International Journal of Sediment Research最新文献

The current paper deals with the navigable section of the Volga River tailwater of the Nizhny Novgorod Hydropower Plant. To ensure navigation in this area, the existing navigation structures are being reconstructed and an additional chamber of the Gorodets Navigation Lock is to be constructed, and an extended navigable channel in the Volga River is to be created. To assess the impact of the planned measures on hydrological and riverbed regimes, the hydromorphological situation has been analyzed, and the flow parameters and the water level regime in the tailwater pool of the hydro system have been studied. An analysis of the changes in the riverbed along the design channel path was done and the kinematics of the flow and sediment transport parameters were studied under conditions of unsteady water movement resulting from the daily regulation of the river flow. Numerical experiments have revealed the peculiarities of river sediment movement under conditions of unsteady water movement, and recommendations for modeling river bed deformation under such conditions have been developed. It was found that with daily and weekly regulation of river, discharge bedload transport becomes more active at the moment when a wave of daily release from the upper reaches of the hydrosystem passes through. The basic characteristics of bedload transport, i.e., dune velocity and bedload rate, increase in comparison with a steady water flow. Under the conditions of unsteady water movement due to the daily regulation of the river flow, there are no strong changes in the size of the bottom dunes, while their velocity, and, consequently, the bedload rate increases significantly during periods when the wave of daily water releases from upstream passes. The results obtained indicate that for hydraulic calculations of the characteristics of water movement and sediment transport, it is necessary to use data from hourly observations of flow rates and water levels. It was found that during the passage of release waves under daily flow regulation, there are short periods of increase in Froude number values. Similarly, the bedload rate increased during these periods compared to the average daily values. The result of this comparison ultimately led to the recommendation that daily flow regulation should be abandoned in order to reduce the intensity of bedload transport rate and channel erosion in the tailwater of the hydrosystem.

Sediment leads to problems with navigation, agricultural productivity, and water pollution. The study of sediment flow in river reaches, which is a non-linear and complex process, is, thus, essential to addressing these issues. The application of artificial neural networks (ANN) to such problems needs to be investigated. For unsteady flow in a river system, river reach storage is an important variable that needs to be considered in data-driven models. However, previous research on sediment modeling did not involve the explicit use of storage variables in such models as is investigated in the current study. In the current study, storage variables have been explicitly (Model 2) used to predict the output state of the system at time ‘t + 1’ from the input state at time ‘t’ using ANNs. Sediment discharge at six gaging stations on the Mississippi River system, USA, has been considered as the state variable. The model has been compared with a model considering implicit variation of the storage parameter in the river system (Model 1). Dynamic ANNs are used for time-series datasets, which are more suitable for incorporating the sequential information within the dataset. Focussed gamma memory neural networks have been used in the current study. The numbers of hidden layers and hidden nodes, activation function, and learning rate have been varied step by step to obtain the optimal ANN configurations. The best selected input–output variables are those used in Model 2 as it performed slightly better than the other model in terms of Nash–Sutcliffe efficiency coefficient (CE) values. Model performance evaluated using normalized root mean square error (NRMSE) and CE shows satisfactory results. NRMSE was < 10% for all the outputs except for the Venedy and Murphysboro locations and CE values for sediment loads were > 0.45 for all locations except Murphysboro indicating acceptable performance by both the models. The models proved highly efficient (CE > 0.80, i.e., very good predictions) for predicting sediment discharge at locations along the main river channel with acceptable accuracy (CE > 0.45) for other locations and the storage change for the river system. These models can be used for real-time forecasting and management of sediment-related problems.

