Sediment Grain Size Distribution Research Articles

BEMMA: A Hierarchical Bayesian End-Member Modeling Analysis of Sediment Grain-Size Distributions

Sediment grain-size distributions provide rich information about sedimen- tary dynamics and potentially about environmental and climatic changes. However, entrainment, transport, and deposition, as a sequence of sorting process, modify original grain-size distributions of source materials, thereby resulting in complex distribution forms that are commonly multimodal and asymmetrical. However, nei- ther traditional descriptive statistics nor curving fitting methods are able to address this complexity fully. End-member modeling analysis, essentially based on polytope expansion, stands out as a flexible and robust method for the unmixing of sediment grain-size distributions. Yet there are still several key issues that remain unresolved. Here a hierarchical Bayesian end-member modeling analysis of grain-size distribu- tions, fully subject to the non-negative and unit-sum constraints on the distributions, is presented. Within the Bayesian framework, the number of end members, as well as the end-member spectra and fractions can be inferred sequentially using a reversible- jump Markov chain Monte Carlo algorithm in conjunction with Gibbs samplers. Test runs using both a synthetic and a real-world dataset from a small playa located on the southern margin of the Badain Jaran Desert, NW China, reveal that this model

Grain-size distribution of volcaniclastic rocks 2: Characterizing grain size and hydraulic sorting

Quantification of the grain size distribution of sediments allows interpretation of processes of transport and deposition. Jutzeler et al. (2012) developed a technique to determine grain size distribution of consolidated clastic rocks using functional stereology, allowing direct comparison between unconsolidated sediments and rocks. Here, we develop this technique to characterize hydraulic sorting and infer transport and deposition processes. We compare computed grain size and sorting of volcaniclastic rocks with field-based characteristics of volcaniclastic facies for which transport and depositional mechanisms have been inferred. We studied pumice-rich, subaqueous facies of volcaniclastic rocks from the Oligocene Ohanapecosh Formation (Ancestral Cascades, Washington, USA), Pliocene Dogashima Formation (Izu Peninsula, Honshu, Japan), Miocene Manukau Subgroup (Northland, New Zealand) and the Quaternary Sierra La Primavera caldera (Jalisco State, Mexico). These sequences differ in bed thickness, grading and abundance of matrix. We propose to evaluate grain size and sorting of volcaniclastic deposits by values of their modes, matrix proportion (<2mm; F-1) and D16, instead of median diameter (D50) and standard deviation parameters. F-1 and D16 can be uniformly used to characterize and compare sieving and functional stereology data. Volcaniclastic deposits typically consist of mixtures of particles that vary greatly in density and porosity. Hydraulic sorting ratios can be used to test whether mixed clast populations of pumice and dense clasts are hydraulically sorted with each other, considering various types of transport underwater. Evaluation of this ratio for our samples shows that most studied volcaniclastic facies are deposited by settling from density currents, and that basal dense clast breccias are emplaced by shear rolling. These hydraulic sorting ratios can be applied to any type of clastic rocks, and indifferently on consolidated and unconsolidated samples.

The phosphorus speciations in the sediments up- and down-stream of cascade dams along the middle Lancang River

We detected the longitudinal variability of phosphorus speciations and its relation to metals and grain size distribution of sediments in three cascade canyon reservoirs (Xiaowan, Manwan and Dachaoshan) along Lancang River, China. Five phosphorus speciations including loosely bound P (ex-P), reductant soluble P (BD-P), metal oxide-bound P (NaOH-P) calcium-bound P (HCl-P) and residual-P were extracted and quantified. Results showed that in Manwan Reservoir HCl-P accounted for the largest part of total phosphorus (TP) (49.69%), while in Xiaowan and Dachaoshan reservoirs, NaOH-P was the most abundant speciation which accounted for 57.21% and 55.19% of total phosphorus respectively. Higher contents of bio-available phosphorus in Xiaowan and Dachaoshan reservoirs suggested a high rate of P releasing from sediments. Results also showed ex-P and HCl-P had positive correlation with Ca. Total phosphorus was positively correlated with Fe. The silt/clay contents of the sediments had close relationship with ex-P (r=0.413, p<0.05), NaOH-P (r=0.428, p<0.05) and BAP (r=0.458, p<0.05). The concentration of Ca, Mn and silt/clay speciation in the sediments explained 40%, 10% and 4% of the spatial variation of phosphorus speciations, respectively.

