Showing papers on "Siltation published in 2022"

Impact of the Three Gorges Dam on riverbed scour and siltation of the middle reaches of the Yangtze River

Abstract: This study investigated the influence of the Three Gorges Dam (TGD) on the evolution of nearby channels in the Yangtze River (Changjiang River) system in order to better understand the environmental impact of large‐scale reservoir operations. From 2003 to 2017, the amounts of runoff and sediment transport in the Yangtze River were reduced by 3.3–14.5% and 67.8–92.7%, respectively, relative to 1955–2002 before the TGD was operational. Topographic measurements of the middle reaches (Yichang to Hukou) of the Yangtze River were analyzed from 1975 to 2017, during which time the cumulative erosion of the flood channel was 22.78 × 108 m3, and the dry channel accounted for 90.3% of the erosion. Following commissioning of the TGD, the scouring intensity of the sandy gravel section near the dam initially increased then decreased, whereas the scouring intensity of sandy sections away from the dam continued to increase. Beaches on convex banks of curved sections were scoured, and deep channels on concave banks became silted. In braided sections, the braids tended to shrink, and the diversion ratio of the main branch during dry seasons reduced, resulting in frequent branch alternation. Compared to changes in the downstream river channels of other large reservoirs worldwide, scouring from the TGD is extensive. The findings of this study are significant for river channel regulation and waterway planning in the Yangtze River and worldwide.

Temporal and spatial hydrological variations of the Yellow River in the past 60 years

Abstract: Understanding water and sediment variations of the Yellow River is significant for future management of the river from perspectives of water resources utilization and ecology protection. The study characterized the temporal and spatial variations in its flow and sediment load based on the daily datasets of the past 60 years from all main-stem hydrological gauge stations, and proposed a new five-segment river reach division scheme instead of the traditional three- or four-segment division. Furthermore, we distinguished the key factors and their contributions that caused the flow and sediment variations, offering reference for the river management strategies as per the local conditions. A significant decreasing trend in annual flow and sediment load was observed in the river basin, except for certain reaches in Segment 1 (upstream of Tangnaihai). The maximum flood-event duration decreased significantly in Segment 2 (from Tangnaihai to Shizuishan). The flood volume and sediment load during the maximum flood event also showed significant downward trend in Segments 2–5 (downstream of Tangnaihai). Operation of cascade reservoirs, especially in Segment 2, has smoothed the annual flow process in the upstream. Sediment trapping in the reservoirs had little effect on the change in sediment load, but reservoir regulation of flow greatly influenced siltation in the upstream river channel. Water abstraction has also played a significant role in flow variation. Consequently, the ratio of flood sediment load to annual sediment load decreased significantly in the upper reach, but increased significantly in the lower reach. Temporally, the recorded period was divided into three periods using change point detection: pre- 1987, 1987–1999, and post 1999. The ratio of flood volume to annual flow decreased significantly in the first two periods, and generally increased post 1999, with the impact of aforementioned anthropogenic activities.

Influence of check dams on flood hydrology across varying stages of their lifespan in a highly erodible Catchment, Loess Plateau of China

Abstract: Check dams are applied worldwide as an effective approach for soil conservation. Its effect on hydrology, however, has not been fully understood. As a critical step towards revealing the lumped effect of check dams at the regional scale, this study explored the modified flood hydrology induced by check dams in a representative highly erodible catchment in the Loess Plateau, China. A coupled hydrological and hydraulic modeling approach was applied to simulate the flooding process in different stages of deposition and topographic changes in a check dam reservoir. The results suggest a paradigm shift of the dam effect on flood attributes, which transits from a total interception in the early stage of the dam to peak reduction and flood detention, rather than a complete loss of flood control functions, when it approaches the maximum capacity of sedimentation. Under the same level of deposition, the reduction of a minor flood by a check dam was higher than that of a major flood. The topographic changes contributed to the reduction of flood peak appreciably by reducing the flow velocity and retarding the flood propagation. Noticeably, this reduction was augmented with the advancement of siltation and topographic changes, indicating the persistence of the hydrologic effects of check dams in the long run. As a result of these hydrological changes, the reduction in the flood flow velocity induced by check dams suggests substantial reductions in sediment transport and channel erosion during floods. In addition, a dam system containing multiple, cascading check dams exhibits much more significant effect in modifying both hydrologic and hydraulic properties of flood than individual dams. The current research provides a mechanistic understanding of the effects of check dams on catchment hydrology under heavy rainstorm conditions in small catchments.

