Reservoir Sedimentation Research Articles

Spatiotemporal patterns and co-occurrence patterns of dissimilatory nitrate reduction to ammonium community in sediments of the Lancang River cascade reservoirs.

Dissimilatory nitrate reduction to ammonium (DNRA) is an important nitrate reduction pathway in freshwater sediments. Many studies have focused on the DNRA process in various natural habitats. However, the joint operation of cascade reservoirs will affect the physical and chemical properties of sediments, which may change the DNRA process and bacterial community pattern in the surface sediments of cascade reservoirs. Our study was the first to investigate the spatiotemporal distribution patterns of potential DNRA rate, nrfA gene abundances, and DNRA bacterial community diversity in surface sediments of the Lancang River cascade reservoirs. The results of slurry incubation experiments combined with the 15N isotope tracer experiment ascertained that the potential rates of DNRA were 0.01-0.15 nmol-N cm-3 h-1, and qPCR results indicated that the abundance range of nrfA was 1.08 × 105-2.51 × 106 copies g-1 dry weight. High throughput sequencing of the nrfA gene revealed that the relative abundance of Anaeromyxobacter (4.52% on average), Polyangium (4.09%), Archangium (1.86%), Geobacter (1.34%), and Lacunisphaera (1.32%) were high. Pearson and RDA correlation analysis exhibited that nrfA gene abundance was positively correlated with altitude, pH, OC, and sand concentration. Anaeromyxobacter was positively correlated with reservoir age and DNRA potential rate. The deterministic environmental selection process plays a crucial role in the formation of the DNRA bacterial community. Network analysis displayed that the dominant DNRA genus was the key population of the DNRA microbial community in the sediments of Lancang River cascade reservoirs. This study reveals that the variation of DNRA bacterial activity and community structure is largely driven by the construction of cascade reservoirs, and provides a new idea for further understanding the characteristics of the DNRA community in the cascade reservoir ecosystem.

Formation mechanism of high-quality Palaeogene “compositional mixing” reservoirs in lacustrine rift basins: A case from the offshore Bohai Bay Basin, China

Mixed siliciclastic-carbonate reservoirs are currently considered a unique type of reservoir for hydrocarbon exploration. However, to date, research on “compositional mixing”, an important type of mixed sediment, remains relatively scarce. In recent years, significant breakthroughs in hydrocarbon exploration have been achieved in compositional mixing deposits, which are found within a lacustrine setting of the offshore Bohai Bay Basin in China. Despite the lack of systematic studies on their formation mechanisms, these discoveries are insightful case for understanding compositional mixing reservoirs (CMRs). In this study, an interdisciplinary approach that integrates petrographic analysis, cathodoluminescence analysis, reservoir characterization, and high-pressure mercury injection tests from drilled wells, cores, and thin sections, is employed to elucidate the characteristics of CMRs and the factors controlling reservoir modifications. Our findings reveal that compositional mixing deposits predominantly occurred with fan delta systems adjacent to uplifts. Two distinct reservoir types, namely, mixed slide deposits and mixed sandy debris flow deposits, were formed by coexistence of fan delta front siliciclastics with shallow-water biota (carbonate components). A comparative analysis of reservoir properties suggests that CMRs are of equivalent or superior quality to fan delta systems, challenging existing paradigms. This superiority is attributed to two primary factors: (1) the unique internal texture of compositional variations significantly influences subsequent diagenesis, leading to the extensive development of selective dissolution porosity and a reduction of clay cementation; and (2) distinctive early diagenetic events, including micrite envelopes and early dolomitization, markedly enhance the reservoirs’ resistance to compaction. In contrast, fan delta deposits lacking carbonate components do not undergo selective dissolution and their siliciclastic composition facilitates clay cementation. This study offers novel insights and a model for understanding the formation mechanism of CMRs, providing implications for future global petroleum exploration in mixed sediment reservoirs.