FIELD MEASUREMENTS OF BEACH-DUNE DYNAMIC PROFILES TO ASSESS EROSION HAZARD ON THE COAST OF NSW, AUSTRALIA

The coast of New South Wales (NSW), Australia is about 2000 km long and consists of 721 sandy beaches (68%), rock coastline (32%), and more than 185 estuaries. It is most populated in Australia and one of the NSW greatest assets with significant economic, social and environmental values. The NSW coast has epsodically been ravaged by severe storms together with large ocean waves and high water levels, resulting in severe dune-beach erosion/recession, damaging coastal infrastructure and properties and degrading coastal ecosystems. With potential changes to storm-wave climate and rising sea level, coastal erosion hazards on the NSW coast are likely to worsen in the future. This study was undertaken to collect essential field data on beach-dune profiles and sediment grain-size distributions over more than 200 sandy beaches to assess NSW coastal erosion hazard. For each of the selected beaches, three beach-dune profiles of shore-normal transects at 50m apart were surveyed by RTK-GPS, and three sediment samples only on the first transect line were also colleced from the dune, dry beach/berm and swash zone by using a simple hand grabbing method. A sediment grain size analyzer, Malvern Mastersizer 2000E, was used to obtain sediment grain size distributions. It is found that the 618 sediment samples analysed consist of fine sand (10%), medium sand (82%) and coarse sand (8%), and that the dune sand d50 correlates well with the dry-beach sand d50 and is about 8% smaller, but less well correlates with the swash sand d50 and is about 15% smaller. The beach orientation was estimated from the direction of the shore-normal transect lines and generally ranges from 90o to 150o. The beaches surveyed are found to have erosion problems when they were directly exposed to predominant waves in the south-east direction and also when the dune toe elevations were lower than 3~3.5m (AHD). A conceptual model is also developed to assess likelihood storm erosion of a beach-dune system.

Morphological and sedimentological response of a mixed-energy barrier island tidal inlet to storm and fair-weather conditions

Abstract. The environment of ebb-tidal deltas between barrier island systems is characterized by a complex morphology with ebb- and flood-dominated channels, shoals and swash bars connecting the ebb-tidal delta platform to the adjacent island. These morphological features reveal characteristic surface sediment grain-size distributions and are subject to a continuous adaptation to the prevailing hydrodynamic forces. The mixed-energy tidal inlet Otzumer Balje between the East Frisian barrier islands of Langeoog and Spiekeroog in the southern North Sea has been chosen here as a model study area for the identification of relevant hydrodynamic drivers of morphology and sedimentology. We compare the effect of high-energy, wave-dominated storm conditions to mid-term, tide-dominated fair-weather conditions on tidal inlet morphology and sedimentology with a process-based numerical model. A multi-fractional approach with five grain-size fractions between 150 and 450 μm allows for the simulation of corresponding surface sediment grain-size distributions. Net sediment fluxes for distinct conditions are identified: during storm conditions, bed load sediment transport is generally onshore directed on the shallower ebb-tidal delta shoals, whereas fine-grained suspended sediment bypasses the tidal inlet by wave-driven currents. During fair weather the sediment transport mainly focuses on the inlet throat and the marginal flood channels. We show how the observed sediment grain-size distribution and the morphological response at mixed-energy tidal inlets are the result of both wave-dominated less frequent storm conditions and mid-term, tide-dominant fair-weather conditions.

Microelements in fractions of bottom sediments of the Amur River

The grain size distribution and fraction composition of bottom sediments (BS) in the Amur River are studied. The total composition of sediments, their clay subfractions—water-peptizable (WPC) and aggregated (AC) clays—and residue after clay elutriation were analyzed. It has been established that contents of Fe, Mn, Zn, and Cu in the clay subfractions are 2–4 times higher than in common bottom sediments and their residue left after the extraction of clay subfractions, confirming a significant role of clay minerals in the accumulation of elements mentioned above. It is shown that diatom algae participate in the accumulation and transport of Fe, Mn, Zn, and Cu in the river water-suspended matter-bottom sediments (clay minerals) system. It has been established that high Zr concentrations in bottom sediments are related to the presence of zircon therein.

