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Among the most characteristic offsite effects of soil erosion, reservoir sedimentation is one of the longlasting problems pertaining to storage capacity and dam operations. In this work, sedimentation rates of the Fortezza reservoir in South Tyrol (Italian Alps) are analysed for nine erosive events over 14 months. Sediment rates are calculated through a modified Universal Soil Loss Equation (USLE) combined with a Sediment Delivery Ratio (SDR) module at 2.5 m resolution. The topographic factor is adjusted to the Alpine terrain, and an additional factor is included to consider the nonerodible parts of the basin. The results are validated against a 14-month sediment yield obtained by high-resolution (0.2 m) reservoir bathymetric analyses. The calculated sediment yield has an absolute deviation of 4.8% from the measured one. The results indicate that these methods can be successfully applied for the estimation of reservoir sedimentation rates in the Alps. [ABSTRACT FROM AUTHOR]

We investigated the magnitude of reservoir sedimentation upstream and its sediment-shadow effect on a large downstream-linked lake, Lake Tana (Ethiopia). To better understand the reservoir-induced reductions of fluvial sediment downstream, sediment budgets for two scenarios were developed. The results indicated that reservoir sedimentation is extremely high, with annual storage capacity losses (1–4.2%) exceeding the global average. Consequently, unless mitigation measures are implemented, the reservoirs' dead storage volume will be filled with sediment in less than 42% of their designed life expectancy. The sediment-trapping conditions created by the reservoirs will also reduce sediment inflow from tributaries into the lake by 15–52%, overbank sedimentation on floodplains by 33–48%, and sediment deposition in Lake Tana by 16–54% annually. Overall, under the current environmental set-up, the ongoing and planned reservoir constructions could extend the expected useful lifetime of Lake Tana from 918 to 2022 years. [ABSTRACT FROM AUTHOR]

This study aimed to investigate the critical suction velocity required for lifting sediment off the bed under hydrosuction. Flow characteristics below the suction pipe under unbound and bound conditions were studied experimentally and numerically, and the effects of various parameters on the critical suction velocity, such as suction pipe diameter, suction discharge, suction inlet height, and sediment size, were investigated. The results showed all the parameters significantly affect the critical suction velocity, with suction inlet height being the most influential. Unbound and bound conditions yielded divergent flow characteristics beneath the suction pipe, revealing distinct flow patterns. Interdependency among the parameters affecting critical suction velocity have been studied statistically, and empirical relations are developed to compute the critical suction velocity, its resultant centerline velocity, and the associated flow profile. A proposed relation for critical suction velocity showcased a ±10% error margin, while equations computed resultant centerline velocities and flow profiles with ±15% accuracy, representing satisfactory agreement. These findings can also be helpful in designing efficient suction systems in various sediment-laden water environments. Practical Applications: The current study on hydrosuction sediment removal presents practical solutions for addressing the persistent challenge of reservoir sedimentation. Hydrosuction is an efficient and cost-effective method of sediment removal with minimum disturbance to the connecting structures and surrounding ecosystem and aquatic life, ensuring sustainable reservoir management. The study focuses on providing detailed insight into the behavior of the flow below the suction pipe under varying conditions, which is responsible for sediment movement during hydrosuction. The study also investigates the minimum suction velocity required to lift the sediment off the bed surface. Applicability of hydrosuction is not limited to the desilting of a reservoir; it also holds potential for various applications in multiple fields, such as river and canal restoration, dredging of navigational channels, irrigation canal cleaning, dewatering and slurry removal, contaminant cleanup, trench excavation, flood control, and wastewater management. [ABSTRACT FROM AUTHOR]

• A coupled model is proposed integrating a 2D double layer-averaged model for reservoir and a 1D model for bottom tunnel. • High water level, small cover layer thickness, short tunnel length and steep tunnel slope favour the occurrence of sediment flushing. • Low water level, large cover layer thickness, long tunnel length and gentle tunnel slope are more likely to cause tunnel blockage. Sediment flushing through a bottom tunnel is one of the most effective methods to alleviate reservoir sedimentation. However, the multi-physical hydro-sediment-morphological processes of reservoir sediment flushing through an initially covered bottom tunnel intake have remained poorly understood, and a physically enhanced and practically viable mathematical model is required. The present study uses an integrated model to resolve sediment flushing through an initially covered bottom tunnel intake. The proposed model couples a two-dimensional double layer-averaged model for the reservoir with a one-dimensional model for the bottom tunnel by means of numerical fluxes calculations at the inner boundary. The governing equations are solved synchronously using a well-balanced finite volume method. Several parameters related to boundary resistances and sediment exchange flux need to be determined for model closure. The model is tested against data from a series of physical experiments on reservoir sediment flushing, with sound agreement achieved between computed and measured scour hole geometries. Moreover, the present model successfully predicts the occurrence of tunnel blockage. The results prove that reservoir sediment flushing is best accomplished for high reservoir water level, small cover layer thickness, short tunnel length and steep tunnel slope. The present model facilitates reservoir design and operation to help preserve reservoir capacity. [ABSTRACT FROM AUTHOR]

Reservoir sedimentation is of worldwide concern, involving not only reservoir benefits, but environmental and ecological issues. Sediment flushing is one of the most common and effective methods for alleviating sedimentation and its negative impacts. To date, however, it has remained unknown whether sediment flushing can be accomplished through a bottom tunnel once its intake has been covered by sediment deposits. Here, a total of 65 runs of flume experiments were conducted to investigate the threshold conditions for occurrence of sediment flushing. The results demonstrate that higher water level, smaller cover layer thickness, shorter tunnel length, and steeper tunnel slope favor the occurrence of sediment flushing. Physically, as the gate located at the tunnel intake is opened, the sediment deposits immediately upstream of the intake collapse and enter the tunnel, entraining water. A mass of hyperconcentrated water-sediment mixture is generated, in which the amount of sediment depends on the cover layer thickness. If the water level is sufficiently high, the cover layer thickness is low, the tunnel slope is steep, or the tunnel is short, the pressure difference between the inlet and outlet of the tunnel suffices to drive the water-sediment mixture to move down to the outlet, overcoming the resistance due to the tunnel boundary. This way, sediment flushing is accomplished. Based on dimensional analysis of the observed data, a threshold condition is proposed for the occurrence of sediment flushing. It is also shown that an equilibrium scour hole is formed immediately upstream of the tunnel intake, and an empirical model for the scour hole geometry is proposed. The present findings can be exploited to improve sediment-rich reservoir planning, design, and operation for benefit maximization and eco-environmental wellbeing. [ABSTRACT FROM AUTHOR]