Assessing the effectiveness of reservoir operation and identifying reallocation opportunities under climate change

Reservoirs are designed and operated to mitigate hydroclimatic variability and extremes to fulfill various beneficial purposes. Existing reservoir infrastructure capacity and operation policies derived from historical records are challenged by hydrologic regime change and storage reduction from sedimentation. Furthermore, climate change could amplify the water footprint of reservoir operation (i.e. non-beneficial evaporative loss), further influencing the complex interactions among hydrologic variability, reservoir characteristics, and operation decisions. Disentangling and quantifying these impacts is essential to assess the effectiveness of reservoir operation under future climate and identify the opportunities for adaptive reservoir management (e.g. storage reallocation). Using reservoirs in Texas as a testing case, this study develops data-driven models to represent the current reservoir operation policies and assesses the challenges and opportunities in flood control and water supply under dynamically downscaled climate projections from the Coupled Model Intercomparison Project Phase 6. We find that current policies are robust in reducing future flood risks by eliminating small floods, reducing peak magnitude, and extending the duration for large floods. Current operation strategies can effectively reduce the risk of storage shortage for many reservoirs investigated, but reservoir evaporation and sedimentation pose urgent needs for revisions in the current guidelines to enhance system resilience. We also identify the opportunities for reservoir storage reallocation through seasonal-varying conservation pool levels to improve water supply reliability with negligible flood risk increase. This study provides a framework for stakeholders to evaluate the effectiveness of the current reservoir operation policy under future climate through the interactions among hydroclimatology, reservoir infrastructure, and operation policy.

115 years of sediment deposition in a reservoir in Central Europe: Effects of the industrial history and environmental protection on heavy metals and microplastic

AbstractHumans have considerably influenced accumulation rates and sediment composition in lake deposits. Due to near‐continuous accumulation, lakes and reservoirs are an excellent archive of these anthropogenic influences. The Urft Reservoir in the Eifel Mountains, western Germany, provides a unique record of the human influence on the landscape for the past 115 years. In 2020 and 2021, 24 cores and 23 surface samples were obtained from the bottom of the, by that time drained, reservoir. Grain size, heavy metals, weathering signatures and microplastic were analysed. For the chronology, caesium‐137 and microplastic were used. Using the first occurrence of microplastic as well as different plastic types for dating was not successful. However, a distinct layer with a high number of microplastic particles could be traced back to a fire in 1991 and was used as an additional stratigraphic marker in the age‐depth model. A period of relatively high accumulation rates in the reservoir occurred in the mid‐1950s and was related to enhanced construction works in the local valleys. Analysis of heavy metal content in the reservoir sediments shows a strong connection to historical changes in ore industry in the valley of the Urft. Stricter environmental protection laws and the decline of the metal processing industry resulted in a reduced input of lead, copper and zinc in the reservoir until the mid‐1980s. Since then, heavy metal content has remained relatively constant. A major flooding event in July 2021 did not lead to the remobilisation of older contaminated deposits as indicated by low heavy metal content in flood deposits. Accordingly, also the microplastic content is not increasing following the extreme event. Due to the degree of weathering of the flood deposits, it is argued that mainly hillslope material was transported into the Urft and subsequently into the Urft Reservoir during this event.

Elevated lead mobility in sediments of a eutrophic drinking water reservoir during spring and summer seasons: Insights from isotopic signatures

Lead (Pb) pollution in sediments remains a major concern for ecosystem quality due to the robust interaction at the sediment/water interface, particularly in shallow lakes. However, understanding the mechanism behind seasonal fluctuations in Pb mobility in these sediments is lacking. Here, the seasonal variability of Pb concentration and isotopic ratio were investigated in the uppermost sediments of a shallow eutrophic drinking lake located in southeast China. Results reveal a sharp increase in labile Pb concentration during autumn-winter period, reaching ∼ 3-fold higher levels than during the spring-summer seasons. Despite these fluctuations, there was a notable overlap in the Pb isotopic signatures within the labile fraction across four seasons, suggesting that anthropogenic sources are not responsible for the elevated labile Pb concentration in autumn-winter seasons. Instead, the abnormally elevated labile Pb concentration during autumn-winter was probably related to reduction dissolution of Fe/Mn oxides, while declined labile Pb concentration during spring-summer may be attributed to adsorption/precipitation of Fe/Mn oxides. These large seasonal changes imply the importance of considering seasonal effects when conducting sediment sampling. We further propose a solution that using Pb isotopic signatures within the labile fraction instead of the bulk sediment can better reflect the information of anthropogenic Pb sources.