Shape optimization of egg-shaped sewer pipes based on the nondominated sorting genetic algorithm (NSGA-II)

Abstract: Overflow pollution is an undesired issue that commonly occurs in combined sewers under wet weather conditions. There is a lack of existing studies on the structural optimization of sewers to prevent siltation, and no previous study on egg-shaped sewers with this purpose has confirmed satisfactory anti-sedimentation performance. To achieve reduced sedimentation and lower energy loss under low- and high-flow conditions, respectively, the nondominated sorting genetic algorithm (NSGA-II) was adopted in this study based on a constant full filling discharge capacity equal to that of a 300 mm (diameter) circular sewer. The results showed that egg-shaped sewers with bottom and top arc radii of 58.3 and 116.6 mm, respectively, and a height of 408.1 mm performed significantly better than circular sewers (d = 300 mm). Notably, at a low flow ratio below 0.2, the shear stress of the optimized egg-shaped sewer was 5.2%-20.6% higher than that of the circular sewer. At a flow ratio of 0.2-0.6, both the egg-shaped and circular sewers were capable of maintaining a balanced amount of sediment between deposition and erosion. As the flow ratio increased to 0.6-1, both types of sewers completely scoured sediments: in this situation, the shear stress of the egg-shaped sewer was 5.5%-10.1% lower than that of the circular sewer, thus exhibiting reduced energy loss. This study indicates that egg-shaped sewers have an attractive future in replacing circular sewers for sedimentation prevention and cost control.

Channel morphodynamics and sediment budget of the Lower Ganga River using a hydrogeomorphological approach

Abstract: The Ganga River is one of the largest river systems in the world that has built extensive alluvial plains in northern India. The stretch of the Lower Ganga River is vulnerable to siltation because of: (a) the naturally low slope in the alluvial stretch; (b) the confluence of several highly sediment‐charged rivers such as the Ghaghra, Gandak, and Kosi; and (c) the reduction in non‐monsoon flows because of upstream abstractions of both surface and groundwater. Additionally, the Farakka barrage has impacted the morphology of the Ganga River significantly, both upstream and downstream of the barrage. Large‐scale siltation in several reaches has reduced the channel capacity, leading to catastrophic floods in this region even at low discharges. This work has utilized historical remote sensing data and UAV surveys to reconstruct channel morphodynamics and compute sediment volumes accumulated in the channel belt along the Lower Ganga River between Buxar and Farakka. The work was carried out by dividing the total length of the river into four continuous stretches: (a) Buxar–Gandhighat (GW1, 160 km); (b) Gandhighat–Hathidah (GW2, 106 km); (c) Hathidah–Azmabad (GW3, 182 km); and (d) Azmabad–Farakka (GW4, 132 km). We document that major ‘hotspots’ of siltation have developed in several reaches of the Lower Ganga during the last four to five decades. Sediment budgeting using planform maps provides estimates of ‘extractable’ volumes of sediment in GW1, GW2, GW3, and GW4 as 656 ± 48, 706 ± 52, 876 ± 71, and 200 ± 85 Mm3, respectively. These estimates are considerably lower than those computed from the hydrological approach using observed suspended sediment load data, which assumes uniform sedimentation between two stations. Further, our approach provides reach‐scale hotspots of aggradation and estimates of extractable sediment volumes, and this can be very useful for river managers to develop a strategic sediment management plan for the given stretch of the Ganga River.

Predicting Daily Suspended Sediment Load Using Machine Learning and NARX Hydro-Climatic Inputs in Semi-Arid Environment

Abstract: Sediment transport in basins disturbs the ecological systems of the water bodies and leads to reservoir siltation. Its evaluation is crucial for managing water resources. The practical application of the process-based model can confront some limitations noticed in the lower accuracy during the validation process due to the lack of reliable physical datasets. In this study, we attempt to apply machine-learning-based modeling (ML) to predict the suspended sediment load, using hydro-climatic data as input variables in the semi-arid Bouregreg basin, Morocco. To that end, data for the years 2016 to 2020 were used for the training process, and the validation was performed with 2021 data. The results showed that most ML models have good accuracy, with a Nash–Schiff efficiency (NSE) ranging from 0.47 to 0.80 during the validation phase, which indicates satisfactory performances in predicting the SSL. Furthermore, the models were ranked against their generalization ability (GA), which revealed that the developed models are good to excellent in terms of GA. Overall, the present study provides new insight into predicting the SSL in a semi-arid environment, such as the Bouregreg basin.

