Sediment Transport Mechanisms Research Articles

Determinant factors of sediment graphs and rating loops in a reforested watershed

Summary Sediment graphs and hydrographs were analyzed in a small, high-gradient reforested watershed in Mie Prefecture, Japan, which was instrumented to measure discharge and sediment transport. The study was conducted to recognize determinant factors on sediment graphs and hydrographs relationships, and corresponding rating loops using eight storm events that occurred in 2004. A cause-and-effect analytical approach was employed in the investigation of factors controlling individual hydrological events. The results showed that in a single event, the sediment peak preceded the peak discharge, following a clockwise hysteretic loop. The rating loops exhibited hysteresis with a greater suspended sediment concentration for a given discharge occurring on the rising limb rather than on the falling limb. Furthermore, the reduction in suspended sediment concentrations in a sequence of events and the falling limbs could mainly be associated with sediment exhaustion and contributions of throughfall and throughflow. We concluded that a combination of sediment availability, rainfall characteristics, soil water repellency, and contribution of different flow components control the variations in event-wise sediment graphs and rating loops in the study area. Understanding the relationship between controlling factors and sediment yield dynamics in the study area should greatly improve our understanding of sediment transport mechanisms and predictions in reforested watersheds, which should ultimately lead to better management of forest ecosystems.

Scour and flow field around a spur dike in a 90° bend

Spur dike is an important element in river training that creates rapid variations in flow field, sediment transport and bed topography. The mechanism of flow and sediment transport in a channel bend is very complex, especially when a spur dike is constructed in a bend. Most of previous investigations on flow behavior and scour around spur dike were carried out in straight channels. In this paper results of experiments on flow field and scour around a spur dike in a 90 degree channel bend are presented. Sand with uniform grain size was used as the bed material. Experiments were conducted for different locations and different lengths of spur dikes at the bend with different values of discharge. The three dimensional flow fields around a spur dike were investigated. The maximum depth of scour was correlated to the Froude numbers, lengths and the locations of spur dike in the bend.

Early Miocene sequence development across the New Jersey margin

ABSTRACTSequence stratigraphy provides an understanding of the interplay between eustasy, sediment supply and accommodation in the sedimentary construction of passive margins. We used this approach to follow the early to middle Miocene growth of the New Jersey margin and analyse the connection between relative changes of sea level and variable sediment supply. Eleven candidate sequence boundaries were traced in high‐resolution multi‐channel seismic profiles across the inner margin and matched to geophysical log signatures and lithologic changes in ODP Leg 150X onshore coreholes. Chronologies at these drill sites were then used to assign ages to the intervening seismic sequences. We conclude that the regional and global correlation of early Miocene sequences suggests a dominant role of global sea‐level change but margin progradation was controlled by localized sediment contribution and that local conditions played a large role in sequence formation and preservation. Lowstand deposits were regionally restricted and their locations point to both single and multiple sediment sources. The distribution of highstand deposits, by contrast, documents redistribution by along shelf currents. We find no evidence that sea level fell below the elevation of the clinoform rollover, and the existence of extensive lowstand deposits seaward of this inflection point indicates efficient cross‐shelf sediment transport mechanisms despite the apparent lack of well‐developed fluvial drainage.

NUMERICAL MODELLING OF BEDFORM DEVELOPMENT

Formation of the initial bedforms, known as sand-wavelets, developing from a flat erodible bed in hydrodynamically smooth flow has been studied numerically. The computational model consists of hydrodynamic and sediment transport equations. The unsteady hydrodynamic equations are solved by Large-Eddy-Simulation (LES) and Immersed-Boundary-Method (IBM) on a non-bodyfitted grid. Dynamics of individual grains are neglected and only bedload is considered, as calculated from Van Rijn's equation (1984, Journal of Hydraulic Engineering, Vol. 110) . Two-dimensional (2D) bed evolution is coupled with the above 3-dimensional (3D) hydrodynamic equations. The computed wavelength agrees with linear stability analysis and with experimental data in the literature. The length of the sand-wavelets is observed to correspond to the disturbance region of bed shear stress caused by discontinuities on the bed surface, as postulated by Yalin (1977, Mechanics of Sediment Transport, Pergamon press) .