Grain-size effect of biogenic silica in the surface sediments of the East China Sea

Biogenic silica (BSi) is an important parameter for understanding biogeochemical processes and paleoceanographic records in the ocean, but this proxy still has many challenges when used to reconstruct changes in the paleoproductivity and evolution of the environment, one of which is the grain size effect. We analyzed the BSi distribution in different size fractions from 8 surface sediments collected in the East China Sea (ECS). We observed the particulate characteristics of diatoms in the water and assessed the grain size effect of BSi. The results suggest the following conclusions: (1) the BSi content of the surface sediments in the ECS is generally below 1%, and the BSi content of different size fractions varies significantly, with largest fraction <16μm, which is approximately 1.1–1.8 times that in the bulk sediments. (2) The variation in the BSi content in different size fractions is largely controlled by the species of diatoms and their cell sizes. In the East China Sea, nano-diatoms are the dominant species, with dominant cells of 2–14μm, resulting in a high BSi content in the fraction <16μm. (3) The hydro-dynamic condition affects the diatom cell size distribution and sediment character, and, thus, has a significant influence on the BSi content of different size fractions. Our research suggests that the grain size distribution of the bulk sediment should be considered when using BSi as a proxy for diatom primary production.

Impact of storm-induced remobilization on grain size distribution and organic carbon content in sediments from the Blanes Canyon area, NW Mediterranean Sea

Interactions between organic carbon and mineral particles are known to play a key role for the preservation of organic carbon in marine sediments. In this study we combine meteorological, hydrological and oceanographic information with grain size and geochemical analyses (organic carbon, OC, its stable isotope δ13C, and the nitrogen to OC ratio) in different grain size fractions of surface sediments to determine the impact of a major storm on organic matter redistribution in a deep continental margin of the NW Mediterranean Sea. Sediment cores were obtained from the continental shelf, along the axis of the Blanes submarine canyon (BC), and in the adjacent southern open slope before (November 2008) and after (February and May 2009) the occurrence of one of the most severe coastal storms recorded in the last century in the study area.The storm occurred the 26 of December 2008 and dramatically impacted the North Catalan coast and its offshore area. The high shear stress produced by this event on the shelf floor led to sediment erosion and, subsequently, to its transport and deposition into BC. While sand and gravel-sized particles up to 2500μm in diameter were deposited at 300m of water depth in the canyon head, the finer particles (<4μm) were transported farther offshore towards the deep margin reaching at least 1500m depth.The results obtained show that the hydrodynamic processes that controlled the sorting and differential deposition of particles according to their grain size also controlled the redistribution of particles with different loads of OC. Thus, whereas coarse particles transported to the canyon head were poorly loaded with OC, fine particles reaching greater depths were relatively rich in OC, a significant proportion of which was of marine origin (i.e. derived from marine primary production). This adds to evidence that the physical action of waves and currents during storms plays a key role in determining the amount of OC stored in surface sediments not only in the shelf but also in the deep margin and possibly deeper.

Elemental and isotopic records in recent fluvio-lacustrine sediments in karstic river Krka, Croatia

A history of metal input and changes in the environmental conditions of the karst ecosystem of Krka River during the last several decades was investigated by statistical evaluation of the combination of several proxies including sediment content of minor (Pb, Zn, Cu, As, Cr, Ni, Co, V, Ga, Y, Rb, Sr) and major (Ca, K, Fe, Mn, Ti, Al) elements, concentrations of sedimentary organic carbon (Corg) and nitrogen (N), their stable isotope compositions (δ13Corg and δ15N), as well as stable isotope composition of sedimentary carbonate (δ13Ccarb and δ18Ocarb). The results show that measured trace and major elements are predominately of terrigenous origin, reflecting the geological background of the area, whereas the sedimentary organic matter is predominantly authigenic. Although Zn amounts in sediments are results of anthropogenic pressures, its distribution in investigated sediment columns is influenced by natural processes, i.e. occasional re-sedimentation and self-purification of the water column caused by the particulate Zn deposition in lakes formed by the tufa barrages. Levels of Pb in sediment columns are low except in the K2 which is close to the crossroads, especially in the period of lead fuel use. Trace element concentrations in sediment columns are compared to consensus-based sediment quality guidelines (SQG) for freshwater ecosystems. Some of the As, Zn and particularly Ni sediment concentrations were found to be above threshold effect concentration (TEC) range.The stable isotopes of Corg and Corg/N ratios suggest that the sedimentary organic matter is predominately of algal and bacterial origin, while the carbonate is a mixture of terrigenic detritus and in-situ precipitates. The average grain size distribution and carbonate content of sediment indicate decreasing input of terrestrial non-carbonate fraction, as a consequence of afforestation and land use changes in studied area.

