Coarse Sediment Research Articles

Analysis of Coupled Coordination and Driving Factors of Urbanization, Ecosystem Services, and Human Well-Being in the High and Coarse Sediment Yield in the Middle Yellow River

The ecological and social problems brought about by China’s rapid urbanization process threaten sustainable development. Taking the cities with high and coarse sediment yield in the middle reaches of the Yellow River as the research objects, this study constructed a complex analytical framework composed of urbanization, ecosystem service value (ESV), and human well-being (HBW) subsystems. The equivalent factor method (EFM) was used to evaluate the ESV. The entropy weight method (EWM) and the comprehensive nighttime light index (CNLI) were adopted to assess the level of urbanization and human well-being. Then, coupled coordination degree (CCD) and grey relational modes were developed to reveal the level and major obstacles of synergistic development. The results showed that, from 2000 to 2020, urbanization, ESV, and HWB increased. The average value of CCD grew from 0.34 to 0.56, with higher coordination levels in the northeast and south. The coordinated development is influenced by multiple factors, with the intensity of the driving factors decreasing in the order of industrial structure, population quality, economic drive, government regulation, and scientific and technological support. The results indicate that it is essential to formulate differentiated regionally coordinated development strategies and establish cross-regional cooperation mechanisms to achieve interactive urban economic development, ecological protection, and well-being improvements in the middle reaches of the Yellow River.

Beach nourishment with coarse sediments: An in-situ investigation on the issues of abrasion and chipping

Beach nourishment with coarse sediments: An in-situ investigation on the issues of abrasion and chipping

Four new free-living marine nematode species (Plectida: Ceramonematidae) and two new species records (Desmodorida: Desmodorinae) from Rangitāhua / Kermadec Islands, Aotearoa / New Zealand

Kermadec Islands is a remote subtropical island arc in the Southwest Pacific Ocean located 800–1000 km northeast of New Zealand’s North Island. Until now, no data was available on the nematode fauna living in the seafloor environments surrounding these islands. A single sample of subtidal coarse sediments from the Raoul Island coast yielded four new ceramonematid species: Ceramonema taikoraha sp. nov., C. taiora sp. nov., Metadasynemoides taihua sp. nov. and Pselionema huakita sp. nov. This new discovery is the first addition to the family since 2008 and brings the total number of valid ceramonematid species globally to 67 species. Dichotomous identification keys are provided for valid species of Ceramonema, Metadasynemoides and Pselionema. The desmodorid species Acanthopharynx dormitata and Desmodora bilacinia, were also recorded from Raoul Island some 1350 km away from their type locality in Wellington Harbour (New Zealand’s North Island). These species may have a relatively widespread distribution but testing this hypothesis will require further morphological comparisons and analyses of molecular sequence data to confirm the status of the Kermadec specimens.

Fluvial erosion in a catchment cultivated with no-tillage at the edge of the southern Brazilian plateau.

The dynamics of fluvial erosion responds to soil erosion and surface runoff on hillslopes due to land use and environmental fragility, conditioned by the soil, geology, relief, and rainfall rate. Despite the increasing problems associated with fluvial erosion in Brazil, little information is available on bedload transport in headwater catchments under intense agricultural activity. Therefore, this study sought to characterize the fluvial erosion processes and bedload dynamics in an experimental catchment in southern Brazil located at the edge of the Brazilian Meridional plateau, which is representative of a large area of high environmental fragility and intense agricultural activity in Southern Brazil. The Guarda Mor River drains a headwater catchment (18.5 km2) with undulating and hilly terrain with fragile soils and intense agricultural activity controlling fine and coarse sediment supply downstream. During 11 major rainfall-runoff events, monitoring was conducted to measure streamflow, bedload transport rates, sediment size, and hydraulic parameters, such as Manning's n values and viscous layer thicknesses. A rating curve was established based on 40 streamflow and bedload discharge measurements taken at different water levels and stages along the hydrograph. In addition, a river portion (gravel bed) was characterized as well as the granulometric characteristics of its surface and subsurface layers. The results showed that the transport pattern is influenced by factors other than hydraulic parameters alone, including the interdependence between successive events, armoring effects, and hysteresis. These factors are strongly related to the surface runoff and erosion observed on the hillslopes, which define the streamflow energy and the supply of sand fraction, respectively. A discussion is held on the bedload transport dynamics under non-equilibrium conditions in the modeling of fluvial erosion processes.