THE ROLE OF STONES ON THE RIVER BED STABILITY IN STONY BED RIVERS

River cross sectional form had settled in the dynamic equilibrium form at the relationship between discharge, the river bed slope and bed material when river improvement is not progressing so much. But, river channels do not seem to satisfy the equilibrium condition of sediment transport due to river improvement work in progress. As the result, many problems have arisen on river management. Sediment transport became discontinuous by the dam and removing large size of bed materials as stones and gravel from rivers is mentioned. In this study, we describe the bed stabilization mechanism of the stony bed rivers found by the field experiment of the Joganji River and the difference in sediment transport mechanism of the stony bed river and the gravel bed river. Moreover, we insist on the problem on washing out of dam reservoir sediment and the necessity for the new bed variation analysis method in the stony bed rivers.

Modeling sediment transport patterns during an upwelling event

Being one of the most outstanding hydrodynamic processes at ocean margins, upwelling is not only a key factor controlling bioproduction but also acts as a driving mechanism for sediment transport. In order to quantify its capability to erode and transport sedimentary particles without being masked by other oceanographic processes, we present a numerical model only forced by surface wind drag. Thereby, transport of particles is not only controlled by upwelling circulation, but also by their physical properties as well as time and location of release into the water column. The study combines a hydrodynamic finite difference model and Lagrangian particle tracing technique. Model geometry mimics a two‐dimensional profile from the passive margin offshore Walvis Bay, Namibia. Model runs describe a 5‐day wind‐forcing event and a subsequent 20‐day period of relaxation. As our work is also motivated by paleoceanographic questions, a lowered sea level geometry is used simulating Last Glacial Maximum (LGM) conditions. Results suggest the establishment of a long‐lasting circulation comprising an offshore‐directed surface layer and an onshore‐directed bottom current. Shelf currents are vigorous but short‐lasting, allowing transport of particles up to sand size. In contrast, transport at the upper slope is more persistent but restricted to smaller grain sizes. Sea level changes cause a shift of upwelling front in cross‐shelf direction and of sedimentary deposition centers along the slope. Transport paths of surface source tracers show systematic variations, which can be evaluated in terms of grain‐size fractionation as well as temporal and spatial clusters.

Dust provenance in Antarctic ice during glacial periods: From where in southern South America?

The origin of dust deposited in East Antarctica is not fully understood yet. This study demonstrates that, contrary to Nd, Sr isotopes are strongly fractionated in the finest‐sized material representing potential dust sources in South America. Analysis of Nd isotopes suggests that Argentine loess, Southern Ocean sediments and Antarctic dust, may all have a related origin or can represent comparable mechanisms of eolian sediment transport and deposition. This analysis also concludes that the Patagonian signature can explain a high proportion of the isotopic composition of dust trapped in Antarctic ice during glacial periods. The Puna‐Altiplano plateau emerges as the second possible important sediment contributor explaining the crustal‐like signature found in East Antarctic dust and should be considered as a potential source area when Quaternary paleoclimate is reconstructed from Argentine loess, Antarctic dust and South Atlantic sediment records.

Paleocurrent patterns of the sedimentary sequence of the Taitao ophiolite constrained by anisotropy of magnetic susceptibility and paleomagnetic analyses

