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.

Estimation of reservoir sedimentation using bathymetry survey at Shumburit earth dam, East Gojjam zone Amhara region, Ethiopia

Sediment accumulation is a major factor in reducing the useful life of irrigation reservoirs. As a result, information on any changes in storage capacity is required to reduce the reservoir's risk. Therefore, the goal of this research was to identify the amount of sediment accumulated in the reservoir using a bathymetric survey. The bathymetric survey method involves subtracting the two-period capacities (before dam construction and after dam construction). The bathymetric survey of the reservoir was carried out by analyzing volume and surface area using ArcGIS 10.8, and developing the Triangulated Irregular Network (TIN) surface from topo-sheet data. The bathymetric survey showed that 297,000 m³ (362,340 tons) of sediment had accumulated in the reservoir over six years of operation. This means the reservoir's total capacity has been reduced by 7.52% over the last six years. The average rate of sedimentation was assessed to be 49,500 m³/y (60,390 tons/year) and the annual capacity reduction of the reservoir was 1.25 percent. The proportion of dead storage capacity to sediment deposition level was used to calculate the reservoir's useful life. Hence; the sedimentation rates of the dead storage and live storage regions were 31,666.67 m³/year and 48,333.33 m³/year, respectively. Finally, this indicates that the reservoir will not be functional for more than 15 years if the sediment deposition rate remains the same as the previous six years throughout the operation periods.

A MATLAB GUI program for reservoir management to simultaneously optimise sediment release and power generation

Sedimentation affects the normal function of reservoirs and is a decisive factor in the reservoir's service life. Flushing sediment during reservoir operation is an effective non-engineering measure to alleviate reservoir sedimentation; however, lowering water level to discharge more flow conflicts with hydropower generation. In this study, reservoir management software is developed to simultaneously optimise sediment discharge and hydropower generation with the reservoir discharge as the decision variables. The sediment transport rate is calculated by an integral of the vertical distribution of suspended load concentration and flow velocity instead of empirical formulas. The model is solved by the most widely used multi-objective optimisation algorithm NSGA-II, resulting in the optimal schedule corresponding to the maximal sediment discharge and hydropower generation, which can be displayed graphically in the software. The software was developed in MATLAB with a Graphical User Interface (GUI) and applied to a large reservoir and can be generalised to other reservoirs. The results show that within the recommended discharge variation of 5%, the sediment release can be increased by 2.07 × 10⁶ t as a reduction of per 10¹⁰ kW h in annual power generation. Compared with the original scheme, sediment release can be increased most by 3.31% at the cost of 0.03% loss of power generation. Moreover, the dual objective in the flood season was optimised by 7.30% and 3.92%, respectively.

Implementation of comparative detection approaches for the accurate assessment of sediment thickness and sediment volume in the Passaúna Reservoir

Siltation has significant economic and social impacts as it directly reduces the useable amount of water in reservoirs. Giving a solution to the issue of sedimentation is a complicated task and maybe one of the most important engineering and environmental challenges of the 21^st century. The deposited volume and the distribution pattern of the sediment are often unknown and not easy to assess. The sedimentation process is highly dynamic, initially due to the hydrological conditions of the incoming rivers, but also due to common internal phenomena like resuspension or density currents. Sediment remediation measures such as mechanical sediment removal or flushing are planned based on the sediment thickness distribution and the overall sediment volume/mass. Often, the sediment thickness is calculated through topographic differencing between the pre-impoundment reservoir lake bottom and the actual lake bottom. However, it is common that the previous depth distribution map is not available or in insufficient quality. In this regard, alternative measurement techniques have to be taken into consideration. In this study, we assessed the best possible approach depending on the characteristics of the sediment and of the reservoir. We combined three different acoustic systems (a multibeam, a sub-bottom profiler, and a single beam dual frequency system) with sediment coring and dynamic free fall penetrometer measurements for an improved assessment of sediment stock and sediment distribution in the Passaúna Reservoir. Our results showed that topographic differencing could not be applied, as the data for the pre-impoundment lake bottom was insufficiently accurate. The parametric sub-bottom profiler could detect the sediment thickness in high accuracy, but significant limitations were recorded in areas with high gas contents. The dual-frequency echosounder derived the sediment thickness with a normalized mean absolute error of 56% due to the high volumetric gas content in the sediment. The dynamic free-fall penetrometer showed satisfying results compared to the other systems. The normalized mean absolute error was 22%, and sediment thickness could be detected in areas with up to 1.8 m of sediment. Sediment coring is also a reliable technique for sediment thickness determination. However, the results showed that if only traditional coring devices are used (gravity corer), the limited penetration depth of the equipment combined with sampling disturbances often prevent a correct assessment of the sediment thickness. The overall results of this study can help for an improved decision-making regarding reservoir management. The accurate assessment of sediment volume and distribution can reduce costs for sediment removal and assist in having a precise overview of the reservoir lifetime.