Deciphering the CO2 hydrates formation dynamics in brine-saturated oceanic sediments using experimental and machine learning modelling approach

One of the potential strategies for limiting global warming is capturing excess amounts of industry CO2 emissions and sequestrating it inside the oceanic sediments as CO2 hydrates. The seabed consists of sediments with different porosity and sizes, which can significantly impede the kinetics of CO2 hydrates. To address these challenges, in this novel work the CO2 hydrate kinetics and morphologies have been investigated in brine-rich [3.5 wt% NaCl] coarse sediments [porosity = 0.28, Ø =0.5-1.5 mm], granule sediments [porosity = 0.43, Ø =1.5-3 mm] and dual sediments [coarse + granules] inside a high-pressure reactor [3.5 MPa, 1-2 oC] with artificial seabed. According to the key experimental results, the water-to-hydrate conversion [%] was estimated to be 67.24 (± 3.02) % for coarse sediments, 49.58 (± 4.97) % for dual sediments (coarse + granules), and 27.68 (± 5.21) % for granules. In-situ Raman spectroscopy was used to evaluate the real-time solubility of CO2 [0.023 mol/mol], and image processing thresholding was used to identify CO2 hydrate distribution patterns. A mathematical model was proposed as a major contribution to predict CO2 hydrates kinetics in sediments, using 94,203 data points. The model was trained via a supervised machine-learning [ML] algorithm and can predict CO2 hydrate kinetics with an AARD of 7.54-8.47%.

Coupled Impact of Climate Change and Anthropogenic Activity on the Changes of Terrestrial Organic Carbon Accumulation in the River‐Dominated Coastal Margin

AbstractRiver‐dominated marginal seas play a crucial role in the global carbon cycle. However, the centennial burial record of organic carbon (OC) remains unclear. In this study, we conducted a comprehensive analysis of bulk OC, its isotopic composition (δ13C and Δ14C), biomarkers (lignin and n‐alkanes), and sedimentological evolution based on sediment core from the Yellow River‐dominated Bohai Sea (BS). We also compiled several published OC burial records from other river‐dominated coastal margins. Our findings indicated that since the 1950s, the accumulation of terrestrial OC in central BS has shown a concurrent decline, as evidenced by a ∼50% decrease in terrigenous/aquatic ratio of n‐alkanes (TAR), which accompanied by a significant reduction in sediment load due to the watershed human activities. More intense erosion and resuspension due to stronger hydrodynamic condition under the increasing frequency of winter storms could account for the observed sediment coarsening and concomitant increase of the degraded lignin and old‐OC since the 1980s, suggesting that delta erosion‐induced sediment redistribution could influence the selective transport and accumulation of the more woody allochthonous OC components. The temporal profiles of lignin records indicated a spatial heterogeneity of recent terrestrial OC burial among the large river‐dominated coastal margins under the enhanced global delta erosion. Compared to the fluvial input‐dominated OC burial in the Yangtze River, Pearl River and Mississippi River delta margins, a more hydrodynamic forcing impact on the terrestrial OC burial was discerned in the BS due to the coupled effect of recent climate change and substantial decline in sediment load.

Geological and Ecological Insights on the Lake Faro Global Geosite Within the Messina Strait Framework (Italy)

