Bed Stress Research Articles

Vegetation‐Generated Turbulence Does Not Impact the Erosion of Natural Cohesive Sediment

AbstractPrevious studies have demonstrated that vegetation‐generated turbulence can enhance erosion rate and reduce the velocity threshold for erosion of non‐cohesive sediment. This study considered whether vegetation‐generated turbulence had a similar influence on natural cohesive sediment. Cores were collected from a black mangrove forest with aboveground biomass and exposed to stepwise increases in velocity. Erosion was recorded through suspended sediment concentration. For the same velocity, cores with pneumatophores had elevated turbulent kinetic energy compared to bare cores without pneumatophores. However, the vegetation‐generated turbulence did not increase bed stress or the rate of resuspension, relative to bare cores. It was hypothesized that the short time‐scale fluctuations associated with vegetation‐generated turbulence were not of sufficient duration to break cohesion between grains, explaining why elevated levels of turbulence associated with the pneumatophores had no impact on the erosion threshold or rate.

Enhanced flow variability and morphological changes through individual wood placements on a gravel bed

To design and evaluate river restoration projects with wood, it is crucial to understand how the placement of wood will alter flow and bed morphology. This information is still missing. To address this, flume experiments were conducted with a single log placed on a mobile bed with a uniform grain size. The effect of log diameter, log orientation to the flow, and log length on flow variability and bed morphology was studied for various initial flow conditions. Even a single log increased flow variability, herein defined as spatial variability in both velocity and turbulent kinetic energy. For each log placed at the channel center, the logs generated a symmetric scour pattern alongside and downstream of the log due to increased bed shear and turbulence generated by the log. A steady, adjusted cross-section was reached at the cessation of erosion, which occurred when the bed stress decreased below the critical shear velocity. Logs with larger diameter tended to erode more deeply, so that the final, adjusted cross-section was similar for different log dimensions. Changes in bed topography affected wake characteristics. Logs with small changes in bed topography generated longer zones of reduced velocity, which can increase the residence time of organic matter or nutrients in the wake region. Logs with big changes in bed topography created the deepest and longest scour holes, which may locally reduce stream temperature and improve fish habitat during summer.

Interdisciplinary approaches to modeling extraction efficiency and safety for clean energy under coupled multi-field effects

Coal bed methane, a crucial clean energy source, has attracted extensive research attention. Characterized by intricate and rough fracture systems, coal seam is vital for gas migration, which will be influenced by the in situ stress, coal temperature, adsorption–desorption effect, solid deformation, and gas pressure. This paper introduces an innovative, interdisciplinary fractal model that addresses the limitations of current computational models in accurately representing the complex fractures under the coupled multi-field effects. Four novel fractal micro-parameters are introduced to capture the dynamics of rough networks. And rigorous validation against field extraction data reveals that the proposed micro-parameters outperform existing methods in analytical efficacy. Notably, those micro-parameters significantly influence fracture behavior and gas seepage. For instance, a DT increase from 1.2 to 1.8 and an ε rise from 0.06 to 0.18 lead to a respective 29.8% and 22.7% increase in gas pressure. Moreover, alterations in these fractal micro-parameters under coupled multi-field effects markedly impact coal bed stress, raising safety concerns in engineering projects, with a potential increase in coal stress by up to 2.62%. This research offers innovative insights into the complex coupled mechanisms governing rough fractures and significantly advances the understanding of the efficiency and safety in clean energy extraction processes.

Turbulence and fine sediment dynamics in a benthic boundary layer in Fujian Coastal Sea, SE China