Sediment Bypass Tunnels (SBTs) are proven to be an effective measure to reduce or even stop reservoir sedimentation by bypassing sediment laden flows around reservoir dams to the downstream river reach. They are mostly used in Switzerland, Japan, and Taiwan. However, hydraulic and sedimentological operating conditions and the resistance of the invert materials against hydroabrasive erosion affect their cost-effectiveness. Hydroabrasion is a pressing issue at SBTs, other hydraulic structures and steep bedrock rivers exposed to high sediment transport rates under supercritical flow conditions. The present study was therefore conducted to address this issue by aiming at improving knowledge on abrasion mechanics and calibrating a mechanistic saltation abrasion model enhanced by Demiral-Yüzügüllü (2021). To this end, the abrasion resistance of fourteen different invert materials installed at Solis, Pfaffensprung and Runcahez SBTs in Switzerland was quantified by annual 3D laser scanning and the hydraulic conditions and sediment transport rates were regularly monitored between 2017 and 2021. The analysis of invert scans and hydraulic conditions revealed that Prandtl's first and second kinds of secondary currents occurring in the bends and straight sections of the SBTs, respectively, and the observed abrasion patterns were strongly interrelated. The tested potassium aluminate cement and steel fibre concretes, granite, cast basalt and steel plates had better abrasion resistance against impact of sediment-laden flows compared to other materials. Sediment mineralogical composition i.e., bulk hardness relative to the invert material properties significantly affected hydroabrasion. The enhanced abrasion prediction model was calibrated with the present data and a quasi-constant abrasion coefficient of k v = (4.8 ± 2.2) × 104 was obtained. The enhanced model is well-suited for both laboratory and field scales. The present findings will contribute to the sustainable utilization and operational safety of hydraulic structures, optimization of SBT and reservoir operations regarding bypassing efficiency and reservoir lifetime and modelling of bedrock river erosion. [ABSTRACT FROM AUTHOR]

Sedimentation has a prominent impact on the functionality and lifetime of reservoirs and is a growing concern for stakeholders. Various parameters influence sedimentation caused by soil erosion. Here we have examined fifty Italian reservoirs to determine sedimentation rates and storage capacity loss. The reservoirs studied have an average age of 78 years as of 2021, with the highest loss of capacity observed, equal to 100%, for Ceppo Morelli. For the fifty Italian catchments covering north, south, central and islands of Italy, we found the mean annual sediment yield varying between 17–4000 m3/km2. year. Six of fifty reservoirs studied (Quarto, Colombara, Ceppo Morelli, Fusino, Vodo and Valle di Cadore) are already in a very critical situation in terms of storage capacity loss. Out of the fifty reservoirs, half of them will reach their half-life year by 2050. For example, for the Fusino reservoir located in northern Italy, we observed a loss of 90% of the storage volume as of 2020 with respect to its operation year 1974, compared to 6% in 2015 as available in literature. Modelling the sediment delivery ratio (SDR) is an open question, due to the lack of adequate data and uncertainties about the variability in hydrological, geomorphological, climate and landcover parameters. Here, we addressed the issue with a simplified multiple regression approach based on sediment delivery ratio values retrieved by the RUSLE model. We found different multi regressions for reservoirs belonging to the Alpine and Apennine regions. This analysis offers a starting point for the management and prioritization of adaptation and remediation policies necessary to address reservoir sedimentation. [Display omitted] • Reservoir sedimentation, sediment yield and sediment delivery ratio of Italian dams have been investigated. • Sediment yield from the catchment and the sediment deposited have been calculated and compared. • 17 out of 50 reservoirs examined (34%) have already reached their half-life year. • Sediment mitigation measures were not applied in Italy according to past legislations. [ABSTRACT FROM AUTHOR]

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The sediment-carrying capacity is reduced due to the rise of water level and the reduction of velocity, which leads to sedimentation in the reservoir, especially in front of the dam. Therefore, the storage capacity, flood control ability, and power generation are reduced, which affects reservoir functions and economic efficiency owing to reservoir sedimentation. The main measures to solve the problem of reservoir sedimentation include dredging in reservoir, reducing inflow sediment realized by soil protection and vegetation, as well as reservoir operation management such as "storing clear water and discharging muddy flow", and sediment discharging by heterogeneous flow, etc. In this paper, a normal funnel device with the smooth surface that has arbitrary order derivative is designed for discharging sediment in reservoir. The Euler two-phase flow near the normal funnel is numerically simulated with the Realizable k-e turbulence model on rectangle and O-type grids. Numerical model is tested by data on experiment flow that carried out in flume. The flow velocities, vorticities and sediment phase concentrations are analyzed under different boundary conditions. Research shows that velocity gradient is large in normal funnel, and phase concentration contours display the V-shaped asymmetrical distribution. Three large vorticity areas appear in the front edge, the left and right rear sides of the normal funnel. The large flow velocities and small resistance appear near the wall of the normal funnel. The average sediment concentrations at the output of the normal funnel are 0.462, 0.521 and 0.558 under three boundary conditions. The average vorticity is reduced by 25.8%-27.2%, the efficiency of discharging sediment remarkably increases with 15.2%-16.5% in contrast to the cone funnel. It demonstrates that the normal funnel is more favorable than the cone to smoothing flow, reducing energy dissipation, enhancing the capacity of sediment transport and discharging sediment outwards reservoir. [ABSTRACT FROM AUTHOR]

Lago Loíza impounded in 1953 to supply San Juan, Puerto Rico, with drinking water; by 1994, it had lost 47% of its capacity. To characterize sedimentation in Lago Loíza, a study combining land-use history, hillslope erosion rates, and subbasin sediment yields was conducted. Sedimentation rates during the early part of the reservoir's operation (1953–1963) were slightly higher than the rates during 1964–1990. In the early history of the reservoir, cropland comprised 48% of the basin and erosion rates were high. Following economic shifts during the 1960s, cropland was abandoned and replaced by forest, which increased from 7.6% in 1950 to 20.6% in 1987. These land-use changes follow a pattern similar to the northeastern United States. Population in the Lago Loíza Basin increased 77% from 1950 to 1990, and housing units increased 194%. Sheetwash erosion measured from 1991 to 1993 showed construction sites had the highest sediment concentration (61,400 ppm), followed by cropland (47,400 ppm), pasture (3510 ppm), and forest (2050 ppm). This study illustrates how a variety of tools and approaches can be used to understand the complex interaction between land use, upland erosion, fluvial sediment transport and storage, and reservoir sedimentation. [ABSTRACT FROM AUTHOR]