Determination of paleocurrent directions is a robust approach to establish the mechanism of sediment transport and of deposition. However, in tectonically controlled (or affected) basins where rotations are suspected, it is of critical importance to correct previously calculated paleocurrent directions to establish the original (pre-tectonic) transport and deposition patterns. In order to illustrate this, we determined the orientations of the anisotropy of magnetic susceptibility (AMS) fabrics from 14 sites distributed along the sedimentary deposits interbedded within the Late Miocene Main Volcanic Unit (MVU) of the Taitao ophiolite, Southern Chile. Data were used to estimate characteristics (directions and relative velocities) of the sediment transporting paleocurrents and orientations of the depositional slope. Sedimentary deposits consist of a rhythmic sequence of conglomerate (base), sandstone and mudstone (top) beds that present a systematic decrease in grain size up-sequence as a whole, and within individual beds. Sedimentary deposits of the MVU, as well as overlaying effusive volcanic products, are disrupted by small and discrete dextral/normal brittle faults. The distribution of fault planes and their kinematic characteristics suggest the development of a dextral negative flower-like fault zone in the area. Previously reported NW-ward paleocurrent directions obtained from the orientations of sedimentary structures roughly agree with our calculations based on the orientations of AMS fabrics. Calculated paleocurrent directions are sub-parallel to the NW-SE trend of Estero Cono channel, the main lineament in the area. The spatial distribution of AMS axes between and within sites suggest that coarse grain deposits were transported by relatively high velocity flows whereas fine grained deposits were by low to nearly null velocity flows. Previously reported paleomagnetic analyses indicate that counterclockwise rotations affected the whole Taitao ophiolite, including the MVU. By applying the necessary corrections we obtained paleocurrent directions yielding mostly NNW-ward directions. Restoration of the orientations of beddings indicates that the depositional surface dipped steeply at its easternmost edge and shallowly at the westernmost end. We propose here that sediments were transported by turbidite flows generated by slope instabilities in the innermost part of the Estero Cono. These turbidites followed the Estero Cono lineament while they filled the channel and transported sedimentary material (eroded from the upper portions of the ophiolite and nearby units) towards the Pacific Ocean. Sediments were deposited on a NW- to W-facing slope and sorted in decreasing grain size along the channel. The instability of the slope was most probably triggered by the activity of a transtensional fault zone developed along the mid-axis of the Estero Cono channel.

Numerical modeling of heavy metal pollutant transport-transformation in fluvial rivers

On the basis of comprehensively reviewing existing mathematical models, a mathematical model describing heavy metal transport-transformation (dynamics) in its entity in fluvial rivers has been worked out by using basic principals of environmental chemistry, hydraulics and mechanics of sediment transport and recent developments obtained by pioneers, and especially by the authors. Effects of sediment transport on heavy metal pollutant transport-transformation are clarified through two examples. Specifically, the transport-transformation of heavy metal pollutants in a steady, uniform equilibrium sediment-laden flow is calculated by applying this model, and the results are shown to be rational. Results of theoretical analysis and calculations indicate that the transport-transformation of heavy metal pollutants in sediment-laden flows possesses not only the generality of common tracer pollutants, but also characteristics of transport-transformation induced by sediment motion

Spit Growth and Downdrift Erosion: Results of Longshore Transport Modeling and Morphologic Analysis at the Cape Lookout Cuspate Foreland

Abstract This study examined geomorphologic changes, littoral processes and sediment budget along the west side of the Cape Lookout, North Carolina, cuspate foreland, where a prograding spit at right angles to an eroding barrier island forms prominent features. Progradation of the spit is of interest geologically because it occurs within a transgressive, sediment-starved coastal setting in which adjacent barrier island limbs are eroding and in which immediate sediment sources and transport mechanisms are not obvious. Results indicate that as the spit extends to the north, it also widens to the west through swash bar attachment, leaving a well-defined record of dune ridges that mark former shoreline positions. Although northerly longshore transport occurs only under SW wave conditions, the average rate, ∼0.18 × 106 m3/y, is considered high given the sheltered position of the spit within the cuspate foreland. Results of modeling and development patterns of a series of ridge and runnel systems along the spit...

