Transport Fluctuations Research Articles

Transport in cellular aggregates described by fluctuating hydrodynamics

Biological functionality of cellular aggregates is largely influenced by the activity and displacements of individual constituent cells. From a theoretical perspective this activity can be characterized by hydrodynamic transport coefficients of diffusivity and conductivity. Motivated by the clustering dynamics of bacterial microcolonies we propose a model of active multicellular aggregates and use recently developed macroscopic fluctuation theory to derive a fluctuating hydrodynamics for this model system. Both semianalytic theory and microscopic simulations show that the hydrodynamic transport coefficients are affected by nonequilibrium microscopic parameters and significantly decrease inside of the clusters. We further find that the Einstein relation connecting the transport coefficients and fluctuations breaks down in the parameter regime where the detailed balance is not satisfied. This study offers valuable tools for experimental investigation of hydrodynamic transport in other systems of cellular aggregates such as tumor spheroids and organoids. Published by the American Physical Society 2024

Drivers of summer Arctic sea-ice extent at interannual time scale in CMIP6 large ensembles revealed by information flow

Arctic sea-ice extent has strongly decreased since the beginning of satellite observations in the late 1970s. While several drivers are known to be implicated, their respective contribution is not fully understood. Here, we apply the Liang-Kleeman information flow method to five different large ensembles from the Coupled Model Intercomparison Project Phase 6 (CMIP6) over the 1970-2060 period to investigate the extent to which fluctuations in winter sea-ice volume, air temperature and ocean heat transport drive changes in subsequent summer Arctic sea-ice extent. This allows us to go beyond classical correlation analyses. Results show that air temperature is the most important controlling factor of summer sea-ice extent at interannual time scale, and that winter sea-ice volume and Atlantic Ocean heat transport play a secondary role. If we replace air temperature by net shortwave and downward longwave radiations, we find that the sum of influences from both radiations is almost similar to the air temperature influence, with the longwave radiation being dominant in driving changes in summer sea-ice extent. Finally, we find that the influence of air temperature is more prominent during periods of large sea-ice reduction and that this temperature influence has overall increased since 1970.

Interpreting the power spectral density of a fluctuating colloidal current.

The transport of molecules through biological and synthetic nanopores is governed by multiple stochastic processes that lead to noisy, fluctuating currents. Disentangling the characteristics of different noise-generating mechanisms is central to better understanding molecular transport at a fundamental level but is extremely challenging in molecular systems due to their complexity and relative experimental inaccessibility. Here, we construct a colloidal model microfluidic system for the experimental measurement of particle currents, where the governing physical properties are directly controllable and particle dynamics directly observable, unlike in the molecular case. Currents of hard spheres fluctuate due to the random arrival times of particles into the channel and the distribution of particle speeds within the channel, which results in characteristic scalings in the power spectral density. We rationalize these scalings by quantitatively comparing to a model for shot noise with a finite transit time, extended to include the distribution of particle speeds. Particle velocity distributions sensitively reflect the confining geometry, and we interpret and model these in terms of the underlying fluid flow profiles. Finally, we explore the extent to which details of these distributions govern the form of the resulting power spectral density, thereby establishing concrete links between the power spectral density and underlying mechanisms for this experimental system. This paves the way for establishing a more systematic understanding of the links between characteristics of transport fluctuations and underlying molecular mechanisms in driven systems such as nanopores.

Research on the starting-up process of a prototype reversible pump turbine with misaligned guide vanes: An energy loss analysis

With fossil fuel depletion and rising demand for renewable energy, reversible pump turbines (RPTs) are crucial for balancing energy supply. Understanding the dynamics of RPTs during starting-up is essential for optimal performance and stability in renewable energy systems. The unique “S" curve of RPTs can lead to instability under transient conditions. This study aimed to investigate the impacts of misaligned guide vanes (MGV) devices on prototype RPT stability, pressure pulsation, and performance improvement during the starting-up process. Using CFD simulation, flow characteristics and energy loss during start-up were analyzed and validated against experimental data. The results show that MGV enhances stability but induces significant energy loss. The analysis of entropy production, vorticity transport, and pressure fluctuation revealed high energy loss regions and the evolution of vortices during the starting-up process. This study provides new insights into hydrodynamic challenges and energy loss with MGV in prototype RPT start-up, with implications for optimizing pumped storage hydropower operations. Overall, this study has valuable engineering significance for the development and application of RPT in renewable energy systems.