Sediment trap efficiency (TE), defined as the sediment trapped in a reservoir divided by the sediment flowing into the reservoir, or the sediment delivery ratio (SDR=100%-TE), has been studied for small or ponded reservoirs, but for large reservoirs it is still not well understood even though it is important for predicting and managing reservoir sedimentation. In this case study, we analyzed the changes in water and sediment regime caused by the operation of two cascade dams (the Sanmenxia and Xiaolangdi Dams just upstream of the lower Yellow River) during 2000–2020. We found that more than 50% of the water discharge (Q) entering and flowing out of the reservoirs was smaller than 1,000 m3/s , while over 85% of the sediment was transported by larger discharges. Among different ranges of Q , the range 2,000–3,000 m3/s transported the most sediment (30%–50%). Due to the Water and Sediment Regulation Scheme of the dams, the maximum sediment concentration of floods tended to lag behind the maximum water discharge. The longer the lag time, the greater the water volume released to the downstream, resulting in a higher SDR of floods. A calculation method for the SDR at the Xiaolangdi Reservoir during floods and flood seasons was proposed by considering the water retention time in the reservoir. The sedimentation delta in the Xiaolangdi Reservoir changed rapidly after the dam closure and gradually became stable, and its geometric changes also impacted sediment transport through the reservoir. This study may shed light on sediment transport through large reservoirs under the operation of water and sediment regulation. [ABSTRACT FROM AUTHOR]

Sediment deposition at the bottom of artificial reservoirs has become a worldwide problem. This comprises a dual issue that is, in the first place, associated with the reduction in storage capacity and lifetime of large reservoirs. The second aspect comprises the threat that the sediment represents for the dam structure. This research is mainly aimed at identifying and inferring the main sediments' triggering processes through a rigorous analysis of deposition shapes in a large reservoir. For identifying the main deposition shapes, a sequential methodology was designed and developed comprising the following stages. First, an analysis of XYZ cartography from bathymetric development was conducted. Then, a shapes categorization was developed that comprises the identification of six types of shapes based on four parameters: slope continuity, slope break, absolute and relative slope, and arc configuration. The third stage comprised a visualization and spatial calculation of shapes through GIS-based cartography. The fourth stage comprised an interpretation of deposition shapes processes: for that, a dual analysis was developed. First, an analysis based on fluvial sediments transport processes was realized. The second stage implied an analysis of the dam influence on fluvial hydrodynamics and sediments transport. Results comprised a quantitative assessment of each shape as well as physical processes identification and interpretation, generating a robust equivalence between shapes and triggering processes. This research proved successful for the identification and characterization of the main deposition and transport processes that may help to prevent, palliate, and/or correct phenomenon of silting in large reservoirs. This detailed knowledge of deposition forms opens new strategies to release sediments from storage water more effectively. [ABSTRACT FROM AUTHOR]

Abstract: Extreme events, such as large storms and floods, have a major effect on the transport of sediment and contaminants and, hence, on water quality. To be realistic, models designed to predict the short-term transport and fate of sediment and contaminants must include large floods and storms, an idea not incorporated into existing models based on uniform sediment flux. Sediment transport in and to river channels in mountainous terrain is strongly influenced by precipitation, particularly when heavy precipitation triggers a surface landslide in the catchment area. This study provides a quantitative measure of such sediment transport. The Yahagi Dam watershed was modeled using a 50-m grid-based digital elevation model (DEM), and flow directions were determined for each of the slopes present. The study objective was to develop a general algorithm for simulating water flow and sediment movement in a dammed mountain basin using water and sediment dynamics modeling. The nonuniform flow model used for stream channels can incorporate energy loss around dam reservoirs, and the resulting algorithm can estimate all phenomena related to sediment movement. The new method was verified by applying it to the Tokai rainfall disaster and the Yahagi Dam basin, Japan. In this basin, the volume of bedload sediment deposited is 993×103 m3/y, suspended-load sediment is 669×103 m3/y, and wash load sediment is 900×103 m3/y, as determined from sediment production in each tributary of the Yahagi River. Most sediment is probably introduced during extreme flood events owing to lithological factors, and thus extreme rain events that now accompany climate change are very important in sediment supply. During extreme flood events, the river''s effective tractive force (τ*e, d90) exceeds the critical tractive force (τ*c), especially upstream of the Yahagi Dam. Rainfall-related sediment disasters can be accurately predicted using water- and sediment-dynamics models that are founded on realistic geomorphological characteristics and sediment flux of rivers. [Copyright &y& Elsevier]

The Dongsha area, a key region in the northern South China Sea (SCS), features both diffusive deep and seepage shallow gas hydrate reservoirs. Utilizing sediment samples from gas hydrate reservoirs and adjacent layers at sites W08 and W16 in the Dongsha area, this study aims to uncover the sediment property differences between deep and shallow gas hydrate reservoirs and their impact on gas hydrate accumulation through grain size, X-ray diffraction, and specific surface area (SSA) analyses. The findings classify the study intervals into four distinct layers: shallow non-gas hydrate layer (shallow-NGHL), shallow gas hydrate reservoir (shallow-GHR), deep non-gas hydrate layer (deep-NGHL), and deep gas hydrate reservoir (deep-GHR). In the clayey silt sediment reservoirs, grain size has a minor influence on gas hydrate reservoirs. Both shallow and deep NGHLs, characterized by high smectite content and SSA, possess a complex structure that impedes gas and fluid migration and offers limited potential reservoir space. Consequently, both shallow and deep NGHLs function as sealing beds. The deep GHR, having low smectite content and SSA, exhibits a strong capacity for gas and fluid migration and greater potential reservoir space. As a result, sediment properties significantly influence the deep GHR. Seepage primarily controls the shallow GHR. [ABSTRACT FROM AUTHOR]

The study compares the structure of invertebrate communities inhabiting two types of bottom sediments in Krzynia Reservoir, known also as Lake Krzynia (on the Słupia River, northern Poland). The lake is prone to intensive sedimentation because it is limnic. This limnicity is due to the impact of the river, rather than to surface runoff from the catchment area, where the heavy predominance of natural habitats over transformed ones slows the transport of mineral suspensions from adjacent terrain. In the organic-poor sediments dominated by sand and gravel, the mean density of benthic invertebrates was ten-fold higher than in silty, organic-rich sediments, while invertebrate biomass was only slightly and non-significantly higher in organic-poor sediments than in organic-rich sediments. Simultaneously, the mean invertebrate body weight was markedly lower in organic-poor sediments (0.03 g) than in organic-rich sediments (0.18 g). No significant differences in taxonomic diversity (number and Shannon index of taxa) were found between the studied sediment types in the analysed communities. Among the major invertebrate groups, the Oligochaeta and Mollusca significantly differed in density between sediment types, while the Oligochaeta and Chironomidae differed in biomass. An increased percentage of silt fraction in bottom sediments resulted in a drastic decrease in abundance of benthic fauna, but their biomass declined only slightly. Thus, plans to construct new reservoirs should aim to reduce the rate of silt accumulation in them, in order to preserve their primary function and support biodiversity in transformed river valleys. [ABSTRACT FROM AUTHOR]

Presents information on a study which analyzed the uncertainty of annual reservoir sedimentation and accumulated reservoir sedimentation. Estimation of annual and accumulated reservoir sediment load and volume; Uncertainty of reservoir sedimentation; Application; Summary and conclusions.

