Behavior Of Sediment Transport Research Articles

Bridging the gap between turbulence and larger scales of flow motions in rivers

ABSTRACTAlthough flow turbulence in rivers is of critical importance to earth scientists, ecologists and engineers, its relations with larger flow scales are not well understood, thus leaving a fundamental gap in our knowledge. From an analysis of a long time series of the streamwise and vertical flow velocity fluctuations measured in a gravel‐bed river, we show that the signature of the fundamental turbulent flow structures (e.g. ejections and sweeps) is embedded within increasingly larger flow scales in a self‐similar manner. The imbrication of turbulent structures into large flow pulsations of flow acceleration and deceleration covers more than two‐orders of magnitude from a few seconds to nearly 10 minutes. This property is explained by the clustering of turbulent events creating an emergent pattern at larger scales. The size of the larger flow pulsations scales with the spacing of the pools and riffles in the river. This implies a mutual adjustment between turbulence generation mechanisms and long pulsations of flow acceleration and deceleration controlled by the bed morphology. These results bridge a gap in our understanding of flows in rivers and offer a new perspective on the interactions between the turbulent flow with larger scales of flow motion that are critical for sediment transport, habitat selection and fish behaviour. Copyright © 2011 John Wiley & Sons, Ltd.

Computations of suspended sediment transport in a shallow side-cavity using depth-averaged 2D models with effects of secondary currents

Two-dimensional (2D) depth-averaged shallow flow models’ integrated effects of secondary currents are applied to open channel flows with a side cavity, and the accuracy of predictions of the flow structures as well as the behavior of suspended sediment transport is examined. The flow in a side-cavity is characterized by formations of a steady recirculation with a vertical axis and associated secondary current due to a centrifugal force (a secondary current of the first kind). The flow structures become completely three-dimensional and suspended sediment transports are affected by such complicated flow features. In this study, we applied depth-averaged shallow flow equations including effects of secondary currents, instead of 3D flow equations considering computational efficiency. We applied 4 different types of models for shallow flow equations and compared the performances of each model. First, these models were applied to a simple flow in a torus-shape channel with rigidly rotating flow, in order to consider the fundamental characteristics of suspended sediment transport. Then the models were applied to open channel flows with a rectangular side cavity, in order to consider both flow structures and sediment transport affected by the secondary current. The computations were performed under the same conditions of the laboratory tests and applicability of each model was discussed through the comparison between computational and experimental results. The model considering the lag of development of secondary current behind the streamline curvature, and the deformation of mean velocity profiles affected by secondary currents showed excellent performance for predicting flow phenomena with recirculation.

Sediment Trend Analysis in Support of Unexploded Explosive Ordnance Risk Assessment

Abstract For 48 years (1952–2000) the south half of Lac Saint-Pierre, a widening of the St. Lawrence River between Montreal and Quebec, was used as an experimental firing range. Some 500,000 shells were fired into the lake, of which about 8000 are thought to contain energetic material. Approximately 2000 projectiles are considered as unexploded explosive ordnance (UXO). Furthermore, the lake itself is potentially the recipient of other contaminants associated with industrial wastes and agricultural runoff. The Canadian Department of National Defence, in order to research appropriate clearance approaches programs, instigated a large sediment transport study (sediment trend analysis or STA) to determine both sediment and contaminant sources and sinks and to assess the possible environmental consequences of various clearance options. STA is an empirical technique whereby patterns of sediment transport and dynamic behavior are derived from relative changes in grain-size distributions. About 1000 samples were collected at 500 m spacing encompassing the entire firing range. The lake bottom contained a complex number of sediment types ranging from “ancient” late glacial deposits, recent mud and sand, and a variety of mixtures. The STA identified the significance of the late glacial deposits as the dominant source for the sediments contained in the lake. The latter showed complex patterns of transport that originated from the ancient sediments and terminated on well-sorted sand. The findings demonstrated that anthropogenic contaminants were unlikely to be deposited in the lake sediments, or if they were, self-cleaning by natural processes would likely be rapid. Furthermore, trace metals were strongly associated with the ancient sediments evidently originating from an underlying anaerobic black shale formation. The STA provides an important framework to understand the chemistry of the sediments and their dynamics and helps to define the context within which the UXO clearance and environmental remediation options can be developed.

