Grain Size Distribution Analysis Research Articles

Holocene relative sea-level changes on the southern east coast of the Yellow Sea

Projections indicate increasing coastal hazards in the Yellow Sea region owing to relative sea-level rise. Accordingly, regional studies of such change over the Holocene timescale could be helpful in mitigating these threats. Here, we describe the Holocene record of relative sea-level change in the Gochang-Yeonggwang area of South Korea. We compared this record with that of other sites around the coast of the Yellow Sea, including Bohai Bay (China), the Tando Estuary and Gunsan Bay (South Korea), and sites on the east coast of the South Sea and Toyo-oka Basin, Japan. The findings are based on a multi-proxy study, including lithostratigraphy combined with radiocarbon dating and analyses of grain size distribution, magnetic susceptibility, and geochemistry, which facilitated the generation of 89 paleo-sea-level points reconstructed from 28 borehole cores. The early Holocene transgression was found to start later at Gochang-Yeonggwang than at Gunsan Bay, although the distance between them is only 150 km. This result was inferred from substantial differences in sediment delivery owing to Holocene differences in river basin size, topography, and tectonic context. Before ∼7 ka, the slope and highstands from the predicted mean sea-level models at Bohai Bay, Tando Estuary, and Gochang-Yeonggwang were steeper and lower, respectively, from north to south in the Yellow Sea, approaching the slope of the glacio-eustatic sea-level curve. Therefore, the levering effect of transgression in the early Holocene decreased southward in the Yellow Sea. The slight fluctuation in mean sea-level points of the late Holocene resulted from the formation of storm deposit layers in the coastal area, erosion and redeposition in the channel, and geomorphic effects. A partial difference occurred between the relative sea levels at Gochang-Yeonggwang and Toyo-oka. Future relative sea-level research at the southernmost part of the east coast of the Yellow Sea would further improve understanding of the coastal evolution of the region.

Assessing Contaminant Mass Discharge Uncertainty With Application of Hydraulic Conductivities Derived From Geoelectrical Cross‐Borehole Induced Polarization and Other Methods

AbstractA new methodology was developed to support contaminant mass discharge (CMD)‐based risk assessment of groundwater contamination downgradient of point source zones. Geoelectrical cross‐borehole induced polarization (IP) data were collected at a site undergoing in situ remediation of chlorinated solvents for determining 2D hydraulic conductivity (K) distributions with an inversion model resolution of 0.15 m (vertically) x 0.50 m (horizontally) in three control planes from 10 to 20 m depth. Additionally, 18 slug tests and 31 grain size distribution analyses (GSA) from the control planes, were used for K‐estimation. The geometric means and variance of the IP, slug test, and GSA derived K‐estimates were consistent with previously studied sandy aquifers. Furthermore, the vertical variation in K between two geological settings, a sandy till and a meltwater sand formation, was clearly identified by the IP K‐estimates. The vertical variation was backed up by hydraulic profiling tool (HPT) measurements. Random realizations of CMD were simulated based on the cross‐borehole IP derived K‐values. For comparison, the CMD was also estimated with a geostatistical conditional simulation approach, using the data from slug tests and GSAs. The high IP resolution captured the small scale variations in K across the transects and led to CMD predictions with a narrow uncertainty interval, whereas slug test and GSA either under‐ or overestimated the magnitude of the areas with the highest CMD. Applying the geophysical cross‐borehole method for estimating K‐distributions in addition to traditional methods would improve CMD‐based risk assessment and evaluation of remediation performance at contaminated sites.

