Sediment Transport Mechanisms Research Articles

Importance of Sediment in Rivers, and Issues in Sediment Transport Mechanics

Importance of Sediment in Rivers, and Issues in Sediment Transport Mechanics

3D-NUMERICAL MOVABLE BED IN PARALLEL COMPUTATION FOR REAL SCALE SIMULATION OF SEDIMENT FLOW

In the computational mechanics of sediment transport, which approaches to movable-bed phenomena from the viewpoint of a computational dynamics, the numerical movable bed, or the numerical model of granular material, plays an significant role. A numerical movable bed is the tool which is based on the distinct element method analyzing the interparticle collision directly. Although the conventional numerical movable bed has been applied to a comparatively small domain, the application of numerical movable bed to the practical problems of movable-bed phenomena requires the handling of complicated boundary condition and resultant extreme increases of number of the calculation particles. Therefore the parallel computation by the three dimensional numerical movable bed is developed in this study, to enable the calculation with the 1, 000, 000 numbers of the particles. The performance of numerical movable bed is clarified, in a scale of practical problems, through the velocity of sediment in the fill and overflow processes of sediment control dam.

NUMERICAL SIMULATION FOR DEVELOPING PROCESS OF BED LOAD PARTICLES LAYER

Flow/particle and particle/particle interaction are key-factors to investigate internal structure of the bed load layer from the viewpoint of computational mechanics of sediment transport. In addition, interaction between bed load layer and fluctuating free water surface is also key-factor under high bottom shear condition. In this paper, the particle/ particle interaction was evaluated by the Distinct Element Method (DEM), the flow/particle interaction was evaluated by drag force and the free water surface was traced by the Lagrangian massless marker particles. Developing process of the bed load layer was simulated by the Euler-Lagrange coupling model, furthermore, the internal structure of the bed load layer was discussed by the velocity profile, ineter-particle force and so on.

浜崖形成予測のための擬似三次元海浜変形モデルの開発と応用に関する基礎的研究

Beach and dune scarps can be seen often after a typhoon, hurricane and northeaster. In general, beach scarp and resulted shoreline recession are computed using a cross-shore sediment transport model in EU countries and US. On the other hand, beach scarp due to longshore sediment transport mechanics has been often estimated using N-line model in Japan. There is still unknown factor to distinguish which process is dominant to cause beach and dune scarp in natural condition. Here, field studies were conducted after typhoons to clarify that at least longshore process could cause beach scarp in sandy beach. Then, a quasi 3-D model to simulate a beach scarp generation is developed based on longshore sediment transport calculation and resulted shoreline position. The new concept is applied to an idealized bathymetry condition.

Characteristics of the bedload and suspended sediment in a small torrent Evidence from intense field sampling for understanding sediment transport mechanism

Characteristics of the bedload and suspended sediment in a small torrent Evidence from intense field sampling for understanding sediment transport mechanism

Measuring flow velocity and sediment transport with an acoustic Doppler current profiler

An acoustic Doppler current profiler (aDcp) measures three-dimensional velocity profiles within the water column using the Doppler shift principle, whilst the bottom tracking function and acoustic backscatter can be used to measure bed load velocity and estimate suspended sediment concentration. The aDcp offers many advantages over traditional single-point current meters and sediment samplers, including deployment from a moving launch, a single instrument for both velocity and sediment transport measurements, profiles of three-dimensional velocity and suspended sediment and the ability to map an entire flow field. Limitations of aDcps include a large sampling diameter close to the bed, coarse measurement of vertical velocity, fragmented bottom track records, a poor understanding of the relation between bottom tracking and the mechanisms of sediment transport, and sensitivity of the acoustic backscatter to particle size.