Chemical Analysis and Agricultural Utilizations of Dams Reservoir Sediments

Sediment samples were collected from King Talal Dam (KTD) and subjected to chemical, physical, and mineralogical study to test their suitability as soil replacement. The concentration of Cadmium (Cd), Chromium (Cr), Copper (Cu), Iron (Fe), Lead (Pb), Nickel (Ni), Arsenic (As), Cobalt (Co), and Zinc (Zn) was obtained by atomic absorption spectrophotometer, and the elements phosphorus (P) and boron (B) were determined by using the spectrophotometric technique. These elements concentration was compared with the average concentration of soil in near by area of Jordan valley, It was found that the concentration of Cu and Zn, P, Fe, Mn, and K is within the standard range for soil fertility, and the concentration of Pb, Ni, As, Co, and Cd is within the standard range of agricultural human health safety regulation’s, the highest concentration on of iron (Fe) and manganese (Mn) in sediment were found to be 1.48% and 15.0 ppm respectively. The lowest average concentration in KTD sediment was identified for cadmium (Cd) and arsenic (As) and was found to be <0.01 and <0.05 ppm respectively. The Laboratory tests carried out indicate that the water of King Talal Dam is free of heavy metals pollutants, and reservoir sediments can be used for soil replacement and amendment. It is highly recommended to keep the percentage of sediments of KTD to soil used in agriculture within the range of 25%-50%.

Seeding Density Alters the Assembly of a Restored Plant Community after the Removal of a Dam in Southern Wisconsin, USA

Recently exposed reservoir sediments, prone to colonization by invasive species, provide novel settings to test hypotheses related to soil conditions and propagule supply as potential drivers of plant assembly in disturbed ecosystems. We used a dam removal site in southwestern Wisconsin to examine the relationship between the physiochemical properties of dewatered sediments, seeding density, and plant community assembly. The plant communities from five seed densities (1000, 500, 250, 125, and 0 seed m−2) were annually assessed over four years. We hypothesized (1) that the native aboveground biomass and the proportion of native to invasive (non-seeded species) aboveground biomass would increase with the seeding density and (2) that the diversity of seeded native species would increase with a higher seeding density. We found evidence that sowing at least 500 seeds m−2 of prairie species increased their abundance, establishment, and plot diversity compared to non-seeded plants that persisted four years after seeding (p < 0.05). The seeding density treatments led to the assembly of two distinct communities: “native” and “invasive”. The “native” community, assembled in plots seeded with at least 500 seeds m−2, had a greater aboveground biomass and diversity (i.e., richness) of seeded plants compared to plots with lower seed densities, and its productivity was positively related to this richness. In the “weedy” community, the diversity of invasive species had no relationship to their aboveground biomass, likely because these species share similar traits (i.e., redundancy) and may have performed similar functions within the plant community. These findings suggest that the seeding density interacted with the disturbed soil resources to increase the diversity and productivity of seeded native species and may serve as a positive feedback mechanism for the establishment of native communities in dewatered sediments.

Microplastic Evaluation in Water and Sediments of a Dam Reservoir–Riverine System in the Eastern Carpathians, Romania

The complex aquatic system of dam reservoirs is known to trap emerging pollutants as microplastics (MPs) in sediments and water column. Considering the knowledge gaps in this type of environment, we investigated the amount and distribution of MPs in the surface water layer, as well as in the surface and deep sediments of the Siriu Reservoir in the Buzău River system, which is located in the southern area of the Eastern Carpathians, Romania. There was a discrepancy between MP abundancy in both water and sediment samples collected near the reservoir (5.3 MPs/m3, 315.5 MPs/kg) and at several kilometers downstream of the dam (1.4 MPs/m3, 132.5 MPs/kg). The chronological accumulation of MPs in the lacustrine sediments was determined by analyzing 5 cm intervals of a 50 cm length core extracted from the reservoir bed. By comparing the concentration of MPs identified in each interval with the solid debit volumes registered in the last decade, we found that flood events could be traced easily due to abundant MP accumulation. Morphologically, the particles were observed mainly as fibers and fragments. Fourier transform infrared spectroscopy (FT-iR) investigations identified most of the MPs as polypropylene (28%), polyethylene (26%), and polyethylene terephthalate (19%).