Existing methods for estimating critical bed shear stress are limited and usually do not consider local scour processes; therefore, the accuracy of numerical modeling is usually compromised. This paper presents the results of physical experiments on scour downstream of low-head weirs and proposes new equations for estimating the critical bed shear stress in scour holes. The experiments were done using a camel hump weir and coarse-bed materials which are applicable to steep streams with coarse-bed materials (gravel, rocks, etc.). The critical shear stress was regarded as the bed shear stress within the scour holes, which was determined using the three-dimensional (3D) flow field and bed morphology measured at the equilibrium scour state. The influence of the sediment size also was investigated. The experimental results showed that a scour hole can be divided into three zones: Zone-I for the upstream scour slope, Zone-II for the downstream scour slope, and Zone-III for the downstream slope of the sediment deposit downstream of the hole. The relation between the critical shear stress of the bed surface in Zone-I, the flow rate, and bed position was established, and the new equations yielded better accuracy than existing methods. In addition, the relation between the critical shear stress and the slope of the bed and the ratio of the local water depth to the particle size in Zone-II and Zone-III were established. After verification, the calculation results of the newly proposed equations were in good agreement with the standard values of the dimensionless critical Shields parameter obtained through processing the experimental results in the current study. Further discussion is provided regarding the integration of the parameters calculated applying the new parameters in the numerical models.

Wildfires are an increasingly alarming phenomenon that affects forests and agroecosystems, generating several cascade effects among which soil erosion is one of the most deleterious. A robust body of data-based evidence on post-fire soil erosion and sediment yield at the watershed scale is, thus, required, especially when dealing with areas where wildfires are particularly frequent, such as the Mediterranean basin. This study analyzes the impact of the first rains after a large wildfire in terms of soil erosion and sediment yield at the watershed scale in a Mediterranean area, the Pisan Mountains, central Italy. Here about 1,000 ha of olive groves, maquis, maritime pine, and chestnut forests, all on steep slopes, burned in 2018. Fire (or burn) severity was mapped by remote sensing and checked by a field survey. Sediment yield was assessed by sampling earthy materials deposited upstream of a check dam at the outlet of the studied watershed. Finally, a hydrological model was developed in the hydrologic engineering center–hydrological modelling system (HEC–HMS) environment to explore the relationship between the erosion–deposition events observed in the watershed and the rainfall-induced hydrological processes. The first two post-fire rainy events relocated a high mass of sediment, mostly non-organic and characterized by light color, perhaps already in the stream before fire, while the subsequent four rain showers deposited materials rich in pyrogenic organic matter. Overall, the soil erosion caused by these six major rainfall events–the larger of which had a return time of one year–was estimated to amount to 7.85 t/ha (0.26 mm in the watershed), corresponding to 42% of the watershed average annual potential erosion rate in unburned conditions. This value is lower than expected, and, overall, moderate if compared to other Mediterranean case studies, possibly because of the nature of soils in the watershed, i.e., shallow and stony, thus, poor in fines prone to erosion.

Due to the complex nature of coastal dynamics, several models were developed to estimate longshore sediment transport (LST) rates. The Coastal Engineering Research Center (CERC), Kamphuis, and Bayram models are among the most used and were applied to three east–west oriented sandy urban beaches from Rio de Janeiro (southeastern Brazil): Macumba, Recreio, and Barra da Tijuca. The wave record from oceanographic buoys and the WaveWatch III (WW3) model were used to identify fairweather periods and increased storminess between 2016 and 2018. Eastwards, median sediment grain-size grades from coarse to medium sand, and is associated with decreasing beach slopes in the same direction. The magnitude of the estimated LST rates varied significantly between the models (Δ ≈ 1,000 m³/day, p < 0.001), although the time-averaged LST rates obtained from the four models indicate eastward transport. The four models identified areas of convergence and divergence of sediment transport with increasing rates to the east. The modeling results may support effective coastal management initiatives when integrated with topographic profiles, numerical modeling, satellite imagery, and historical information.