MinSORTING: An Excel® worksheet for modelling mineral grain-size distribution in sediments, with application to detrital geochronology and provenance studies

MinSORTING is an Excel® spreadsheet devised to model mineral distributions in different size-classes within sediment samples and maximize mineral recovery during separation procedures for single-grain analysis. It is based on the physical laws of settling by tractive currents, both in water and air, applied to the different minerals in silt to sand-sized terrigenous sediments. Input values are: (i) grain size and sorting, (ii) depositional medium (seawater, freshwater or air), (iii) sediment composition (selected from a range of tectonic settings).The software's output includes the distribution of 27 different detrital components contained in sediments in size intervals of 0.25, 0.5, or 1 phi. Researchers can thus select the most appropriate size window for their own analyses and obtain valuable information on the amount of minerals lost in finer and coarser grain-size classes, allowing to accurately constrain the significance of their results.

Factors structuring the spatial distribution of Chironomidae larvae community in the floodplain of the northern Pantanal, Brazil

AIM: In order to identify the factors structuring the spatial distribution of the Chironomidae community, we analyzed the effects of water and sediment characteristics, flood duration and type of phytophysiognomic unit on the taxa composition of this community. MÉTODOS: O estudo foi realizado em 26 parcelas do Sítio de Amostragem de Longa Duração (SALD), localizado em uma área sazonalmente inundável na parte norte do Pantanal (Brasil). The study was conducted at 26 plots of the Long-Term Sampling Site, located in a seasonally flooded area in northern Pantanal (Brazil). Sediment samples were taken from each plot to analyze the Chironomidae community, organic matter and particle size; limnological parameters (depth, water temperature, dissolved oxygen, electrical conductivity and pH); besides the duration of flood. RESULTS: We identified 432 individuals distributed into 21 Chironomidae taxa. Polypedilum and Ablabesmyia were the most frequent spatially. The multivariate multiple regression analysis showed that the community composition was significantly related to the flood duration and grain size characteristics, but not to phytophysiognomic units and limnological parameters. Plots with predominantly sandy substrate and flooded for a shorter time presented a greater occurrence of taxa. CONCLUSIONS: The gradient produced by the spatial distribution of sediment grain size and flood duration determined the distribution of Chironomidae taxa. Changes in the hydrological functioning of the Pantanal, such as those from reservoir construction in the uplands region and roads and dykes construction in the floodplain could then impact the local chironomid biodiversity.

Biofilms and Size Sorting of Fine Sediment During Erosion in Intertidal Sands

The goal of this research was to investigate size-specific retention of clay and silt-sized grains by biofilms in sandy intertidal sediments. Sediment cores were collected from an intertidal flat in Cole Harbour, NS, and eroded at increasing shear stresses (0.08–0.60 Pa) with a Gust microcosm. Half of the cores were eroded without undergoing prior treatment, while sodium hypochlorite was added to the other cores to destroy biofilms. The disaggregated inorganic grain size distribution of sediment resuspended by the Gust microcosm was then obtained with a Multisizer™ 3 Coulter Counter®, and each treated core was compared with its corresponding untreated core. Overall, significantly less total sediment mass was resuspended from untreated cores than from treated cores. At intermediate shear stresses, the sediment resuspended from treated cores contained a greater proportion of fine and medium silts than the sediment resuspended from untreated cores. Very fine silts and clays were not retained preferentially by biofilms. The results show that biofilms stabilize the sediment, but they do not necessarily enhance the proportion of finest sediment sizes, as previously proposed.

SIZE DISTRIBUTION AND SORTING DEGREE OF THE RECENT SEDIMENTS OF AL-KHAZER RIVER/ NORTHERN IRAQ

Mechanical and statistical analysis were conducted to determine the grain size distribution and sorting degree of the recent sediments of Al-Khazer river at northern of Iraq in order to clarify the sedimentation processes along this river. The study was carried out after the annual flood time of the river. Sediment size distribution was determined mechanically for major grain size fractions [gravel (-2 phi to 0 phi), sand (0 phi to +4 phi), silt (+5 phi to +7 phi)] and clay (+8 phi)] using sieve and pipette technique. Numerical treatment of grain size curves has been done on the basis of some quintiles read from the empirical cumulative distribution function (CDF) curve: phi median size, sorting, skewness and kurtosis. The results showed that the sediments of this river were relatively coarse grained textured (Ø50=0.35 to 1.2 phi), very poorly sorted, showing strongly coarse skewed towards fine particles and are platy to leptokurtic. The increase in phi toward outlet plain is equivalent to decrease in grain diameter and is indicative of a lower energy and higher depositional environment. For this reason, there are some relatively lower phi values in the inlet plain in comparison with the phi values of outlet plain. On the basis of this assumption, the change in sediment-size distribution along the river is a non-linear fashion and can be inferred from that to which the present deposition takes place by bed or graded suspended load.