Variability of Indonesian Throughflow Transport in the Indonesian Seas during Triple-Dip La Niña

Abstract The Indonesian seas serve as an important conduit for the Indonesian Throughflow (ITF), a critical system of warm water and freshwater transport connecting the Pacific and Indian Oceans. El Niño-Southern Oscillation (ENSO) significantly impacts the regional variability in the Pacific and exerts a profound influence on water mass transport from the Pacific to the Indian Oceans. This study focuses on the influence of triple-dip La Niña events during two distinct periods (1998 – 2001 and 2020 – 2023) on the variability of ITF transport in key areas of the Indonesian seas, including the Makassar, Lombok, and Ombai Straits, Flores and Banda Seas, and Timor Passage. Using forecast and reanalysis data from the Copernicus Marine Environment Monitoring Service (CMEMS), transport calculations are conducted across cross-sectional profiles representing the study areas. The results reveal that the volume transport is significantly higher at depths ranging from ∼50 – 300 m. It also shows seasonal fluctuations in volume transport across all regions, with lower transport during winter (DJF) and higher transport during summer (JJA) and transition period (SON) in the Makassar and Lombok Straits. In the Flores and Banda Seas, Ombai Strait, and Timor Passage, the surface volume transport follows seasonal patterns, while at deeper depths (∼50 – 300 m and 300 – 800 m) exhibits distinct fluctuations compared to the surface. Triple-dip La Niña events lead to an overall increase in volume transport during both periods, with the 2019 – 2023 period showing larger values (∼19 Sv) compared to the 1997 – 2001 period (∼14 Sv). The strengthened ocean current during triple-dip La Niña events results in more significant variations in total transport, leading to larger differences between minimum and maximum transport during these periods. The volume transport in Makassar Strait is further analyzed using the Empirical Orthogonal Function (EOF) that identifies three dominant modes (86.4%), revealing the influence of semi-annual, interannual, and decadal factors on the observed variability.

Performance limiting inhomogeneities of defect states in ampere-class Ga2O3 power diodes

Impacts of spatial charge inhomogeneities on carrier transport fluctuations and premature breakdown were investigated in Schottky ampere-class Ga2O3 power diodes. Three prominent electron traps were detected in Ga2O3 epilayers by a combination of the depth-resolved capacitance spectroscopy profiling and gradual dry etching. The near-surface trap occurring at 1.06 eV below the conduction band minimum (EC), named E3, was found to be confined within a 180 nm surface region of the Ga2O3 epilayers. Two bulk traps at EC − 0.75 eV (E2*) and at EC − 0.82 eV (E2) were identified and interconnected with the VGa- and FeGa-type defects, respectively. In the framework of the impact ionization model, employing the experimental trap parameters, the TCAD simulated breakdown characteristics matched the experimental breakdown properties well, consistently with inverse proportionality to the total trap densities. In particular, the shallowest distributed E3 trap with the deepest level is responsible for higher leakage and premature breakdown. In contrast, Ga2O3 Schottky diodes without E3 trap exhibit enhanced breakdown voltages, and the leakage mechanism evolves from variable range hopping at medium reverse voltages, to the space-charge-limited conduction at high reverse biases. This work bridges the fundamental gap between spatial charge inhomogeneities and diode breakdown features, paving the way for more reliable defect engineering in high-performance Ga2O3 power devices.

Surface Water (SW) and Shallow Groundwater (SGW) Nutrient Concentrations in Riparian Wetlands of a Mixed Land-Use Catchment