A short-term (∼ 25 h) intensive field observation focused on flow, turbulence, suspended sediment concentration, and floc/particle size distributions was conducted in a coastal benthic boundary layer to understand flocculation and erosion/deposition processes of fine sediment. Observations showed suspended particles were mainly flocculated into the size range of microflocs (50-200 μm). Macroflocs (> 230 μm) occurred at slack tides and SPM (suspended particulate matter) concentrations decreased as they settled faster. Twofold intra-tidal variations in floc size and settling velocity were observed with a larger medium floc size of 140 μm and a settling velocity of 1.3 mm/s occurring at slack tides than peak flood/ebb tides with their corresponding values of 70 μm and 0.7 mm/s. This variability can be ascribed to the turbulence modulation in that low turbulence enhances aggregation to form large flocs but they turn to break down by high turbulence. The inverse relationship was found between floc size and turbulence shear rate, SPM concentration, and their product. SPM concentration covaried with bed stress and the low critical erosion stress (∼ 0.02 Pa) indicates that the cohesive bed was primarily eroded as aggregates which formed a fluff layer above the consolidated bed. The fine sediment dynamics behave as a loop from resuspension, through to breakup and re-aggregation, and to settling in response to intro-tidal turbulence variability. This looped behavior is vital for understanding the sequestration of organic carbon and pollutants in fine sediments, awaiting more detailed and longer-term field observations and the assemblage into numerical models.

Dynamic impact of sleeper unsupported defects on the heavy haul locomotive-ballasted track coupling system

In this study, a dynamics analysis framework combining the multi-body dynamics method (MBD) and the discrete element method (DEM) capable of accounting for sleeper unsupported defects is proposed. Different sleeper hanging defects (e.g., number of unsupported sleepers and the hanging height) is an input for an analysis of the heavy haul locomotive dynamic behavior and related ballasted bed degradation in terms of ballast particle wear. Simulated results indicate that the sleeper hanging defects dramatically increase the average contact stress underneath the well-supported sleeper adjoining the sleeper hanging area and the corresponding ballasted bed stress, indicating that the unsupported height should be suppressed within 5 mm even if only there exist single or two consecutive unsupported sleepers. When occurring minor sleeper unsupported defects and not being maintained in time, the vibration acceleration and dissipation energy of the ballasted bed will further increase sharply, which will aggravate the wear of the ballast particles and may change the particle size distribution of the ballasted bed or even pollute it.

The influence of tides on the North West European shelf winter residual circulation

Tides contribute to the large-scale residual circulation and mixing of shelf seas. However, tides are typically excluded from global circulation models (GCMs) so their modelled residual circulation (and mixing) in shelf seas may be systematically wrong. We focus on circulation as it is relatively unexplored, and affects shelf temperature and salinity, potentially biasing climate impact studies. Using a validated model of the North West European Shelf Seas (NWS), we show the essential role of tides in driving the residual circulation, and how this affects the NWS temperature and salinity distribution. Over most of the NWS, removing the tides increases the magnitude of residual circulation while in some regions (such as the Irish Sea) it leads to a reduction. Furthermore, we show that modelling the NWS without tides leads to a cold fresh bias in the Celtic Sea and English Channel (of >0.5°C, and >0.5 psu). This shows that NWS tidal dynamics are essential in the transport of heat and matter, and so must be included in GCMs. We explore two processes by which the tides impact the residual circulation and investigate whether these could be parameterised within non-tidal GCMs: (1) Enhancing the seabed friction to mimic the equivalent energy loss from an oscillating tidal flow; (2) Tidal Phase-driven Transport (TPT), whereby tidal asymmetry drives a net transport due to the phase between tidal-elevation and velocities (equivalent to the bolus term in oceanographic literature). To parameterise TPT, we calculate a climatology of this transport from a harmonic analysis from the tidal model and add it as an additional force in the Navier Stokes equations in the non-tidal model. We also modify the bed drag coefficient to balance the bed stress between the simulations – hypothesising that using this modified drag coefficient will simulate the effect of the tides. This tends to improve the mean and variability of the residual circulation, while the TPT improves the spatial distribution and temporal variability of the temperature and salinity. We show that our proof-of-concept parameterisation can replicate the tidally-driven impact on the residual circulation without direct simulation, thus reducing computational effort.