Spatio-temporal variability of hydraulic conductivity in the floodplain riverbank of a hyporheic zone

A detailed measurement of floodplain riverbank sediment hydraulic conductivity (K) is the first step in analyzing the hydraulic relationship between streams and aquifers. Poyang Lake is the largest freshwater lake in China and has historically been a flood-prone area. A detailed field experiment was conducted in the Xiushui River, China, before and after the flood season. The grain-size distribution analysis method was used to determine the hydraulic conductivity of the riverbank. A total of 116 sediment samples were collected from the test site. An in situ permeameter test was used to confirm the consistency of the spatial distribution trend of the estimated hydraulic conductivity based on the grain-size distribution analysis. A Kruskal–Wallis test of the riverbank sediment K values obtained before and after the flood season showed that they belong to different populations. The probability distribution illustrated the strong heterogeneity of the riverbank sediments before and after the flood season. Previous studies have confirmed the presence of strong hyporheic exchange and biological processes at this study site. Following the flood season, the K values had a higher mean, median, and coefficient of variation, confirming that the flood process enhanced the heterogeneity of the riverbank sediments. Overall, the hydraulic conductivity of the three areas changed with distance from the river; the closer the area was to the river, the smaller the hydraulic conductivity and weaker the heterogeneity. An experimental framework of spatio-temporal variability in riverbank sediments is proposed, providing recommendations for researchers on conducting an integrated experiment.

Early Cretaceous (late Barremian−Early Albian) expanding aeolian activity in East Asia: Evidence from the stratigraphic evolution of aeolian deposition in the Baiyin-Jingyuan Basin, northern China

Cretaceous aeolian deposition is important for understanding variations of the tropical−subtropical atmospheric circulation under the greenhouse background. Nevertheless, detailed records revealing the response of aeolian deposition to rising atmospheric CO2 are still lacking. To shed light on their linkage, we examined the Early Cretaceous stratigraphic evolution of the Hekou Group in the Baiyin-Jingyuan Basin, northern China, using multiple methods. The results indicate that the lower Hekou Group is characterized by a transition from submerged alluvial to shallow lacustrine deposition. Overlying these facies is loess-like deposition without bedding, distinguished by grain-size distribution, surface microtextures, and geochemical analysis, which is followed by palustrine deposition. In the palustrine deposition, aeolian sand-dune and sand-sheet deposition can be identified, which gradually developed upward in the Hekou Group, with intercalated wet interdunes, damp interdunes, and pebble-sand sheetflood beds. Finally, aeolian sand dunes and sand sheets dominated the upper Hekou Group. Overall, the upward-changing facies indicated the development of aeolian deposition from an aqueous environment, revealing a long-term paleoclimatic shift from semi-humid, semi-arid to extremely arid, which reflects the expanded aeolian activity in East Asia during the Early Cretaceous. Furthermore, various records were combined to investigate the expanding aeolian activity in East Asia, which indicates a west−east-trending arid belt that was wider than the modern counterpart during the late Barremian−Early Aptian and late Aptian−Early Albian, respectively. These stepwise expansions of aeolian activity were mainly driven by rising atmospheric CO2, which strengthened the subsiding branches of Hadley circulation and the subtropical high, eventually leading to intensified aridification.

Prevention of well clogging during aquifer storage of turbid tile drainage water rich in dissolved organic carbon and nutrients

Well clogging was studied at an aquifer storage transfer and recovery (ASTR) site used to secure freshwater supply for a flower bulb farm. Tile drainage water (TDW) was collected from a 10-ha parcel, stored in a sandy brackish coastal aquifer via well injection in wet periods, and reused during dry periods. This ASTR application has been susceptible to clogging, as the TDW composition largely exceeded most clogging mitigation guidelines. TDW pretreatment by sand filtration did not cause substantial clogging at a smaller ASR site (2 ha) at the same farm. In the current (10 ha) system, sand filtration was substituted by 40-μm disc filters to lower costs (by 10,000–30,000 Euro) and reduce space (by 50–100 m2). This measure treated TDW insufficiently and injection wells rapidly clogged. Chemical, biological, and physical clogging occurred, as observed from elemental, organic carbon, 16S rRNA, and grain-size distribution analyses of the clogging material. Physical clogging by particles was the main cause, based on the strong relation between injected turbidity load and normalized well injectivity. Periodical backflushing of injection wells improved operation, although the disc filters clogged when the turbidity increased (up to 165 NTU) during a severe rainfall event (44 mm in 3 days). Automated periodical backflushing, together with regulating the maximum turbidity (<20 NTU) of the TDW, protected ASTR operation, but reduced the injected TDW volume by ~20–25%. The studied clogging-prevention measures collectively are only viable as an alternative for sand filtration when the injected volume remains sufficient to secure the farmer’s needs for irrigation.