Sheet flow and suspended sediment due to wave groups in a large wave flume

A series of sand bed experiments was carried out in the Large Wave Flume in Hannover, Germany as a component of the SISTEX99 experiment. The experiments focussed on the dynamic sediment response due to wave group forcing over a flat sand bed in order to improve understanding of cross-shore sediment transport mechanisms and determine sediment concentrations, fluxes and net transport rates under these conditions. Sediment concentrations were measured within the sheet flow layer (thickness in the order of 10 grain diameters) and in the suspension region (thickness in the order of centimetres). Within the sheet flow layer, the concentrations are highly coherent with the instantaneous near-bed velocities due to each wave within the wave group. However, in the suspension layer concentrations respond much more slowly to changes in near-bed velocity. At several centimetres above the bed, the suspended sediment concentrations vary on the time scale of the wave group, with a time delay relative to the peak wave within the wave group. The thickness of the sheet flow changes with time. It is strongly coherent with the wave forcing, and is not influenced by the history or sequence of the waves within the group. The velocity of the sediment was also measured within the sheet flow layer some of the time (during the larger wave crests of the group), and the velocity of the fluid was measured at several cm above the sheet flow layer. The grain velocity and concentration estimates can be combined to estimate the sediment flux. The estimates were found to be consistent with previous measurements under monochromatic waves. Under these conditions, without any significant mean current, the sediment flux within the sheet flow layer was found to greatly exceed the sediment flux in the suspension layer. As a result, net transport rates under wave groups are similar to those under monochromatic waves.

Langmuir Supercells: A Mechanism for Sediment Resuspension and Transport in Shallow Seas

Recent measurements at a cabled sea-floor node in 15 meters of water off the coast of New Jersey suggest that Langmuir supercells, Langmuir circulations that achieve vertical scales equal to the water depth under extended storms, are an important mechanism for major sediment resuspension events on the extensive shallow shelves off the eastern U.S. coast. Because sediment resuspension is a prelude to transport, supercell events are a necessary condition for major sediment transport. Such events may also contribute to shelf-sea exchange and to offshore gradation of benthic community structure in shallow seas.

River sediment flux and shelf sediment accumulation rates on the Pacific Northwest margin

To test the generality of insight obtained from recent STRATAFORM studies of northern California's Eel margin, river sediment sources and continental shelf sinks were examined on the Pacific Northwest margin from 38° to 44.5°N. River discharge and sediment concentration data acquired by the US Geological Survey were used to update long-term annual suspended-sediment loads for 17 rivers that range in basin area from 635 to ∼22,000 km 2. Resulting annual load estimates vary over 3 orders of magnitude (0.065–18×10 9 kg/yr), with major suspended-sediment fluxes supplied by, in decreasing order, the Eel, Klamath/Trinity, Mad, Rogue, Umpqua and Russian rivers. Down-core profiles of excess 210Pb and 137Cs were used to estimate sediment accumulation rates (SARs) at prescribed depths (70 and 110 m) and distances (0–40-km north and south along-shelf) from each of the major rivers. SARs were found to vary much less than the river flux estimates, and are mostly in the range of 1.5 to 6 mm/yr. Most significantly, shelf SARs on the other Pacific Northwest margins are only slightly less than those observed on the Eel shelf, implying that much higher proportions of riverine sediment are retained on those shelves. Likely reasons that the Eel dispersal system exhibits greater off-shelf transport are (1) the narrower and steeper shelf geometry, and (2) the existence of a newly documented cross-isobath sediment transport mechanism that involves wave-modulated fluid mud flows. Testing whether the fluid mud flows are a consequence of the Eel River basin's high sediment yield, and are thus unique to the Eel, or are caused by intense wave energy during discharge events, and hence are operative on many other margins, awaits future bottom-boundary layer measurements.

Turbulence Characteristics and Interaction between Particles and Fluid in Particle-Laden Open Channel Flows

Mechanism of sediment transport is composed of complicated interactions between turbulent flow, particle motion, and bed configurations. Of particular significance is the interaction between turbulence and particle motion, although turbulence measurements of particle-laden two phase flow have been a problem for a long time, especially in the near-wall region. In this study, simultaneous measurements of both the particles and fluid (water) were conducted in particle-laden two phase open channel flows by means of a discriminator particle-tracking velocimetry. The mean velocity and turbulence characteristics for fluid and particles each were examined in comparison with those in clear-water (particle-free) flow, together with previous existing data measured by laser Doppler anemometer and phase Doppler anemometer. The relative velocity and the turbulence modulation, which are the most important topics in two phase-flow approach, were revealed by varying the particle diameter and specific density. The fluid-sweeps are more contributory to the motion of particles than the fluid ejections in the near-wall region. In turn, the particle-sweeps transport the high momentum to the carrier fluid and enhance the turbulence intensities of fluid.

