Bed Sediment Research Articles

Selective aggregation of hematite from quartz using charged polyacrylamides: In-situ sizing

Recovering fine valuable material has been a significant challenge in minerals processing. This primarily stems from insufficient collision opportunities between fine particles and bubbles, specifically when utilising traditional recovery methods such as froth flotation. This research endeavours to address this challenge by exploring the use of various commercially available, high molecular weight polymers- specifically polyacrylamide (PAM)- to selectively aggregate hematite from quartz. Investigation of the conditions in which selective aggregation of hematite over quartz occurs, with the goal to achieve aggregate sizes of between 20 and 150 μm, were undertaken. Targeting this size range ensure that conditions are optimised for future use in conventional mechanical flotation cells as an efficient mineral recovery method. The study investigates the selectivity of charged PAM towards hematite and quartz at pH 10, using adsorption studies. Anionic PAM demonstrates a preference for hematite, with a range of charge density studies, highlighting stronger adsorption capacity with lower charge density polymers. Using BlazeMetrics probe technology for in-situ sizing and imaging, it was observed that conditioning 50–300 g APAM per t of hematite resulted in aggregates in the range of 30–230 μm after 5 min. Interestingly, the charge density of APAM does not markedly affect the size of the hematite aggregates, although altering the charge density of the polymer impacts the selectivity towards quartz. Additionally, cationic PAM results in the simultaneous aggregation of both hematite and quartz, not demonstrating selectivity. To mitigate heterocoagulation between minerals, a common dispersant, sodium hexametaphosphate (SHMP), was employed, which notably prevents quartz aggregation. Separation of aggregated hematite from quartz by sedimentation was only slightly effective with the majority of quartz settling with the hematite. Adding up to 1000 g of dispersant per t of hematite reduced the amount of quartz in the sediment bed by 23 %. The dispersant increased the magnitude of the negative zeta potential of the two minerals, particularly hematite, lowering heterocoagulation and improving effectiveness of hematite flocculation.

Experimental investigation of flow and sediment transport on an equilibrium beach formed by surging breaker

Flow patterns and sediment transport induced by breaking waves on a beach profile have been studied in a laboratory wave flume. Two parallel experiments were conducted on the same profiled rigid bed and sediment (mobile) bed, each subjected to identical wave forcing. The rigid bed experiments include measurements of bed shear stress and water-surface elevation, while the sediment bed experiments involve the measurement of pore-water pressures. The beach profile features a beach step leading to the generation of a hydraulic jump at the end of the rundown phase and accordingly forms a backwash vortex. The bed shear stress measurements revealed that the maximum onshore directed mean bed shear stress occurs after the wave breaking whereas the offshore directed one occurs during rundown at the same section. The increase of the bed shear stress with respect to that in the approaching wave boundary layer during the wave breaking and hydraulic jump processes can be by as much as a factor of 13 and 4, respectively. The effect of the wave breaking on turbulence is also more pronounced by a factor of 2.2 than the hydraulic jump in terms of r.m.s values of the fluctuating component of the bed shear stress. The pore pressure measurements showed that the vortex generated during the wave breaking creates an impulsive, upward-directed pressure gradient force that can be up to 1.2 times the submerged weight of the sediment. The sediment transport occurs in the sheet flow regime in a large part of the beach profile. The turbulence generated by the backwash vortex and the hydraulic jump at the beach step directly impacts the local sediment suspension which serves as the source for the advected sediment in the swash zone. The results are compared with a previous study that used the same methodology but involved plunging waves and the initial bed profile.

The effect of imbrication on the porosity value of uniform gravel bed

Existing empirical relations used to predict the porosity of gravel beds are mainly derived from laboratory-generated sediment beds with random grain packing. However, such relations could not adequately describe beds with non-random grain arrangements that occur widely in fluvial deposits. In this work, the effect of grain imbrication on gravel-bed porosity has been quantified using beds with variable strengths of imbrication generated by flume experiments. Mono-sized ellipsoids with specific shapes were used in experiments to remove particle size and sorting effects on porosity. Random bed packings were generated by settling of ellipsoids in still water whilst imbricated beds generated under flowing water. Beds were frozen using liquid nitrogen before extraction. A new relatively simple and time-saving workflow was developed to measure the orientation of particles and quantify the degree of grain imbrication in frozen beds from X-ray Computed Tomography images. Beds with the strongest grain fabric display a ca. 0.03 absolute reduction of porosity value on average (8–10% relative reduction) compared to that of random packing for undisturbed beds. Further, results were obtained for beds deposited under still-water conditions subject to disturbance by shaking, to mimic the potential effect of vibrations from currents, waves or other sources in the environment. A reduction in bed porosity of ca. 0.014–0.018 (ca. 5% relative reduction) is observed between beds with the strongest grain fabric and those with random packing that had undergone shaking after deposition. Hence, a significant proportion (> 50%) of the porosity loss observed for imbricated beds may be attributable to tighter packing due to turbulence-related vibrations from the flow. The small decrease in porosity value despite the formation of strong imbrication is considered to be due to the limited improvement in grain organization, as the results show that the flat shape of the ellipsoids and the uniformity of their size promote the formation of a stacking structure under gravity, leading to a similarly highly ordered grain organization in random packings compared to the imbricated packings.Graphical abstract