Prediction of River Sediment Transport Based on Wavelet Transform and Neural Network Model

Abstract: The sedimentation problem is one of the critical issues affecting the long-term use of rivers, and the study of sediment variation in rivers is closely related to water resource, river ecosystem and estuarine delta siltation. Traditional research on sediment variation in rivers is mostly based on field measurements and experimental simulations, which requires a large amount of human and material resources, many influencing factors and other restrictions. With the development of computer technology, intelligent approaches have been applied to hydrological models to establish small information in river areas. In this paper, considering the influence of multiple factors on sediment transport, the validity of predicting sediment transport combined with wavelet transforms and neural network was analyzed. The rainfall and runoff cycles are extracted and decomposed into time series sub-signals by wavelet transforms; then, the data post-processing is used as the neural network training set to predict the sediment model. The results show that wavelet coupled neural network model effectively improves the accuracy of the predicted sediment model, which can provide a reference basis for river sediment prediction.

Long-Term Geomorphological Evolution of the Mouth Bar in the Modaomen Estuary of the Pearl River over the Last 55 Years (1964–2019)

Abstract: Based on mass bathymetric data and remote sensing data in the Modaomen Estuary, this study explored the long-term evolutionary characteristics of the mouth bar in the Modaomen Estuary of the Pearl River from 1964 to 2019. In the past 55 years, due to the impact of human activities, such as shoal reclamation and estuarine regulation in the Modaomen Estuary, the river mouth moved out of the shallow sea covered by several islands and faced the South China Sea directly. Therefore, the mouth bay became a siltation center in the estuarine region and expanded outwards, gradually evolving a geomorphic pattern with three shallow shoals and two distributary branches; a west branch as the main branch accompanied by a small east branch. Over the past decade, high-intensity sand dredging activities in the mouth bar have led to a considerable deepening of the water depth and a significant refinement of bed sediments, forming a discharge pattern of a wide and shallow channel flowing into the sea. Therefore, the evolutionary characteristics of the mouth bar have become abnormal in recent years, so additional field bathymetric data and hydrological data are required for further research regarding the subsequent evolution of the mouth bar, against the background of a significant reduction of sediment discharge and high-intensity human activities.

Downstream semi-circular obstacles' influence on floods arising from the failure of dams with different levels of reservoir silting

Abstract: Dam-break wave propagation in a debris flood event is strongly influenced by accumulated reservoir-bound sediment and downstream obstacles. For instance, the Brumadinho dam disaster in Brazil in 2019 released 12 × 106 m3 of mud and iron tailings and inflicted 270 casualties. The present work was motivated by the apparent lack of experimental or numerical studies on silted-up reservoir dam-breaks with downstream semi-circular obstacles. Accordingly, 24 dam-break scenarios with different reservoir sediment depths and with or without obstacles were observed experimentally and verified numerically. Multiphase flood waves were filmed, and sediment depths, water levels, and values of front wave celerity were measured to improve our scientific understanding of shock wave propagation over an abruptly changing topography. Original data generated in this study is available online in the public repository and may be used for practical purposes. The strength of OpenFOAM software in estimating such a complex phenomenon was assessed using two approaches: volume of fluid (VOF) and Eulerian. An acceptable agreement was attained between numerical and experimental records (errors ranged from 1 to 13.6%), with the Eulerian outperforming the VOF method in estimating both sediment depth and water level profiles. This difference was most notable when more than half of the reservoir depth was initially filled by sediment (≥0.15 m), particularly in bumpy bed scenarios.

Development of Ceramic Tiles from Philippine Nickel Laterite Mine Waste by Ceramic Casting Method

Abstract: Siltation is the biggest environmental challenge associated with nickel laterite mining in the Philippines. The amount of silt generated is huge and one mitigation strategy currently employed by the mining companies is the construction of siltation ponds where the bulk of the clayey- and silt-sized surface runoffs is collected. However, this poses several serious environmental hazards such as landslides due to heavy rainfall and the potential release of hazardous heavy metals. A promising approach to reduce the risks associated with long-term storage of nickel mine waste (NMW) is to employ circular economy by repurposing it for ceramic applications. While generating useful materials with economic value out of a mine waste, it will result in a reduction in volume of waste for disposal. In this study, the method employed to produce NMW-based ceramic wall and floor tiles is slip casting as it is the most appropriate method in forming tiles with complex surface features. Five formulations of NMW-based slips were made for the casting of ceramic tiles and each slip was characterized for its suitability as raw material. The results of NMW characterization show that NMW could be utilized as raw materials for both ceramic wall and floor tiles and the addition of feldspar can enhance casting and physical properties.