Sediment dynamics in tidally dominated environments controlled by transport and turbulence: A case study for the East Frisian Wadden Sea

Various tide‐related sediment transport mechanisms near a barrier island coast are addressed on the basis of analytical theory and numerical simulations carried out with a three‐dimensional (3‐D) numerical model. The theory proposed gives an explanation of the observed “twin peaks” in the concentration of suspended sediment; however, it does not adequately describe the whole variety of sediment responses. The major idea in this paper assumes that it is not only the change of the level of turbulence and advection individually but also the correlation between the two which controls the concentration of sediment. Because the horizontal gradients of sediment concentration are created not only by sources at the coast (e.g., fluxes from rivers) but also by the transport and turbulence, the system is highly nonlinear. Several simulations have been carried out aimed at revealing the nonlinear mechanisms in the sediment transport and individual contribution of turbulence and velocity field. An experiment in which the sediment model is driven only by turbulent kinetic energy fails to simulate most of the characteristic temporal and spatial sediment patterns. With another experiment it is demonstrated that the velocity gradient in the vertical gives an important mechanism shaping the patterns of temporal and spatial variability (and tidal asymmetries). Without shear diffusion, the sediment dynamics are largely governed by the “displacement mechanism”; that is, the system is more linear. One practical conclusion from the numerical simulations is that using simplified (2‐D) dynamics in sediment transport modeling along with externally prescribed turbulence characteristics could lead to large artifacts.

Dynamics of suspended sediment transport at field‐scale drain channels of irrigation‐dominated watersheds in the Sonoran Desert, southeastern California

AbstractSuspended sediment is a major source of pollution in irrigation‐dominated watersheds. However, little is known about the process and mechanisms of suspended sediment transport in drain channels directly connected to agricultural fields. This paper explains sediment dynamics using averaged 5 min flow discharge Q (m3 s−1) and suspended sediment concentration C (mg l−1) collected during one crop season in a small catchment containing a first‐order drain channel and its connected six agricultural fields within the Salton Sea watershed. The statistical properties and average trends of Q and C were investigated for both early (i.e. November) and late (i.e. January) stages of a crop season. Further in‐depth analysis on sediment dynamics was performed by selecting two typical single‐field irrigation events and two multiple‐field irrigation events. For each set of irrigation events, the process of suspended sediment transport was revealed by examining hydrograph and sediment graph responses. The mechanisms underlying suspended sediment transport were investigated by analysing the types of corresponding hysteresis loop. Finally, sediment rating curves for both hourly and daily data at early and late stages and for the entire crop season were established to seek possible sediment‐transport predictive model(s). The study suggests that the complicated processes of suspended sediment transport in irrigation‐dominated watersheds require stochastic rather than deterministic forecasting. Copyright © 2007 John Wiley & Sons, Ltd.

Sediment transport event analysis on the western Adriatic continental shelf

The sediment-transport mechanisms that contribute to and redistribute the modern sediment deposits on the western Adriatic continental shelf were evaluated utilizing data collected from two instrumented benthic tripods deployed at 12-m water depth, one in the northern Adriatic basin on the Po River subaqueous delta, and the other in the central Adriatic basin on the Pescara River shelf. Sediment-resuspension events driven by cold, northeasterly Bora winds dominate the along-shelf transport climatology at both tripod locations, but at the Po delta site, the southwesterly Scirocco wind events also play a significant role. At the Pescara shelf site, interaction between Bora wind-driven currents and the Western Adriatic Coastal Current strongly contributes to the resuspension and advection of suspended sediment. Interannual variability of the forcing mechanisms (including strength, frequency, and relative mix of Bora and Scirocco wind events) is evident in the three winters of data collected on the Po River subaqueous delta. In both types of wind events, and throughout all years of data collection, the net along-shelf sediment transport is significantly larger than the net across-shelf transport at the 12-m sites. This may be characteristic of low-energy environments, where sediment resuspension and transport occurs in such shallow water that it is not subjected to strong downwelling features characteristic of higher-energy environments.

