International Journal of Sediment Research最新文献

Spur dikes are structures built along riverbanks that serve two purposes: stabilizing the banks and minimizing erosion risk by controlling water flow in the river channel. The current study used L-shaped spur dikes in an alluvial channel to analyze the bed morphology and flow pattern in the spur dikes zone with the influence of no-seepage and two distinct seepage velocities, V_S1 = 0.075 mm/s and V_S2 = 0.15 mm/s near the channel bed z/h < 0.2. The experimental study was also done to examine and compare the transformation in the local scour depth for the seepage condition. According to the study results, downward seepage movement causes significant modification in the channel's bed elevation and the development of scour depth. Observations indicate that the maximum local scour occurs at the first spur dike's leading edge. Seepage velocity V_S1 results in a 16.1% increase in the maximum scour depth compared to the no-seepage scenario. In comparison, seepage velocity V_S2 causes an increase of 25.2% in the maximum scour depth. Due to downward seepage, the flow distribution is shifted down near the channel's boundary. With an increase in the seepage rate, the magnitude of velocity, Reynold shear stress, turbulent kinetic energy, and bed shear stress also rise close to the channel's boundary. The current study also examined bursting events near the channel's bed under seepage and no-seepage conditions. These events included outward interaction, inward interaction, ejection, and sweep. Quadrant analysis of velocimeter data revealed that ejection and sweep were the dominant events contributing to the production of Reynolds shear stress in seepage and no-seepage flows. Meanwhile, outward interactions and inward interactions made minor contributions compared to ejection and sweep events to the Reynolds shear stress.

Hydromorphodynamic interactions with vegetation are a part of fluvial biomorphodynamics in actively meandering rivers. Using palynofacies and grain size from sub-urban to urban reaches across the river valley, the spatial patterns of organic matter behavior are examined in a 38 km reach of the Gomati River in Lucknow District, Uttar Pradesh, India. This is done to understand how they respond to the alteration, preservation, and degradation after getting transported and deposited in sediment. Thirteen surface sediment samples of the Gomati River floodplain were analyzed for palynofacies and grain size to ascertain its fate in this reach, which comprises the big picture for past human settlement. The shifts in the proportions of palynofacies associations, i.e., phytoclasts, palynomorphs, and amorphous organic matter (AOM) along with grain size, are considered to visualize the depositional process. The CONISS cluster analysis revealed four zones reflecting high degradation and alteration of palynofacies in the urban regime compared to the sub-urban reaches where the low interference with natural settings illustrates the low deterioration of palynofacies. The relation between grain size and palynofacies was obtained using Principal Component Analysis (PCA) to emphasize the correlation with palynofacies in the meandering fluvial system of the Gomati River. In the floodplain deposits, the behavior of palynofacies, allows for the distinction of the regional aspects of fluvial sediment disposition. The current study compares urban and sub-urban settlement premises of today's communities and contributes to the understanding of the growth, dispersal, and decline of earlier human settlements.

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.