The Lake Faro brackish basin (Sicily, Italy) was established as a Global Geosite as a key locality of tectonic coastal lakes, but little research has been devoted to this rare geological and ecological framework. To fill this gap, the main stratigraphical, sedimentological, ecological, morpho-bathymetric, and structural features were reported, linking geodiversity with biodiversity. In Lake Faro, a shallow platform develops alongside a deep funnel-shaped basin, reaching a maximum depth of 29 m. A NNW-SSE trending steep cliff, representing the abrupt transition from the platform to the basin, was interpreted as a dextral transtensional fault (Lake Faro Fault), presumably active since the middle-late(?) Pleistocene. The switches of the steep cliff NW-wards, acquiring an E-W trend, was interpreted as being due to the occurrence of the normal Mortelle Fault, cut by the Faro Lake Fault. Bottom terrigenous deposits consisted of coarse- to fine-grained quartzo-lithic rich sediments deriving from high-grade metamorphic and igneous rocks, whereas bioclasts mainly derived from clam farming actives for several centuries up until today. The Quaternary shallow platform, from top to base, includes the following: (i) soft cover composed of coarse terrigenous and prevalent bioclastic deposits; (ii) hard conglomerates cemented by carbonates; and (iii) siliciclastic coarse deposits of the Messina Fm. In the deep basin, siliciclastic silty loams with minor amounts of bioclastic deposits prevailed in the soft cover. Substrate heterogeneity coupled with brackish-marine gradients are the main factors responsible for an articulate patchiness of different lagoon habitats and related benthic associations, which, according to the Barcelona Convention classification, can be summarized as follows: (i) MB1.541 (marine angiosperms or other halophytes), (ii) MB1.542 (Fucales), (iii) MB5.543 (photophilic algae, except Fucales), (iv) MB5.544 (Facies with Polychaeta), and (v) MB5.545 (Facies with Bivalvia). Typical marine associations, such as rhodolite beds, also occur. Finally, the lake, which has been exploited since the prehistoric age because of its high biodiversity and productivity, maintains some evidence of millennial relationships with the resident human cultures, attracted there by the favorable geomorphological and ecological peculiarities.

Predicting flow resistance in rough‐bed rivers from topographic roughness: Review and open questions

AbstractMost ways of predicting flow resistance in shallow rivers with a partial or complete cover of coarse sediment use a bed‐sediment grain diameter as a roughness length scale. However, beds with the same grain size distribution differ in roughness and flow resistance depending on how the larger grains are arranged, the nature of any bedforms and the possible complications of bedrock or rough banks. This has led to interest in predicting flow resistance using metrics of the topographic roughness of the bed. Some researchers have used the standard deviation of bed elevation as a roughness length scale. An alternative for channels containing boulders is to regard the bed as an array of large roughness elements. Fluvial research to date using these two approaches is limited and inconclusive. We review potentially relevant findings from the much more extensive literature in boundary‐layer meteorology and various branches of engineering and note links between the distribution‐statistics and element‐array approaches. The skewness of the elevation distribution is widely seen as important but it is unclear how best to use it for flow prediction. Other open questions include the scale dependence of topographic metrics, and what type of flow resistance equation to use them in. Calibration and testing of new prediction methods require flow data from reaches with known roughness statistics. This need should be met partly by measurements at field sites or in flume models of them, but also by flume experiments and numerical simulations using synthetic roughness.

Groundwater flow and salinity dynamics in swash Zones: Combined effects of Evaporation, Waves, and geologic heterogeneity

This study employs groundwater simulations to examine the combined effects of waves, evaporation, and geologic heterogeneity on the flow and salinity dynamics in a shallow beach environment. The modeling results reveal that wave motion generates a saline plume beneath the swash zone, with hypersalinity near the surface triggered by evaporation. Geologic heterogeneity critically controls the temporal and spatial patterns of evaporation, moisture content, and salinity in the swash zone. Heterogeneous capillarity creates localized moisture hotspots within the unsaturated zone, which support enhanced evaporation and therefore facilitate salt accumulation at the surface, even when the overall moisture conditions along the swash zone are not conducive to high rates of evaporation. The formation of capillary barriers allows these moisture hotspots to persist over tidal cycles, leading to the retention of saltwater pockets within heterogeneous unsaturated finer sediments. As the swash zone recedes and evaporation intensifies, salt begins to accumulate near the beach surface. The moisture hotspots create preferential pathways that facilitate the penetration of hypersaline water into deeper, saturated sediments. In contrast, within the saturated zone, groundwater flow and salt transport are predominantly driven by preferential flow within high-permeability coarse sediments where capillarity is relatively low. Such transport and capillary mechanisms are crucial for a better understanding of coastal groundwater flow, interstitial habitats, biogeochemical conditions, and consequent nutrient cycling and contaminant transport in coastal zones. This highlights the necessity of considering integrated coastal physical drivers when investigating flow and transport processes in coastal swash zones.