Precipitation patterns, water flow direction, and local land-use practices affect surface water (SW) and shallow groundwater (SGW) nutrient concentrations in riparian wetlands. Given physical process complexities, spatiotemporal quantification of nutrients and physical factors influencing nutrient concentrations are needed to advance wetland water resource management. To address these needs, a study was conducted in riparian wetlands of a mixed land-use catchment in West Virginia (WV), USA. Observed data included SW–SGW levels and nutrient concentrations, including nitrate (NO3-N), nitrite (NO2-N), ammonium (NH4-N), orthophosphate (PO43-P), total nitrogen (total_N), and total phosphorus (total_P) from January 2020 to December 2021. Water samples were collected monthly from stream gauge sites (n = 4) and co-located piezometers (n = 13). Results showed that, on average, gaining stream conditions were observed in upstream sites, and losing stream conditions were observed in downstream sites. Observed nutrient profiles between SW and SGW included SW exhibiting a higher average NO3-N concentration (0.42 mg/L), while SGW displayed an elevated NH4-N concentration (1.55 mg/L) relative to other nitrogen species. Significantly high (p < 0.05) SW NO3- concentrations in summer and fall were attributed to increased precipitation and corresponding water level and, therefore, pressure head and transport fluctuations. Principal Component Analysis (PCA) showed differences in nutrient concentrations based on the water source type and catchment land use, explaining 65% of data variability. Spearman correlation analysis illustrated the correlation among nutrient concentrations, land use, and water level changes in SW and SGW environments. This study provides needed baseline data on nutrient dynamics for a riparian wetland in a mixed land-use catchment, supplying science-based information to advance land and water management practices in the study watershed and similar physiographic watersheds globally.

Periodic Fluctuations of Transport as a Determinant of Inland Waterway Shipping Competitiveness

Periodic Fluctuations of Transport as a Determinant of Inland Waterway Shipping Competitiveness

Entanglement Rényi entropies from ballistic fluctuation theory: The free fermionic case

The large-scale behaviour of entanglement entropy in finite-density states, in and out of equilibrium, can be understood using the physical picture of particle pairs. However, the full theoretical origin of this picture is not fully established yet. In this work, we clarify this picture by investigating entanglement entropy using its connection with the large-deviation theory for thermodynamic and hydrodynamic fluctuations. We apply the universal framework of Ballistic Fluctuation Theory (BFT), based the Euler hydrodynamics of the model, to correlation functions of branch-point twist fields, the starting point for computing Rényi entanglement entropies within the replica approach. Focusing on free fermionic systems in order to illustrate the ideas, we show that both the equilibrium behavior and the dynamics of Rényi entanglement entropies can be fully derived from the BFT. In particular, we emphasise that long-range correlations develop after quantum quenches, and accounting for these explain the structure of the entanglement growth. We further show that this growth is related to fluctuations of charge transport, generalising to quantum quenches the relation between charge fluctuations and entanglement observed earlier. The general ideas we introduce suggest that the large-scale behaviour of entanglement has its origin within hydrodynamic fluctuations.

Bedload transport fluctuations, flow conditions, and disequilibrium ratio at the Swiss Erlenbach stream: results from 27 years of high-resolution temporal measurements

Abstract. Based on measurements with the Swiss plate geophone system with a 1 min temporal resolution, bedload transport fluctuations were analysed as a function of the flow and transport conditions in the Swiss Erlenbach stream. The study confirms a finding from an earlier event-based analysis of the same bedload transport data, which showed that the disequilibrium ratio of measured to calculated transport rate (disequilibrium condition) influences the sediment transport behaviour. To analyse the transport conditions, the following elements were examined to characterise bedload transport fluctuations: (i) the autocorrelation coefficient of bedload transport rates as a function of lag time (memory effect), (ii) the critical discharge at the start and end of a transport event, (iii) the variability in the bedload transport rates, and (iv) a hysteresis index as a measure of the strength of bedload transport during the rising and falling limb of the hydrograph. This study underlines that above-average disequilibrium conditions, which are associated with a larger sediment availability on the streambed, generally have a stronger effect on subsequent transport conditions than below-average disequilibrium conditions, which are associated with comparatively less sediment availability on the streambed. The findings highlight the important roles of the sediment availability on the streambed, the disequilibrium ratio, and the hydraulic forcing in view of a better understanding of the bedload transport fluctuations in a steep mountain stream.

