International Journal of Sediment Research最新文献

In the present era, sedimentation and bed morphology near hydraulic structures is a great concern as it affects the flow configuration and reduces the discharge capacity. Experimental investigations were done to study the flow field in the vicinity of one cycle, two cycle, and three cycle piano key weirs (PK weirs) with noses to monitor the sediment passing capacity of the weir. The three dimensional (3D) velocities were measured close to the inlet and outlet keys using an acoustic Doppler velocimeter (ADV). The velocity fields near both the inlet and outlet keys were examined, and it was found that there were considerable increases in the vertical (v-component) and lateral (w-component) velocities. Added to this, it was seen that the mean vertical velocity was more in front of the outlet key as compared to the front of the inlet key while the mean lateral and longitudinal velocities were low. The maximum velocity was observed in front of the inlet key which resulted in an increase of sediment movement over the PK weir. Furthermore, 2.6%–5.2% of the total sediment passing over the upstream portion of the weirs are flushed over the inlet key by self-cleaning. The flow field over the keys could not be accurately estimated using an ADV, possibly due to the flow complexity so computational fluid dynamics (CFD) simulations were done to understand the complex flow field for all the three PK weirs using a CFD solver which needs less computational cost and space. The widely used standard k–ε turbulence model (an eddy-viscosity model) was applied in the current numerical investigations. The numerical investigation shows that the magnitude of the velocity components was increased because of the vertical contraction in front of the sloped keys owing to very high flow immediately downstream of PK weirs.

Grainsize is among the most important parameters in aeolian research as it controls the sediment mobilization and the mode of transport. Therefore, it is a critical parameter for instance in wind erosion modeling studies which are used to analyze sand and dust storms in the Tarim Basin, an important natural hazard in this region. Spatially explicit parameterization of grain size is difficult, as the texture of the topsoil is not homogeneous across a landscape. Thus, a geomorphological classification of a Tugai landscape in the eastern Tarim Basin is developed and a stratified analysis of the grain-size distributions and the corresponding threshold friction velocities is presented. The results show that transversal dunes have the coarsest sediment in this landscape, while vegetated patches within the alluvial plain of the Tarim River are characterized by the finest sediments. The other landform classes open space, channel, and nebkha have properties between these two landform types. It is concluded that the surface sediment of a Tugai landscape in the eastern Tarim Basin shows a considerable heterogeneity. The landform-based stratification for grain-size analysis is an appropriate solution for an assessment of sediment grains.

Sediment pulses can cause relevant geomorphological alteration and fine sediment deposition, affecting the entire river ecosystem. Quantifying the impact of these events is becoming increasingly important, as sediment disturbance is predicted to intensify because of growing anthropogenic activity combined to the effects of current climate change. In this paper, the effects of a sediment pulse event on riverbed morphology and, in turn, on fish habitat availability are assessed in a regulated river, located in the southern European Alps. Specifically, four target fish species were selected: marble (Salmo marmoratus) and brown trout (Salmo trutta), grayling (Thymallus thymallus), and bullhead (Cottus gobio). The investigated event was triggered by the effects of localized landslides combined with an uncontrolled sediment release from a hydropower reservoir. A pre-post event integrated approach was applied, involving topographic surveys, fine sediment deposition measurements, and eco-hydraulic modeling. Habitat availability analysis was done using two-dimensional steady-state channel-flow simulation and standard habitat suitability modeling, with univariate suitability curves as biological input. The investigation focused on two river reaches, quite close each other, but characterized by significantly different streamflow patterns, due to the layout of the local hydropower system. The results showed that noticeable riverbed fining occurred at both study reaches after the pulse, with the maximum thickness of the fine sediment deposits up to one meter, and higher amounts of deposition detected in areas wetted at baseflow. Moreover, for both reaches, recovery to the pre-event riverbed grain-size condition occurred in about a year, but with differences related to different streamflow patterns. Pre-post topographic comparison showed that areas of erosion prevailed over areas of deposition, and fine sediment deposition was not systematically associated to the riverbed elevation raise. Simulated habitat availability was affected by both topographic changes and fine sediment deposition, with the latter predominantly impacting juveniles and the most bottom dwelling species, i.e., the bullhead. The study confirms the need for improving the management of the hydro-sedimentary regime of regulated rivers to preserve the habitat of fish species, particularly of those characterized by conservation status already endangered.