Soil-sediment connectivity through Bayesian source tracking in an urban naturalised waterway via microbial and isotopic markers

Riverine sediments are important habitats for microbial activity in naturalised waterways to provide potential ecosystem services that improve stormwater quality. Yet, little is known about the sources of these sediment microbes, and the factors shaping them. This study investigated the dominant source of sediments in a tropical naturalised urban waterway, using two Bayesian methods for microbial and isotopic 13C/15N markers concurrently. Additionally, key factors shaping microbial communities from the surrounding landscape were evaluated. A comprehensive two-year field survey identified source land covers of interest based on topology and soil context. Among these land covers, riverbanks were the dominant source of sediments contribution for both edaphic and microbial components. The physico-chemical environment explains most of the variation in sediment communities compared to inter-location distances and microbial source contribution. As microbes provide ecosystem services important for rewilding waterways, management strategies that establish diverse sediment microbial communities are encouraged. Since riverbanks play a disproportionately important role in material contribution to sediment beds, management practices aimed at controlling soil erosion from riverbanks can improve overall functioning of waterway systems.

Mechanisms and Morphological Time Scales of Avulsed Channel Process on the Modern Yellow River Delta

AbstractThe modern Yellow River Delta (YRD) has witnessed frequent channel avulsions followed by the morphological processes of “wandering‐ short‐lived braiding ‐merging” in history. However, process‐based investigation and relevant physics behind these processes remain poorly constrained. The present study complements the understanding for this evolution through numerical experiments. This is achieved by applying a two‐dimensional (2‐D) depth‐averaged fully coupled morphodynamic model that considers the feedbacks of sediment‐laden flow and bed deformation to a schematized fan‐shaped YRD. Under an uneven bed of uniform sediment, the avulsed processes of “wandering‐ short‐lived braiding ‐merging” during new channel routing on the YRD are satisfactorily reproduced by the present modelling in terms of the time evolution of planar channel pattern and channel length. Moreover, the mechanisms of the avulsed evolution have been elucidated through factor analysis on delta slope, incoming flow‐sediment conditions and artificial trenching, etc. It is suggested that morphological stability could be reached when a single meandering channel is finally sustained, of which the time scale is 4–8 years echoing the documented data of abandoned channels on the YRD. In addition, quantitative criteria have been advocated to help distinguishing the sub‐stages of the avulsed evolution together with the characteristic pattern judgement. Based on the above, implications for the YRD management are discussed.

A synopsis of the Early Miocene St Bathans Fauna of New Zealand

The St Bathans Fauna, from sites near the village of St Bathans, Central Otago, South Island, is the first substantive pre-Quaternary terrestrial vertebrate fossil fauna discovered in New Zealand. This fauna derives from 33 sites or discrete sedimentary beds located in the lower 50 m of the lacustrine Bannockburn Formation, Manuherikia Group, and is generally accepted as local stage Altonian (19–15.9 Ma; Burdigalian, Early Miocene) in age. Investigations since 2001 have revealed an abundant and diverse fauna from over 9000 catalogued lots that is herein reviewed. Invertebrates notably include eight genera and species of terrestrial molluscs. Among vertebrates, freshwater fish remains dominate with 17 species evidenced by 16,500 analysed otoliths (genera Neochanna, Galaxias, Prototroctes, and Mataichthys) and many thousands of bones. Birds (minimally 45 species, several thousand bones) are the most common non-fish vertebrates, among which waterfowls dominate all assemblages (10 species). Co-occurring with these was a diverse herpetofauna, including undetermined crocodylians and a terrestrial turtle, both absent in Recent faunas. Significantly, the St Bathans Fauna evidences that Zealandia already had all of New Zealand’s ‘old’ endemic Recent taxa (sphenodontids, leiopelmatids, dinornithiforms, apterygids, aptornithids, strigopoid parrots, acanthisittids, and mystacinids) during the Early Miocene. Furthermore, it includes Australasia’s oldest ardeids, two flightless rallids, a novel higher landbird family, a greater diversity of bats, and terrestrial mammals. All sites reflect a single fauna, except that the ducks Manuherikia lacustrina (stratigraphically lower in section) and M. primadividua (higher) have a mutually exclusive distribution that is not yet correlated with any other biotic distribution differences.