Chronostratigraphy of the Periglacial Loess-Paleosol Sequence in Zaprężyn, SW Poland

Abstract Loess-paleosol sequences (LPS) are an exceptional source of palaeoclimate data constituting an indirect record of changing environmental and climatic conditions that prevailed during loess deposition and early diagenesis. The Zaprężyn LPS (17°11′52″E, 51°14′44″N) is situated in the southern morphological edge of the Trzebnica Hills. This sequence provides an opportunity to reconstruct past climate conditions in this part of SW Poland. In this work, we used two sets of optically stimulated luminescence (OSL) dating from the Gliwice and Bayreuth laboratory to establish a chronological framework for this profile. The results of 14C accelerator mass spectrometry (AMS) dating were also used. The dating results are accompanied by detailed analyses of grain-size distribution and colour. The applied methodology allowed us to distinguish four litho-pedotratigraphic units: modern soil S0, L1LL1 loess unit, L1LL2 loess unit and fossil S1 soil. The OSL dating results indicate an unconformity representing a time interval &gt;30 ka between the L1LL1 and L1LL2 units. This unconformity explains the lack of a L1SS1 soil unit. In Zaprężyn LPS, the presence of the ice wedge pseudomorph in L1LL2 loess unit was recorded. Ice wedge pseudomorph approves the presens the permafrost conditions during the Lower Plenivistulian in Trzebnica Hills.

Response of modern fluvial sediments to regional tectonic activity along the upper Min River, eastern Tibet

Abstract. The deposition of fluvial sediments in tectonically active areas is mainly controlled by tectonics, climate, and associated Earth surface processes; consequently, fluvial sediments can provide a valuable record of changes in regional climate and tectonic activity. In this study, we conducted a detailed analysis of the grain-size distribution in modern fluvial sediments from the upper Min River, eastern Tibet. These data, combined with information on regional climate, vegetation, hydrology, geomorphology, lithology, and fault slip rate, indicate that modern regional tectonic activity along upper Min River can be divided into three segments. Specifically, fluvial sediments in the Minjiangyuan–Diexi segment are dominated by silts (&lt; 63 µm, 70.2 %), agreeing with low runoff, low rainfall, and high vegetation cover and revealing a windblown origin influenced by the arid and windy climate. These observations are consistent with the low hillslope angle and low relief, all indicating weak activity along the Minjiang Fault. The coarse-grained fraction (&gt; 250 µm) of fluvial sediments in the Diexi–Wenchuan and Wenchuan–Dujiangyan segments increases stepwise downstream, although runoff and rainfall do not change significantly. These patterns correlate well with increases in both regional relief and hillslope angles. Together, these observations imply that regional tectonic activity along the Maoxian–Wenchuan Fault becomes more pervasive downstream along the Min River. The occurrence of well-sorted and well-rounded pebbles of fluvial sediments downstream of Dujiangyan must be related to the long-time scouring and sorting by rivers. This study marks the first development of a new research approach that can characterize regional tectonic activity by analysis of grain-size distribution of fluvial sediments collected from tectonically active regions.