Sedimentation assessment and effects in Venda Nova dam reservoir (Portugal)

Sediment quantity and quality in dam reservoirs is often neglected and usually only water quality is monitored in many countries, such as in Portugal. Nevertheless, there are risks associated to sedimentation in dam reservoirs, particularly considering that many dams in the world are ageing into an overextended lifetime. The present study was conducted with the goal to understand how sedimentation monitoring in Portuguese dam reservoirs could be attained, as that information is essential to tackle any sedimentation effects that may have occurred. A dam reservoir in the north of Portugal, the Venda Nova reservoir, was selected for the study. First, historical relevant data was compiled and then new data for sediment and water quality analysis was collected. The results show that since the reservoir was filled, after the dam construction conclusion in 1951, the reservoir morphology has been affected by the sedimentation at different rates and underwent multiple transformations from 1946 (before the dam construction) to 2004 (last topobathymetric survey). An expressive morphology transformation detected in 1984 led to an intervention to remove some of the accumulated material. It was verified, as well, that the asymmetrical contamination of the sediments and water is still strongly influenced by a nearby mine even if now inactive. The methodology applied, if periodically used, will be helpful to understand the sedimentation phenomena in Portuguese and other European dam reservoirs where reservoir sedimentation has been overlooked, and can play a key role to improve river continuity and to comply with the Environmental Quality Standards under the EU Water Framework Directive (2000/60/EC).

Specific sediment yield model for reservoirs with medium-sized basins in Spain: An empirical and statistical approach

The world's reservoirs are losing capacity at a rate of 0.5-1% a year due to sedimentation processes. The strategies to reverse this trend must include an accurate estimation of sedimentation rates in terms of Specific Sediment Yield (SSY). This research develops an empirical and statistical model based on data from the CEDEX (Spanish Studies and Experimentation Centre for Public Works). From an initial number of 131 reservoirs studied in the period 19,672,004, a group of 26 reservoirs with medium-sized basins (750 to 1750 km²) was selected for analysis. Reservoir catchments were described with 11 explanatory variables, representing the production, transport and deposition of sediment, although the calibrated model considers only six of these variables: Fournier Index (Rainfall Torrentiality), Drainage Length, Reservoir Coefficient (relation between reservoir capacity and area), C (USLE Land Cover Factor), Yearly Average Rainfall and Slope. SSY and the explanatory variables were transformed during the calibration process, and the resulting model shows a non-linear relation between them. Compared to other models calculated with CEDEX data with a determination coefficient of between 17% and 80%, this model has a determination coefficient of 84%, is statistically consistent, validated by means of a jackknife analysis and contrasted with other models. However, the model is not cross validated with information on additional reservoirs, and shows substantial uncertainty and instability deriving from the definition of the explanatory variables and the quality of the data set, so extrapolation to other reservoirs is only possible under supervision and local calibration. However, the model reveals a strong non-linear relation between land cover, rainfall amount and torrentiality, slope gradient, drainage length and reservoir depth with SSY, and provides valuable information for exploring the effect of watershed alterations on sedimentation.

From waste to resource: Cost-benefit analysis of reservoir sediment reuse for soil fertilization in a semiarid catchment

Reservoir networks have been established worldwide to ensure water supply, but water availability is endangered quantitatively and qualitatively by sedimentation. Reuse of sediment silted in reservoirs as fertilizer has been proposed, thus transforming nutrient-enriched sediments from waste into resource. The aim of this study is to assess the potential of reusing sediment as a nutrient source for agriculture a semiarid basin in Brazil, where 1029 reservoirs were identified. Sedimentation was modelled for the entire reservoir network, accounting for 7 × 10⁵ tons y^-1 of sediment deposition. Nutrients contents in reservoir sediments was analysed and compared to nutrients contents of agricultural soils in the catchment. The potential of reusing sediment as fertilizer was assessed for maize crops (Zea mays L.) and the sediment mass required to fertilize the soil was computed considering that the crop nitrogen requirement would be fully provided by the sediment. Economic feasibility was analysed by comparing the costs of the proposed practice to those obtained if the area was fertilized by traditional means. Results showed that, where reservoirs fall dry frequently and sediments can be removed by excavation, soil fertilization with sediment presents lower costs than those observed for application of commercial chemical fertilizers. Compared to conventional fertilization, when using sediments with high nutrient content, 25% of costs could be saved, while when using sediments with low nutrient content costs are 9% higher. According to the local conditions, sediments with nitrogen content above 1.5 g kg^-1 are cost efficient as nitrogen source. However, physical and chemical analyses are recommended to define the sediment mass to be used and to identify any constraint to the application of the practice, like the high sodium adsorption ratio observed in one of the studied reservoirs, which can contribute to soil salinization.