Climatic and tectonic controls on deposition in the Heidelberg Basin, Upper Rhine Graben, Germany

The Upper Rhine Graben in Central Europe, and notably its depocentre in the Heidelberg Basin, is an archive of complex and long-lasting deposition throughout the Quaternary. A new drill core, 136 m long, from the southern Heidelberg Basin is investigated by characterising sedimentary facies, sediment provenance, as well as analysing the pollen and mollusc content. The chronological framework is based on post-infrared infrared-stimulated luminescence dating, and complemented with amino acid geochronology. The sediment sequence consists of fluvial, colluvial, and palustrine deposits that represent at least the last ∼500 ka, interrupted by some, major and minor, hiatuses. In the lower part, fluvial gravel and colluvial diamicts of a lateral alluvial fan into the Upper Rhine Graben prevail. The central part of the succession consists of a large-scale fining upward cycle that contains increasing amounts of material from the Alps delivered by the Rhine river. This sequence terminates with palustrine fines with rich mollusc and pollen assemblages that allow for a detailed reconstruction of environmental conditions. The results of pIRIR dating place the palustrine deposits in marine isotope stage 11. However, the pollen profile shares similarities with the Mannheim Interglacial that has previously been assigned to the Cromerian, a correlation that is supported by the amino acid geochronology, which poses a chronostratigraphical problem. In the upper part, Alpine sediments are progressively replaced by a new alluvial fan from the graben margin with striking variations in grain size. Overall, the diverse succession is the result of an interplay of tectonic activity and climatic factors. While subsidence triggers the generation of accommodation space and river deflection, pulses of coarse sediment are probably related to periglacial weathering, mass wasting and short-scale transport during cold periods.

Numerical modeling of graded sediment transport with limited bed material supply – A case study in the Middle Yangtze River

Suspended sediment concentration in the Middle Yangtze River (MYR) has reduced sharply below the Three Gorges Dam since 2003, and led to significant channel degradation and riverbed coarsening. However, the complex interaction between riverbed coarsening and sediment transport is essential for channel evolution processes but is poorly understood. This paper presents a one-dimensional model for simulating graded sediment transport when the bed material supply is limited. The model takes account of the effect of bed material composition (BMC) and threshold probability on graded sediment transport, and thus improve the accuracy of recovery coefficient. The modeling results of graded sediment concentrations was calibrated and then satisfactorily verified using field observations in the JingJiang Reach from 2015 to 2016. The results showed bed material composition affect the simulated fine, medium, and coarse sediment concentrations, and the improvements of normalized errors are 17.6%, 45.9%, and 330.7%, respectively. With the BMC effect considered, the recovery factor of fine and medium were far less than coarse sediment in the MYR. In addition, the recovery ratios of medium and coarse sediment concentrations along the MYR were improved by 88.6%-265.4% and 75.1%-105.9% respectively, owing to the bed material supply after the TGD operation; however, there was no significant effect on fine sediment, with a low recovery ratio of 8.3%-8.6%. Therefore, the effect of bed material composition on graded sediment transport and channel evolution needs to be considered for higher simulation accuracy in the MYR.

Influence of Oxygen Exposure on Lignin Preservation during Sediment Lateral Transport in the Ocean: Insights from Molecular Dynamics Simulations.

Understanding the fate of terrestrial organic carbon (terrOC) preservation in the marine environments is critical for deciphering the biogeochemical processes associated with the global carbon cycle and the Earth's climate change. The mechanisms controlling terrOC preservation are not completely understood, while lateral oxygen exposure time (OET) is considered as a critical controlling factor. Here, we first utilized molecular dynamics simulations to investigate the structural properties of lignin under anoxic, suboxic, and oxic conditions for understanding the mechanisms of terrOC preservation during sediment lateral transport in the ocean. Our finding suggested that oxygen exposure was indispensable for terrOC degradation through influencing the structural stability and reactivity of lignin. Our simulated results showed that in suboxic environments, prolonged OET may enhance terrOC preservation. Our organic geochemical results suggested that terrOC preferably preserved in coarse silts (20-63 μm) than fine silts (<20 μm) in suboxic environments, largely due to hydrodynamics-driven prolonged OET in coarse sediments, which may efficiently reduce CO2 emissions. Overall, our study sheds new light on the mechanisms of lateral OETs on terrOC preservation in suboxic conditions and, from a unique molecular structural perspective, provides insights into the impact of prolonged OETs on terrOC oxidative degradation in the marine environment.