Numerical analysis of train-track-subgrade dynamic performance with crumb rubber in ballast layer

• Coupled DEM-FDM-MBD model to study train-track-subgrade dynamic responses. • Crumb rubber (or tire-derived aggregates) effects on train-track-subgrade dynamic responses. • Crumb rubber size effects on the train-track-subgrade dynamic responses presented by ballast friction energy dissipation. Crumb rubber (CR) has been proposed to apply in the ballast or sub-ballast layer for ballast degradation mitigation and vibration (noise) reduction. The CR can change the ballast layer stiffness, which can affect the train-track-subgrade dynamic performance and cause travel comfort and safety issues. Towards this, this study aims at confirming 1) how much the CR application can affect the dynamic performance of train and ballast layer; 2) to what extent the CR-ballast layer can distribute the train loadings to reduce subgrade surface stress. To achieve this aim, a whole train-track-subgrade system model was built by coupling multibody dynamics (MD), discrete element method (DEM) and finite difference method (FDM). The MD was used to build the train, including one vehicle body, two bogies and four wheelsets. The DEM was used to build the ballasted track, including rail, sleepers and ballast layer. The FDM was used to build the subgrade. Using the coupled model, the dynamic performance of train and track were studied, including the vehicle body acceleration, wheel-rail force, rail dynamical bending moment, sleeper acceleration, sleeper displacement and ballast acceleration. In addition, the energy dissipation of the ballast bed was also presented. For the subgrade, the subgrade surface acceleration and surface stress were measured and analysed. In the model, different CR size and percentage were considered. Results show that using the CR in ballast layer can increase the accelerations of sleeper, rail and train. But it can decrease the ballast degradation, subgrade surface acceleration and subgrade surface stress. CR helps consume train loading energy, reducing the energy that has to be consumed by ballast friction. Small size CR (8–22.4 mm) has greater influence on dynamic performance of the whole train-track-subgrade system than big size CR (9.5–63 mm). In summary, 10% percentage of CR-ballast mixture is recommended, and for CR size it is difficult to give a recommendation. Small size CR increase ballast acceleration more than big size CR, but small size CR are better at improving sleeper displacement, subgrade stress and ballast bed stress.

Study on the Methods to Estimate the Bed Shear Stresses in Yangtze Estuary

High spatial and temporal current resolution velocities were measured by use of an acoustic Doppler velocimeter (ADV) in the tidal bottom layer of North Passage Deep water Navigational Channel (DNC) of Yangtze Estuary in China. And the vertical profiles of mean velocity were measured by using a pulse-coherent acoustic Doppler profiler (PC-ADP). The measured data show the vertical profiles of mean velocity are almost logarithmic. For discussing, the turbulent kinetic energy (TKE) method based on the velocity for ADVs-based is chosen and a new logarithmic profile (LP) method ignoring the influence of complex variation of the parameter Z 0 is suggested to estimate the bed shear stresses respectively. The calculated results show that the bed shear stresses estimated by LP method are larger than the ones estimated by TKE method mostly in Yangtze estuary. At the moment of ebb peak, the maximum error can be about twice occurring and the error of ebb tide is almost greater than the one of flood tide. These phenomena are explained in this study by the deduction of formula for steady and unsteady flow. It is concluded that the vertical logarithmic profile of velocities will be influenced by unsteady flow and there is not the same bed shear stress even if there is the same logarithmic velocity profile in vertical. Generally, this has led to a necessary adjustment of the drag coefficient Cd of steady flow in flow model to avoid overestimating the bed stress for unsteady flow in estuary based on the observed data.