Local scour at group of bridge piers founded in gravel bed in staggered arrangement

Local scour around bridge piers is the common cause of undermining of bridge foundation. Many researchers have studied local scour around bridge pier in finer sediment bed. The literature survey reveals a limited information is available on scour around a group of bridge piers with and without collar founded in gravel bed. Keeping this in view, an attempt is made to examine experimentally the efficacy of the collar around a group of three piers placed in a staggered triangular arrangement in the gravel bed of 3 mm effective size. The data on temporal variation of scour depth, scour hole characteristics, and areal extent of scour hole were collected. The collected experimental results infer very prominent information to the bridge engineers. The obtained results on equilibrium scour depth and areal extent of scour help in knowing the extent up to which bridge pier protection can be provided against scour. It is found that at longitudinal pier spacing (Z) equals to 8.45 times the pier diameter (b), the equilibrium scour depth at nose of the piers was maximum than that of at other pier spacing. At pier spacing Z/b > 8.45, the equilibrium scour depth at downstream pier without collar decreases with increasing pier spacing and reaches to minimum at Z/b = 40. At pier spacing 8.45 < Z/b < 10, the equilibrium scour depth at upstream pier without collar decreases because of decreasing effect of reinforcement and this effect disappears at pier spacing Z/b > 10. The present study reveals that bridge piers without collar became free of mutual interference effect on local scour at centre-to-centre (c/c) pier spacing of 60 times the pier diameter. It is worth mentioning that for piers with collar, the effect of mutual interference vanished at c/c longitudinal pier spacing of 20 times the pier diameter. The results show that additional shielding of collar makes the piers free from mutual interference effect at lesser spacing than the pier group without collar. This concludes that the present investigation is very useful when limited space is available between the existing and newly proposed bridges.

Experimental Investigation on the Effect of Sequences of Unsteady Flows on Bedload Sediment Transport

Flash floods in ephemeral streams are rare, short and difficult to forecast and thus to monitor. During these events, bedload transport reaches very high rates and most sediment transport occurs within a limited number of hours during the course of a year. Because monitoring of bedload in ephemeral rivers is challenging, here we present the results of a series of flume experiments designed to simulate short, flashy floods. Since most flume experiments usually involve single events, here we add to existing evidence by testing the effects of sequences of multiple floods in rapid succession. The flume is 10 m long, 0.3 m wide and 0.5 m deep. Two bed sediment mixtures (well sorted and poorly sorted) with similar median grain size but a different standard deviation were used. Bedload was monitored continuously during each hydrograph, but no sediment was fed. The flume experiments used six triangular hydrographs with peak flows ranging from 0.0147 to 0.02 m3s−1 and durations ranging from 150 to 400 s. Results indicate that the sediment transport rate decreases progressively from the first to the third hydrograph, and that this pattern is consistent for all permutations of peak discharge and flood duration. In all of the runs, the sediment transport rate at a specified flow was higher during the rising limb than the falling limb of the hydrograph, indicating clockwise hysteresis. Furthermore, in the subsequent repetitions of the same hydrograph, the degree of hysteresis generally diminishes in magnitude from the first to the last repetition for all the experiments, irrespective of their magnitude and duration.

Assessment of Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) Mass Flux and Distributions in a Lake System Using Sediment Bed Passive Flux Meters and Ceramic Dosimeters