One-Dimensional Numerical Model for Nonuniform Sediment Transport under Unsteady Flows in Channel Networks

In this study, the proposed one-dimensional model simulates the nonequilibrium transport of nonuniform total load under unsteady flow conditions in dendritic channel networks with hydraulic structures. The equations of sediment transport, bed changes, and bed-material sorting are solved in a coupling procedure with a direct solution technique, while still decoupled from the flow model. This coupled model for sediment calculation is more stable and less likely to produce negative values for bed-material gradation than the traditional fully decoupled model. The sediment transport capacity is calculated by one of four formulas, which have taken into consideration the hiding and exposure mechanism of nonuniform sediment transport. The fluvial erosion at bank toes and the mass failure of banks are simulated to complement the modeling of bed morphological changes in channels. The tests in several cases show that the present model is capable of predicting sediment transport, bed changes, and bed-material sorting in various situations, with reasonable accuracy and reliability.

Variations in natural levee morphology in anastomosed channel flood plain complexes

Natural levees are common features of alluvial river systems, yet their origin and evolution are poorly understood. In this paper, we present morphologic and sedimentologic data from two anastomosed rivers and offer a hypothesis of natural levee growth in these systems based on mechanisms of sediment transport. In settings where floodbasins fill at the same rate as the channel, levees form by turbulent diffusion of suspended sediment away from a high-velocity thread into a floodbasin of relatively stagnant water. The theory of diffusive transport suggests that these levees should be narrow, steep, and display abrupt decreases in grain size due to rapid decreases in carrying capacity with distance into the floodbasin. In environments where an appreciable water surface slope is maintained between the main channel and the floodbasin, levees form by advection of sediment out of the channel and into the floodbasin. Advective transport theory indicates that these levees should be broad and gently sloped, with grain sizes gradually decreasing away from the main channel. Natural levees occurring in the anastomosed reach of the upper Columbia River in SE British Columbia display significantly different morphology from levees in the Cumberland Marshes region of the lower Saskatchewan River in east-central Saskatchewan. At the upper Columbia site, the rise in stage of the floodbasin water nearly keeps up with that of the channel water because of good communication between channel and floodbasins through crevasses. This inhibits the establishment of a water surface slope, and the bulk of the escaping sediment is deposited close to the channel. In contrast, the Cumberland Marshes region is characterized by wide and volumetrically large floodbasins. These conditions keep floodbasin water surface elevations relatively low and maintain an appreciable elevation head between the channel and its floodbasin, fostering levee growth by advective transport.

Material Analysis and Radioisotope Studies of Spent Nuclear Fuel found in the Marine Environment

AbstractSand-sized particles of spent nuclear fuel have been found in the environment at Dounreay, north Scotland. These particles, believed to have been discharged during early stages of site operations in the 1960s, have been recovered from areas within the Site, coastal foreshore adjacent to the Site and offshore (marine) environments. As part of the Dounreay Site Restoration Plan, a significant program of work is being undertaken to establish the fate of the Dounreay particles in the marine environment. This program includes materials analysis of particles and particle analoguesThe diverse depositional environments, coupled to relatively long fuel-particle residence times, allow insight to be gained into the behaviour of the particles in the natural environment. This includes the effects of physical, chemical and mechanical processes such as particle erosion and corrosion, particle abrasion and particle-saltwater interaction. Using materials analysis techniques including Scanning Electron Microscopy, Energy Dispersive X-Ray Analysis, Electron Probe Microanalysis, together with 137Cs activity measurement we review the fuel-particle mechanical and chemical stability and current radio-isotope composition. These studies allows us to identify the fuel provenance, fuel treatment history and draw some general conclusions of the ultimate fuel particle residence time. We demonstrate that radioisotope modelling strongly indicates that particle break-up is limited by the chemical effects of the marine environment coupled with the native oxide films present on the metallic fuel. Particle transport and distribution are governed by environmental effects on the sediment and sediment transport mechanisms such as tidal currents and storm events.This study is unique in that it involves the potential effects of long-term saltwater and abrasion interactions with fuel matrixes containing both U-Al and U-Mo of which there is little literature available.

