Effects of sediment flushing operations versus natural floods on Chinook salmon survival

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 ($\textit{Oncorhynchus tshawytscha}$) 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

Study on farmers' Pest management strategy, knowledge on pesticide safety and practice of pesticide use at Bhaktapur district, Nepal

Nepal being an agricultural country, the majority of the population is involved in agriculture. Pests are one of the major problems in the agriculture sector. Pesticides are being widely used to encounter this problem. This study aims to determine the knowledge level of farmers on pesticides and their use. Specifically, it investigates the pest management strategy, pesticide use, and safety measures used by the farmers. For this study, four municipalities of Bhaktapur district were selected with 100 respondents using a random sample design. Primarily, data collection was done through field observations, interviews, and questionnaires. Various secondary data were also used. Our study showed that farmers in this region were mainly dependent on chemical methods. For technical guidance, several applications, and use rates, farmers preferred the agro vets available in their area. As less care and concern were given to safety measures, it was not given top priority. The study showed that the knowledge of various aspects of pesticides, their use, and safety was inadequate among the farmers

Numerical modelling of sediment erosion from reservoirs

Sedimentation in reservoirs causes loss in the useful water storage volume, threatening the long term sustainability of dams. The number of rivers being dammed worldwide is increasing rapidly, but the favorable locations for dam sites are decreasing with it. So it is necessary to establish some efficient measures to manage the problem of sedimentation in existing reservoirs. Recently, sediment flushing operations have been more common to release part of the stored sediment in reservoirs. However, there is no clear methodology for the study and optimization of sediment removal from reservoirs. Sediment removal from reservoirs is a complex process involving the interaction between hydrodynamic and morphodynamic phenomena, which makes it difficult to study it analytically. Recent advancements in numerical models have provided some opportunity to study such complex processes. One-dimensional numerical models have been employed in some reservoir sediment removal process, but their performance is just limited to longitudinal processes and cannot give information at the cross-sectional scale. So two-dimensional models are necessary to get sufficient information in both the reach scale and cross-sectional scale. But the performance of two-dimensional models in predicting the sediment removal from reservoirs has not yet been verified. Recently some dam removal events are being extensively monitored to document the river morphological changes due to the exposure of large amounts of sediment from the reservoir. Dam removal can be considered analogous to large scale sediment flushing operations, as both activities involve release of large amounts of sediment from the reservoir towards downstream. Similar is the case of Marmot Dam Removal. The Marmot Dam located across the Sandy River in the Oregon State of USA, was removed in 2007, due to economic and environmental considerations. This exposed a large amount of sand and gravel deposited in the reservoir to be eroded by the natural flow of the Sandy River. The removal of the Marmot Dam was monitored extensively for the erosion, transport and downstream deposition of the impounded sediment. This provided an opportunity to conduct a research on the morphological changes occurring in reservoirs during the sediment removal. This study considers the case of Marmot Dam Reservoir Erosion to evaluate the performance of a 2D numerical model built in the open source software Delft3D. The modelling work includes simulation of the cofferdam breaching process and the erosion process of graded non-cohesive sediment from the reservoir after the dam removal. The model results are analyzed to validate the model considering any shortcomings. The validated model is then used to study and optimize the sediment flushing operations in the same reservoir. The results indicate that Delft3D is indeed a good tool in simulating sediment erosion process from reservoirs. There are some limitations in the quantification of the removed sediment amounts, but the overall morphological development is quite well represented by the model. This evaluates Delft3D as a useful tool to simulate different sediment flushing operations, in view of optimization and selection of the best scenario for flushing

Effects of sediment flushing operations versus natural floods on Chinook salmon survival

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.ISSN:2045-232

Effects of sediment flushing operations versus natural floods on Chinook salmon survival

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.Rivers, Ports, Waterways and Dredging EngineeringEnvironmental Fluid Mechanic