A model for linking grain-size component to lake level status of a modern clastic lake

Grain-size distributions of fluvial, eolian and marine sediments were explicated decades ago. For lake sediments, however, there is still great uncertainty in explaining the genesis of grain-size components due to the inherent complexity of their polymodal distributions. In this study, the grain-size components of the surface sediments of Daihai Lake, Inner Mongolia, were partitioned using a lognormal distribution function and the relationship between the identity of each component and the specific sedimentary environment was investigated. The data indicate that the modern clastic sediments of Daihai Lake contain five distinct unimodal grain-size distributions representing five grain-size components. Each of the components retains its identity including modal size, manner of transportation and environment of deposition although the relative percentage varies with the hydraulic condition throughout the lake. These components are specified from fine to coarse modes as long-term suspension clay, offshore-suspension fine silt and medium-to-coarse silt, and nearshore-suspension fine sand and saltation medium sand. The percentage of the components interpreted as an indication of nearshore environments displays a negative correlation with water depth across the modern lakebed, suggesting a model for linking the nearshore components in sediment cores to the lake level status in the geological past. The model was applied to a sediment core from the lake where high percentages of the nearshore components in the core sediments were correlated with low regional precipitations reconstructed on the pollen profile of the same core. The coincidences between two independent proxies do not only demonstrate the validity of lognormal distribution function in partitioning polymodal sediments but also reveals the potential of the grain-size component–lake level status model for lake’s paleohydrological reconstruction.

Detrital grain size distribution of the surface sediments and sedimentary processes in Lake Kizaki, central Japan

This study presents that the grain-size distribution obtained by the hydrometer method is affected by diatom frustules in bulk sediments finer than 6 phi in median size. This fact implies that detrital grains need to be extracted from bulk muddy sediments to clarify sedimentary processes of detrital particles. This paper reveals that detrital grain size distribution of the surface sediments and sedimentary processes in Lake Kizaki, central Japan. Detrital grains were extracted by removing diatom frustules composed of amorphous opal, and were measured by laser grain size analyzer. Surface sediments can be divided into four groups. The Group I and II sediments are classified into clayey silt, and distribute in the offshore zone. The Group III sediments are of sandy silt or sand-silt-clay, and form transitional zone between the offshore and following nearshore. The Group IV sediments are of silty sand or sand, and their distributions almost correspond to nearshore zone. Boundaries between the Group III and Group IV cross the counter line toward offshore at the river mouth, implying that flood event plays an important role of sedimentation in Lake Kizaki. Sandy and gravelly sediments of the nearshore zone can be regarded as residual sediments by wave action. Sediments composed of transitional zone transported by waves and spread from the river mouth during flooding event and normal periods transported toward offshore, and then settle down quickly according to Stokes’ law. Moreover, fine sediments composed of the offshore zone remained suspended and appeared to be controlled by surface current.

利根川汽水域における細粒底質の輸送に関する数値シミュレーション

This paper discusses the motion of fine bed sediment in Tone River Estuary by a vertical 2-D numerical simulation. The simulation model for stratified flow computation is based on k-..turbulent model which was already verified in the estuary by some existing studies. The model for SS transport contains a new formulation of SS concentration at the estuary bed which was deduced from a unified processing of some published flume experimental results and some field data collected in the estuary. Calculation result suggests that fine sediment transported from the river mouth by estuarine circulation becomes dominant in the lower estuary under saline wedge stagnation during non-flood season. On the other hand, the sediment from the upstream watershed may be still dominant in the upper estuary. The result is consistent with some published data about seasonal change of grain size distribution and chemical characteristics of the bed sediment collected in the estuary.