Atlas of pacific sand lance (Ammodytes personatus) benthic habitat – Application of multibeam acoustics and directed sampling to identify viable subtidal substrates

Defining and delineating species distribution and habitat is critical to informed management and conservation. This process is complicated in marine environments, where detection of marine taxa and characterization of marine habitat is more difficult. Small pelagic fishes and forage fishes are particularly challenging, though insights may be more accessible in species highly dependent on particular habitat. Pacific sand lance (Ammodytes personatus) is a common and ecologically-important pelagic fish that relies on specific benthic sediments for rest and refuge from predation. We applied multibeam echosounder (MBES) bathymetric data to develop high-definition benthic habitat maps and implemented multiyear sampling to assess potential habitat for sand lance via in situ sampling of sediments. We also applied acoustic Doppler current profilers (ADCP) data and tidally-driven volume-based ocean models to measure current strength and to visualize currents. We leveraged this data to further define and describe habitat for this important forage species. Sediment transport processes were mapped and areas of dispersal, embedment, and accumulation were evaluated. Dynamic bedform habitats, banner banks and glacial banks were identified as potential habitat and sampled for fish presence, density and sediment composition. In the central Salish Sea, approximately 25% of benthic substrates represent potential sand lance habitat. Sand lance prevalence and density correlated with substrate type and sediment coarseness. Densities were highest in areas of coarse grain sediments and presence was limited by fine particulates, such as silt and mud. Tidal currents appear important. Presence and densities of sand lance were correlated with current velocity and distance from current flow path. Nearly all viable sites were located on the immediate margins of high flow (<0.16 km from tidal currents with max speed of 1.72–2.58 m/s). While both flood and ebb were important, processes related to flood were dominant. Viable habitat was not constrained by depth. These results inform a developing atlas for sand lance in the central Salish Sea, provide new insights to subtidal sand lance habitat, characterize conditions that create and maintain subtidal benthic habitat, and provide a template for mapping habitat for this species in the coastal Pacific Ocean.

Disruption of natural disturbance regime decouples habitat and life stage in a keystone species

AbstractAnthropogenic disturbance of stream ecosystems, often chronic in nature, has been studied extensively. However, when disturbance is driven by more than one resource policy over many decades, feedback between habitat evolution and biological adaptation can be disrupted and ecological function affected in unforeseen ways. We analyzed over 100 years of Chinook salmon (Oncorhynchus tshawytscha) length frequency trends associated with fisheries management and changes in available spawning substrate (habitat) linked to flow regulation in a highly altered California river. Over time, salmon lengths generally decreased, fluctuating with exploitation (ocean harvest) and hatchery production rates. Female size reduction, coupled with a degrading and coarsening channel, and perching peripheral habitat related to past mining activity, indicates available spawning substrate may be too large to support the current salmon population. Assuming a salmon can move material ~10% of her body length, length frequency data and current substrate size distribution suggest that increasing salmon sizes to historic distributions could increase available spawning habitat by as much as 13%. Alternatively, decreasing spawning substrate size could support a greater portion of the current population. To test the latter hypothesis and inform future management actions, we monitored two spawning riffles where large and small gravel was placed on top of a cobble. We observed an immediate spawning activity increase that was more pronounced where smaller gravel was deposited. Following a decade of habitat decline, the two sites were both replenished with medium gravel. Elevated spawning use occurred immediately at both sites, commensurate with this intermediate size, further supporting our hypotheses. Sediment coarsening and habitat disconnect below dams, combined with reduced salmon size, indicate the natural spawning process may be decoupled from available habitat below dams in the foreseeable future without continuous intervention. Actively managing salmon population demographics through modified hatchery and size‐selective harvest practices and developing a coarse sediment budget with size‐appropriate material for regulated anadromous rivers could produce immediate benefits for ecosystem services, including salmon populations. However, these management actions will require continued maintenance and informed socio‐ecological goals to remain successful.