Morphological Response of the Pussur River, Bangladesh to Modern-Day Dredging: Implications for Navigability

Abstract: Mongla Port is the second gateway of Bangladesh, situated at the bank of the Pussur River some 131 km upstream from the Bay of Bengal. In this study, the available hydrographic survey charts of Pussur River were collected from Mongla Port Authority (MPA) and analyzed to assess the trends of morphological changes at the potential sites. The causes of navigation problems, previous interventions of MPA, dredging history of the river, and effectiveness of dredging was analyzed. It is found that in the upstream portion of the navigation route at the MPA jetty area and its approaches was slightly scour-prone from 2010 to 2013; however, after capital dredging in 2014, the channel was found highly siltation-prone, having a rate of 0.1–0.6 m/year between 2015 and 2017. Before dredging the channel, the Inner bar area (downstream of Mongla Port) did not show any significant change in the bed topography between 2010 and 2013, however, a high backfilling rate (up to 1.5 m/year) was observed after the capital dredging. The channel was sufficiently deep and quite stable from Joymonirgol to Hiron point. The current navigational channel at the outer bar (at the mouth of the river) is silted by 0.1–0.25 m/year. Reduction of upstream flow, numerous shipwrecks at different positions, and human disturbances were identified as the causes behind the navigational problems in the river route.

Estimation of sedimentation in Manwan and Jinghong reservoir of the Lancang river

Abstract: The Lancang reservoirs play an essential role in the national economy and life, and the study of reservoir siltation is of great significance to ensure its sustainable service for the nation and people. In this paper, reservoir sedimentation is quantified in stages by empirical models and theoretical methods using reservoir information and sediment data to reveal the latest status of siltation in Lancang reservoirs. Results show that the storage capacity loss of Manwan and Jinghong reservoirs has reached 51.4% and 1.54% until 2019, which illustrates that the situation of siltation is serious. The theoretical trapping efficiency of Manwan reservoir was about 69% and the estimation result of Brune method performed best with a value of 67.5% among the empirical methods. Brune method was then modified with a correction coefficient and the revised Brune method can be used for the estimation of trapping efficiency in other reservoirs. Overall, this paper can present relatively accurate information for managers to understand the current state of siltation in Lancang reservoirs, and provide scientific guidance and data support for them to take measures to reduce sedimentation and ensure the sustainability of reservoirs.

Impact of upstream reservoirs on geomorphic evolution in the middle and lower reaches of the Yangtze River

Abstract: Intensive human activities in river basins have substantial effects on fluvial hydrological and morphological processes and developmental trends. In this study, we assessed the evolutionary characteristics and developmental trends of the fluvial morphological system of the Three Gorges Dam (TGD) downstream to understand the impacts of human activities on the fluvial hydrological–morphological system of the region. The results indicated that the Three Gorges Project (TGP) and other upstream cascade reservoirs promoted sediment retention, which in turn aggravated the trend of declining sediment load and cumulative scouring over a long distance in the channels downstream of the TGD. Approximately 84% of scouring occurred in the low‐water channels, along with thalweg scouring and riverbank collapse. Notably, extended operation of the TGP and other upstream cascade reservoirs has resulted in gradual changes in the downstream riverbeds. Specifically, we found that the most intensive scouring zone shifted downstream from the near‐dam Yichang–Chenglingji reach (0–408 km) to the Chenglingji–Hankou reach. The scouring–silting distribution in swales has also changed: the scouring of deep swales and silting on floodplains prior to the TGP have been replaced by the scouring of both deep swales and floodplains. The scouring intensity of riverbeds in the Yichang–Hukou reach (954 km) was slightly higher than that forecasted prior to the TGP; the scouring intensities increased significantly during 2013–2021, which could be attributed to the TGP and other upstream cascade reservoirs; river sand mining and waterway dredging are secondary factors. We conclude that, owing to the effects of sediment retention by cascade reservoirs in the upper reaches of the Yangtze River (or Changjiang River), the cumulative scouring intensities of riverbeds in the middle and lower reaches of the Yangtze River will continue to increase in future decades.