Investigating Asphaltene Precipitation and Deposition in Ultra-Low Permeability Reservoirs during CO2-Enhanced Oil Recovery

CO2 flooding is an economically feasible and preferred carbon capture, storage, and utilization technology. Asphaltene deposition is a common problem in the process of CO2 injection because it may cause reservoir damage. The mechanism of asphaltene precipitation damage to the formation remains elusive. Experiments were conducted to reveal the pore-scale formation damage mechanism in ultra-low permeability reservoirs caused by asphaltene precipitation during CO2 flooding. Initially, the precipitation onset point for asphaltene within the crude oil-CO2 system was determined using a high-pressure tank equipped with visual capabilities. Subsequently, CO2 flooding experiments were conducted on ultra-low permeability cores under miscible and immiscible conditions, with the support of nuclear magnetic resonance (NMR) to quantitatively evaluate the impact of asphaltene precipitation on ultra-low permeability reservoirs. The results indicate that within the pressure range from the asphaltene precipitation onset point to the minimum miscibility pressure (MMP). The level of asphaltene precipitation rises as CO2 injection pressure increases. In the miscible flooding stage, asphaltene precipitation can still occur, but to a lesser extent. Notably, asphaltene deposition predominantly occurs in larger pores; above the MMP, the permeability decreases significantly as asphalt particles agglomerate, resulting in notable pore-throat blockages. While asphaltene deposition has a minimal impact on porosity, the bridging effect of asphaltene particles reduces permeability.

Synergies of storing hydrogen at the crest of CO2${\rm CO}_{2}$ or other gas storage

AbstractThere are mutual benefits in storing in sedimentary reservoirs jointly with another gas serving as a cushion gas, such as the of a carbon capture and storage (CCS) operation or the natural gas of seasonal storage or left in a depleted hydrocarbon reservoir. When occupies the crest of the reservoir, the presence of either gas is beneficial to the other. reinforces the sealing efficiency of the caprock due to its very favorable interfacial properties with respect to brine and rock‐forming minerals. storage safety and capacity are also increased with cushion gases such as , which alleviate the buoyancy pressure at the top of the gas column. The potential drawback of this storage scheme is gas/gas mixing, which can, however, be strongly reduced if, by an appropriate choice of well completion and placement, is positioned in the upper zones of the reservoir, and its injection rate is kept below a critical value corresponding to the incipient fingering instability of the gas mixing zone. This value, which depends on reservoir permeability, the dip angle of the mixing front, and how density varies with viscosity in the mixing front, turns out to be well above practical injection rates. Therefore, dispersive mixing is the only cause of front spreading, which is acceptable for not‐too‐heterogeneous reservoirs. The mutual benefits identified in this study are the strongest when the cushion is made up of dense , which suggests that the crest of offshore storage reservoirs is a good candidate for storage. © 2024 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Seasonal hypoxia enhances sediment iron-bound phosphorus release in a subtropical river reservoir

Dams worldwide commonly accelerate the eutrophication of reservoirs. While the seasonal hypoxia in deep reservoirs is widely acknowledged, there is limited research on its impact on benthic phosphorus (P) cycling and P fraction release from the reservoir sediments. Here we show that seasonal hypoxia enhances sediment P release and P fluxes at the sediment-water interface (SWI) which might alter P dynamics in deep reservoirs. We conducted a detailed measurement of sediment P fractions through the SEDEX approach, combined with a labile P gradient analysis using the diffusive gradients in thin films (DGT) technique to understand P cycling patterns in sediments during the transition period from spring (oxic) to late summer (hypoxic) conditions. The sediment P pool was predominantly composed of iron-bound phosphorus (Fe−P, 76–80 %), primarily due to the widespread occurrence of lateritic red soil (rich in Fe2O3/MnO2) in subtropical areas. More organic-P was observed in summer compared to spring. A significant increase in labile P occurred at the depth of 0–4 cm and 0–1 cm in spring and summer, respectively, where sediment P release was primarily governed by the reduction of Fe−P and the generation of S2−. A higher apparent fluxes of phosphate across the SWI were observed in summer characterized by higher temperature and lower oxygen levels. The current results suggest that seasonal hypoxia was a crucial factor affecting P cycling and diffusion in deep reservoirs. These findings present important implications for the ecology and management of the watershed-coast ecosystem.