Study on Grain Size, Physical Properties and Organic Matter Characteristics of Tidal Flat Surface Sediments: May 2022 Hwangdo Tidal Flat Dataset, Cheonsu Bay

This study analyzes the geological and geochemical features of surface sediments in the Hwangdo Tidal Flat, located on Korea’s West Coast. The tidal flat experiences semi-diurnal tides, impacting organic matter decomposition and nutrient cycling. Ninety one sediment samples were collected and analyzed for physical and chemical properties including grain size, density, water content, organic carbon, and nitrogen. Sediments consist mainly of sand and silt, with coarser sediments near the main channel and finer sediments towards the west. Sediment grain size averages 4.12 Φ with a sorting coefficient of 1.96 Φ, indicating diverse energy environments. Total organic carbon and nitrogen correlate positively with grain size and density, reflecting sediment origin and environment. Kriging maps sediment grain size distribution, while correlation and linear regression analyses show relationships between variables. High correlations exist between various parameters, aligning with tidal flat characteristics and aiding understanding of sediment transport and deposition. The study provides baseline data for understanding the tidal flat’s geological, geochemical, and physical aspects, valuable for remote sensing validation and environmental monitoring. The dataset is freely available for research and management purposes.

Investigation of shaft and segment failure in co-rotating twin-screw extruder

This research is focused on identifying the reasons behind the failure of the shaft and screw segment associated with the ABS plastic material co-rotating twin-screw extruder. These steel parts failed during production and after only 12 days of operation. To understand the failure causes, their chemical composition was examined initially. The results showed that the chemical composition of the analyzed parts meets the design and standard of BS EN 4957 (2018). Subsequently, a detailed study of the fracture cross-section of both pieces was conducted, along with microscopic and mechanical tests, to gather comprehensive information on the failure reasons. The findings indicate that the initiation of the cracks is linked to the presence of coarse carbide deposits, leading to micro-cracks on the surface of the shaft. Under torsional-bending cyclic loading, these cracks expanded as low-cycle fatigue fractures. When the fatigue crack reached its critical extent, rapid and abrupt crack propagation and failure ensued. Furthermore, defects like holes and coarse sediments within the segment also contributed to the conditions conducive to rapid crack growth and failure. In conclusion, to prevent future accidents and issues with comparable equipment, various safety measures and recommendations were introduced.

Sources and transport of organic matter in the Ganges-Brahmaputra-Meghna River system of Bengal Basin, Bangladesh

This study aimed to understand the sources and transport mechanism of organic matter (OM) in the Ganges-Brahmaputra-Meghna (GBM) river system in Bangladesh. We conducted analyses of total organic carbon (TOC), total nitrogen (TN), their stable isotopes (δ13C and δ15N), and sediment grain size. The results reveal a heterogeneous mixture of OM derived from terrestrial plants, aquatic environments, and anthropogenic sources. The Brahmaputra River exhibited higher concentrations of TOC and TN, with δ13C and δ15N values indicating that the OM is primarily sourced from C3 plants. Conversely, the Ganges River demonstrated lower TOC levels and higher isotopic values, reflecting significant anthropogenic inputs. The Lower Meghna showed a mixture of terrestrial and marine sources. Variations in the TOC/TN ratios across the river system underscore the complex interplay between natural and anthropogenic factors. Additionally, sediment grain size plays a crucial role, with finer sediments in the Brahmaputra River associated with increased OM concentrations, while coarser sediments in the Ganges River correlate with lower TOC and TN levels.

