Composition Of Bed Load Research Articles

Fluvial and permafrost history of the lower Lena River, north‐eastern Siberia, over late Quaternary time

ABSTRACTArctic warming and permafrost thaw visibly expose changes in the landscape of the Lena River delta, the largest Arctic delta. Determining the past and modern river regime of thick deltaic deposits shaping the Lena River mouth in north‐eastern Siberia is critical for understanding the history of delta formation and carbon sequestration. Using a 65 m long sediment core from the delta apex a set of sedimentological techniques is applied to aid in reconstructing the Lena River history. The analysis includes: (i) grain‐size measurements and the determination of the bedload composition; (ii) X‐ray fluorescence, X‐ray diffractometry, and magnetic susceptibility measurements and heavy mineral analysis for tracking mineral change; (iii) pH, electrical conductivity, ionic concentrations, and the δ18O and δD stable isotope composition from ground ice for reconstructing permafrost formation. In addition; (iv) total and dissolved organic carbon is assessed. Chronology is based on; (vi) radiocarbon dating of organic material (accelerator mass spectrometry and conventional) and is complemented by two infrared – optically stimulated luminescence dates. The record stretches back approximately to Marine Isotope Stage 7. It holds periods from traction, over saltation, to suspension load sedimentation. Minerogenic signals do not indicate provenance change over time. They rather reflect the change from high energy to a lower energy regime after Last Glacial Maximum time parallel to the fining‐up grain‐size trend. A prominent minimum in the ground ice stable isotope record at early Holocene highlights that a river arm migration and an associated refreeze of the underlying river talik has altered the isotopic composition at that time. Fluvial re‐routing might be explained by internal dynamics in the Lena River lowland or due to a tectonic movement, since the study area is placed in a zone of seismic activity. At the southern Laptev Sea margin, onshore continental compressional patterns are bordering offshore extensional normal faults.

Bedload composition, transport and modification in rivers of Westland, New Zealand, with implications for the distribution of alluvial pounamu (jade)

River bedload surveyed at 50 sites in Westland is dominated by Alpine Schist or Torlesse Greywacke from the Alpine Fault hanging wall, with subordinate Pounamu Ultramafics or footwall-derived Western Province rocks. Tumbling experiments found ultramafics to have the lowest attrition rates, compared with greywacke sandstone and granite (which abrade to produce silt to medium-sand), or incompetent schist (which fragments). Arahura has greater total concentrations (103–105 t/km2) and proportions (5–40%) of ultramafic bedload compared with Hokitika and Taramakau catchments (101–104 t/km2, mostly <10%), matching relative areas of mapped Pounamu Ultramafic bedrock, but enriched relative to absolute areal proportions. Western Province rocks downthrown by the Alpine Fault are under-represented in the bedload. Enriched concentrations of ultramafic bedload decrease rapidly with distance downstream from source rock outcrops, changing near prominent ice-limit moraines. Bedload evolution with transport involves both downstream fining and dilution from tributaries, in a sediment supply regime more strongly influenced by tectonics and the imprint of past glaciation. Treasured New Zealand pounamu (jade) is associated with ultramafic rocks. Chances of discovery vary between catchments, are increased near glacial moraines, and are highest near source-rock outcrops in remote mountain headwaters.

Mineral Magnetic Characterization of the Godavari River and Western Bay of Bengal Sediments: Implications to Source to Sink Relations

Abstract Godavari Drainage Basin (GDB) represents the largest Peninsular source of sediment flux into the Bay of Bengal. Mineral magnetic measurements are carried out on the GDB sediments along with the surface sediment cores from the adjoining western Bay of Bengal to explore the relationship between the two. The sediments from Godavari river transact shows varied floodplain to bed load composition with the former dominated by unimodal soft ferrimagnetic, Deccan basalt source. Whereas, the bed load show polymodal composition of mixed nature dominated by silici-clastic sediments derived from the Precambrian granites and Proterozoic sequences. The surface sediment cores (~100-300 cm) from the Bengal fan region off the Godavari delta broadly display an increasing trend in ferrimagnetic mineralogy towards top. Based on mineral magnetism the ferri- and para-magnetic susceptibilities are assigned to the basaltic and siliciclastic sources, respectively which also represents the low and high rain fall zones. The increasing upwards trend of the ferrimagnetic minerals in the western Bay of Bengal sediments, therefore, can be related to the predominance of basaltic source over the siliciclastic/cratonic source from the GDB. These controls of magnetic susceptibility in the Bengal fan sediments are assigned to the fluctuation of the two sources as a result of differential weathering in response to monsoonal variability.