Primitive and gravity modulation of periodical heat transfer along magnetic-driven porous cone with thermal conductivity and surface heat flux

The significant goal of present problem is to evaluate oscillating frequency and amplitude in heat and magnetic flux with thermal conductivity and heat flux effects across magnetized porous cone under lower gravitational region. To maintain heat transport efficiency, the thermal conductivity of conducting fluid is assumed as temperature dependent. The magnetic-driven cone is placed in lower gravitational region. To improve the heating rate across the magnetic-driven porous cone, the convective heating conditions are applied. The implications of thermal conductivity, surface heat flux, and reduced gravity on the periodic behavior of convective heat transfer characteristics of conducting fluid across porous gravity-driven cone is the novelty of this research. Using the appropriate non-dimensional variables, the model's nonlinear governing partial differential has been converted into a dimensionless form. The proposed model is solved later with the help of finite difference approach. The dimensionless form of the equations is reduced to the convenient form for numerical algorithm. The influence of adjusting parameters, such as thermal conductivity parameter ζ, reduced gravity parameter Rg, Biot number Bi, Prandtl number Pr, mixed convection parameter λ, porosity parameter Ω, magnetic prandtl number γ and some others stationary parameters has been highlighted. By using FORTRAN tool, the numerical outcomes are explored in graphs and tabular form. It is depicted that magnetic field enhances as Biot number decreases because magnetic field insulates the excessive heating across the magnetic-driven porous cone. It is noticed that significant amplitude in fluid velocity is noticed with prominent changes as reduced gravity increases. It is concluded that fluctuations in heat transport increases as parameter γ decreases under thermal conductivity because magnetic material insulates the extreme heat across the porous cone. The recent thermal conductivity and lowered gravity problem is significant in the fields of radioactive waste, cooling electronic components, storing nuclear, catalysts, heat exchangers, the underground storage of radioactive or nonnuclear materials, aircrafts and space vehicles.

Long radial coherence of electron temperature fluctuations in non-local transport in HL-2A plasmas

The dynamics of long-wavelength (kθ < 1.4 cm−1), broadband (20 kHz–200 kHz) electron temperature fluctuations ( T∼e/Te ) of plasmas in gas-puff experiments are observed for the first time in HL-2A tokamak. In a relatively low density (n e(0) ≃ 0.91 × 1019m−3–1.20 × 1019m−3) scenario, after gas-puffing the core temperature increases and the edge temperature drops. On the contrary, temperature fluctuation drops at the core and increases at the edge. Analyses show the non-local emergence is accompanied with a long radial coherent length of turbulent fluctuations. While in a higher density (n e(0) ≃ 1.83 × 1019 m−3–2.02 × 1019 m−3) scenario, the phenomena are not observed. Furthermore, compelling evidence indicates that E × B shear serves as a substantial contributor to this extensive radial interaction. This finding offers a direct explanatory link to the intriguing core-heating phenomenon witnessed within the realm of non-local transport.

Decadal Variability in Subsurface Nutrient Availability on the Scotian Shelf Reflects Changes in the Northwest Atlantic Ocean

AbstractSubsurface nutrients on the Scotian Shelf, an ocean region at the convergence of the subpolar and subtropical western boundary currents (i.e., Labrador Current (LC) and Gulf Stream (GS)), are chiefly modulated by upstream shelf and slope waters. Yet little is known about long‐term fluctuations in the advective transport of nutrients to the shelf. Here, we synthesized nutrient and hydrographic data from 1975 to 2020 to characterize decadal changes in upper (60–150 m) and lower (150–200 m) subsurface nutrient availability across the shelf. Hydrographic and nutrient anomalies highlight a transition from a warm, saline, high‐nutrient GS‐dominated system in the 1980 to colder, fresher, lower‐nutrient LC‐dominated conditions in the late 1980 and early 1990. In the mid‐1990, an increase in excess silicate and phosphate (relative to nitrate) marks a shift in the LC contribution toward a strengthening of the inner shelf component relative to the outer shelf‐edge branch. Since 2010, rapid increases in subsurface temperature and salinity indicate the transition back to a GS‐dominated system, albeit with a distinct decline in dissolved nutrients relative to the previous warm period, which diverges from the nutrient increase generally associated with GS water. Declines in lower subsurface nutrient concentrations since 2010 are likely driven by a density‐related shift in the GS source water toward a less dense, lower‐nutrient GS contribution. This shift is evidenced by a decrease in excess phosphate amidst minor declines in excess silicate, reflecting the characteristic density‐related patterns of excess nutrients in GS waters.