The roles of geometry and viscosity in the mobilization of coarse sediment by finer sediment

In rivers, the addition of finer sediment to a coarser riverbed is known to increase the mobility of the coarser fraction. Two mechanisms have been suggested for this: a geometric mechanism whereby smaller sizes smooth the bed, increasing near-bed velocity and thus mobility of the larger sizes, and a viscous mechanism whereby a transitionally smooth turbulent boundary layer forms, rendering the coarser grains more mobile. Here, we report on experiments using two sediment mixtures to better understand these proposed mechanisms. In Mixture 1, we used 0.5 and 5 mm grains, and in Mixture 2, we used 2 and 20 mm grains. If the entrainment of coarse gravel by finer sediment is a purely geometric effect, then the addition of finer material should produce the same effect on the mobility of the coarser material for both mixtures because they have the same size ratio. We show that addition of finer material has a different effect on the two mixtures. We observed an increase in the mobility of the coarse fraction for both mixtures, but the increase in coarse fraction mobility for Mixture 1 was almost twice that for Mixture 2. Our experiments show that in addition to the geometric effect, enhancement of coarse gravel transport by finer sediment is also driven by a viscous effect.

Imbrication fabric as a diagnostic feature for the genetic classification of gravels deposited by fluid‐gravity versus sediment‐gravity subaerial flows

AbstractGravel transport in subaerial environments occurs through different flows that are conveniently classified as debris flows, debris floods and water flows based on their distinct morpho‐sedimentary dynamics and different implications for geomorphic hazard. Because distinctive features allowing gravelly sedimentary bodies to be ascribed to related genetic process are still a matter of discussion, this study aims to establish whether imbrication fabric represents a sedimentological fingerprint potentially applicable towards a more robust genetic classification of gravels. We analysed the fabric of 1007 imbricated clasts from modern and ancient deposits. Our results highlight statistically significant differences between imbrication fabrics in gravels deposited by different flows. Particles imbricated by water flows are typified by low imbrication angles (median of 35°) and elongated clasts oriented perpendicular to the flow. In contrast, debris‐flow gravels exhibit high imbrication angles (median of 65°) and elongated clasts oriented parallel to the flow. Debris‐flood deposits display elongated clasts both parallel and transverse to the main flow and intermediate values of imbrication angle (median of 47°). We propose that imbrication angles result from the combination of stability‐driven selection—a process acting under tractional transport and promoting the remobilization of high‐angle imbrication fabrics—and shear‐stress‐driven overriding—a mechanism leading to the formation of the higher imbrication angles—with the first dominating in water flows and the latter being effective in mass transport processes. The progressive change in imbrication fabrics from fluid‐gravity to sediment‐gravity flow deposits offers easily quantifiable sedimentological evidence to help in distinguishing genetic processes that contribute to the accumulation of gravels in alluvial and colluvial settings. Analysis of imbrication fabric can add valuable information, particularly as regards the classification of (1) coarse deposits in stratigraphic records and (2) modern debris flood deposits.

The Genus Chaetogaster Baer, 1827 (Annelida, Clitellata) in Switzerland: A First Step toward Cataloguing Its Molecular Diversity and Description of New Species on a DNA Sequence Basis.

The genus Chaetogaster belongs to the subfamily Naidinae (Naididae); it includes mostly species of small size and is diverse and abundant in surface coarse sediments in streams. The aim of the present study is to initiate an inventory of lineages (=species) of Chaetogaster in Switzerland. We used 135 specimens collected at 6 sites in 4 streams of 4 cantons. We sequenced the cytochrome c oxidase (COI) gene from all specimens and ITS2 and rDNA 28S from all or several specimens of each lineage that was delimited using COI data, and preserved, for morphological identifications, the anterior part of almost all sequenced specimens. We were able to delimit, based on the calculation of genetic distances and analyses of single-locus data, one lineage for Chaetogaster diaphanus (Gruithuisen, 1828), three within Chaetogaster diastrophus (Gruithuisen, 1828), one for Chaetogaster langi Brestcher, 1896, one for Chaetogaster setosus Svetlov, 1925, and three unidentified Chaetogaster spp. Two lineages of Chaetogaster spp. could correspond to a new morphological group, but this should be confirmed in more specimens. We proposed a new identification key of the nominal species and described the three C. diastrophus lineages and two Chaetogaster spp. as new species. The prospects of the present work are to complete the data of the molecular diversity of this genus in Switzerland and to describe the newly found Chaetogaster species on a molecular/morphological basis.