Revisiting the rocks discovers turbidites and a new exploration play in the Penola Trough, Otway Basin, South Australia

As part of an integrated geological, geochemical, and geophysical study, conventionally cored intervals from the lower-most stratigraphic section of the onshore South Australian part of the Penola Trough, Otway Basin have been reviewed and reinterpreted. Three conventionally cored wells; Bungaloo-1, Jolly-1ST1 and Sawpit-2, were described using a quick-look approach to determine gross lithological and sedimentological information. Seven lithology types were observed: clay-siltstone with horizontal lamination, diamictite (claystone with horizontal lamination and dropstones), clay-siltstone with massive-horizontal lamination, silt-sandstone with horizontal lamination, sandstone with ripple lamination, sandstone with parallel lamination and sandstone with massive bedding ± rip-up clasts. The first two lithologies are interpreted as representing deposition in a cold climate, profundal-lacustrine depositional environment. The first, containing interpreted varves is the most distal and the second, containing dropstones indicates relatively colder freeze/thaw conditions. The last five lithologies are interpreted as lacustrine turbidite depositional environments, specifically Bouma sequences Te, Td, Tc, Tb and Ta, respectively. Seismic facies interpretation has identified canyon-like and mound features and interpreted them as slope-feeder channels and fan/s respectively, at and away from well control. This rock-based turbidite interpretation provides an alternative to the current alluvial channel/crevasse splay/floodplain lake model for reservoir deposition and as such presents a new exploration play. Further the reservoirs’ immediate lateral association with gas/oil-prone source rocks and seals in the clay-rich lithologies potentially provide for stratigraphic trapping in addition to the previously targeted fault-controlled structural traps.

Predicting scale deposition in oil reservoirs using machine learning optimization algorithms

Scale deposition, a form of formation damage, not only affects the reservoir but also damages the well and equipment. This phenomenon occurs due to changes in temperature, pressure, and the injection of incompatible salt water, leading to ionic reactions. This study investigated permeability reduction due to scale deposition and examined how parameters such as temperature, pressure drop, and ion concentration affect the prediction accuracy. The scale deposits investigated in this study include CaSO4, BaSO4, and SrSO4. This paper uses Python to employ different machine-learning algorithms to predict the results. Each machine learning model has certain hyper-parameters that need adjustment. Failure to do so will result in reduced accuracy and incomplete interpretation of input data. The accuracy of the support vector regression (SVR) algorithm was significantly affected by the variation of the epsilon parameter in the dataset used. Therefore, before hyperparameter optimization, SVR had the lowest accuracy at 0.575. After adjusting the hyper-parameters, our findings show that SVR had the highest increase in R-squared value, which was 0.900, and the most minor growth in KNN, which went from 0.995 to 0.996. Additionally, the highest accuracy value for K-Nearest Neighbor is 0.996. Furthermore, most errors were related to SVR and XGBoost algorithms, while the most negligible errors were for the Decision Tree and KNN algorithms.

Responses of ancient reservoir sediments to water level fluctuations, human activity and climatic changes over the past ∼700 yr in Guizhou Province, southwestern China