Geomorphological dynamics of tidal channels and flats in mangrove swamps

Abstract: The tidal channels and mudflats of estuarine wetlands have vital ecological functions for the habitat of aquatic organisms and the exchange of nutrients and sediment. Understanding the spatiotemporal dynamics of their morphology helps to formulate an appropriate management plan for mangrove swamps. The water levels, wetland topography, suspended sediment concentration, sediment particle size, and mangrove stand characteristics were investigated in the Tanshui River estuary to verify the geomorphological model established in this study. The decoupled model includes a hydrodynamic module, a sediment module, and a mangrove module to integrate the effects of tidal hydroperiods and river currents. The simulation results revealed that the model could effectively predict the tidal channel morphodynamics, but the siltation of tidal mudflats was overestimated. Sensitivity analysis results showed that the soil properties and erosion rate had the highest impacts. The flow velocities and bed shear stress of the tidal channels were several times those of the mudflats. The hydrodynamics of ebb tide were more extensive than those of flood tide. The asymmetry between ebb and flood tides contributes to maintaining a self-sustaining tidal channel. The deposition rate during the flood period was 20–80 times that of the normal period. The results indicated that river floods entrained large amounts of sediment to accelerate wetland siltation. The reduced tidal range scenarios and the corresponding suspended sediment input suggested an alternative for preventing channel deposition in the wetland. This study offers a quantitative tool for delivering effective maintenance of tidal channels and tidal flats that may enhance the integrated and adaptive management of mangrove swamps.

Experimental study on static characteristics of the Yellow River silt under (triaxial) consolidated undrained conditions

Abstract: The siltation of reservoirs and rivers in the Yellow River Basin and the occurrence of flood disasters are mainly caused by silt transportation. The Yellow River silt being applied to practical production will help solve the problem of siltation in the Yellow River. In this paper, the consolidated undrained (CU) test of the Yellow River silt is carried out by using GDS triaxial apparatus. The effects of confining pressure, relative density and loading rate on the stress-strain relationship, strength parameters, generalized shear stress ratio and pore water pressure variation of the Yellow River silt are explored. The results show that, under the same axial strain, the deviator stress increases with the increase of confining pressure and relative density, and decreases with the increase of loading rate; compared with the relative density, the effect of the confining pressure and loading rate on the generalized shear stress ratio is insignificant. The friction angle is affected by relative density and confining pressure in varying degrees, but less by loading rate. The secant modulus and initial tangent modulus increase with the increase of confining pressure and relative density. The pore water pressure first increases to the positive peak value, and then decreases to a negative value gradually with the increase of axial strain under most conditions. The pore water pressure variation is affected by the confining pressure and relative density obviously, while affected by the loading rate insignificantly.

Study on High-Resolution Suspended Sediment Distribution under the Influence of Coastal Zone Engineering in the Yangtze River Mouth, China

Abstract: In this study, we analyzed the influence of coastal zone engineering on suspended sediment concentration (SSC) in the Yangtze River mouth based on HY-1C and Chinese GaoFen (GF) satellite data. The results show that: (1) SSC in the Yangtze River mouth is mainly in the range of 200–1300 mg/L, which changes due to natural factors as well as coastal zone engineering; (2) The sand blocking engineering effectively protects the channel from siltation, and SSC in the central line of the channel is 100–300 mg/L lower than that near the dikes in the north and south; (3) Qingcaosha Reservoir plays a stabilizing role in the hydrodynamic force, promoting the deposition of suspended matter, so that the water in the reservoir is clear. SSC inside the Qingcaosha Reservoir, located in the center of the estuary, is 100–500 mg/L lower than the high SSC water outside the reservoir; (4) The bridges interact with the current, facilitating the transport of local sediment, resulting in the increase in SSC downstream of these bridges, and obvious vortexes appear with the length up to nearly 2400 m; and (5) Corresponding protection and development suggestions were put forward as follows. A linear array of wind power plants can be built near and parallel to dikes and leading jetties; to increase the depth of the southeast end of the reservoir; and the location of the bridge should be selected in the place of river where siltation is easy. This study aimed to study the SSC distribution under the complex impact of coastal zone engineering with satellite and to provide possible improvement suggestions.