AbstractPer‐ and polyfluoroalkyl substances (PFAS) are persistent in the environment due to their chemical stability and can spread quickly in a lake system due to mixing. Passive samplers allow for time‐weighted average concentration monitoring and the ability to detect low concentrations, which are difficult to measure with conventional grab sampling. This study demonstrates the feasibility of deploying both ceramic dosimeters and Sediment Bed Passive Flux Meters (SBPFMs) to assess time integrated PFAS concentrations and fluxes, respectively, at a historically contaminated PFAS lake near Baden‐Baden, Germany. Long‐term surface water grab samples resulted in the detection of PFBA, PFPeA, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFBS, PFHxS, and PFOS at a total concentration of approximately 1 μg/L. Dosimeters were deployed for 66 and 126 d, resulting in detected concentrations ranging from approximately 250 to 380 ng/L and 120 to 460 ng/L, respectively. The 66 d deployment resulted in the detection of PFPeA, PFHxA, PFHpA, and PFOA, whereas the 126 d deployment additionally detected PFBA, PFNA, PFDA, PFUnDA, PFDoDA, PFTeDA, PFBS, PFPeS, PFOS, PFNS, and PFDS. SBPFMs resulted in the detection of PFBA, PFPeA, PFHxA, PFHpA, PFOA, PFNA, PFUnA, PFTrDA, and PFBS and the determination of a total mass discharge of 5.6 g/d into the lake. Overall, dosimeters and SBPFMs are more sensitive than grab samples at detecting PFAS at low concentrations and can be used to better understand spatial distribution of PFAS in a lake system.

Exploring historical changes in mountain river hydrodynamics induced by human impact

During the 20th-century many mountain rivers in Europe were subjected to intensive human impacts which substantially modified their channel morphology. How these changes affected river hydrodynamics and response to floods remains uncertain. In this work, we perform hydraulic modelling using data from archival aerial photos to explore relations between hydraulic parameters of floods and human-induced channel incision occurring on the Czarny Dunajec River (Polish Carpathians) between 1964 and 2012. Data on vertical position of the channel used for two-dimensional modelling of flood flows were extracted (as Digital Elevation Models DEMs) from archival aerial photos from 1964 and 1983 and ALS (Airborne Laser Skanning)-derived DEM from 2012. Water depth, flow velocity, bed shear stress, and sediment critical diameter were modelled for four flood scenarios (2-year, 5-year, 20-year, and 50-year floods) as well as the extent of flooded area and additionally the grain size of channel sediment was calculated. The values of water depth, flow velocity, bed shear stress and sediment critical diameter increased significantly between 1964 and 1983, especially for 20-year and 50-year floods. Only the flow velocity within the floodplain zone did not increase for the two largest flood scenarios due to the expansion of riparian forest in the second half of the twentieth century. The increase in flow rate was accompanied by a progressive reduction of the extent of flooded area, especially between 1964 and 1983, as well as by increase in mean grain size of channel sediment. Between 1983 and 2012 changes in hydraulic parameters were less pronounced, and coarser and well packed channel sediment dominated on the river bed. Our work demonstrates that reconstruction of past river hydrodynamics, rather than river state at time horizons, can give essential insights into functioning of the river channel and floodplain during the intensification of human impacts after 1950s.

Salt marshes for nature-based flood defense: Sediment type, drainage, and vegetation drive the development of strong sediment beds

In face of sea-level rise and increasing risks for storm impacts on shorelines, there is a growing demand for developing nature-based flood defenses, for example by restoring or creating salt marshes in front of engineered structures such as dikes. However, salt marshes can only optimally provide flood defense if their sediment beds are erosion resistant, even under very high flow velocities. It remains unknown how fast sediment strength develops in marshes restored or created for nature-based flood defense. Therefore, this study investigated how 1) sediment type, 2) tidal drainage depth and duration, and 3) pioneer vegetation species drive the development rate of sediment strength. A controlled experiment was set up with pots filled with two sediment types, which were either left bare or planted with Spartina anglica or Scirpus maritimus, two dominant salt marsh pioneers in NW Europe. All treatments were subjected to four different tidal regimes with different tidal drainage depth and duration. The results showed that sandy mud (with a 37% silt and clay fraction) led to much stronger sediments than fine mud (with a 77% silt and clay fraction). Sediment strength was higher in the treatments with deeper tidal drainage depth and longer drainage duration. The presence of vegetation increased sediment strength and this effect was stronger with Scirpus maritimus than with Spartina anglica. Plant roots increased sediment strength directly, and the presence of vegetation also seemed to increase sediment strength through enhanced evaporation and transpiration. From these results it can be concluded that to restore or create erosion resistant salt marshes for flood defense, it is essential to ensure that marshes can form at relatively high elevations from well-draining sand-mud mixtures, thereby also ensuring vegetation growth.