This study aimed at determining the rate of sedimentation in Opa Reservoir since impoundment in 1978 with a view of ascertaining how much of its volume has been silted up. The volume of sediment trapped in the reservoir since impoundment was estimated using the bathymetric survey method. Based on reservoir profiling, the volume and mass of deposited sediments since impoundment in 1978 were estimated at 1 313 397m³ and 893 096 tonnes, respectively. The rate of sedimentation and specific sediment yield from the reservoir catchment were estimated at around 38 000t/yr and 575t/km²/yr, respectively. The annual rate of sedimentation obtained in this study highlights the need for more frequent bathymetric surveys especially after large floods to provide more information on the magnitude of annual sedimentation, and to help planners determine how to prolong the economic life of reservoirs. [ABSTRACT FROM AUTHOR]

Reservoir sedimentation is a long-term process with various risks, such as reduction of storage capacity, hydropower generation and flood control functions throughout a reservoir's lifespan. For reservoirs facing loss of function with severe deposition, sediment flushing is an approach to preserving long-term storage. Here, submerged vanes combined with pressure flushing operations are proposed to facilitate scouring of such deposition. Experiments with various submerged vane heights, inclination angles on the flow direction, spacing, and divergent and convergent arrangements were conducted to investigate the scour cone volume and removal efficiency. Results showed that two divergent submerged vanes at an angle of 20° on the flow direction and optimum vane spacing ratios removed the deposited sediment significantly. In the presence of the submerged vanes, final sediment flushing volume and efficiency increased by a factor of 48 compared to the reference test. Using the experimental results, three equations were proposed to estimate the scour cone dimensions (length, width and depth). Additionally, a non-dimensional equation was developed for estimating the scour cone volume, using a polynomial correlation with vane spacing. The results of this study recommend the best installation of submerged vanes for releasing deposited sediments in the region near the bottom outlets of reservoirs. [ABSTRACT FROM AUTHOR]

The storage volume of reservoirs is highly susceptible to the effects of sedimentation. This study investigated the Wushe reservoir in Taiwan, the volume of which dropped to just 31.9% of its original capacity in 2016. Our focus is the improvement of desilting operations via scale model experiments involving a sediment bypass tunnel. The experiment parameters included inflow and outflow boundary conditions, sediment discharge, and the shape of particles used in the scale model (i.e., matching those in the real-world location). This study also performed numerical modeling to select upstream inflow boundary conditions for the experiment. The results revealed that the proposed desilting strategy using a sediment bypass tunnel would not only be more effective than the existing outlet in terms of sediment release, but would also provide economic benefits over dredging. The return on investment should be evident within 8–21 years, depending on the scale of the typhoons that strike. There seems little doubt that a sediment bypass tunnel is the best approach to preserving reservoir storage capacity and relieving concerns of an impending water crisis. [ABSTRACT FROM AUTHOR]

Energy and water are the two most important natural resources in the globe. In this regard, dams and reservoirs are the critical hydraulic structures that store water and, above all, provide energy required by humanity. However, water storage and the provision of energy by reservoirs and dams have been disrupted by significant environmental changes taking place in the catchment areas and the reservoir environment. These disruptions are brought about by climatic parameters and sediment transport by different eroding agents. One such environmental problem is soil erosion, whose effect is reservoir sedimentation. Consequently, a part of the transported sediment is deposited at the catchment outlet, which serves as the reservoir inlet. This study was carried out to establish the physicochemical characteristics of the deposited sediment at the reservoir inlet. The following parameters were analyzed: particle size distribution, organic matter content, bulk density, porosity, electrical conductivity, penetration resistance, hydraulic conductivity, pH, and nutrients (nitrogen, phosphorous, and potassium) using standard laboratory procedures. The study established that the deposited sediments were predominantly sand particles with mean values of 50.60% and 58.60% for the surface (0–10 cm) or sub-surface horizons (10–20 cm), respectively. The average values for sediment pH, organic matter, porosity, bulk density, electrical conductivity, penetration resistance, hydraulic conductivity, and nutrients were 6.30 and 6.61; 1.91 and 1.80%; 54.10 and 57.10%; 1.22 and 1.14 g·cm−3 for the surface and sub-surface horizons, respectively. The most variable parameters were silt content (sub-surface horizon), hydraulic conductivity, penetration resistance, electrical conductivity, nitrogen content (surface horizon), and phosphorous (surface horizon) content with CV >0.35. Based on the present study results, the deposited sediments at the reservoir inlet were found to have low concentrations of nutrients and high sand proportions. Therefore, the deposited sediments appear to have great potential to reclaim the immediate barren dam environment upon enrichment and to promote sand harvesting programs for economic benefits. [ABSTRACT FROM AUTHOR]

After the impoundment of the Three Gorges Reservoir,the increase of water depth caused the increase of the lag time of the sediment peak behind the flood peak in the flood propagation. The asynchronous movement characteristics between the flood and sediment peaks can be utilized to conduct the sediment peak regulation,one of the crucial measures to reduce the reservoir sedimentation. The three-dimensional numerical model named SCHISH (Semi-implicit Cross-scale Hydroscience Integrated System Model) was adopted to study the asynchronous movement characteristics of the flood and sediment peaks in the Three Gorges Reservoir. The reliability and accuracy of the model were verified by comparing the water and sediment transport during the flood season in 2013,then the asynchronous movement characteristics of the flood and sediment peaks under different impounded levels were preliminarily analyzed based on the model results. The results showed that the numerical model could accurately simulate the long-distance propagation of the flood and sediment peaks. The impounded level in front of the dam had little influence on the propagation time of the flood peak,but had a significant effect on the propagation time of the sediment peak. The increase of the impounded level resulted in the decrease of the flow velocity and the sand-carrying capacity,so the propagation speed of the sediment peak was slower and the sediment peak value was smaller,which led to the longer lag time of the sediment peak behind the flood peak along the reservoir. The research results can provide scientific evidence for further optimization of the operation of the sediment peak regulation in the Three Gorges Reservoir. [ABSTRACT FROM AUTHOR]