Controls on Sediment Bed Erodibility in a Muddy, Partially-Mixed Tidal Estuary

The objectives of this study are to better understand controls on bed erodibility in muddy estuaries, including the roles of both sediment properties and recent hydrodynamic history. An extensive data set of erodibility measurements, sediment properties, and hydrodynamic information was utilized to create statistical models to predict the erodibility of the sediment bed. This data set includes >160 eroded mass versus applied stress profiles collected over 15 years along the York River estuary, a system characterized by “depth-limited erosion,” such that the critical stress for erosion increases rapidly with depth into the bed. For this study, erodibility was quantified in two ways: the mass of sediment eroded at 0.2 Pa (a stress commonly produced by tides in the York), and the normalized shape of the eroded mass profile for stresses between 0 and 0.56 Pa. In models with eroded mass as the response variable, the explanatory variables with the strongest influence were (in descending order) tidal range squared averaged over the previous 8 days (a proxy for recent bottom stress), salinity or past river discharge, sediment organic content, recent water level anomalies, percent sand, percent clay, and bed layering. Results support the roles of 1) recent deposition and bed disturbance increasing erodibility and 2) cohesion/consolidation and erosion/winnowing of fines decreasing erodibility. The most important variable influencing the shape of the eroded mass profile was eroded mass at 0.2 Pa, such that more (vs. less) erodible cases exhibited straighter (vs. more strongly curved) profiles. Overall, hydrodynamic variables were the best predictors of eroded mass at 0.2 Pa, which, in turn, was the best predictor of profile shape. This suggests that calculations of past bed stress and the position of the salt intrusion can serve as useful empirical proxies for muddy bed consolidation state and resulting erodibility of the uppermost seabed in estuarine numerical models. Observed water content averaged over the top 1 cm was a poor predictor of erodibility, likely because typical tidal stresses suspend less than 1 mm of bed sediment. Future field sampling would benefit from higher resolution observations of water content within the bed’s top few millimeters.

Gravity-driven sediment flows on the shallow sea floor of a muddy open coast

Wave and/or current supported gravity-driven sediment flows (GDSFs) cause substantial sediment movement across the continental shelf, contributing to morphological evolution in many regions worldwide. However, they appear to occur episodically and ephemerally and, therefore, it remains a challenge to document them in detail with in situ measurements. Here we present solid evidence of frequent generation of such flows over the shallow sea floor of a muddy open coast. They were triggered by wave resuspension and/or sediment settling from the overlying water column, maintained by wave- and/or current-induced bed stress, and terminated due to upward spreading of bottom sediment within the high concentration layer. Randomly selected GDSF events were analyzed to realize parameterization with a buoyancy-friction model; the resultant bed drag coefficient for two of the cases is higher than the value of 0.003, which is attributed to the additional drag at the interface between the overlying flow and the moving GDSFs. Although short-lived, the observed GDSF events in the three different seasons indicate that they occur more frequently than previously thought.

Bed and Bank Stress Partitioning in Bedrock Rivers

AbstractApproximation of bed and wall (bank) stresses in confined, narrow bedrock rivers is key to accurately assessing hydraulic roughness, sediment transport, bedrock erosion and the morphodynamics of bedrock channels. Here, we partition bed and wall stresses using the ray‐isovel model (RIM) and field observations. We used the RIM to calculate the distribution of shear stress across an idealized trapezoidal channel and found that the ratio of wall to bed stress () grows slightly with increasing bank angles, but exponentially declines with increasing width‐to‐depth ratio. We applied the RIM to 26 of the canyons along the Fraser River and found that RIM predicts 0.60 0.98. We used field observations of bed and total stress to calculate wall stress in each canyon. The total stress was calculated from 1D momentum balance (depth‐slope product). The distribution of bed stress was calculated from near‐bed velocity profiles and showed that bed stress spikes as water enters constriction‐pool‐widening (CPW) sequences. For the majority of studied canyons, the observed wall stress is larger than the total stress and the observed bed stress. The maximum observed bed stress through a CPW sequence is ∼7.5 times the mean bed stress and ∼4.9 times the total stress. Compared with the observed stresses, the model systematically over‐predicts the observed bed stress and under‐predicts the observed wall stress by ∼55%. Our results reveal that the complex flow structure in bedrock canyons influences the distribution of bed and wall stresses and that bedrock walls contribute more hydraulic roughness than predicted with RIM.