Laboratory validation of acoustic Doppler current profiler (ADCP) techniques for suspended sediment investigations

The aim of this paper is to validate a method of investigating the grain size distribution of suspended sediments using acoustic Doppler current profilers (ADCPs) and to compare different calibration strategies of the ADCP backscattering power, which can be correlated with the concentration of corresponding sediments. Over the last two decades, the methods for suspended sediment investigation using the backscattering power of ADCPs have been gaining increasing acceptance within the river engineering community. This acceptance is due to acoustic backscattering providing the opportunity to indirectly quantify suspended sediment by non-intrusive measurements with high temporal and spatial resolution. We have already presented the method using two ADCPs working at different frequencies (600 and 1200 kHz) on the same water column to profile the concentration and grain size in the Paraná River (Argentina). The present work, as a complement to the Paraná application, demonstrates the reliability of the method by a laboratory validation of monitored concentrations and known grain size distributions. The context of our research is introduced in a discussion of the topic of river sediment transport measurement using ADCPs. Then, the underwater acoustic physics is briefly presented with a focus on acoustic backscattering (ABS) methods for the estimation of suspended sediment grain size. The laboratory tests, which were conducted at the sediment tower-mixing facility using four different known distributions of sand in the range 50–700 μm, are described in detail to demonstrate the reliability of the ABS methods and to justify the adopted experimental strategies to overcome undesired air bubble interference with the ABS due to injected sand.

Patterns of grain-size temporal variation of sediment transported by overland flow associated with moving storms: interpreting soil flume experiments

Abstract. This study describes and interprets the evolution of grain-size distribution of sediment yields generated in an experimental soil flume subjected to downstream and upstream moving rain storms. Results of laboratory experiments show that downstream moving storms cause more soil loss than do upstream moving storms. The pattern of sediment grain-size evolution in time during a runoff event exhibits a clear dependence on the direction of storm movement. A strong relationship between overland flow discharge and mean sediment size is found. Nevertheless, the mean grain-size of sediments transported during the rising limb of the hydrograph is coarser than during the recession limb of the hydrograph. This is more marked for downstream moving storms.

How important is it to integrate riverine suspended sediment chemical composition with depth? Clues from Amazon River depth-profiles

The vertical variability in mineralogical, chemical and isotopic compositions observed in large river suspended sediments calls for a depth-integration of this variability to accurately determine riverine geochemical fluxes. In this paper, we present a method to determine depth-integrated chemical particulate fluxes of large rivers, based on river sampling along depth-profiles, and applied to the Amazon Basin lowland tributaries. The suspended particulate matter (SPM) concentration data from depth-profiles is modeled for a number of individual grain size fractions using the Rouse model, which allows to predict the grain size distribution of suspended sediment throughout the whole river cross-section. Then, using (1) the relationship between grain size distribution and the Al/Si ratio (2) relationships between the Al/Si ratio and the chemical concentrations, the chemical composition of river sediment is predicted throughout the river cross-section, and integrated to yield the depth-integrated chemical particulate flux for a number of chemical elements (e.g. Si, Al, Fe, Na, REEs, …). For elements such as Al, Fe, REEs, Th, the depth-integrated flux is around twice as high as the one calculated from river surface sample characteristics. For Na and Si, the depth-integrated flux is three times higher than the “surface” estimate, due to the enrichment of albite and quartz at the bottom of the river. Depth-integrated 87Sr/ 86Sr composition of suspended sediment, also predictable using this method, differs by more than 10 −3 from the surface sample composition. Finally, potential implications of depth-integrated estimates of Amazon sediment chemistry are explored. Depth-integration of particulate 87Sr/ 86Sr isotopic ratios is necessary for a reliable use of Sr isotopes as a provenance tracer. The concept of steady-state weathering of a large river basin is revisited using depth-integrated sediment composition. This analysis shows that, in the Amazon Basin river, the previously observed discrepancy between (1) weathering intensities of channel surface sediment and (2) silicate-derived dissolved fluxes is only slightly accounted for by the vertical variability of suspended sediment weathering intensities. This observation confirms that most large rivers basins are not eroding at steady-state.

A new framework for modeling sediment fining during transport with fragmentation and abrasion

A new framework for modeling sediment fining during transport is proposed. The model is based on a physical description of both abrasion and fragmentation processes. It attempts to describe and explain the changes in the grain-size distribution of sediments during their transport in a water flow. Abrasion and fragmentation are modeled through the use of breakage frequency functions and daughter size distributions. These functions are determined experimentally for the specific case study which is presented. Comparison between the predictions from the model and experimental data from the case study show that 1)both abrasion and fragmentation are involved in the degradation 2)fragmentation eciency decreases during the experiment 3)the shape of the pebbles provides feedback between abrasion and fragmentation. This type of model is therefore appropriate for explaining degradation mechanisms, which is promising for a future application to rivers.