Morphometric parameters of sulphide ores as a basis for selective ore dressing

To assess the possibility of selective disintegration and reduction of overgrinding of hard-to-reproduce ores, optical microscopic and X-ray microtomographic studies were carried out and quantitative characteristics of morphological parameters of disseminated and rich cuprous ore samples from Norilsk-type Oktyabrsky deposit were identified. Among quantitative morphological parameters the most informative are area, perimeter, edge roughness, sphericity, elongation and average grain spacing for disseminated copper-nickel ores; area, perimeter, edge roughness and elongation for rich cuprous ores. The studied parameters are characterized by increased values and dispersion in ore zones, which is especially important for fine-grained ores, which are difficult to diagnose by optical methods. Three-dimensional modelling of the internal structure of sulphide mineralisation samples was carried out using computed X-ray microtomography, which allows observation of quantitative parameters of grains, aggregates and their distribution in the total rock volume and interrelationship with each other. The evaluation of rock pore space by computer microtomography made it possible to compare the results obtained with the strength characteristics of rocks and ores, including those on different types of crushers. The obtained quantitative characteristics of structural-textural parameters and analysis of grain size distribution of ore minerals allow us to evaluate the possibility of applying selective crushing at various stages of ore preparation

The 2.6-2.3ka explosive eruptive period of the Pululahua dome complex, Ecuador: insights from pyroclast analysis.

Pululahua is an active volcano located 15 km north of Quito, Ecuador, that comprises sixteen dacitic-andesitic lava domes and a 13 km2 sub-rectangular depression formed between ~ 2.6 and ~ 2.3 ka. We use a detailed study of 70 flow and fall deposits that make up the pyroclastic sequence to show that the depression, previously classified as a caldera, was formed by numerous Vulcanian to (sub-) Plinian eruptions that destroyed both earlier and co-eruptive lava domes. We support this interpretation with field work, analysis of grain size distributions, density and components of 24 key deposits, supplemented by textural and petrologic analyses of 16 juvenile pyroclasts from throughout the pyroclastic sequence. These data document an alternation of (sub-) Plinian and Vulcanian eruptions dominated by denser juvenile material that preserves microtextural variations indicating changes in shallow level magma storage accompanying Vulcanian explosions. An exploratory examination of phenocryst textures and plagioclase and amphibole rim compositions suggests that much of the eruptive activity was driven by repeated inputs of less evolved magma into the Pululahua magmatic system. The inferred sequence of events provides a new hypothesis for the formation of the current morphology of Pululahua, including multiple episodes of both effusive and explosive eruptions accompanied by vent migration. Our findings offer an important insight into Pululahua’s potential future hazard scenarios, which could affect millions of people.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00445-022-01590-4.

Numerical methodologies and tools for efficient and flexible unmixing of single-sample grain-size distributions: Application to late Quaternary aeolian sediments from the desert-loess transition zone of the Tengger Desert

Parametric curve-fitting techniques are routinely adopted to derive the components of individual grain-size distributions of sediment samples. However, due to the lack of methodologies and calculation platforms to efficiently generate reproducible unmixing solutions, and appropriate statistical techniques to analyze and visualize unmixing results characterized by complex and variable structures, the technique has seldom been extended to the systematic analysis of massive grain-size distributions in sedimentological and paleoenvironmental research. In this study, we use numerical techniques to develop a novel strategy enabling the efficient and flexible unmixing of single-sample grain-size distributions. The method was applied to a large set of grain-size distributions of late Quaternary aeolian sediments from the desert-loess transition zone around the Tengger Desert in North China. The potential structures of the pooled unmixed subpopulations were derived using a finite mixture model, taking variances into account, together with a pattern recognition (clustering) method. Five unimodal endmembers were identified by the finite mixture model and four prevalent structural patterns were recognized by the clustering method. The significances of the endmembers and structural patterns are discussed in detail. We demonstrate that the single-sample unmixing method can be used to complement the widely applied end-member modelling to reveal potential provenances of large datasets of grain-size distributions, and that the component structures of grain-size distributions may contain valuable information that can be used to constrain the associated paleoenvironmental conditions.