Effect of typhoon on suspended sediment concentration, bed erosion and sediment transport in the Yangtze Estuary

Annually 5–6 typhoons strike the Yangtze Estuary (YE) as extreme events. However, their high energy and importance for sediment transportation and geomorphic changes are still not fully understood. In this study, high-resolution observations of wind, wave, flow velocity, and suspended sediment concentration (SSC) at two in-situ stations were carried out during the 2022 Hinnamnor typhoon. Additionally, we simulated the change in SSC, estuarine bed erosion/deposition, and flow and sediment transport with and without a typhoon in the YE using MIKE3 numerical model. The findings revealed that the Hinnamnor typhoon-induced waves increased the SSC of the turbidity maximum zone (TMZ) by a factor of 5.6 times (maximum is 2.8 kg/m3). The TMZ area also extended by 2.68 times (maximum is 7880km2, 70.4% of YE) in the YE. Moreover, the typhoon caused a dramatic change in sediment transport and bed erosion/deposition in the YE. First, in the delta front area where the mean water depth is >5 m, the typhoon significantly increased the southward flux of residual flow and sediment, causing sediment transport into Hangzhou Bay to abruptly increase 26.3 times (increase of 52 million tons, accounting for 1/3 of the present annual flux of the Yangtze River (150 million tons)) during a single spring-neap period. The net erosional area and volume extended to 6770km2 (60.4% of YE) and 91.18 × 106 m3. Second, in the delta shoals (where the mean water depth is <5 m, including east Chongming Shoal, Hengsha Shoal, Jiuduansha Shoal, and east-south Nanhui Shoal), residual flow and sediment flux decreased northward from the typhoon and resulted in the erosion of the shoal. Third, in channels with trumpet-shaped mouths (North Branch (NB), North Channel (NC) and South Passage (SP), except for North Passage (NP)), the upward flux of residual flow and sediment increased due to the typhoon, resulting in bed deposition in these channels (NB, NC and SP). This study highlights the important influence of typhoons on flow and sediment transport and bed erosion in estuarine areas.

Experimental study of gravity currents moving over a sediment bed: suspension criterion and bed forms

Experimental study of gravity currents moving over a sediment bed: suspension criterion and bed forms

Experimental Investigation of Local Scour Around Inclined Bridge Piers on Clay-Sand Mixed Cohesive Sediment Bed in Clear-Water Conditions

Experimental Investigation of Local Scour Around Inclined Bridge Piers on Clay-Sand Mixed Cohesive Sediment Bed in Clear-Water Conditions

Field observations on the characteristics of sand ripples on tidal flats

Ripples on tidal flats significantly influence bedform roughness and near-bed turbulence, yet their dynamics in sandy and muddy environments remain incompletely understood. While laboratory studies have elucidated the effects of mud content and extracellular polymeric substances (EPSs) on ripple formation and stability, the interactions between ripple characteristics, EPS, and mud content in natural settings are more complex and not thoroughly explored. To address this gap, we conducted field studies on sand ripples at the central Jiangsu coast, China, using high-resolution drone photogrammetry to accurately measure ripple dimensions. We performed bedload sampling at both ripple crests and troughs to analyze median grain size, mud content, and EPS concentrations. Our results show that the investigated ripple wavelengths range from 34 to 46 mm, and heights vary between 2.7 and 5.3 mm. Ripples developed near tidal creeks show pronounced asymmetry. Significantly, EPS concentrations are markedly higher at the ripple crests than at the troughs, and it follows a power-law relationship with the median grain size. These results highlight the complex intricate relationship between the ripple morphology, environmental forcing conditions, and EPS content. Our findings enrich the understanding of ripple development and the factors influencing current-dominated ripples in natural environments. This research also adds to the better prediction of the bedform roughness and quantification of the near bed sediment transport.

Fine Sediment in Mixed Sand‐Silt Environments Impacts Bedform Geometry by Altering Sediment Mobility

AbstractGeometric characteristics of subaqueous bedforms, such as height, length and leeside angle, are crucial for determining hydraulic form roughness and interpreting sedimentary records. Traditionally, bedform existence and geometry predictors are primarily based on uniform, cohesionless sediments. However, mixtures of sand, silt and clay are common in deltaic, estuarine, and lowland river environments, where bedforms are ubiquitous. Therefore, we investigate the impact of fine sand and silt in sand‐silt mixtures on bedform geometry, based on laboratory experiments conducted in a recirculating flume. We systematically varied the fraction of sand and silt for different discharges, and utilized an acoustic Doppler velocimeter to measure flow velocity profiles. The final bed geometry was captured using a line laser scanner. Our findings reveal that the response of bedforms to an altered fine sediment percentage is ambiguous, and likely depends on, among others, bimodality‐driven bed mobility and sediment cohesiveness. When fine, non‐cohesive material (fine sand or coarse silt) is mixed with the base material (medium sand), an increased dune height and length is observed, possibly caused by the hiding‐exposure effect, resulting in enhanced mobility of the coarser material. However, weakly cohesive fine silt suppresses dune height and length, possibly caused by reduced sediment mobility. Finally, in the transition from dunes to upper stage plane bed, there are indications that the bed becomes unstable and dune heights vary over time. The composition of the bed material does not significantly impact the hydraulic roughness, but mainly affects roughness via the bed morphology, especially the leeside angle.