Methods for medium-term prediction of the net sediment transport by waves and currents in complex coastal regions

Medium-term prediction of sediment transport and morphological behaviour in the coastal zone is becoming increasingly important as a result of human interference and changing environmental conditions. The interaction of waves and tides is shown to play a pivotal role in the net (annual) sediment transport and morphodynamics of the coastal zone. The Telemac Modelling System has been applied to the Dyfi Estuary and neighbouring coastline, mid Wales, to recreate the annual wave–current conditions and the resulting sediment fluxes. ‘Input reduction’ methods have been required to produce realistic schematisations of events in practical computation times. A field campaign carried out in 2006 provided data for validation of the flow module ( Telemac-2D) and also observations to verify the patterns predicted by the wave module ( Tomawac). To improve model accuracy refinements were implemented with regard to the sand transport formulation used in the sand transport module ( Sisyphe). Here, a parameterisation of the results from the UWB 1DV sand transport ‘research’ model, for the conditions in the Dyfi Estuary, has been introduced, allowing Sisyphe to provide greater realism in the morphological predictions. The model predictions are presented along with a discussion of the success/failure and limitations of the modelling methods applied.

Persistent sand bars explained by geodynamic effects

[1] The persistent nature of intertidal sand bars has been the subject of much speculation concerning the hydrodynamic mechanisms involved, but its origin remains enigmatic. Here, we introduce salient geophysics in contrast to the physics of fluids above the sediments. The geophysical evidence combined with theoretical modeling and analysis demonstrates that the geodynamic processes ensuing during exposure periods have a profound impact, yielding the persistent nature of the intertidal bars under severe hydrodynamic forcing which would otherwise lead to unstable bar behavior. The feedback between the effects of the dynamics of suction, i.e. negative pore water pressure relative to atmospheric air pressure, and sediment transport and morphology is found to play a crucial role in the intertidal bar morphodynamics. Our finding may fundamentally alter the current perspective, leading to a new level of understanding, of sediment transport and bar behavior at waterfronts that are ubiquitous in rivers, estuaries, and coastal seas.

Sediment Transport and Contaminant Behavior in the Buffalo River, New York: Implications for River Management

Abstract The lower 9 km of the Buffalo River that flows into the eastern end of Lake Erie has been designated by the International Joint Commission as a Great Lakes area of concern (AoC) because of poor water quality, degraded riparian and river habitat, and contaminated sediments—impairments related to a long history of contamination from the industrial legacy of the past century. As a designated AoC, attention is presently focused on sediment remediation, an endeavor requiring an assessment of the relationship between sediment transport processes and sediment contaminant concentrations. In 1990 a pilot sediment trend analysis (STA) revealed an upriver return of sediments from the mouth of the Buffalo River as far as 5 km inland. A complete STA conducted in 2004 confirmed the upriver transport regime. Examination of river discharge and Lake Erie water levels demonstrated that lake seiches occur at far greater frequencies than river discharges of a magnitude capable of transporting sediment. Thus the rive...

Characteristics of turbulent boundary layers over a rough bed under saw-tooth waves and its application to sediment transport

A large number of studies have been done dealing with sinusoidal wave boundary layers in the past. However, ocean waves often have a strong asymmetric shape especially in shallow water, and net of sediment movement occurs. It is envisaged that bottom shear stress and sediment transport behaviors influenced by the effect of asymmetry are different from those in sinusoidal waves. Characteristics of the turbulent boundary layer under breaking waves (saw-tooth) are investigated and described through both laboratory and numerical experiments. A new calculation method for bottom shear stress based on velocity and acceleration terms, theoretical phase difference, φ and the acceleration coefficient, a c expressing the wave skew-ness effect for saw-tooth waves is proposed. The acceleration coefficient was determined empirically from both experimental and baseline k– ω model results. The new calculation has shown better agreement with the experimental data along a wave cycle for all saw-tooth wave cases compared by other existing methods. It was further applied into sediment transport rate calculation induced by skew waves. Sediment transport rate was formulated by using the existing sheet flow sediment transport rate data under skew waves by Watanabe and Sato [Watanabe, A. and Sato, S., 2004. A sheet-flow transport rate formula for asymmetric, forward-leaning waves and currents. Proc. of 29th ICCE, ASCE, pp. 1703–1714.]. Moreover, the characteristics of the net sediment transport were also examined and a good agreement between the proposed method and experimental data has been found.