Reservoir sediment flushing, one of the most effective strategies for alleviating reservoir sedimentation, involves discharging sediment-laden flows downstream through bottom tunnels. However, whether flushing can be accomplished if the intake of a bottom tunnel is initially covered by cohesive sediment remains poorly understood. Here, flume experiments were done to investigate cohesive sediment flushing in a reservoir. It is demonstrated that cohesive sediment in a reservoir is harder to flush than non-cohesive sediment. A higher water level in the reservoir, initially smaller cover layer thickness, and lower dry density of the sediment favor the occurrence of sediment flushing. The flushing process of cohesive sediment is significantly affected by seepage. Under the combined action of gravity erosion and water erosion, the scour hole upstream of the dam is characterized by angular and broken edges. The threshold conditions for flushing of non-cohesive and cohesive sediments are evaluated. Empirical formulas applicable to both non-cohesive and cohesive sediment are proposed to estimate the equilibrium scour depth immediately upstream of the bottom tunnel intake. Also, empirical models are proposed for the time variation of sediment position in the bottom tunnel. The current findings are significant for informing the design and operation of reservoirs on rivers carrying fine-grained cohesive sediment in support of reservoir benefits and capacity preservation.

River sediment is comprised of complex mineral systems composed by different kinds of organic and inorganic matter, and thus, is difficult to characterize. Besides, some standard techniques, such as X-ray diffraction (XRD), energy dispersive X-ray (EDX), optical and scanning electron microscopy, Fourier transmission infrared spectroscopy, inductively couple plasma-mass spectrometry (ICP-MS), and simultaneous Thermogravimetric Analysis – Differential Thermal Analysis (TGA-DTA), Mössbauer spectroscopy and magnetometry can provide substancial information about the compositional, physical, and chemical characteristics. In the current study, the versality of these methods is tested and the information provided by these methods for eight sediment samples, collected from the Moquegua River, Peru is compared. Qualitative analysis indicates that the samples consist of sand grains with different shapes, sizes, and colors coexisting with the presence of some diatoms. The chemical and mineralogical analysis reveal that the samples are composed mainly of silicon (Si), aluminium (Al), sodium (Na), potassium (K), aluminon–silicates, and carbonates, typical for river sediment. More detailed information obtained by these techniques include the discovery of adsorbed oxygen–hydrogen (O–H), carbon–H (C–H) and C, from organic matter, the thermal reactions and decomposition of the components, and the identification of the minor iron–oxides components. Further, other properties such as magnetic interaction are also analyzed in detail.

Annual sediment load transportation in the Indus River varies from 0.725 to 1.0 Mt/d and varies from 260 to 300 Mt/y. Sediment accumulation upstream of the inline structure (Guddu Barrage) has frequently increased. Consequently, the sediment accumulation reduces the intake canal supply, design withdrawals, and flood-carrying capacity of the Guddu Barrage. Furthermore, the Indus River changes its behavior, channel dimensions, pattern, and flooding frequency with respect to temporal and spatial morphology with braided high to low meanders. In the current study, the Hydrologic Engineering Center- River Analysis System (HEC-RAS) model, in combination with ArcGIS, were used to study sediment dynamics, analyze flood profile/water surface elevation, and assess erosion and deposition of sediment. In addition to this, a quasi-unsteady flow analysis method was used to simulate sediment transport from July to September 2010. It was found the invert change due to sediment transport maximum aggradation was 6.40 ft (1.950 m), and the maximum degradation was 30 ft (9.144 m), which further varies with the hydraulic conditions of the model. Cumulative mass bed change, sediment transport aggradation was 10.50 million tons (9.53 million t), and degradation was 3.7 million tons (3.3 million t). Moreover, it was found cumulative longitudinal mass change, sediment transport aggradation was 155 million tons (140.62 million t), and degradation was 10 million tons (9.07 million t). The cumulative mass inflow was 320 million tons (290.3 million t). Whereas the model revealed that the flood level upstream and downstream of the hydraulic structure was 264 ft (80.467 m) and 260.29 ft (79.34 m), respectively. Therefore, the HEC-RAS model accurately represents the sediment transport and water levels observed at a gated weir, which is an inline structure.