Sediment Transport and Bed Topography for Realistic Unsteady Flow Hydrographs of Varying Length in a Laboratory Flume

Sediment Transport and Bed Topography for Realistic Unsteady Flow Hydrographs of Varying Length in a Laboratory Flume

Effects of small rock check dams on channel bed, vegetation diversity, and soil properties 10 years after a severe wildfire in forest micro‐catchments of central eastern Spain

AbstractOne of the most common post‐fire management techniques is the construction of check dams in catchments. However, little research has explored how small check dams impact on bed profile, vegetation diversity and soil properties several years after their construction in Mediterranean ephemeral channels draining micro‐catchments burned by severe wildfires. To fill this gap, this study has evaluated the effects of rock check dams installed about 10 years ago in small catchments after a wildfire in Castilla La Mancha (Central Eastern Spain). The changes in channel morphology, plant diversity as well as a large dataset of soil properties have been analysed close to the check dams in comparison to burned but non‐regulated channels. Neither the channel slope nor bed sediment size significantly changed downstream and upstream of the structures in comparison to the non‐regulated channels. Thanks to the unexploited retention capacity, the studied control works are still able to store the mobilised sediments on occasion of the most intense floods. Higher species richness in both upstream and downstream sections (+64%) was detected, while the plant evenness was not altered compared to the non‐regulated channels. Regarding the soil properties, increases in organic matter (over 200%), nutrients (+72% for N and +152% for P) and some cations (+29% for Ca and +86% for Mg) were measured. Overall, the presence of the rock check dams in the burned micro‐catchments, although improving many key plant and soil characteristics, did not sharply change the hydrological, geomorphological and ecological conditions compared with those of the non‐regulated catchments.

Sugarcane cultivation as a major surface source of sediment in catchments from a coastal zone of Pernambuco, Brazil

ABSTRACT Identifying sediment sources is fundamental for protecting and improving soil and water quality. Conventional fingerprinting studies have often collected sediments at the overall watershed outlet only, resulting in an important spatial scale dependency. This study aimed to identify and to assess the delivery patterns of sediment sources within three priority sub-catchments (Sapocaji, Piedade and Minas) located in the downstream portion of the Ipojuca River watershed in Brazil. This research would build on understanding sediment sources in the studied watershed by elucidating source type contributions on a sub-catchment basis. Both bed and suspended sediment samples were collected in these sub-catchments, and two types of sources were sampled: surface and subsurface sources. A total of 21 geochemical tracers were measured. The tracers were evaluated for their conservation and discriminatory ability (Kruskal-Wallis test and linear discriminant analysis), and the best set of tracers was selected for source apportionment modeling using MixSIAR. Surface (i.e., sugarcane croplands) sources contributed the highest to suspended sediments in the Piedade and Minas sub-catchments. There was a difference in the quality of riverbank management, which helped explain contrasts in the importance of this source. Overall, sub-catchment-specific sediment control management measures, such as the revegetation of riparian forests, need to be implemented, mainly in the Sapocaji sub-catchment. These results underscore the importance of connectivity for surface source contributions.

Presence of Frozen Fringe Impacts Soft‐Bedded Slip Relationship

AbstractGlaciers and ice streams flowing over sediment beds commonly have a layer of ice‐rich debris adhered to their base, known as a “frozen fringe,” but its impact on basal friction is unknown. We simulated basal slip over granular beds with a cryogenic ring shear device while ice infiltrated the bed to grow a fringe, and measured the frictional response under different effective stresses and slip speeds. Frictional resistance increased with increasing slip speed until it plateaued at the frictional strength of the till, closely resembling the regularized Coulomb slip law associated with clean ice over deformable beds. We hypothesize that this arises from deformation in a previously unidentified zone of weakly frozen sediments at the fringe's base, which is highly sensitive to temperature and stress gradients. We show how a rheologic model for ice‐rich debris coupled with the thermomechanics of fringe growth can account for the regularized Coulomb behavior.