Origin of Morphodynamic Stability of Intertidal Sand Bars

The persistent nature of intertidal sand bars has been the subject of much speculation concerning the hydrodynamic mechanisms involved, but its origin remains enigma. Here, we aim to resolve this question by introducing salient geophysics to the analysis of intertidal sediments, in contrast to the physics of fluids above the sediments. The geophysical evidence combined with theoretical modeling and analysis demonstrates that the feedback between the effects of suction dynamics and sediment transport and morphology plays a crucial role in the intertidal bar morphodynamics, yielding the persistent nature of the intertidal sand bars. Our finding may fundamentally alter the current perspective, leading to a new level of understanding, of sediment transport and bar behavior at waterfronts that are ubiquitous in rivers, estuaries, and coastal seas.

An Interface between the Agricultural Non-Point Source (AGNPS) Pollution Model and the ERDAS Imagine Geographic Information System (GIS)

The U.S. Department of Agriculture developed the Agricultural Non-Point Source (AGNPS) pollution model. The AGNPS pollution model simulates the behavior of runoff, sediment, and nutrient transport from watersheds that have agriculture as their prime use. This model has been used extensively by scientists conducting hydrologic or water quality analyses using computer modeling in an attempt to further understand the complex problem of managing non-point sources of pollution in a watershed hydrology domain. A difficulty with AGNPS is creating and formatting all of the data necessary to execute the model to conduct landscape modeling and watershed analyses. A unique Windows-based program, the AGNPS Data Generator (ADGen), has been developed to simplify the task of preparing and creating the input for AGNPS through an interface with ERDAS Imagine (a Leica Geosystems product). Because of the complexity and quantity of the input required and the nature of the output text file produced by AGNPS, ADGen is a helpful tool for the researcher who is trying to analyze non-point source pollution.

Modelling the long‐term fluvial erosion of the River Somme during the last million years

AbstractA process‐based model that simulates fluvial erosion in the River Somme Valley over the last million years is presented here. The model takes into account lithology and climatic influences and allows the simulating of undercapacity and overcapacity sediment transport behaviour. The model has been calibrated to a family of terraces within the River Somme Valley. When matched to this field data, simulation trials suggest that bedrock incision occurred principally from 120 to 60–40 kyr during the last climatic cycle and before the last glaciation. The impact of a progressive tectonic uplift (c. 60 m over c. 1 million years) on the River Somme has also been studied here. Extended over a longer period of time, the simulations suggest that 1 million years ago the profile of the River Somme had a lower slope gradient than today, with little relief throughout the Paris Basin.

CHARACTERISTICS OF TURBULENT BOUNDARY LAYER OVER A ROUGH BED UNDER CNOIDAL WAVES MOTION

Ocean waves in reality often have a strong non-linear shape when propagating to shallow water, and the gap from the sinusoidal wave become remarkable. Therefore, it is generally believed that wave boundary layers, bottom shear stress and sediment transport behaviors actualizing the effect of non-linearity are different from sinusoidal wave. However, the example of research treating such turbulent boundary layer and sediment transport characteristic is very few. Therefore, the accuracy of bottom shear stress and amount of sediment transport used to evaluate the beach morphological change obtained from the wave motion model of sinusoidal is necessary to be clarified by that of non-linear. In this present study, turbulent boundary layer characteristics for asymmetric or non -linear waves according to the non-linearity effect is examined through both experimental and BSL turbulence model. Moreover, a new calculation method of botto m shear stress based on incorporating acceleration and velocity terms is used to examine both the experimental and BSL model results of bottom shear stress.