Meteorological and land use controls on past and present hydro‐geomorphic processes in the pre‐alpine environment: an integrated lake–catchment study at the Petit Lac d'Annecy, France

AbstractA wide range of environmental records is integrated in order to reconstruct the mechanisms of flooding and sediment transport within the 170 km2 Petit Lac catchment, Annecy, France, over time scales of 10−1 to 102 years. These records include sequential lake sediment trap samples and cores, floodplain stratigraphies, dated landform assemblages, hydro‐meteorological records, and documented histories of river channel and land‐use change. Mineral magnetic measurements are used as the basis for classifying catchment sediment sources and tracing sediment movements through time. Records of magnetic susceptibility for monthly sediment trap samples (1998–99) track seasonal discharge, peaking in winter and spring. Magnetic records in lake sediment cores are compared against and tuned to precipitation records to provide dated proxy records for past discharge spanning sub‐annual to decadal time scales back to 1826. Calculated sediment accumulation rates in lake sediment cores are used as proxies for time‐averaged catchment sediment load. Analysis of the results reveals that climate and land‐use controls on the hydrological and sediment system are complex and vary according to the time scale of observation. In general, cycles of agricultural expansion and deforestation appear to have been the major cause of shifts in the sediment system through the late Holocene. Deforestation in the 18th century may have caused a number of high‐magnitude flood and erosion events. As the time scale of observation becomes shorter, changes in climate and hydro‐meteorological conditions become progressively more important. Since the mid‐19th century, smoothed records of discharge roughly follow annual precipitation; this is in contrast to sediment load, which follows the trend of declining land‐use pressures. Episodic erosion events during this recent period seem to be linked to geomorphic evidence for slope instability in the montane and sub‐alpine zones, triggered by intense summer rainfall. At the annual scale, changes in seasonal rainfall become paramount in determining sediment movement to downstream locations. The study demonstrates that the connections between forcings and responses span a four‐dimensional array of temporal and spatial scales, with strong evidence for dominantly nonlinear forcing–response mechanisms. Copyright © 2003 John Wiley & Sons, Ltd.

Sand Transport on Steeply Sloping Plane and Rippled Beds

Experiments on sand transport have been carried out in the Sloping Sediment Duct at HR Wallingford. The aim of the experiments was to investigate sediment transport mechanisms, for sand of varying degree of grading, on sloping beds. The Sloping Sediment Duct is a steady flow, recirculating duct, capable of generating mean flow speeds of up to 1 m/s and tilting to +/-30°. Twenty-two tests with two different sediments were conducted. Both sediments had a median grain size of about 0.23 mm but different standard deviations. Bed slopes up to +/-20° were used in the experiments. The results show that bedforms have a significant effect on the transport rate. Since the bedforms, in turn, are affected significantly by the slope, the relation between transport rate and slope is not a monotonic function. Maximum suspended transport rates were attained for downslope flows at angles of about 10°. The transport rate for widely graded sediment was significantly larger than that for well-sorted sediment for almost all flows and slopes.

Sediment source strength, transport pathways and accumulation patterns on the Siberian–Arctic's Chukchi and Laptev shelves

In this study, we estimate sediment source strength and determine sediment transport/accumulation patterns on the Chukchi and Laptev shelves for lithogenic material from several key source regions. In the western Laptev Sea, sediments from the Siberian flood basalt province account for <20% of surface sediments, while detritus from the eastern Laptev's Lena and Yana Rivers constitutes as much as 40% of seafloor sediments. Eastern Laptev Sea sediments similarly reflect inputs from multiple sources; here, however, local inputs from the Lena and Yana Rivers are most prevalent. Chukchi Sea sediments are more homogeneous in composition, with sediments originating from the Okhotsk-Chukotsk volcanic belt and Bering Strait inflow comprising over 60% of surface sediments except near Wrangel Island and along sections of the Siberian coast. In the Chukchi portion of our study area, the dominant sediment dispersal pathways are from the Bering Strait region to the north and from Long Strait to the east, into the central Chukchi Sea. There appears to be comparatively minimal sediment transport parallel to the Siberian coast at our sample sites (>40 m water depth). In contrast, at our Laptev Sea sites (typically >20 m water depth) sediments supplied by the Lena and Yana Rivers move primarily eastward/northeastward parallel to the coast, with relatively minimal transport to the west/northwest or north to the central shelf. Good correspondence between sediment accumulation patterns and currents on the Chukchi shelf indicates water circulation is an important sediment transport mechanism in this marginal sea. In the Laptev Sea, a combination of river outflow, cyclonic water circulation and the Siberian Coastal Current controls sediment distribution. We speculate that sediment ice rafting may also affect shelf sedimentation patterns through a combination of factors. Our findings have implications for the fate of particle reactive contaminants released to the Siberian continental margin.