How stable are twentieth-century calibration models? A high-resolution summer temperature reconstruction for the eastern Swiss Alps back to AD 1580 derived from proglacial varved sediments

We found a significant positive correlation between local summer air temperature (May-September) and the annual sediment mass accumulation rate (MAR) in Lake Silvaplana (46°N, 9°E, 1800 m a.s.l.) during the twentieth century (r = 0.69, p < 0.001 for decadal smoothed series). Sediment trap data (2001-2005) confirm this relation with exceptionally high particle yields during the hottest summer of the last 140 years in 2003. On this base we developed a decadal-scale summer temperature reconstruction back to AD 1580. Surprisingly, the comparison of our reconstruction with two other independent regional summer temperature reconstructions (based on tree-rings and documentary data) revealed a significant negative correlation for the pre-1900 data (ie, late ‘Little Ice Age’). This demonstrates that the correlation between MAR and summer temperature is not stable in time and the actualistic principle does not apply in this case. We suggest that different climatic regimes (modern/‘Little Ice Age’) lead to changing state conditions in the catchment and thus to considerably different sediment transport mechanisms. Therefore, we calibrated our MAR data with gridded early instrumental temperature series from AD 1760-1880 (r = -0.48, p < 0.01 for decadal smoothed series) to properly reconstruct the late LIA climatic conditions. We found exceptionally low temperatures between AD 1580 and 1610 (0.75°C below twentieth-century mean) and during the late Maunder Minimum from AD 1680 to 1710 (0.5°C below twentieth-century mean). In general, summer temperatures did not experience major negative departures from the twentieth-century mean during the late ‘Little Ice Age’. This compares well with the two existing independent regional reconstructions suggesting that the LIA in the Alps was mainly a phenomenon of the cold season.

Mobile Measurement of Tidal Current over Wide Area by ADCP and Behavior of Sand Wave in Bisanseto Fairway

In the present paper, the mobile measurement of tidal current was conducted by using the developed system with ADCP and D-GPS over the wide area in Bisanseto fairway. On the basis of the measurement, the sediment transport mechanism and the dynamics of sand wave formation is understood, and then, the fundamental information is obtained for the maintenance of the fairway. In the flood and ebb tides, the amplitude of tidal current in the upper layer is similar to it in the lower layer, but the current direction is slightly different near the deeper part.

Field measurement and quantification of longshore sediment transport: an unattainable goal?

Abstract Longshore sediment transport (LST) is an important component of many coastal systems. This common term is frequently expressed as an annual volume, either net or gross, passing a particular point. In spite of its importance as a factor in coastal morphodynamics it has received relatively little attention as a process. We review the mechanisms of longshore sediment transport and current approaches to its assessment using field measurement and geoindicators. We conclude that approaches to date have never succeeded in quantifying longshore sediment drift rates in an adequate way. This shortcoming is attributed to the spatial and temporal variability of contributing processes coupled with a technical inability to quantify simultaneously longshore sediment transport in all its modes. We also conclude that research involving the measurement of longshore transport can enhance our understanding of the processes of transport and rates of transport under certain conditions. However, the accurate quantification of drift rates as annual volumes is an unattainable goal. We propose that such quantifications be replaced by qualitative values (small, medium, large) or measures of the relative importance of longshore transport in effecting morphological change (e.g. drift-dominated, cross-shore dominated, mixed processes).

Sediment Transport From Beach Around Coastal Structures To Submarine Canyon

Sediment carried by longshore transport moves to deep sea through the submarine canyons. The investigation of submarine topography had been conducted by narrow-multi beam echo-sounders system. Detailed bathymetric changes shows the mechanism of sediment transport from beach around coastal structures to submarine canyon. This sediment transport mainly occurs at storms related with several typhoons. Large amount of sediments appear to be migrated by sediment gravity flow at storms submarine topography and distribution pattern of bottom materials have been changed due to this processes.