Diagenetic facies classification and characterization of a high-temperature and high-pressure tight gas sandstone reservoir: A case study in the Ledong area, Yinggehai Basin

A high-temperature and high-pressure tight gas sandstone reservoir, located in the Ledong area of the Yinggehai Basin, shows considerable resource potential. However, the diagenetic characteristics of this reservoir remain unclear. In this study, reservoir petrology and diagenetic characteristics were systematically investigated through different experimental methods. Further, the porosity and diagenetic evolution models were constructed. The methods employed included petrophysical analysis, observation of core and thin sections, and grain-size distribution analysis. Furthermore, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, cathodoluminescence, carbon and oxygen isotope analysis, digital image analysis, mercury injection capillary pressure tests, and micro-CT imaging were also employed. In the study area, gravity-flow channel sediments composed of feldspathic litharenite primarily developed in a restricted submarine canyon. And tight sandstones in target formation have an average porosity of 9.34% and permeability of 1.79 mD. Mechanical compaction and cementation (mainly illite and ferrocalcite) were regarded as crucial roles in the densification process. Further, the diagenetic evolution model indicated that the reservoir underwent a high degree of thermal diagenetic evolution. Five diagenetic facies types were classified and characterized at multiple scales. Moreover, a vertical distribution model of different diagenetic facies was also developed considering the overpressure, depth of formation, thickness and frequency of mudstones, and injection concentration of the CO2. This study helps to improve the understanding of the diagenetic characteristics and diagenetic facies types in high-temperature and high-pressure clastic reservoirs.

Modeling of Polycrystalline Material Microstructure with 3D Grain Boundary Based on Laguerre-Voronoi Tessellation.

Voronoi tessellations are shown to be statistically representative of polycrystalline microstructures, which have been widely accepted for the modeling of microstructures of metallurgic and ceramic materials. In this paper, a new implementation of the Voronoi diagram in Laguerre geometry is presented for the generation of numerical models of polycrystalline microstructures, where the size and shape of the grains can be controlled, and the 3D grain boundaries can be modeled with a specified thickness. The distribution of grain sizes in the models is fitted to a lognormal distribution, compared with the normal distribution in the Voronoi tessellation methods. Finally, statistical analyses of grain face and grain size distribution are performed with the models, and the macroscopic elastic properties of polycrystalline ceramic materials are simulated to verify the capability of the presented method.

A ∼28-kyr Continuous Lacustrine Paleoseismic Record of the Intraplate, Slow-Slipping Fuyun Fault in Northwest China

The Fuyun Fault is a typical intraplate, slow-slipping fault, but has been repeatedly ruptured by surface wave magnitude (Ms) ≥ 8.0 earthquakes. The 11 August 1931 Ms 8.0 Fuyun earthquake resulted in more than 10,000 casualties in the sparsely populated Fuyun area. Cosmogenic 10Be dating of offset landforms produced by prehistoric Ms ≥ 8.0 earthquakes yields an average recurrence interval of 9,700 ± 3,300 years, much longer than previously estimated 2,000–4,500 years, clouding our understanding of the timing and recurrence behavior of past earthquakes originating from the Fuyun Fault. Reflection seismic data reveal widely distributed subaquatic faults in Yileimu Lake, implying high sensitivity of the lake sediments to paleoearthquakes originating from the Fuyun Fault. Two new long sediment cores (Y20A: 267 cm; Y20B: 890 cm) together with previously published two short cores (Y19: 71 cm; Y20: 31.5 cm) from the depocenter and nearshore zone of Yileimu Lake are used for stratigraphic correlations and analyses of sedimentary structures, grain-size distributions, magnetic susceptibility, elemental composition and carbon content. The mass-wasting deposits with underlying soft-sediment deformation structures (SSDS) in the Y20B core indicate six siliciclastic-enriched sandy sediment fluxes from earthquake-triggered landslides of granitic rocks, and isolated SSDS record three additional earthquake-induced in situ deformations. Turbidite-like deposits with sorting indices &amp;gt;3 and Si contents &amp;gt;700 counts per second (cps) are comparable to those of the seismic mass-wasting deposits, and are thus interpreted as seismites from earthquake-induced re-deposition of nearshore sediments. There are a total of 20 seismic events recorded by the Y20B core. Seismic intensity calculation results, combined with historical seismic data, provide potential magnitudes of Ms ≥ 8.0, Ms ≥ 7.0, and Ms ≥ 5.5 for the earthquake-triggered mass-wasting deposits, SSDS, and turbidite-like seismites, respectively, in Yileimu Lake, generally consistent with previously published magnitude thresholds. Radiocarbon dating and stratigraphic correlations constrain the timing of these past earthquakes to ∼28 cal kyr BP. This unique, long lacustrine paleoseismic record suggests a weakly periodic pattern with recurrence intervals between 2,317 and 7,830 years and an average of 5,303 years for potential Ms ≥ 8.0 earthquakes, and reveals an unprecedented high frequency of potential Ms ≥ 7.0 earthquakes originating from the Fuyun Fault in the last 5 kyr, demonstrating the urgent need for an improved assessment of seismic hazards and risks in the Fuyun Fault zone.