Sediment accumulation in reservoirs across the world diminishes reservoir benefits, increases maintenance costs, and damages the environment. United States regulatory policy requires State and Federal permits for discharging sediment into waterways, which has tended to discourage alternatives that pass sediment downstream of the dam. The cost of methods that remove the sediment from the fluvial system is usually prohibitive and is a major factor precluding sustainable sediment management in the United States and other countries with similar sediment reduction policies. This paper challenges the notion that US policy requires sediment removal and terrestrial storage by highlighting seven case studies in the United States where reservoir sediment management included downstream sediment discharge. Where technically feasible, these types of solutions can lead to greater economic and environmental sustainability of US reservoirs. [ABSTRACT FROM AUTHOR]

Reservoir sedimentation management has become an important topic for large dams in the United States due to their historical design, current age, and increased environmental regulation. Less attention has been paid to small dams (hydraulic size < 0.01) in remote mountains with urgent sedimentation problems. In drier climates, such reservoirs may be frequently drained and trans-catchment flows routed over their sediment deposits heading from one mountain tunnel to another. This study asked an unexplored scientific question focusing on this special setting: how do different amounts of water transfers interact with different reservoir stages to affect sediment erosion and its redistribution in the backwater zone? Mindful timing and magnitude adjustment of water transfer, involving water diverted across watersheds by tunnels, through a reservoir were hypothesized to strategically redistribute sediment erosion for sites with water transfer/diversion facilities in the main channel. For a study site in the north-central Sierra Mountains of California, 2D hydrodynamic modeling revealed that sediment erosion within the backwater zone increased by > 100% when water transfer was maximized, involving a flow 12 times higher than mean annual discharge. With reservoir stage drawdown, the increment of sediment erosion was further increased by > 50% compared with water-transfer-only scenarios. The natural upstream inflow with daily flow occurrence of 5–25% was the optimal water transfer to avoid disturbing sediment. These results indicated that water transfer and stage drawdown optimization is a promising strategy to promote or abate redistribution of deposited sediment through a smaller reservoir. [ABSTRACT FROM AUTHOR]

Authors propose a beneficial methodology for hydrological planning in their study. Prospective evaluations of the basins' net capacity can be done using the technique presented. The HEC-HMS (Hydrologic Modelling System) software can be used to estimate in a basin, the sediment emitted. For a certain precipitation, this methodology allows estimating, within a certain range, the gradual blockage of a reservoir, and even a projected date for total blockage. This has some applications to adopt corrective measures that prevent or delay the planned blockage deadlines. The model is of the semi-distributed type, estimating the generation and emission of sediments by sub-basins. The integration of different return periods in HEC-HMS with a semi-distributed model by sub-basins and the application of a mathematical model are the differentiating element of this research. The novelty of this work is to allow prognosing the reservoir sedimentation rate of basins in a local and regional scale with a medium and large temporary framework. The developed methodology allows public institutions to take decisions concerning hydrological planning. It has been applied to the case of "Charco Redondo" reservoir, in Cádiz, Andalusia, in southern Spain. Applying the methodology to this case, an average soil degradation of the reservoir basin has been estimated. Therefore, it is verified that in 50 years the reservoir is expected to lose 8.4% of its capacity. [ABSTRACT FROM AUTHOR]

Water systems contain fine sediment as a natural phenomenon. The main cause of ecological deterioration is undesirable fine sediment. Fine sediment comprises inorganic particles of sand, silt, and clay that are less than 2 mm in diameter. Fine sediment is considered to be pollutant in water bodies (rivers, streams, lakes, and reservoirs). Sediment has various channel and nonchannel sources. Fine sediment can alter the hydraulics of rivers and their morphology. It has detrimental effects on reservoir storage and function, can degrade the food chain, and affects the reproduction of aquatic animals, which ultimately affects river ecosystems. The deposition of fine sediment renders waterways difficult to navigate and hinders recreational uses. It degrades the water quality for drinking purposes, wildlife, and affects the land surrounding the bodies of water. The intrusion of fine sediment is a global problem, and with time, researchers will understand the effects on biological and physical processes. Comprehension may help scientists minimize the effects of fine sediment on river ecosystems by considering various disciplines such as ecology, hydraulics, fluvial geomorphology, and hydrochemistry. Many researchers have investigated fine sediment in rivers. This paper reviews the literature, focusing on the consequences of intrusion on the river ecosystem, river health, and associated hydraulic parameters. The study of FSI is a major application in applied environmental science. It is applied to gain as complete an understanding of the physical and biological processes as possible. It has been applied to estimate the trap efficiency and to determine the live storage capacity of reservoirs. Excess fine sediment is a primary reason for the failure of water bodies, so it is important to control it and maintain necessary fluid properties such as viscosity, specific weight, and the optical properties of water. Proper control of fine sediment in bodies of water maintains both the survival and growth of plants, and the food chain of aquatic animals. It is important to control excess fine sediment to avoid turbidity in water. In brief, the topic has applications in various fields such as evaluating river health, live reservoir capacity, growth of flora and fauna, and water quality standards. [ABSTRACT FROM AUTHOR]

Sediment flushing from dams can help desilting reservoirs and reinstate the longitudinal sediment transfer continuity in rivers, but negative ecological impacts might arise. We monitored the ecological effects of a flushing event conducted from 27th May to 14th June 2019 in the Rienz River, in South Tyrol (NE Italy). Using a multi-habitat scheme approach, we collected macroinvertebrates 10 days before, and then 40 and 74 days after the completion of the operations. We selected seven biological traits—organism size, life cycle duration, mobility (dispersal and locomotion), feeding type, substrate and current velocity preference—to characterize and compare the invertebrate communities before and after the sediment pulse disturbance. Turbidity was recorded continuously for the entire duration of the event. Results indicate that invertebrate assemblages exhibited a general decrease in taxonomic richness and Shannon diversity 40 days after the event, but density and richness recovered over time. Shifts in species composition were observed in post flushing samples, with a reduction in density of sensitive species (Heptageniidae) and shredders. Post-flushing samples were generally characterized by sediment-tolerant taxa, able to cope with the new habitat conditions. Altered taxonomic and functional community composition following the flushing prevented the full functional recovery to pre-disturbance conditions. [ABSTRACT FROM AUTHOR]