Geomorphology of the Ganges fluvial system in the Himalayan foreland: an update

The Ganges is one of the largest fluvial systems in the world rising from the loftiest Himalaya and draining into the Bay of Bengal. Together with the Brahmaputra, it also constitutes the largest delta in the world before finally meeting the sea. The Ganges system passes through a variety of terrain from the rugged mountains through the flat alluvial plains and the sea margin, and also transects variable climatic zones. As a result, the processes, landforms and stratigraphy are strikingly different in different zones of the system. This paper attempts to provide an update on our understanding of this very large and diverse system. A global effort has been made in the last few decades, and the research has focused on a variety of themes. The mountainous catchments have attracted attention in view of the extent of glaciation and extensive erosional processes. The alluvial plains of the Ganges symbolizes the life line of one of the world's largest population. Consequently, a number of studies have been carried out on the morphology, hydrology including flooding history and sediment transport behaviour of the river system. The alluvial stratigraphy of the large valleys and the interfluves in the plains has provided insight about the sedimentation pattern and response to climate change. The deltaic plain is the final destination of this huge sediment dispersal system before it drains into the sea, and it also records the influence of sea level changes apart from the upstream catchment controls over a period of time.

Supply limited sediment transport in a high‐discharge event of the tropical Burdekin River, North Queensland, Australia

AbstractInteractions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m3 s−1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m3 s−1). About 3·7 × 106 tonnes of suspended sediment and 3 × 105 tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river.

Two-phase modeling of suspended sediment distribution in open channel flows/ Modélisation diphasique de la distribution de sédiments en suspension dans un écoulement à surface libre

The transport of suspended sediment in open channel flows is of key interest to the area of fluvial hydraulics. Traditionally, the phenomenon is analyzed as a one-phase composite system for simplification. In reality, two separate phases of liquid and solid are present. In this study, the two-phase approach of particle group model is adopted to quantify the behavior of suspended sediment transport in open channel flows. The analysis reveals a drift current due to the gradient of turbulence fluctuation, in additional to the Fickian-type dispersion due to the concentration gradient. In the vertical direction, the resulting equation from the analysis is similar to the classical convection-diffusion equation such as the Rouse model. However, the incorporation of the additional dispersion terms predicts more accurately the sediment concentration distribution along the water depth. In the horizontal direction, the analysis shows that the mean fluid velocity is always larger than that of the sediments. The inter...

Two-phase modeling of suspended sediment distribution in open channel flows

The transport of suspended sediment in open channel flows is of key interest to the area of fluvial hydraulics. Traditionally, the phenomenon is analyzed as a one-phase composite system for simplification. In reality, two separate phases of liquid and solid are present. In this study, the two-phase approach of particle group model is adopted to quantify the behavior of suspended sediment transport in open channel flows. The analysis reveals a drift current due to the gradient of turbulence fluctuation, in additional to the Fickian-type dispersion due to the concentration gradient. In the vertical direction, the resulting equation from the analysis is similar to the classical convection-diffusion equation such as the Rouse model. However, the incoiporation of the additional dispersion terms predicts more accurately the sediment concentration distribution along the water depth. In the horizontal direction, the analysis shows that the mean fluid velocity is always larger than that of the sediments. The interaction between the sediment and fluid and the effects of sediment inertia are also addressed. Experiment data from the literature are used to verify the results.

MODELING OF TOTAL SEDIMENT LOAD TRANSPORT IN ALLUVIAL RIVERS

Study of total sediment load transport is an important consideration of the fluvial process in many nowaday rivers. In order to improve the prediction of sediment transport and morphological behavior of the channel bed, a mathematical model taking bed load, suspended load and wash load into account has been developed. In contrast to previous researches, wash load has been especially accentuated from the perspective of cohesive sediment transport. For a better simulation of the interrelationship between different sediment loads and the river bed, cohesive and non-cohesive regimes were distinguished for the bed depending on the critical clay content. Two cases of with and without wash load have been performed using a 1D numerical modeling approach. The comparison between the computational results and field data exhibited its advantages over the conventional models.

