynamics in the seasonal storage reservoir Gigerwald in the Swiss Alps is investigated using 1D numerical modelling. The associated modelling challenges related to hydraulic jump and strongly varying geometry at the inflow section are identified and overcome by a careful model setup and selection of numerical parameters. Two distinct periods of sedimentation are referred to for model calibration and validation. A set of modelling parameters is obtained that produces a good agreement between modelled and monitored sedimentation processes. The validated model provides a basis for further investigation of management strategies along with climate change impacts.
  • Sedimentation Management in Alpine Reservoirs for Sustainable Hydropower under Climate Change
    Item type: Monograph
    Dahal, Sudesh (2025)
    VAW-Mitteilungen
  • 1D numerical modelling of sedimentation propagation in a narrow reservoir
    Item type: Other Conference Item
    Dahal, Sudesh; Evers, Frederic M.; Boes, Robert; et al. (2022)
    7th IAHR Europe Congress: Innovative Water Management in a Changing Climate. Abstract Book
  • Effects of sediment flushing operations versus natural floods on Chinook salmon survival
    Item type: Journal Article
    Panthi, Manisha; Lee, Aaron A.; Dahal, Sudesh; et al. (2022)
    Scientific Reports
    Flushing is a common measure to manage and reduce the amount of sediment stored in reservoirs. However, the sudden release of large volumes of sediment abruptly increases the suspended solids concentration and alters the riverbed composition. Similar effects can be produced also by natural flood events. Do flushing operations have more detrimental impacts than natural floods? To answer this question, we investigated the impact of flushing on the survival of the Chinook salmon (Oncorhynchus tshawytscha) in the Sandy River (OR, USA), assuming that sediment is flushed from hypothetical bottom gates of the, now decommissioned, Marmot Dam. The effects of several flushing scenarios are analyzed with a 2D morphodynamic model, together with habitat suitability curves and stress indicators. The results show that attention has to be paid to duration: the shorter the flushing operation, the lesser the stresses on fish survival and spawning habitats. Flushing causes high stress to salmon eggs and larvae, due to unbearable levels of suspended sediment concentrations. It also decreases the areas usable for spawning due to fine-sediment deposition, with up to 95% loss at peak flow. Without the dam, the corresponding natural flood event would produce similar effects, with up to 93% loss. The study shows that well-planned flushing operations could mimic a natural impact, but only partly. In the long-term, larger losses of spawning grounds can be expected, since the removal of fine sediment with the release of clear water from the reservoir is a lengthy process that may be undesirable due to water storage reduction.
  • Sedimentation in a narrow reservoir under climate change and sediment bypass tunnel operation scenarios
    Item type: Conference Paper
    Dahal, Sudesh; Evers, Frederic M.; Boes, Robert; et al. (2024)
    Proceedings of the 10th International Symposium on Hydraulic Structures (ISHS 2024)
    Reservoir sedimentation is one of the main issues interfering with the sustainable operation of many hydropower reservoirs as it causes a reduction of storage capacity and may also affect dam safety. The rate of sedimentation is anticipated to change due to changes in flow following climate change. Although quantifying the variation of sedimentation is subject to high uncertainty, sediment yield is mainly expected to increase for Alpine catchments due to retreating glaciers and thawing permafrost. Sediment Bypass Tunnels (SBTs) are hydraulic structures to counteract sedimentation problems as they allow for bypassing incoming sediment (bedload and parts of the suspended load) around the dam into the tailwater reach. This study deals with the SBT in Solis reservoir (Switzerland) where an SBT has been operating for more than a decade to counteract sedimentation. The aim is to evaluate the performance of the Solis SBT under climate scenarios by conducting 1D numerical simulations. The HydroCH2018-Runoff ensemble is referred for the future projection of river inflows into the Solis reservoir. In absence of quantified impact on the sediment input, it is computed from calibrated sediment transport equations based on monitored data. The model is found useful to simulate future sedimentation and to compare scenarios of SBT operation during flood events. Considering the impact of SBT operation on energy generation due to water losses, SBT operation during two different floods is more effective than its operation in a single flood for a given total duration of operation.