Characteristics of Beaver Ponds and Landforms Induced by Beaver Activity, S Part of the Tuchola Pinewoods, Poland

Currently, there are only two species of beavers described—the North American beaver (Castor canadensis) and Eurasian beaver (Castor fiber). Their natural habitats are confined to the northern hemisphere but instances of beaver introduction to regions of the world they do not normally inhabit have also been recorded. The activity of beavers leads to changes in the natural environment linked to hydrological and geomorphological and plant cover transformations. Beavers live in natural and artificial water reservoirs and rivers. If the water level in the river is too low, they build dams to create a comfortable living environment. This paper aims to present changes in the relief of the valley inhabited by beavers in which sediments accumulate. During the field study, detailed measurements of dams and of the spatial range of beaver ponds were made, and the thickness and spatial distribution of accumulated sediments were determined. In addition, measurements of geomorphological forms in beaver ponds were also made. The samples of sediments were subject to grain-size distribution analysis, the results of which allowed calculating sediment parameters. Beavers appeared in the Gajdówka valley in the southern part of the Tuchola Forest (Poland) in 2008. In 2008–2011 they built 17 beaver dams that impounded ponds. The beaver ponds and beaver dams were of different sizes. They either flooded the whole flat bottom of the valley or only raised the level of water in the riverbed. A characteristic feature of beaver ponds is that they capture sediments. Different landforms were created in the course of the formation and disappearance of beaver ponds. It was established that these include alluvial fans, levees, sand shadow dunes and microterraces formed by deposition and erosion. They do not occur in all ponds. Points at which mineral sediments are supplied to the watercourse, including beaver burrows and erosion hollows, are presented together with the points at which sediments are transferred from ponds upstream to ponds downstream the watercourse. Beaver activity during valley colonization shows changes in the landscape caused by their presence and in particular their impact on the relief and deposition of sediments. Analysis of contemporary changes in the morphology of the Gajdówka Valley leads to the conclusion that beaver activity has had an intense impact on the terrain relief of the valley inhabited by beavers.

Regional hydroclimates regulate the Holocene aeolian accumulation processes of the Qinghai Lake basin on the northeastern Tibetan plateau