Sediment transport in Yalu River estuary

Tidal cycle measurements of tidal currents, salinity and water temperature, and suspended sediment concentrations were measured at four stations, together with surveys along two profiles short core collection within the Yalu River estuary. Grain size analysis of the three core sediment showed that: 1) the sediment from B1 to B3 became finer, worse sorting and positively skewed; 2) the diversification of matter origin became more and more evident from east to west; 3) the sediments over the region were of the same origin, as indicated by their similar colors and grain sizes. The data indicated that stratification occurred in the flood season, from upstream to downstream, and a salt wedge was formed. The water column was well mixed, but the longitudinal gradient of the salinity was larger on spring tide. The results also showed that the dominating mechanism of suspended sediment transport in the Yalu River estuary was T1, T2, T3 and T5. The non-tidal steady advection transport was restricted by the net transport of suspended sediment induced by mass Stoked drift directed to landwards, then the net sediment transport rate were decreased and the turbidity maxima was also favored to forming and extending.

Heavy metals assessment in Oum Er Rbia bassin using WD-XRF and ICP-AES

The analysis of water, suspended particulate matter (SPM) and sediment collected from Oum er Rbia Moroccan basin for the content of six elements has been performed utilizing ICP-AES and WD-XRF methods. Sediment have been analyzed by WD-XRF; Water and SPM by ICP-AES. The detection limits are sufficient for most of the elements to be determined. Results were assessed for the accuracy by the analysis of a multi-elemental standards. The results for most of the elements obtained were accurate to about 2-15%. The objective of the analysis were to determine the geochemical areas on this bassin, to explain the mechanisms of sediment transport and nevertheless, to assess the pollution of the bassin by heavy metals generated by human activities. This work interests only the last aspect. It has done one 30 samples of water and SPM, and about 80 sediments. Samples were collected along the river, between its source in meddle Atlas and the Atlantic ocean at Azemour city. The results obtained on the basin were normalized to the intemational norms.

TRANSPORT BEHAVIOR OF 3,3′-DICHLOROBENZIDINE IN A FRESHWATER ESTUARY

Like many hydrophobic organic compounds, 3,3′-dichlorobenzidine (DCB) partitions preferentially to (sediment) particles in lake systems. As such, the behavior of DCB in these systems is substantially affected by the movement of sediments. A field investigation of DCB distribution in sediments of Lake Macatawa (Holland, MI, USA) was initiated. The pattern of DCB distribution within the lake was found to display an oscillatory pattern that was consistent with a wind-driven mechanism of sediment transport. Numerical modeling of seiching behavior supported the hypothesized importance of this mechanism of sediment transport and redistribution. The dynamic behavior of sediment-associated DCB within Lake Macatawa seems to be strongly influenced by phenomena that are common to many freshwater estuaries. As such, the behavior of this system is expected to represent a reasonable model of the dynamic behavior of hydrophobic contaminants in other freshwater estuaries.

Transport behavior of 3,3′‐dichlorobenzidine in a freshwater estuary

Like many hydrophobic organic compounds, 3,3'-dichlorobenzidine (DCB) partitions preferentially to (sediment) particles in lake systems. As such, the behavior of DCB in these systems is substantially affected by the movement of sediments. A field investigation of DCB distribution in sediments of Lake Macatawa (Holland, MI, USA) was initiated. The pattern of DCB distribution within the lake was found to display an oscillatory pattern that was consistent with a wind-driven mechanism of sediment transport. Numerical modeling of seiching behavior supported the hypothesized importance of this mechanism of sediment transport and redistribution. The dynamic behavior of sediment-associated DCB within Lake Macatawa seems to be strongly influenced by phenomena that are common to many freshwater estuaries. As such, the behavior of this system is expected to represent a reasonable model of the dynamic behavior of hydrophobic contaminants in other freshwater estuaries.