  • Efficiency evaluation and simulation of sediment bypass tunnel operation: Case study Solis reservoir
    Item type: Conference Paper
    Dahal, Sudesh; Maddahi, Modammad Reza; Albayrak, Ismail; et al. (2023)
    Role of Dams and Reservoirs in a Successful Energy Transition
    Hydropower is the major source of electricity in Switzerland contributing about 57% (36 TWh/yr) of the total annual generation. Therefore, water storage in hydropower reservoirs is crucial to balance the electricity demand over variable river flow. With the increase in storage demand and climate-related stress it becomes important to sustain the existing reservoir storage capacities. Sedimentation impairs the sustainable operation of reservoirs by reducing the storage volume and may also cause dam safety related issues by the interference of sediment deposits with dam outlets. Sediment Bypass Tunnels (SBTs) are an effective countermeasure to reduce or even stop sedimentation and contribute to a sustainable use of reservoir storage capacity. This study investigates the performance of an SBT constructed at Solis reservoir in the Swiss Alps, operated by ewz. The SBT was commissioned in 2012 to mitigate continuous propagation of sediment aggradation towards the dam since its construction in 1986. As the inlet of the SBT is located within the reservoir and therefore typically submerged, optimized reservoir operation is required during the intended period of sediment bypassing. Annual field measurements were conducted to measure the reservoir bathymetry, sediment concentrations, transport rate and sediment particle sizes on the bed to derive the reservoir’s sediment balance. The measurements between October 2018 and August 2019 are analyzed to investigate bypass efficiencies of the SBT. The results indicate that the efficiency of the SBT was 80%, and thus considerably higher than the previous efficiency rate of 17%, due to adaptation of the reservoir operation to a lower water level during SBT operation. This implies that with proper synchronization of SBT and reservoir operation, this type of SBT can be highly efficient. Furthermore, a 1D numerical model is applied to investigate the processes of sedimentation and sediment management for the Solis reservoir. The data from the field measurements is used to set-up, calibrate and validate the model aiming at investigating the performance of the SBT. The model can reproduce the sedimentation as well as SBT operation in terms of longitudinal bed profile evolution and deposition volume. Moreover, the model also allows for simulating additional scenarios, including e.g. no SBT operation, to compare the effects of different operation modes.
  • 1D numerical modelling of Sediment-Bypass-Tunnel operation in a narrow reservoir.
    Item type: Conference Paper
    Dahal, Sudesh; Evers, Frederic M.; Boes, Robert; et al. (2023)
    Proceedings of the 40th IAHR World Congress
    Sustainable operation of many hydropower reservoirs is impacted by reservoir sedimentation which is not only causing a reduction of storage capacity but may also impair the functionality of dam outlets thereby implicating dam safety issues. Sediment Bypass Tunnels (SBTs) represent an effective countermeasure to mitigate sedimentation problems as they allow for bypassing incoming sediment (bedload and parts of the suspended load) around the dam into the tailwater reach. This study considers the Solis reservoir (Switzerland) which has effectively implemented an SBT with intake within the reservoir (type-B SBT) for managing high sediment inflow. The aim is to evaluate the performance of the Solis SBT by conducting 1D numerical simulations. The results show that a 1D model can closely reproduce the processes of sedimentation and SBT operation. Furthermore, the effect of the reservoir water level on SBT efficiency is assessed by conducting a scenario simulation. We found that the reservoir water level has a major impact on the SBT efficiency. For type-B SBTs, synchronous lowering of water level is recommended during SBT operation to ensure effective sediment bypassing.
  • Bypassing efficiency evaluation and optimization of sediment bypass tunnel operation in a narrow reservoir using 1D numerical modelling
    Item type: Journal Article
    Dahal, Sudesh; Boes, Robert; Evers, Frederic M.; et al. (2025)
    Journal of Hydro-environment Research
    Sediment Bypass Tunnels (SBTs) are hydraulic structures designed to mitigate reservoir sedimentation, mainly used in Switzerland, Japan, and Taiwan. The SBT efficiency depends on multiple factors including the incoming sediment yield, SBT inlet location, reservoir water level, and the timing of SBT operation relative to a flood event. To evaluate bypassing efficiency and optimize SBT operation, a methodology is developed to integrate sediment bypassing processes in a 1D numerical model for simulating the sedimentation dynamics under varying hydraulic conditions. The well-monitored case of the Solis reservoir in Switzerland with SBT operation serves as a good basis for this study. After calibrating and validating the model with the field data, three different categories of SBT operation scenarios are studied for a 5-year flood event: i) no SBT operation, ii) the effect of reservoir water level during SBT operation, and iii) the optimum duration of SBT operation. The simulations reveal that SBT operation is highly effective in reducing the amount of sedimentation by 89% compared to no SBT operation. This equals to the prevention of a 9% loss in active storage volume. However, the SBT efficiency is highly sensitive to reservoir operation. A maximum efficiency is achieved at a lower reservoir water level below the minimum operating level for energy generation, while it also releases higher sediment concentrations to the downstream reach. Furthermore, a longer duration of SBT operation increases the bypassing efficiency and minimizes the loss of active storage volume but goes along with a reduction in hydropower generation. Thus, a comparison of net benefits for different scenarios is suggested to derive an optimal SBT operation mode for similar situations. Overall, the applied methodology serves as a useful basis for evaluating and optimising the sediment management efficiency of SBTs and can thus contribute to improving the sustainable operation of reservoirs.
Publications1 - 9 of 9