Long-term and short-term assessments of sediment yield from river basins are required in planning soil conservation measures, water storage in reservoirs and prediction of morphological responses in rivers. In present study, detailed quantification of land-use land-cover and climate variabilityin two contrasting subcatchments of Upper Tapi basin, India, are reported for period 1980–2010. The pattern of measured sediment concentration, decadal and intra-annual variations of sediment yield at the outlets of boththe sub-catchments are presented and attributed with land-use land-cover and climate variability of the relevant periods. Higher sediment and water yields observed at the outlet of Burhanpur sub-catchment are due to steep and hilly topography, excessive deforestation and relatively high simple daily monsoonal intensity index (SDMII) within the sub-catchment versus Purna sub-catchment. In both the sub-catchments, maximum and minimum sediment concentrations are in July and September months respectively. Higher sediment concentration at the outlet of Purna sub-catchment is due to excessive alluvial deposits, extensive agricultural activities and generation of correspondingly less runoff within the sub-catchment. The observed reduction in sediment yield pattern in recent years is due to development of minor hydraulic structures (water area), decrease in scrub lands, increased planned agricultural land and decrease in the SDMII within both sub-catchments. The correlation analyses have indicated that total monsoonal basin average rainfall (TMBAR), rainy days (RD) and SDMII have been found to be significantly correlated with sediment yield for Burhanpur sub-catchment vis-à-vis SDMII and TMBAR for the Purna sub-catchment. Also, the sediment yield from the sub-catchments in the past have been found to be excessively higher than planned reservoir sedimentation of Hatnur reservoir, located at the outlet of the Upper Tapi basin. [ABSTRACT FROM AUTHOR]

Floods rarely occur in hyper-arid deserts and little is known about the magnitude and frequency of sediment delivery from their basins, despite their importance to changes to the landscape, infrastructures and engineering activities. Sediment yield from the Nahal Nehushtan watershed (11.9 km2) located in the Timna Valley in southern Israel was determined by assessing stratigraphic sections in its 60-year reservoir deposits. Stratigraphic correlation between event couplets allowed for quantification of sediment yields representing 13 former flow and flood events. Based on the sediment volume in the reservoir, the 29.8 t km−2 year−1 average specific sediment yield is one of the lowest among other studied warm deserts. Among the event layers, the thickest layer, deposited by a flash flood caused by a single short rain event, contributed 31% of the total sediment yield. Based on event reservoir sedimentation from watersheds located in several hyper-arid areas in the Middle East and North America, we demonstrate that sediment yield increases with drainage area as expected and mean annual sediment yield increases in hyper-arid areas with flood frequency. Our quantitative results, together with previous studies of hyper-arid areas, provide complementary evidence of fluvial sediment transport—the main landscape designer in hyper-arid fluvial landscapes. [ABSTRACT FROM AUTHOR]

Sedimentation is one of the main eco-morphological and technological challenges associated with reservoirs. Sedimentation not only reduces the functional capacity of a reservoir by filling it, but also changes downstream sediment dynamics and habitat availability for the aquatic biota. Additionally, dams hinder free bi-directional fish passage, emerging as a major threat to species of migratory fish. In the past decades, mitigation measures aimed at reducing such environmental and technological impacts have been developed. Sediment bypass tunnels (SBTs) have been shown to successfully help prevent reservoir sedimentation, whereas fish passages have been found to be potential solutions to facilitate bi-directional passage of fish. However, the construction of such structures, in particular of SBT, can be extremely costly. The development of design solutions that can function both for downstream sediment transport and up- and downstream fish passage should be considered as they can mitigate ecological deficiencies of reservoir operations while accounting for economic feasibility. Possibilities and challenges of combining SBT and fish passage were explored by bringing together a team of interdisciplinary specialists on hydraulics, sediment transport and continuity, bypassing, hydraulic structures, hydropower engineering, aquatic biology, and fish passage in a two-day workshop. Here, we present potential solutions identified during the workshop for integrating SBT and fish passage. [ABSTRACT FROM AUTHOR]

Abstract: Bathymetric and sedimentation surveys were conducted with a sonar‐based depth gauge system in two small reservoirs (Selamko and Shina) in the data‐scarce upper Blue Nile Basin, Ethiopia. Bathymetric survey data were merged within geographic information system (GIS) software to provide detailed visualization and analyses of current depths. Preimpoundment topographic information was obtained from dam owners and designers. Bathymetric differencing was used to investigate and quantify the distribution and estimate volumes of deposited sediment, long‐term average annual sediment fluxes and remaining water storage capacity. Calculated long‐term average sediment accumulation rates were used to estimate remaining lifetimes of each reservoir. Results from the depth sonar surveys and GIS analyses suggest a projected lifetime of ~7 years for Shina Reservoir, compared to a projected lifetime of ~22 years for Selamko Reservoir. It also indicated the annual average sedimentation rate for both reservoirs was greater than the global average of one per cent (1.67% for Shina Reservoir; 2.295% for Selamko Reservoir). The specific sediment yield (SSY) is relatively larger for both watersheds (2,499 and 4,333.6 t km−2 year−1 for Shina and Selamko, respectively) indicating the watersheds are degraded by any global standard. The Revised Universal Soil Loss Equation (RUSLE) model‐generated erosion magnitude was only slightly larger from the watersheds of both reservoirs, compared to the bathymetric estimates. These data and analyses also provide a baseline relevant to understanding sedimentation processes and are necessary for developing long‐term management plans for these reservoirs and their watersheds. [ABSTRACT FROM AUTHOR]

Sedimentation in dam reservoirs can cause problems that lead to loss of storage capacity and decrease in the flood control volume. Hydrosuction sediment removal is one of the methods used to remove sediments from within a reservoir using the suction energy provided by the effective head. In this study, a new tool has been developed by attaching the reservoir to a suction pipe intake point and using a simple fan blade mechanism for the hydrosuction sediment removal system. This mechanism is used to create a vortex flow to suspend the settled particles. This paper investigated the effects of the fan blade angles, effective head, and inlet height from the surface of layer particles on the performance and efficiency of fan blades hydrosuction sediment removal (FBHSSR) and hydrosuction sediment removal (HSSR) systems based on the geometric scour hole parameters. Results from the experimental tests indicated the effectiveness of the FBHSSR system, with the fan blade angles of 30°, 45°, and 60° leading to approximately 800%, 200%, and 117%, respectively, removed particles greater than those of the HSSR system. Furthermore, the maximum depth and diameter of the scour hole were increased by 206%, 200%, and 137% and 135, 112%, and 117%, respectively, for each angle. The effective head or experiment time also enhanced system performance by increasing the suction discharge, but no change was observed in terms of efficiency. The critical inlet heights for the FBHSSR and HSSR systems are 1 time and 2.54 times, respectively, more than the diameter of the suction pipe. Thus, it can be concluded that using fan blades in HSSR systems is a good approach to improve the properties of the scour hole. [ABSTRACT FROM AUTHOR]