Спутниковые оценки вариаций увлажненности внутреннего Тянь-Шаня в период 1982–2022 гг. на примере хребта Ат-Баши (Киргизия)

Introduction. Western atmospheric jet streams carry moisture over the Northern Tian Shan ridges into the Inner Tian Shan. Long-term weather changes in the Inner Tian Shan diagnose climatic fluctuations in atmospheric moisture transport. This is of considerable interest for understanding the direction of modern climate change in Central Asia. Materials. The area of the At-Bashi Ridge (2700 км2) in Inner Tian Shan, including a large high-altitude Lake Chatyr-Kul (170 km2, 3530 m), were considered. The ridge, with a peak of 4786 m, has a length of 135 km and is characterized by an Alpine type of relief. The initial information related to the period 1982-2022. The list of data used included: FLDASSnowDepth_inst; JRC Monthly Water History, v1.4; ECMWF/ERA5_LAND/MONTHLY (precipitation); NOAA/NCEP_DOE_RE2 (cloud fraction). Also, to diagnose the mountain glaciation state of the At-Bashi Ridge, three satellite images of sub meter resolution dated 09/21/2002, 07/27/2013 and 09/13/2021 were used. The purpose of the work. The aim of the research was to study the humidity regime of the At-Bashi Ridge area during 1982-2022. Research methodology. The change in the average annual snow depth in the test area in the period 1982-2022 was analyzed, as well as the degree of correlation of this parameter with other weather characteristics.Fischer’s criterion acted as a quantitative basis for estimates. Satellite images of sub meter resolution were used to analyze long–term changes in the area of rock outcrops from ice on the slopes of glacial chambers, which diagnoses the glacier state (degradation – stability – growth). Satellite data recorded the process of mountain glaciation growth above the height of 4000 m in the period between 2002–2013. Research results. In order of increasing correlation (F-criterion), with the snowiness of the cold period (October-May) of the At-Bashi Ridge area, the parameters considered were arranged as follows (F=2.11; α=0.01): cloud fraction, F= 3.02; precipitation, F=12.42; the Lake Chatyr-Kul area, F= 12.53; the balance of snow deposits between the windward (north-western) and leeward (south-eastern) At-Bashi Ridge slopes, F= 22.01. According to satellite data, the growth of mountain glaciation in the zone with an altitude above 4000 m in the period 2002-2013 was recorded. Discussion. The At-Bashi Ridge area has three different periods in terms of humidity: dry in 1982-1999 with an average annual snow depth of 0.099± 0.051 m; wet in 2000-2012; 0.192± 0.074 m; and moderately humid in 2013-2022; 0.133± 0.058 m. Ultra-high-resolution satellite images recorded the growth of the mountain glaciation of the At-Bashi Ridge at altitudes of more than 4000 m in the period from 2002 to 2013. This confirms the actual existence of the wet period of 2001-2012. Conclusion. Thus, the current trend of humidity changes of the At-Bashi Ridge does not have an unambiguous direction, but in general, for the period 1982-2022, it is aimed at a certain increase in the snow content in mountains and the area of the Lake Chatyr-Kul. Apparently, the current climate change is accompanied by an increase in the ability of western atmospheric jet fluxes to transfer moisture over the front Northern Tian Shan ranges into the Inner and Central Tian Shan ridges, which may even be accompanied by an increase in the mountain glaciation of these territories in certain periods of time.

Observation of suppressed viscosity in the normal state of 3He due to superfluid fluctuations

Evidence of fluctuations in transport have long been predicted in 3He. They are expected to contribute only within 100μK of Tc and play a vital role in the theoretical modeling of ordering; they encode details about the Fermi liquid parameters, pairing symmetry, and scattering phase shifts. It is expected that they will be of crucial importance for transport probes of the topologically nontrivial features of superfluid 3He under strong confinement. Here we characterize the temperature and pressure dependence of the fluctuation signature, by monitoring the quality factor of a quartz tuning fork oscillator. We have observed a fluctuation-driven reduction in the viscosity of bulk 3He, finding data collapse consistent with the predicted theoretical behavior.

Phase slip from weak links formed in artificially-stacked NbSe2

The rich nature of van der Waals interactions between artificially-stacked atomic layers has been demonstrated by various quantum states and resonant tunneling transport in low-dimensional materials. However, the role of topological fluctuations in quantum transport through artificially-stacked junctions of 2D superconducting materials, and the resulting energy dissipation, remain elusive. In this research, unique phase-slip centers are designed in artificially-stacked junction areas, where nonequilibrium quasiparticles are formed and relaxed with energy dissipation. The phase slips are observed as voltage steps (peaks or valleys) in transport measurements across a junction between two exfoliated NbSe2 flakes, and at a distance of 4 μm from the junction using local and nonlocal chemical potential probes. Accordingly, two types of energy dissipation modes are newly identified in the artificially-stacked NbSe2 when subjected to an in-plane magnetic field, which implies distinct vortex formation and current flow in the superconducting junction under magnetic fields.