Sustainability evaluation of Mornos Lake/Reservoir, Greece

The modern climate trend and population growth have dramatically increased the need for maximization of the net benefit from the existing storage space in freshwater reservoirs. However, sedimentation in reservoirs through physical deposition and/or slope failures is a major threat to their productivity and life expectancy. In this context, the sedimentation impact on the sustainability of Mornos Lake/Reservoir, which is exceptionally vital for the ~ 3.1 million inhabitants of Athens, had to be evaluated. Therefore, a meticulous geophysical survey of the reservoir bed was conducted in 2015 for the very first time. Bathymetric, sidescan sonar, and seismic profiling datasets, all integrated with real-time kinematic (RTK) positioning, were analyzed for a realistic evaluation of the storage capacity loss. Approximately 18.2 × 10⁶ m³ of lacustrine sediments derived through physical wedge-type deposition process and ~ 800,000 m³ of material produced by slope failures have covered the bottom since reservoir commissioning in 1981. This configures an average storage capacity loss of ~ 0.07% per year, which, however, is one of the lowest rates worldwide. Moreover, the 108-m-deep reservoir basin can presently accommodate a maximum active water volume of ~ 740 × 10⁶ m³. The siltation pattern and sediment transport pathways in the reservoir are principally controlled by vigorous turbidity underflows, which deliver sediment mainly to the dam area (deposition thickness up to ~ 7 m) as well as to the pumping area (deposition thickness up to ~ 4 m) posing there a future risk; nevertheless, according to the predicted lake bathymetry, this risk will be negligible till 2045.

Reservoir Sedimentation and Upstream Sediment Sources: Perspectives and Future Research Needs on Streambank and Gully Erosion

The future reliance on water supply and flood control reservoirs across the globe will continue to expand, especially under a variable climate. As the inventory of new potential dam sites is shrinking, construction of additional reservoirs is less likely compared to simultaneous flow and sediment management in existing reservoirs. One aspect of this sediment management is related to the control of upstream sediment sources. However, key research questions remain regarding upstream sediment loading rates. Highlighted in this article are research needs relative to measuring and predicting sediment transport rates and loading due to streambank and gully erosion within a watershed. For example, additional instream sediment transport and reservoir sedimentation rate measurements are needed across a range of watershed conditions, reservoir sizes, and geographical locations. More research is needed to understand the intricate linkage between upland practices and instream response. A need still exists to clarify the benefit of restoration or stabilization of a small reach within a channel system or maturing gully on total watershed sediment load. We need to better understand the intricate interactions between hydrological and erosion processes to improve prediction, location, and timing of streambank erosion and failure and gully formation. Also, improved process-based measurement and prediction techniques are needed that balance data requirements regarding cohesive soil erodibility and stability as compared to simpler topographic indices for gullies or stream classification systems. Such techniques will allow the research community to address the benefit of various conservation and/or stabilization practices at targeted locations within watersheds.

Assessment of the caesium-137 flux adsorbed to suspended sediment in a reservoir in the contaminated Fukushima region in Japan

We estimated the flux of caesium-137 adsorbed to suspended sediment in the Kusaki Dam reservoir in the Fukushima region of eastern Japan, which was contaminated by the Fukushima Nuclear Power Plant accident. The amount and rate of reservoir sedimentation and the caesium-137 concentration were validated based on the mixed-particle distribution and a sediment transport equation. The caesium-137 and sediment flux data suggested that wash load, suspended load sediment, and caesium-137 were deposited and the discharge and transport processes generated acute pollution, especially during extreme rainfall-runoff events. Additionally, we qualitatively assessed future changes in caesium-137 and sediment fluxes in the reservoir. The higher deposition and discharge at the start of the projection compared to the 2090s are most likely explained by the radioactive decay of caesium-137 and the effects of reservoir sedimentation. Predictions of the impacts of future climate on sediment and caesium-137 fluxes are crucial for environmental planning and management.