Aeolian sediments are widely distributed in the Qinghai Lake basin on the northeastern Tibetan Plateau (TP), and have been extensively investigated for the palaeoclimate reconstruction. However, due to the relatively scanty of dating materials and applicable techniques, and also the commonly existed hiatus in aeolian sequences, the details of the Holocene aeolian activities in the Qinghai Lake basin remain unclear. In this study, we applied the optically stimulated luminescence (OSL) dating technique to a 3.6 m thick loess-palaeosol sequence (QH-18) located on the southern bank of the Qinghai Lake to establish a high-resolution chronology of the aeolian accumulation history. The quartz OSL chronology demonstrated that the loess accumulation commenced since the last deglaciation and continued during the Holocene (∼14.6–0.5 ka), characterized with a remarkably low sedimentation rate period from 6 ka to 2 ka. Similar phenomenon has been recorded in previously investigated aeolian sequences not only in the Qinghai Lake basin but also across the northeastern TP, which was likely owing to the optimal environment conditions that have suppressed the dust emission from the source areas in the extensive arid and semiarid areas. The end-member (EM) analysis of grain-size distribution in QH-18 yielded an optimal model with three EMs, i.e., EM1 (6 μm, background dust), EM2 (56 μm, fraction of medium or long-distance transportation) and EM3 (447 μm, fraction from proximal or local source). The EM3 fraction can only be transported with saltation process in short distances that derived from proximal or local sources. In addition to the regional climatic conditions, according to the variation and contribution of the EMs, we consider that the Holocene aeolian activity in the Lake Qinghai basin was also modulated by the lake-level fluctuations of Lake Qinghai, in the way of changing the supply of sandy materials.

Reworking activity of the thalassinidean shrimp Trypaea australiensis Dana, 1852 (Decapoda: Pleocyemata: Callianassidae) in an Australian estuary: A pilot study

Abstract Trypaea australiensis Dana, 1852 is the predominant bioturbating thalassinidean shrimp on the east coast of Australia. It is, like other large bioturbators, generally considered an important ecosystem engineer. The sediment particle reworking rate of thalassinideans, a key parameter in benthic biogeochemical modelling, nevertheless remains unknown. We have for the first time quantified particle reworking by a population of T. australiensis living in fine estuarine sand. The particle reworking rate was monitored for 18 days using coarse sand as a new tracer approach followed by analyses of grain-size distribution in the sediment to a depth of 24 cm. Burial depth (BD, cm) over time (t, days) followed the relationship BD = 0.3002 × t for an average population density of 96 individuals m–2, equivalent to a sediment displacement of 1.1 m3 m–2 yr–1 (11.4 liters individual–1 yr–1). The individual-specific particle reworking by this thalassinidean is higher than that reported for most other key bioturbators, and its engineering impact on estuarine sediments therefore has potentially large consequences for biodiversity and ecosystem functioning.

Experimental Study on the Riverbed Coarsening Process and Changes in the Flow Structure and Resistance in the Gravel Riverbed Downstream of Dams

Following the construction of a reservoir, sediment is intercepted, which greatly reduces the sediment concentration in the discharged flow. This reduction causes riverbed scouring and flow structure adjustments downstream, thereby impacting the river habitat. This study used the generalized flume test with different bed sand compositions and discharge rates to simulate the scouring adjustment process of a sand pebble riverbed channel downstream of a reservoir. The results show that the bed sediment composition affects the water surface gradient, scour depth, turbulence intensity, and sand resistance directly after final scouring. Coarse-grained bed sediment demonstrated the greatest final turbulence intensity and sand resistance, while bed sediments with reduced coarseness exhibited a greater scouring degree; the resistance for sand grains of moderate coarseness showed the greatest change. Sand resistance was exponentially and positively correlated with the median grain size and the fractal dimension of bed sediment mass. The mass fractal dimension expression was suitable for the analysis of bed sand grain-size distribution; it contributed to the calculation of grain resistance with fewer hydraulic parameters. The relationship between the mass fractal dimension and the adjusted grain resistance was also established, which can aid the calculation of the resistance changes in sandy gravel-bed river reaches downstream of reservoirs, enabling the prediction of their effects on aquatic habitats.