Nonequilibrium fluctuations in boson transport through squeezed reservoirs

We investigate the effect of quantum mechanical squeezing on the nonequilibrium fluctuations of bosonic transport between two squeezed harmonic reservoirs and a bosonic site. A standard full counting statistics technique based on a quantum master equation is employed. We derive a nonzero thermodynamic affinity under an equal temperature setting of the two squeezed reservoirs. The odd cumulants are shown to be independent of squeezing under symmetric conditions, whereas the even cumulants depend nonlinearly on the squeezing. The odd and even cumulants saturate at two different but unique values which are identified analytically. Under maximum squeezing of one bath, the saturation value of the cumulants is solely governed by other bath’s properties. Further, squeezing always increases the magnitude of the even cumulants in comparison to the unsqueezed case. The saturation values of the even cumulants become independent of squeezing as soon as one bath is squeezed to its limit. This is in contrast to what is observed for the odd cumulants. The even cumulants are symmetric with respect to exchanging the left and right squeezing parameters while the affinity is found to be antisymmetric.

Characteristics of sand transport fluctuation in near-neutral atmospheric surface layer

Characteristics of sand transport fluctuation in near-neutral atmospheric surface layer

Causes for Atlantic Freshwater Content Variability in the GECCO3 Ocean Synthesis

AbstractRegional freshwater content (FWC) changes are studied over the period 1961–2018 using the GECCO3 ocean synthesis. In four dynamically distinct regions of the Atlantic, the study identifies causes for FWC variability with a focus on interannual and decadal time‐scale changes. Results show that in each region, it is a combination of the surface freshwater flux and the net freshwater transport across the region's boundaries that act jointly in changing the respective FWC. Surface flux mainly contributes to the FWC variability on multi‐decadal time scales. The impact of surface flux also increases toward the tropics. On shorter time scales, it is especially horizontal transport fluctuations, leading to FWC changes in mid and high latitudes. Going from north to the south, the transport across a single meridional boundary becomes less correlated with the FWC changes but the net transport across both boundaries plays an increasingly important role. Moreover, the subpolar box is mainly gyre driven, which differs from the other two, essentially overturning driven, North Atlantic boxes. In the tropical Atlantic, the shallow overturning cell and the deep overturning contribute about equal amounts to the freshwater variations.

The effect of stress history on fluctuation of bedload transport rate and bed topography in gravel-bed streams

Mountainous gravel-bed stream characterized by large-scale roughness generally goes through the seasonal flow, including flood and interevent periods. Recent studies suggest the effect of the interevent period (termed “stress history”) is significant for the bedload incipient motion and bed topography evolution. However, the research on bedload transport fluctuation and protruding microtopography (a fundamental element in the riverbed) under the effect of stress history is limited, especially for quantitative description. As such, this study evaluated experimentally the effect of stress history on bedload transport and bed topography in gravel-bed streams. The long-duration interevent period restrains the variation of bedload transport fluctuation by decreasing the microtopography spatial density, while the large-magnitude interevent period enhances the variation of bedload transport fluctuation through the increasing spatial density. Meanwhile, the grain distribution of transported bedload is significantly affected by the magnitude of the interevent period, rather than the duration. Bed topography becomes smoother through an intermittent adjustment of structure-scale bed topography in the interevent period. Moreover, the decreasing bedload cumulative transport rate and lower variation coefficient of bedload transport fluctuation after short-duration and small-magnitude interevent period are attributed to the stable riverbed with the decreasing microtopography spatial density of less flow drag. An assumption of the larger rate and higher fluctuation after the long-duration and middle/large-magnitude interevent period is proved that the magnitude of the interevent period can promote the microtopography spatial density to generate large flow drag and carry coarse particles of microtopography to randomly hit the bed to generate abundant sediment ejection and transport. Additionally, the relations among microtopography spatial density, characteristic grain sizes, and statistical parameters of bed topography at the different scales are further explored, indicating bed topography at the grain-scale has more relation with bed topography.