Freshwater reservoirs are an important source of the greenhouse gas methane (CH4) to the atmosphere, but global emission estimates are poorly constrained (13.3–52.5 Tg C yr−1), partially due to extreme spatial variability in emission rates within and among reservoirs. Spatial heterogeneity in the availability of organic matter (OM) for biological CH4 production by methanogenic archaea may be an important contributor to this variation. To investigate this, we measured sediment CH4 potential production rates, OM source and quantity, and methanogen community composition at 15 sites within a eutrophic reservoir in Ohio, USA. CH4 production rates were highest in the shallow riverine inlet zone of the reservoir, even when rates were normalized to OM quantity, indicating that OM was more readily utilized by methanogens in the riverine zone than in the transitional or lacustrine zones. Sediment stable isotopes and C:N indicated a greater proportion of terrestrial OM in the particulate sediment of this zone. Methanogens were present at all sites, but the riverine zone contained a higher relative abundance of methanogens capable of acetoclastic and methylotrophic methanogenesis, likely reflecting differences in decomposition processes or OM quality. While we found that methane potential production rates were negatively correlated with autochthonous carbon in particulate sediment OM, rates were positively correlated with indicators of autochthonous carbon in the porewater dissolved OM. It is likely that both dissolved and particulate sediment OM affect CH4 production rates, and that both terrestrial and aquatic OM sources are important in the riverine methane production hot spot. [ABSTRACT FROM AUTHOR]

Sediments are sinks of nutrients in reservoirs, but may also act as temporary nutrient sources to the water column, leading to eutrophication during the warm season. Several abiotic and biotic factors at the water–sediment interface are known to influence the role of sediments as temporary nutrient sources. This study aimed at quantifying the effects of two factors (i.e., oxygenation and bioturbation) on fluxes of nutrients from the water–sediment interface of the reservoir. An experimental approach was developed in the laboratory to test three fauna conditions (no fauna, presence of tubificid worms, and chironomids larvae) and three conditions of water column oxygenation (aerobic, fluctuating, and anaerobic conditions). Chironomid larvae significantly increased concentrations of N (NH4+ + NO3−) and PO43− released from sediments by 3.7-fold and by 17-fold, whereas tubificid worms had a lesser effect (twofold for N and threefold for PO43−). Anaerobic conditions increased N by 56-fold and PO43− by 102-fold compared to the aerobic treatment. Thus, anaerobic conditions produced greater N and P fluxes than fauna. Nevertheless, fauna and anoxic conditions at the water–sediment interface should not be neglected when quantifying the role of sediments on nutrient dynamics in lakes and reservoirs. [ABSTRACT FROM AUTHOR]

This paper presents a systematic analysis of the hydraulic flushing capacity of a stored volume of water to remove sediments. This analysis is based on 90 laboratory experiments in which the volume of sediment evacuated was measured for varying initial volumes of water, three bed slopes, and three sediment sizes. The experiments consisted of the rapid emptying of a reservoir by means of suddenly opening a tilting gate downstream. This opening produced an accelerated flow which eroded the mobile bed of the reservoir. The efficacy of flushing, herein defined as the ratio of the volume of sediments evacuated to the volume of water released, increased with the initial slope, and decreased as the initial volume of water increased. In relation to the sediment size, while the results obtained for the coarse and medium sands were very similar to each other, the results obtained for the fine sand were affected by the existence of apparent cohesion in the mobile bed. In comparison to the results obtained for the medium and coarse sands, this apparent cohesion reduced the volume of sediment evacuated by a given volume of water and hence, the efficacy of flushing. [ABSTRACT FROM AUTHOR]

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Dam construction is one of the measures that is inevitable in many cases and must be done to supply drinking water, agricultural uses and electricity generation. There are many challenges to a successful dam project, and the managers of each project must consider the appropriate solutions for them. One of the studies that is done in dam design is sedimentation in dam reservoirs. The experimental area-reduction method is a very common technique that obtains the sediment distribution in depth and longitudinal profile. This technique shows that sediment accumulation is not limited to the bottom reservoirs. Sediment accumulation in a reservoir is usually distributed below the top of the protection reservoir or normal water level. In this study, the distribution of sediment in the reservoir of Karun Dam after a period of 65 years has been done using the experimental area-reduction method. Elevation-volume and elevation-area curves of the dam reservoir are obtained after the useful life of the dam and sediment deposition. The results showed that after 65 years, 106.47⋅106 m³ of sediment is deposited in the reservoir of the dam and the useful volume of the reservoir is significantly reduced. Also, up to a height of 36.4 m, the dam reservoir is filled with sediment. Therefore, no valve should be placed up to this height. [ABSTRACT FROM AUTHOR]

Soil erosion is exacerbated by unsustainable land-use activities and poor management practices, undermining reservoir storage capacity. To this effect, appropriate estimation of sediment would help to adopt sustainable land-use activities and best management practices that lead to efficient reservoir operations. This paper aims to investigate the spatial variability of sediment yield, amount of sediment delivery into the reservoir, and reservoir sedimentation in the Koga Reservoir using the Soil and Water Assessment Tool (SWAT). Sediment yield and the amount entered into the reservoir were also estimated using a rating curve, providing an alternative approach to spatially referenced SWAT generated suspended sediment load. SWAT was calibrated from 1991 to 2000 and validated from 2002 to 2007 using monthly observations. Model performance indicators showed acceptable values using Nash-Sutcliffe efficiency (NSE) correlation coefficient (R2), and percent bias (PBIAS) for flow (NSE = 0.75, R2 = 0.78, and PBIAS = 11.83%). There was also good agreement between measured and simulated sediment yields, with NSE, R2, and PBIAS validation values of 0.80, 0.79, and 6.4%, respectively. The measured rating curve and SWAT predictions showed comparable mean annual sediment values of 62,610.08 ton/yr and 58,012.87 ton/yr, respectively. This study provides an implication for the extent of management interventions required to meet sediment load targets to a receiving reservoir, providing a better understanding of catchment processes and responses to anthropogenic and natural stressors in mixed land use temperate climate catchments. Findings would benefit policymakers towards land and water management decisions and serve as a prototype for other catchments where management interventions may be implemented. Specifically, validating SWAT for the Koga Reservoir is a first step to support policymakers, who are faced with implementing land and water management decisions. [ABSTRACT FROM AUTHOR]