Formation and Dispersal of Ash at Open Conduit Basaltic Volcanoes: Lessons From Etna

Open conduit volcanoes are characterized by frequent, small scale explosive eruptions, which have a significant impact. Ash-forming explosions are impacting over larger areas with respect to effusive or poorly explosive events and, consequently, are more significant for hazard assessments. Quantifying the hazard associated with them requires understanding the processes and parameters controlling explosive style, and tephra dispersal and obtaining a comprehensive dataset to constrain syn-eruptive dynamics and particle transport in the volcanic plume. We present a study focused on Etna volcano (Italy), which, despite its continuous outgassing through the summit vents, has very frequent explosive eruptions dispersing ash along the southern Mediterranean area. The goal of this study is to obtain a statistically valid dataset on ash morphology and texture and investigate how various particle types distribute spatially in the tephra blanket. We chose a small scale, ash-forming eruption occurred in May 2016, sampled a few hours after tephra deposition. Analyses of grainsize distribution were coupled with further data on tephra texture and morphology, and numerical simulations. Several components were identified based either on purely textural or purely shape characteristics. Shape parameters related to the form of the grains (aspect ratio) are consistent across grainsizes and components. However, roughness parameters (solidity, convexity, concavity index) vary non-uniformly with particle size and componentry. Ash was formed through complex fragmentation of heterogenous magma, starting in the conduit, extending to the explosion jet, and resulting into a large variability of particle shapes, density and textures which distribute non-uniformly across grainsizes. This variability determines variable traveling potential within the volcanic plume and thus non uniform distribution in the deposit. Componentry variations along the dispersal axis suggest that density is the most effective parameter in controlling particle settling. However, extreme shapes, such as very elongated particles formed by surface tension instabilities in the jet, have the largest potential of being transported in the plume and can disperse downwind up to tens of km. Our results suggest that heterogeneities in textures and morphologies of particles are fundamental characteristics of tephra from frequently erupting volcanoes and should be accounted for plume dispersal modelling and hazard assessment.

QGrain: An open-source and easy-to-use software for the comprehensive analysis of grain size distributions

In the last hundred years, analyses of grain size have been widely performed, and numerous powerful tools have been published. The unmixing of grain size distributions is a popular topic in recent research. As one kind of decomposition method, single-sample unmixing (SSU) is usually regarded as not genetically meaningful with another unmixing method, end-member modeling analysis (EMMA), although it is still widely used. Some recent studies have noted that EMMA also has limits, and different EMMA algorithms may yield various results. However, no research has discussed the similarities and differences between EMMA and SSU and provided objective conclusions regarding their ranges of application. In this study, a detailed discussion is given regarding the similarities and differences of these methods based on the corresponding mathematical models, and detailed explanations of their features are presented based on numerical optimization and information theory. In addition, the ranges of application are objectively constrained, and we suggest that SSU may have greater potential, although the corresponding fitting process is more complex and difficult. We also highlight the importance of a comprehensive analysis. Traditional methods and geological settings are also important for the interpretation of grain size distributions. An easy-to-use software for the comprehensive analysis of grain size distributions called QGrain is introduced; it not only integrates many traditional analysis tools but also provides new high-performance EMMA and SSU algorithms. Compared to other EMMA algorithms, our EMMA algorithm exhibits outstanding performance. Furthermore, our SSU algorithm can overcome the shortcomings of the traditional method and can compete and collaborate with EMMA algorithms.

Laboratory study on the evolution of gravel-bed surfaces in bed armoring processes

Cluster microforms in bed armoring have important implications for bedload transport. In this study, two sets of flume experiments were carried out to study the evolution of the cluster microforms in the bed armoring and their influences on the bedload transport. Dynamically stable armor layers were formed by applying a staged hydrograph on a mobile sediment bed consisted of poorly sorted sand and gravel, thus enabling the investigation of the cluster microforms associated with different flow characteristics simulated in these experiments. The size and the planimetric positions of the large particles on the bed surface were extracted using photogrammetric technique, allowing analysis of surface grain size distribution and particle clustering pattern of the bed. The results showed a tendency towards a coarsened and clustered bed surface, which is likely responsible for the declining bedload transport rate. The cluster microforms evolved with increasing bed shear stress in the following sequence: flat bed - pebble cluster - line cluster - heap cluster - reticulate structure. We found a bed armoring statistics that can partially explain the declining bedload transport rate under constant discharge. Nonetheless, additional parameters are needed to simulate the fluctuation in the bedload transport rate under changing flow conditions.