A GRASS GIS application for vertical sorting of sediments analysis in River Dynamics

The extreme versatility in different research fields of GRASS GIS is well known. A tool for the vertical sorting of sediments in river dynamics analysis is illustrated in this work. In particular, a GRASS GIS python module has been written which implements a forecasting sorting model by Blom&amp;amp;Parker (2006) to analyze river bed composition’s evolution in depth in terms of grainsize. The module takes a DEM and information relative to the bed load transport composition as input. It works in two different and consecutive phases: the first one uses the GRASS capabilities in analyzing geometrical features of the river bed along a chosen river reach, the second phase is the "numerical" one and implements the forecasting model itself, then executes statistical analyses and draws graphs, by the means of matplotlib library. Moreover, a specific procedure for the import of a laser scanner cloud of points is implemented, in case the raster DEM map is not available. At the moment, the module has been applied using flumes data from Saint Anthony Falls Laboratory (Minneapolis, MN) and some first results have been obtained, but the "testing" phase on other flume’s data is still in progress. Moreover the module has been written for GRASS 65 on a Ubuntu Linux machine, even if the debugging of a GRASS 64, Windowsversion, is also in progress. The final aim of this work is the application of the model on natural rivers, but there are still some drawbacks. First of all the need of a high resolution DEM in input, secondly the number and type of data in input (for example the bed load composition in volume fraction per each size considered) which is not easily obtainable, so the best solution is represented by testing the model on a well instrumented river reach to export in future the forecasting method to un-instrumented reaches.

Probabilistic Modeling of Bed-Load Composition

This paper proposes that the changes which occur in composition of the bed load during the transport of mixed-grain-size sediments are largely controlled by the distributions of critical entrainment shear stress for the various size fractions. This hypothesis is examined for a unimodal sediment mixture by calculating these distributions with a discrete particle model and using them in a probabilistic calculation of bed-load composition. The estimates of bed-load composition compare favorably with observations of fractional transport rates made in a laboratory flume for the same sediment, suggesting that the hypothesis is reasonable. The analysis provides additional insight, in terms of grain mechanics, into the processes that determine bed-load composition. These insights strongly suggest that better prediction methods will result from taking account of the variation of threshold within size fractions, something that most previous studies have neglected.

Changes with time of the transport rate of sediment mixtures

A novel sediment trap, designed to enable bed-load activity to be related to elapsed time, is described. An account is then given of its use in a short pilot experiment on the armouring of a mixed grain size bed in a small laboratory flume. Despite the limitations of the experiment, significant information was obtained about the variation with time of bed load transport rate and of the composition of the bed load in the early stages of armouring. The changes of composition are presented in terms of log hyperbolic size distributions. It is contended that, for the purpose of validating numerical models of the armouring process, this information about bed load composition has higher value than data on changes in the static bed population, because the latter involve severe sampling problems. Thus the trap provides new opportunities in the development of knowledge about bed winnowing and armouring.

Bed microtopography and entrainment thresholds in gravel-bed rivers

Although increasingly acknowledged as an important factor in bedload transport, few data on the effects of bed conditions on entrainment in gravel-bed rivers exist. This reflects to some degree the substantial difficulties involved in obtaining accurate threshold measurements in such channels. Using a sensor which allows initial movement of labeled particles to be detected during flood flow, the entrainment thresholds of clasts incorporated into the bed microtopography have been compared with those in exposed, open plane-bed positions. The threshold velocity of clasts from cluster bedforms, the most prevalent type of bed microtopography, is found to be above that of more exposed particles of like size and shape. Measurements indicate that the initial movement of ∼70% of particles in gravel-bed rivers will be directly influenced by the presence of neighboring grains. Bed microtopography plays an important role in delaying incipient motion. The influence of microtopography may be invoked to account for phenomena such as discontinuous particle movement and variations in the composition of bedload during discharge events.