The bottom sediment of reservoirs has many functions. Among them, matter sorption is a very important one, and results in many side-effects on the reservoir sediment forming the water–bottom sediment system. As a result, bottom sediment can also be an indicator of anthropogenic water pollution. There is only very little knowledge of this situation in the study area. The main objective was the analysis of heavy metal accumulation in bottom sediment, as well as their ability to migrate throughout the water–bottom sediment system and their spatial distribution in the Kapshagay Reservoir in Kazakhstan. Heavy metal concentrations, in the both water samples and the bottom sediment, were determined using the atomic absorption spectrophotometric method. Surfer software was used to visualize the processes of migration and accumulation. Another objective was the development of model maps of the spatial distribution of metals in the reservoir water area, which indicated significant anthropogenic loads. It is obvious that both the transboundary inflow of the Ili River and the inflow from small rivers in the territory of Kazakhstan are the reasons for the anthropogenic water and sediment load. The results of the spectrometric analysis verify the water pollution in the reservoir, revealing increased concentrations of zinc reaching up to 10.8 µg/L and lead up to 32.7 µg/L, transported by the transboundary runoff of the Ili River and by the small rivers on the left bank into the Kapshagay Reservoir. Sediment concentrations close to the central part and dam zone of the reservoir reached the following values: zinc up to 37.0 mg/kg and lead up to 8.8 mg/kg. The results of this study indicate a significant anthropogenic load of the ecological conditions of the Kapshagay Reservoir. This is discussed and compared with other relevant studies. [ABSTRACT FROM AUTHOR]

At the intersection of natural and social sciences, interest in river sedimentary fluxes and their alteration by human activities is increasing in the context of general retreat of delta formations. Since the 1950s, the construction of large dams in the main course of rivers has produced, among other impacts, a radical decrease in sedimentary fluxes — a key factor in the worldwide sedimentary crisis. While sediment accumulates in reservoirs, the resulting sediment scarcity downstream contributes to land subsidence and coastal erosion, endangering livelihoods in low lying areas such as river deltas. Focusing on the case of the Ebro River and delta (Spain), this article builds on Katherine Dawson's notion of sedimentary justice to examine the articulation of a social demand to mobilise the sediments trapped in reservoirs to counter delta subsidence and coastal erosion. Using a historical perspective and paying attention to matters of both sediment quantity and quality, we show how this demand has developed hand in hand with the emergence of a scientific interpretation of the Ebro delta's origins that emphasises non-human forces and the importance of sedimentary fluxes before human interferences. Activists have taken up this argument to demand the restoration of sedimentary fluxes, exposing corporate hydropower interests and insisting on the mobilisation of sediment trapped in reservoirs. Overall, by scrutinizing the power relations embedded in the immobilization of river sediments, this article contributes to recent work that integrates the materiality of sediment to the hydrosocial framework of analysis and argues that sedimentary justice provides new avenues to incorporate the non-human world to the literature on environmental and water justice. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sediment bypass tunnels (SBT) deliver sediment downstream of dams. • Benthic invertebrate communities after SBT installation were monitored for 16 years. • A dramatic recovery within a few years was suggested based on community similarity. • Filterer and large-sized taxa were replaced by scraper and small-sized taxa. • Additional actions are needed to maintain habitat heterogeneity in the long-term. A sediment bypass tunnel (SBT) is a dam facility used to mitigate reservoir sedimentation and is expected to contribute to river ecosystem restoration by delivering sediment to the degraded downstream channel. We analyzed past monitoring data of benthic invertebrates sampled downstream of the Asahi Dam in Nara Prefecture Japan to understand the long-term changes in the invertebrate community after the installation of the SBT. Before the SBT installation, the reaches upstream of the dam (one Up site) and downstream of the dam (two Down sites) showed different invertebrate communities, dominated by invertebrates that preferred clean cobble beds, sand and gravel layers, or run habitats in the former and invertebrates that preferred stable or porous beds or pool habitats in the latter. A significant increase in the Up–Down site community similarity within a few years after the SBT installation indicated a rapid recovery of the invertebrate community at the Down sites. During the 16 years of the after-SBT period, the total density and taxon richness at the Down sites increased in the early half, while they decreased in the latter half due to bed changes associated with the continuous accumulation of sediments. The change in dominant invertebrates from taxa of the large sizes to those of the small sizes and the increase in the interhabitat community similarity suggest that the bed of the Down sites became finer and flatter by sand and gravel that filled interstitial spaces of cobbles in the late stage. While the invertebrate community at the Down sites recovered dramatically by the sediment supply through the SBT, additional actions (e.g., cobble augmentation, installation of flow guiding structures) to maintain a balanced sediment size distribution and developed pool-riffle structures would be necessary to maintain the potential habitat heterogeneity and biodiversity in the long term. Our study also demonstrated that invertebrates responded to the sediment supply according to their habitat preferences, which suggests that the classifications of invertebrates by habitat-related traits (e.g., bed-residence types and pool-riffle types in this study) would help to properly understand the sediment and bed status of river reaches under channel degradation or aggradation. [ABSTRACT FROM AUTHOR]

Rich ecosystems such as estuaries and brackish lakes are vulnerable to the effects of human activities and are prone to environmental changes. In particular, the salt environment, which is the backbone of the environment, might be affected not only by direct modifications such as dredging but also in ways that were not initially envisioned. Ichkeul Lake, located in the northern part of Tunisia, is a shallow brackish lake registered as a UNESCO World Natural Heritage Site. The construction of reservoirs upstream of the inflowing river in the 1980s reduced the amount of freshwater inflow. That only had been thought to be the cause of the increase in salinity in Ishkeul Lake. On the other hand, the sedimentation in the reservoirs upstream was remarkable, and the supply of sediment from the reservoirs upstream to the Ichkeul Lake was stopped. Changes in sediment outflow may have reduced lakebed altitude and enhanced seawater intrusion. However, the environmental protection measures for the lake so far have focused only on improving the water budget and have not been quantitatively evaluated for sediment transportation. In this study, we first estimated the water budget of the lake. Then the re-suspension by wind disturbance in the lake was estimated from the correlation with the wind speed based on laboratory experiments and field measurements. The outflow of the sediment estimated with these two models was compared with the sediment volume trapped in the upstream reservoirs that would have flowed into the lake if there had been no construction of the dams. Based on this, we evaluated whether the lake is currently erosive or cumulative. As a result, the estimated annual outflow of sediment to the sea was 4300 tons/year. It was estimated that the construction of the reservoirs upstream changed the trend of lakebed height from accumulation to erosion. [ABSTRACT FROM AUTHOR]