Sediment transport and morphological responses of a silty coast to a cold front event in the southwest Bohai Bay of China

Abstract: Silty coast, as a transitional state between muddy coast and sandy coast, possesses a considerable percentage of the overall coast in China. Huanghua-Binzhou coast in the southwest Bohai Bay in the northeastern China is a typical silty coast which is vulnerable to cold fronts when its seabed is prone to suffer suddenly drastic erosion and deposition due to the sediment there with relatively low incipient velocity and high settling velocity. In this study, a multi-sediment fraction model was established and two-way coupled with the wave-tide-surge model to investigate the responses of the sediment transport and morphodynamics of Huanghua-Binzhou coast to a cold front event during 10–13th Oct. 2003. The model results demonstrate that three sediment fractions, i.e. mud, silt and sand, showed different responses to the storm due to their different spatial percentages and distinct physical properties. The silt fraction, as the primary fraction, played a leading role in the suspension and transport of the total sediment, and showed a pattern of “deposition-erosion-deposition” in the cross-shore direction like the total morphological change. The fine sediment was transported westward and offshore during the storm, which indicated the sediment coarsening in the study area. The depth-induced wave modulation considerably enhanced the resuspension ability of the nearshore waves for sediment and thus played a critical role in intensifying the erosion. The Huanghua waterway siltation mainly resulted from the resuspension of the offshore sediment by the strong wind waves and its westward transport by the wind-induced currents during the storm. The construction of two long jetties in 2006 can significantly reduce the deposition in the waterway by 75%, and the additional large-scale construction in 2016 can further lessen the waterway siltation by intercepting the westward sediment transport and reduce the suspended sediment concentration by 30% by the land reclamation in the study area.

Tracer elements revealed the soil organic carbon sources in a dam-controlled watershed

Abstract: The dynamics of the soil organic carbon (SOC) pool and the mechanisms that underpin those dynamics are at the frontier of terrestrial carbon (C) cycling research. Soil erosion seriously affects the migration and redistribution of SOC, driving a critical part of the C cycle. In this study, soil samples were collected from a 14.69 m sediment profile formed between 1960 and 2017, which was located in a small agricultural catchment on the Loess Plateau in northwest China. Nuclear tracer elements (137Cs, 210Pb) and stable C isotopes (δ13C) from organic matter were used to quantitatively identify the SOC sources in different siltation stages. The method used assumes that the contribution of a source is proportional to the elemental concentration in that source. SOC in soil samples from different erosion source areas (farmland, forest land, grassland, shrub land and ditch wall) were compared against the deposit in the check dam. The concentration of SOC in the sediments of the check dam was between 1.21 g kg−1 and 4.03 g kg−1. Nearly half of the SOC in the check dam sediments was traced back to the ditch wall. Farmland contributed 24.8 %, while shrub land contributed the least SOC at 0.4 %. SOC erosion attributed to farmland decreased gradually, while the relative contribution from the ditch wall increased. Within the agricultural catchment area, there was a net migration of SOC from the various land cover types (the source) to the sedimentary area behind the dam (the sink). These results indicate that a dynamic balance exists between redistribution and heterotopic storage of SOC during sedimentation. This study provides an opinion for sediment source identification and the soil C cycle in watersheds on the Loess Plateau.

Modeling and Mapping of Soil Water Erosion Risks in the Srou Basin (Middle Atlas, Morocco) Using the EPM Model, GIS and Magnetic Susceptibility

Abstract: Abstract The Oued Srou watershed located in the Middle Atlas Mountain of Morocco has been a subject of serious soil erosion problems due to the combination of natural factors and anthropic activities. Therefore, soil erosion hazard assessment and mapping can be handy to initiate remedial measures in the area. In this study, the improved Erosion Potential Model (EPM) integrated with GIS and remote sensing techniques is employed to map and assess the vulnerability of the Oued Srou watershed to the water erosion phenomenon and its impact on the silting of the Ahmed El Hansali dam. The results of the EPM model showed that the maximum annual soil loss rates were in the range of 5-652 m3/km2/year, with an average of 49 m3/km2/year. The delivery coefficient ratio showed that about 34433 t/year of the sediments reach the outlet of the watershed. The correlation analysis between all erosion factors revealed the following order of their importance in the water erosion control: soil sensitivity to erosion, soil protection, slope, erosive state, temperature, and rainfall. The magnetic susceptibility provided results on the evolution of soils; it showed that the most degraded soils had a high erosion rate. Generally, the stable soils not eroded showed an upward increase of magnetic susceptibility values in soil profiles; the evolution of magnetic susceptibility of degraded soils is disturbed. The magnetic susceptibility has also made it possible to highlight the source zones of sediments that reach the outlet of the watershed.