Measuring the grain size distribution (GSD) of unconsolidated particles is critical to understanding coastal spreading, riverbed dynamics, and sediment transport. The current study presents a novel gravel automatic sieving (GAS) method designed to improve the accuracy and reliability of particle size analyses. At the macroscopic, the method utilizes the convex hull property of gravel to define the maximum extent of the searched gravel, effectively reducing over and under-segmentation problems. At the microscopic, the accuracy of gravel segmentation is improved by analyzing the color space characteristics of gravel to identify the pixel patches of gravel accurately. To validate the effectiveness of the GAS method, the proposed method was tested in both the laboratory and the field. In the laboratory, four artificial samples were processed using the GAS method, and the results were compared with those obtained using the traditional sieving method. The results showed that the correlation coefficients between the GAS method and the traditional sieving method ranged from 94.3% to 97.8%, and the relative errors ranged from 5.8% to 20.9%, demonstrating the validity of the GAS method. In addition, the application of ImageJ software to manually identify the particle size method (ImageJ method) was also compared with the mechanical sieving method, and the correlation coefficient between the two methods was greater than 98.2%, and the relative error was less than 10.9%, so the ImageJ method can be used as a standardized method to measure the other methods. In the field, sixteen images taken in four different regions and at different times were analyzed using the ImageJ method as a benchmark. The performance of the automatic with image filtering (AIF), BASEGRAIN, and the GAS methods also were compared. The results show that the relative errors range from 28.1% to 94.6% for the BASEGRAIN, 16.8% to 1003.6% for the AIF method, and only 5.6% to 30.7% for the GAS method. As a result, the GAS method demonstrates higher accuracy and stability in complex environments.

In agricultural landscapes, the use of topographic index (TI) models has been common to predict the presence and extent of ephemeral gullies (EGs). However, these models face two significant challenges: (1) the accurate prediction of EGs relies heavily on a critical threshold (CT) value, which is difficult to determine optimally using existing strategies, and (2) the calibration of TI models limits their applicability on a larger scale. To address these limitations, the current study proposes two methods: (1) the division of the study area into zones based on key factors influencing gully formation, reducing the need for TI model calibration, and (2) a pixel-based binary classification approach coupled with a precision performance metric to identify the calibrated CT value within a watershed. The performance of seven TI models for predicting EG length was evaluated using local validation within zones and transferred validation between zones. Local validation demonstrated that among the TI models, modified stream power index (MSPI), stream power index (SPI), and compound topographic index (CTI), in descending order, yielded the most accurate predictions for EG length. Furthermore, the decrease in accuracy observed in the transferred MSPI model compared to the local MSPI model supported the study's hypothesis that dividing a large-scale area into distinct zones with varying topographic and climatic characteristics enables the determination of a CT value specific to each zone. Soil loss rates due to EGs ranged from 0.36 to 1 kg/m² yr, aligning with findings from similar global studies. These findings offer valuable insight that can be integrated into comprehensive watershed and soil erosion models.

Vegetation-induced sedimentary structures (VISS) are bed features resulting from flow-vegetation feedback and turbulence induced by vegetation at riparian sites. Generally, these sedimentary structures commence at the individual plant level, but sediment accumulation over time may lead to the formation of fluvial islands. Additionally, riparian vegetation exposed to current may provoke local scouring. However, little research has been done on how the permeability of plants affects flow fields and sedimentary processes. Therefore, the main aim of the current study is to experimentally investigate patterns of local scour and deposition around artificial obstacles with six different levels of porosity, using a systematically designed flume experiment. Two experimental setups were implemented, with sediment transport occurring either as bed load or in suspension. Porosity, as a measure of the volume of porous space to bulk volume, was determined using optical porosity which was estimated from digital photographs. Furthermore, VISS of individual P. nigra shrubs were investigated at different seasonal stages (leafless and full foliage) at the Loire River, France, for comparison. Results show that increasing porosity caused a reduction in sediment deposition from suspension and locally scoured bed load sediment. A threshold value for optical porosity was determined, at which point a prolonged area of flow calm develops behind the barrier, promoting the accumulation of suspended sediment and suppressing local scour caused by bed load transport. An empirical model is presented, which assesses sediment erosion and deposition volumes at porous obstructions, considering (a) the flow-exposed surface area, (b) the obstruction's optical porosity, and (c) the intensity of the incoming flow. On the field scale, local scour holes were identified at P. nigra shrubs where flood-caused debris obstructions were stuck at their front, lowering the obstruction's porosity and encouraging local scour processes.