Double-thresholds in scour–fill processes and some implications in channel adjustment

Through an analysis of data collected from the Yellow River and its tributaries on the Loess Plateau of China, the phenomenon of double-thresholds in scour–fill processes of wide-range water-sediment two-phase flows has been shown. Thresholds located in non-hyperconcentrated flows may be called the lower threshold, and that in hyperconcentrated flows the upper threshold. This double-threshold phenomenon leads to complicated sediment transport behavior of heavily sediment-laden rivers. With an increase in suspended sediment concentration, the channel sediment delivery ratio increases initially and becomes higher than 1, followed by a decrease and finally becomes lower than 1 again. Controlled by the double-thresholds in the scour–fill processes, channel adjustment of the lower Yellow River is non-linear and complex. When the suspended concentrations were lower than the lower threshold or higher than the upper threshold, scour or bed downcutting was the dominant channel-forming process. Channel shape tends to be narrower and deeper, and the channel thalweg became more sinuous. When the suspended concentrations lay between the lower and upper thresholds, deposition of sediment was the dominant channel-forming process; channel shape tended to be shallower and wider, and channel thalweg became less sinuous.

Flattened Residue Effects on Wind Speed and Sediment Transport

Mulching with flattened crop residues is widely used to protect soils from wind erosion. Several wind tunnel and field experiments have shown decreased protection of some soil covers with increasing wind speed. In some studies, sediment transport was enhanced with flattened residue as compared with the bare soil condition. The purpose of this article was to determine the behavior of wind speed and sediment transport when the soil surface is covered with randomly applied, flattened crop residues. A literature review was conducted to evaluate recent insights in turbulent flow properties and related sediment transport. A conceptual model was then developed to explain decreasing soil protection of a certain residue quantity when the free stream wind speed increases. The main reason is the change in turbulent flow properties of the near‐surface wind when no‐erodible roughness elements are added to an otherwise smooth surface. The average wind speed is reduced by the roughness but the probability distribution of instantaneous wind speed becomes wider and positively skewed. If free stream wind speed increases, at a certain moment the changed turbulence will cause more wind gusts that exceed the threshold wind speed for soil particle movement than would occur over a bare surface. But it is likely that this only happens when soil cover is less than 10%. For higher soil covers, increasing wind speed will also cause a decrease in soil protection, but natural wind speeds are normally not sufficient to cause enhanced sediment transport, because average wind speed is sufficiently reduced.

OBSERVATION STUDY ON SEDIMENTATION PROCESSES IN A STEEP AND HIGH MOUNTAIN RIVER BASIN

It is very important for the prediction of sediment runoff into reservoirs and downstream of rivers to evaluate the behavior of sediment transport in a river basin located at high mountains. However, the difficulty for this study due to steep topographies and extensive research areas have hindered the accumulation of reliable data.This paper discusses characteristics of production, transportation and deposition processes of sediment in failed slopes and streams, and the overall situation of sediment balance of the basin, based on the on-site data such as aerial photographs and slope erosion data obtained from measuring sticks. The study area, Takase Dam basin, is surrounded by mountains higher than 2800 m in elevation. Characterized by steep topography and weak geology consisting of deteriorated granites, the area is one of the major sediment runoff areas in Japan.

How glaciers entrain and transport basal sediment: Physical constraints

Simple insights from the physics of ice, water and sediment place constraints on the possible sediment-transport behavior of glaciers and ice sheets. Because glaciers concentrate runoff, streams generated by glaciers transport much sediment and may erode bedrock rapidly. Deforming glacier beds also can transport much sediment, particularly in marginal regions. Rapid sediment entrainment producing thick debris-rich basal zones may occur by regelation into subglacial materials, and by freeze-on from rising supercooled waters. Numerous other mechanisms may be important but primarily near ice margins, especially those of advancing or fluctuating glaciers. Several sediment-entrainment mechanisms may be active beneath a single glacier, but one process is likely to be dominant at any place and time.