Short-term soil mixing quantified with fallout radionuclides

Soil mixing plays a significant role in contaminant transport, carbon sequestration, and landscape evolution, yet the rates and driving mechanisms are poorly constrained. Here we use depth profiles and advection-diffusion modeling of fallout nuclides to quantify differences in short-term ( 7 Be and weapons-derived isotopes, and quantify mixing with a steady-state model of vertical 210 Pb transport. On a forested landscape in the Bega Valley in southeastern Australia and on grasslands in Marin County, California, where bioturbation is documented as the dominant sediment transport mechanism, we calculate diffusion-like mixing coefficients of 1–2 cm 2 yr −1 . In montane forest soils of northern New England, we observe little field evidence of short-term mixing, and find that the traditional advection-diffusion model fails to describe 210 Pb profiles. Because nuclide profiles here can be described with a simple model of litterfall, organic matter decay, and radioactive decay, we argue that diffusion-like processes are barely active on short time scales, and that the advection-diffusion model overestimates diffusion-like transport. While animal bioturbation and soil freezing cycles have little effect on the fate of elements in New England, physical soil mixing drives transport at Bega Valley and Marin County. We suggest that the absence of soil stirring that we quantify in New England forests may explain the slow physical erosion here (∼0.2 cm/k.y.) relative to the actively bioturbated soils of Bega Valley and Marin County (5–10 cm/k.y.).

Observations of tidal and springtime sediment transport in the upper Delaware Estuary

Suspended-sediment transport in the upper Delaware Estuary was investigated in Spring 2003 to examine mechanisms of material exchange from the tidal river to the turbidity maximum zone in the lower estuary. Timeseries records of currents and suspended-sediment concentration (SSC) were obtained between 18 March and 10 June 2003 using moored instrumentation deployed at two locations spanning the tidal river−estuary transition: (1) Tinicum Island, 90 km up-estuary of the Delaware Bay mouth in tidal freshwater; and (2) New Castle Flats, 40 km down-estuary of Tinicum in oligohaline waters. Environmental conditions during the observational period were typical for the spring season and included two river peakflows (1000–2000 m 3 s −1) and several moderate remote-wind events. Results indicate that SSC and tidal sediment flux vary spatially in the estuary with local current magnitude and the proximity of patches of easily resuspendable sediment. Landward of the turbidity maximum zone, SSC was not correlated with current velocity due to depletion of bed sediment sources early in the tidal cycle. In contrast, SSC and velocity within the turbidity maximum zone were well correlated due to an abundance of fine sediment generated by resuspension and advection. At both observational sites the depth-averaged residual current (Eulerian mean) and net sediment flux were seaward, and the flux magnitude increased 3–4 fold during river peakflow events on account of elevated ebb flow and bottom scour. The seaward residual current, mostly compensatory flow for Stokes Drift on flood tide, is an important mechanism of sediment transport to the estuarine turbidity maximum zone. Averaged over the 80-day study period, the cross-sectionally averaged sediment flux past New Castle (11 ± 4 × 10 8 kg) was significantly larger than that at Tinicum (4 ± 1 × 10 8 kg), and twofold larger than the estimated influx from river tributaries (5 ± 1 × 10 8 kg). The mass imbalance (∼7 × 10 8 kg) suggests that eroded bed sediment, previously deposited and stored in the upper estuary, was a major source of material to the turbidity maximum zone in Spring 2003.

Markovian–Octant analysis based stable turbulent shear stresses in near-bed bursting phenomena of vortex settling chamber

The coherent structure in near-bed turbulent boundary layer of vortex chamber, particularly the bursting events and their associated shear stresses play the main role in sediment flushing process and consequently the trap efficiency of the vortex settling chamber. Hence, three-dimensional velocity measurements were made at 48 points near the bed of physical model of vortex chamber by using Micro-ADV. The pattern of sediment deposition at the bed of vortex settling chamber reveals three separate regions formed by three predominant currents of inlet flow, flushing flow and outlet over flow. Additionally, due to the instability and three-dimensional nature of the bursting events near the bed of chamber, the new method of Markovian–Octant analysis was applied to study the different classes of near-bed stable shear stresses of vortex chamber in three dimensions. Moreover, the role of each class of stable shear stresses on Sediment transport mechanism at the bed of vortex chamber is investigated.