Influences of multiple anthropogenic disturbances coupled with a tailings dam rupture on spatiotemporal variation in fish assemblages of a tropical river

Abstract: The Mariana disaster, resulting from the rupture of the Fundão tailings storage facility (TSF) in 2015, was a large-scale environmental disaster that drastically affected the entire channel of the Rio Doce, southeastern Brazil. The disaster produced substantial channel siltation, increased turbidity, riparian damage and a massive fish kill. However, the Rio Doce basin has a long history of environmental degradation, including hydroelectric dams. The latter are major threats to the ichthyofauna because they affect flow regimes, trap sediments and large wood, select generalist species, impair fish migrations and favour non-native invasive species. In this study, we analysed the effects of the rupture as well as impacts from the construction of a run-of-the-river hydroelectric dam on native and non-native fish abundances, and species richness and composition in the middle Rio Doce. To do this, we examined fish survey data collected before and after each disturbance upriver of the reservoir, in the reservoir area and downriver of the dam. The data were separated into three temporal intervals: free-flowing river, post-filling reservoir and post-TSF disaster. To assess temporal changes in native and non-native fish species richness, we used a rarefaction method and tested it with two-way ANOVA, which also was used to compare fish abundances. To test temporal changes in fish assemblage composition we used a PERMANOVA test. We observed no significant differences in total native or non-native species richness as a result of the dam or the disaster. However, we did observe increased non-native individual abundances after the TSF rupture as well as native individual abundances, depending on the location studied. The assemblage composition also changed after dam construction and the TSF rupture, except for the upriver reach after dam construction. We conclude that both the dam and the TSF rupture affected fish assemblages, favouring increased non-native abundances, and also changed fish assemblage composition. Our results suggest that a century of landscape and riverscape pressures had extirpated the sensitive fishes from the Rio Doce before the TSF rupture. We conclude that multiple long-term chronic disturbances can be as harmful to aquatic biota as the acute effects of a major environmental disaster.

Constraints to climate change adaptation and livelihood challenges: perspectives from the Sundarbans fishers' community in Bangladesh

Abstract: Fishers' are first-line observers of changes in the Sundarbans region and are among the first to be affected by the changes that occurred. In the Sundarbans fishers' communities, transformations of society have always been a part of life. In contrast, environmental changes were always interim and reversible, allowing them to understand and identify with the Sundarbans ecosystems as food and life providers. In this study, fishers' observations on climate change impacts and their livelihoods were compiled and analysed using a structured questionnaire in accordance with the tenets of grounded theory. The observations of fishers from the region of the Sundarbans demonstrated a rise in the frequency of extreme weather events caused by rising temperatures and changes in the weather pattern. Physical components such as rainfall, coastal erosion, sea-level rise, siltation alterations in fish species distribution ranges, and migratory spawning behaviours were also affected by changes in the region's weather and climate. As salinity levels rose, the diversity and productivity of river ecosystems were affected, particularly in the most vulnerable areas. As a result, river freshwater flow decreased over time. A high rate of siltation in rivers and river mouths was considered another major concern that led to seasonal flooding due to its low freshwater upstream flow rate. The Sundarbans region provides a vast array of resources for diversifying fishers' livelihoods, but climate change is diminishing these alternatives due to more frequent catastrophic events. Specifically, climate change limits the resilience of fishers' communities, restricting opportunities for diversification or forcing them to leave their homes or villages. Climate change generated an environment that was generally unfavourable for all fishing communities. In order to survive in an unfavourable environment, the social well-being of fishers (mostly women and children) was negatively impacted by a variety of challenges, including disease, lack of potable water, malnutrition, sanitary difficulties, lack of electricity, lack of food and clothing, lack of proper medical care, and so on. To evaluate the effects of climate change on fisheries in the study area, the biodiversity, abundance, and production of most freshwater species are drastically reduced due to the destruction of spawning grounds, a transition in the spawning season, and the obstruction of fish migration. The findings of this study show that the climate and livelihood conditions of fishers in the Sundarbans region have changed significantly over the past few decades. Regardless of GOs and NGOs taking the required steps, proper implementation of interdisciplinary adaptive policy and regular monitoring in the Sundarbans fisher's community in Bangladesh could effectively reduce climate change impacts and improve livelihood conditions. Asian J. Med. Biol. Res. 2022, 8 (2), 103-114