Knowledge of the responses of reservoir ecosystems to anthropogenic and climatic impacts on multi-centennial timescales is relatively poor, but is crucial for the management of numerous reservoirs in China. In this study, the history of an ancient reservoir system was reconstructed for the last ∼ 700 yr based on multiple dating methods (210Pb, 137Cs, and 14C) and geochemical data (e.g., the mass accumulation rates, loss-on-ignition values, total organic carbon, total nitrogen, total phosphorus contents, and stable carbon isotope ratios of organic matter) obtained for sediment cores from Gongqing Reservoir in Guizhou Province, southwestern China. Combined with historical literature and regional climatic records, the response of the aquatic ecosystem in Gongqing Reservoir to water level fluctuations, anthropogenic activity, and climatic changes during the last ∼ 700 yr was assessed. The results suggest that dramatic changes in the water level due to dam heightening in 1950 s significantly changed the aquatic ecosystem of Gongqing Reservoir. During periods of low water levels (average water depth = 1–2 m; 1345–1957 CE), aquatic macrophytes contributed more to the sedimentary organic matter, whereas phytoplankton increasingly contributed more during periods of high water levels (∼5 m; 1957–2020 CE). During the late stage (1700–1957 CE) of low water levels, the reservoir ecosystem was significantly affected by human activities. Combined with regional geological and historical records, we consider that increased population and enhanced human activities have markedly affected vegetation conditions in western Guizhou Province since ∼ 1700 CE. After 1957 CE, anthropogenic activities were key in affecting the aquatic ecosystem of Gongqing Reservoir. Especially in the last decade, large housing developments and enhanced agricultural activities in the watershed have led to enhanced catchment erosion and transport of more nutrients into the reservoir, thus promoting the algae productivity. These results provide new insights into how reservoir ecosystems respond to water level fluctuations, anthropogenic activity and climatic changes on multi-centennial timescales.

Residual loads from tilapia farming on the sediment of a Brazilian reservoir

This study investigated the sediment geochemistry of a fish farming area in net cage tanks in the Rosário reservoir, Brazil. Three areas were investigated: reference (RA), fish farming (FFA), and dispersion (DA). The results were analyzed through correlation, similarity, principal component analysis, comparison with legislation, sediment quality guidelines, and sediment pollution indices. The mean concentrations for RA, FFA, and DA areas were respectively: Cu (mg.kg−1) 37.74, 62.23, and 71.83; Mn (mg.kg−1) 22.55, 66.48, and 55.90; Zn (mg.kg−1) 9.13, 114.83, and 94.27; Fe (%) 0.28, 0.40, and 0.43; OM (%) 15.84, 21.95, and 18.45; TOC (%) 1.86, 3.69, and 6.05; TN (mg.kg−1) 2365.00, 5015.00, and 3447.51; TP (mg.kg−1) 780.00, 6896.00, and 2585.50; ORP (mV) −95.50, −135.20, and −127.10; pH 6.60, 6.58, and 6.05; <63 μm 90.59, 78.68, and 87.30. Statistically, the influence of fish farming on sediment, organic matter, and pollutant sedimentation was demonstrated. Cu and Zn concentrations were below sediment quality guidelines. Regarding legal limits (resolution 454/2012/CONAMA), nutrients in the FFA area exceeded by 60% (TN) and 100% (TP), while in DA and RA areas they were 100% lower. TOC was 100% lower in all areas. Organic matter exceeded the limit by 100% in all areas. Pollution indices resulted in: low contamination factor 78%; unpolluted for 87% of pollution load and 83% of combined pollution; moderately polluted for 75% of the Nemerow index. The greatest impacts and influence of farming on pollutant sedimentation were more concentrated in the fish farming area. In terms of legal aspects and pollution indices, fish farming produced low levels of trace metal pollution and nutrient concentrations exceeded legal limits.

CREATION OF AN ALGORITHM FOR THE IDENTIFICATION OF ANALOGOUS OBJECTS IN CARBONATE RESERVOIRS OF THE VOLGA-URAL OIL AND GAS PROVINCE