Local scour at piers is one of the primary causes of bridge failures. The prediction method for the temporal evolution of local scour depth at bridge piers during floods was investigated based on the law of conservation of energy. The energy transfer process between water flow and sediment during the local scour process at bridge piers was theoretically analyzed based on the current understanding of the mechanism of local scour. The results show that there is a dynamic equilibrium relation between the energy loss of water flow and the energy gain of eroded sediment during the local scour process. This relation is applied to establish a mathematical model for predicting the temporal evolution of scour depth. This model has only one parameter, which is the energy transfer efficiency between the water flow and the eroded sediment. The energy transfer coefficient is mainly determined by the flow intensity under clear-water scour conditions, and an empirical formula for calculating it is obtained. The proposed model was evaluated using measured data of local scour depth under both well-controlled flows and natural floods. The results show that the model is able to provide satisfactory predictions and its performance can be further improved by including more sophisticated methods for determining the critical velocity for incipient scour. Meanwhile, the performance of the model is insensitive to the possible uncertainties introduced when determining the energy transfer coefficient. The research results indicate that it is feasible to establish a theoretical prediction model for accurately forecasting the local scour depth at bridge piers.

River levees are subject to bioturbation by various animals which can actively excavate into earthen structures producing an internal erosion that, during the passage of a flood, can grow in time making the levee unstable. This phenomenon can lead to river levee breaching and, as a consequence, collapse, even for relatively minor flood events. A well-known animal burrower is represented by the North American crayfish Procambarus clarkii (P. clarkii), an invasive species in Europe, mainly introduced for commercial purposes, causing a decline in biodiversity and profound habitat changes. The physical damages caused by P. clarkii on levees and banks, such as in rice fields, irrigation ditches, and small channels, have not been fully studied and behavioral components underlying this impact are mostly occasional. To understand the impact of burrowing activity on the seepage process, a field survey was done in a drainage channel in Tuscany, Italy, to evaluate the density and geometry of the internal burrows that were excavated by the crayfish. Based on these observations and some previous laboratory experiments, three dimensional (3D) numerical simulations of the seepage processes were done inside burrowed levees. Numerical results allowed the increase in the hydraulic vulnerability of levees to the process of internal seepage to be disclosed. In particular, for a given river water level, the reduction of the time scale for the phreatic line to reach the levee field side appears to be a function of a quantity here defined as the burrow hydraulic gradient. This quantity is here defined as the ratio between the hydraulic head inside the burrow and the horizontal distance from its end to the field side of the levee. Moreover, a comparison between the 3D with the analogous more common two dimensional (2D) numerical simulations illustrated the schematization which is better suited for describing the seepage processes when animal burrows, not only by crayfish, are present.

The Xiangjiaba and Xiluodu reservoirs, located on the upper Yangtze River, have been operational and the changes in the downstream river regime and waterway conditions have attracted great attention. A two-dimensional horizontal (2DH) flow–sediment mathematical model of the Shuifu to Lanjiatuo reach (Shuilan reach) downstream of the Xiangjiaba Dam was developed. The spatio-temporal characteristics of sediment scouring and siltation after completion of the cascade of reservoirs were simulated using the model, and then the river regime changes in several typical reaches as well as their possible impacts on navigation channels were analyzed. The results show that, after operation of the cascade of reservoirs, the downstream channel is in the state of scour. The general change of the river regime is that the flow in curved reaches tends to be straight though the river pattern still remains curved. Sub-branches tend to experience deposition, while the main channel will be scoured deeper, which can help to increase the navigation depth in the shoals in the transition sections. Deposition in some sections could result in new shoals. In addition, the bottom elevation difference between floodplains and channels will increase, which could aggravate local adverse flow patterns; therefore, navigation conditions in torrential shoals will tend to deteriorate.