For the conditions of deposits confined to the carbonate reservoirs of the Volga-Ural oil and gas province, an algorithm for searching for analogous objects based on the identification procedure is proposed. The principal boundaries of the search for analogous objects confined to various stratigraphic systems have been established through a comprehensive analysis of geological and commercial data. The significant influence of tectonic confinement on the formation of the features of the geological structure of deposits and their distribution in the axes of canonical discriminant functions is shown. The presence of uncertainty in the separation of objects using the tectonic factor is explained by the similarity of objects in the marginal zones of adjacent tectonic elements in both terrigenous and carbonate reservoirs, which in turn must be taken into account when implementing key aspects of field development design. During the simulation, a certain similarity of individual deposits in the carbonate reservoirs of the carboniferous system was established, confined not only to various tectonic elements, but also to various stratigraphic complexes, which makes it possible to expand the search for analogous objects and the possibility of solving various development management tasks in conditions of deterioration of the structure of oil reserves and low technical and technological indicators of well operation. When using pattern recognition methods, in particular, discriminant analysis, dependencies were obtained that make it possible to successfully adapt the accumulated experience of developing an object to other objects while maintaining a high level of compliance with the logical aspects of hydrodynamics and physics of reservoir systems. Further development of the algorithm presented in the work lies in the plane of using a minimum number of parameters for analysis, ensuring a high percentage of separation of objects, taking into account the heterogeneity of their values.

Impact of reservoir construction on the sediment budget of a downstream-linked freshwater lake

ABSTRACT 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.

Predicting reservoir sedimentation using multilayer perceptron – Artificial neural network model with measured and forecasted hydrometeorological data in Gibe-III reservoir, Omo-Gibe River basin, Ethiopia

The estimation and prediction of the amount of sediment accumulated in reservoirs are imperative for sustainable reservoir sedimentation planning and management and to minimize reservoir storage capacity loss. The main objective of this study was to estimate and predict reservoir sedimentation using multilayer perceptron–artificial neural network (MLP-ANN) and random forest regressor (RFR) models in the Gibe-III reservoir, Omo-Gibe River basin. The hydrological and meteorological parameters considered for the estimation and prediction of reservoir sedimentation include annual rainfall, annual water inflow, minimum reservoir level, and reservoir storage capacity. The MLP-ANN and RFR models were employed to estimate and predict the amount of sediment accumulated in the Gibe-III reservoir using time series data from 2014 to 2022. ANN-architecture N4-100-100-1 with a coefficient of determination (R2) of 0.97 for the (80, 20) train-test approach was chosen because it showed better performance both in training and testing (validation) the model. The MLP-ANN and RFR models' performance evaluation was conducted using MAE, MSE, RMSE, and R2. The models’ evaluation result revealed that the MLP-ANN model outperformed the RFR model. Regarding the train data simulation of MLP-ANN and RFR shown R2 (0.99) and RMSE (0.77); and R2 (0.97) and RMSE (1.80), respectively. On the other hand, the test data simulation of MLP-ANN and RFR demonstrated R2 (0.98) and RMSE (1.32); and R2 (0.96) and RMSE (2.64), respectively. The MLP-ANN model simulation output indicates that the amount of sediment accumulation in the Gibe-III reservoir will increase in the future, reaching 110 MT in 2030–2031, 130 MT in 2050–2051, and above 137 MTin 2071–2072.

Comparison of Various Rainfall Erosivity Formulas in the Application of USLE for Erosion Cases in Indonesia

Erosion is a complex process that results in the movement of soil particles from one place to another through a series of processes of detachment, entrainment, transport, and deposition. Erosion can cause serious environmental problems, such as loss of soil fertility, and sedimentation of rivers, lakes, and reservoirs. Therefore, it is important to identify and mitigate erosion problems in the management of a watershed. The erosion prediction model that is popularly used, especially in Indonesia, is the Universal Soil Loss Equation (USLE). One of the parameters in the USLE formula is the rainfall erosivity factor (R). Many formulas have been developed to calculate the R-factor. However, not all formulas are suitable for application in Indonesia. In this study, a comparison of several R formulas from various countries in the Asia Region was carried out. From the results of the comparison, it was found that for Riam Kanan Sub-watershed, the appropriate formula for rainfall erosivity is the Merritt et al. (2003) formula, while for Brantas Sub-watershed erosivity formulas from Utomo (1994) and Babu et al. (2004) is the most suitable. The results of this study can be used as a consideration in determining the appropriate rainfall erosivity formula (R) for a particular case.

Field monitoring and modelling of sediment transport, hydraulics and hydroabrasion at Sediment Bypass Tunnels

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 kv = (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.