Calibrated Suspended Sediment Observations Beneath Large Amplitude Non‐Linear Internal Waves

AbstractWhile it has been recognized for some time that large amplitude non‐linear internal waves (NLIW) can mobilize and transport sediment, quantitative observations of this process are rare. Rarer still are accompanying estimates of suspended sediment mass concentration (SSC) during the passage of NLIW. Here, we present high resolution observations of NLIW and the SSC response within the bottom boundary layer. The observations were made in 250 m of water in a mildly sloping region of the Browse Basin on Australia’s Northwest Shelf. We compare two independent but direct calibration methods, and employ Bayesian methods to estimate the uncertainty in SSC. During a large NLIW event, the peak mean SSC estimate at 0.49 m above the sea bed was 161 mg L−1, with a maximum time‐rate‐of‐change of 0.14 mg L−1 s−1. The unsteady boundary layer forcing under NLIW resulted in a variable time‐height dependent relationship between bed stress and SSC with increasing height above the sea bed. Suspended sediment was restricted to the bottom mixing layer, with sharp vertical gradients of up to 40 mg L−1 m−1 observed at the edge of the layer. The observations presented here are intended to offer guidance to numerical sediment modelers about likely SSC under strong NLIW.

Shallow water model for pollutant distribution in the Ypacarai Lake

In this paper, we analyze the distribution of a non-reactive contaminant in Ypacarai Lake. We propose a shallow-water model that considers wind-induced currents, inflow and outflow conditions in the tributaries, and bottom effects due to the lakebed. The hydrodynamic is based on the depth-averaged Navier-Stokes equations considering wind stresses as force terms which are functions of the wind velocity. Bed (bottom) stress is based on Manning's equation, the lakebed characteristics, and wind velocities. The contaminant transportation is modeled by a 2D convection-diffusion equation taking into consideration water level. Comparisons between the simulation of the model, analytical solutions, and laboratory results confirm that the model captures the complex dynamic phenomenology of the lake. In the simulations, one can see the regions with the highest risk of accumulation of contaminants. It is observed the effect of each term and how it can be used them to mitigate the impact of the pollutants.

DEM simulation of mechanical behavior in one-dimensional compression of crushable ceramic pebble bed

The mechanical behavior in one-dimensional compression of a crushable ceramic pebble bed for the fusion reactor, which is under different crushing ratio assumptions (0% - 2%), was numerically investigated by the discrete element method (DEM) simulation. The crushed particle was modeled by a reduction of the particle radius, therefore, the instantaneous decrease of the local stress could be produced. To guarantee the mass conservation of the particles in the pebble bed, the sub-particle insertion process was conducted. Please note that this paper mainly deals with the post of failure of the particles, hence the pebble failure process was not captured. The results showed that the crushed particles could make the pebble bed softer, and with the same Li4SiO4 pebble bed strain, the bed stress decreased as the crushing ratio grew larger. The impact of the crushing ratio on the radial and axial local packing factor was limited on the oscillating amplitude without affecting the oscillating width. Besides, the effects of the coefficient of friction and restitution under different crushing ratios were investigated. At last, the analysis of changes in the average coordination number with the bed strain under different crushing ratios was performed.

Modeling shallow geological flows on steep terrains using a specific differential transformation

Shallow geological flows such as landslides, debris flows, and floods are frequently modeled mathematically in the depth-averaged framework. As the bed topography becomes steep and irregular, the classical shallow-water model loses accuracy. How to handle bed topography and gravity effects in depth-averaged models becomes important, and the validity of the model needs verifying. In this study, a differential transformation called the G2L transformation linking the global height to the local depth of the surface material is proposed. This transformation helps to reconstruct the landslide/avalanche surface for one-dimensional and two-dimensional terrains that are obviously curved as well as real landslide examples such as the Huangtian and Rongsong landslides with shallowness ratios as high as 1/4. With this G2L transformation and closures of the lateral and bed stress, two versions of a model governing the geological flow dynamics are developed to avoid difficulties in terrain treatment when using differential geometry. Version 1 of the model is a coupled hyperbolic–differential system; version 2 is a modified hyperbolic system. The two proposed models are verified by a simple one-dimensional granular flow problem on a steep slope. Both mitigate problems associated with computational differential geometry of geological flows and the singularity problem of shallow-water models. In particular, the reduced gravity of several current models in landslide dynamics is a special instance of version 2. The two proposed models are well able to mathematically model the gravity geological hazards and may provide the tools required for prediction and mitigation of natural geological hazards.

Shifting Sediment Dynamics in the Coos Bay Estuary in Response to 150 Years of Modification

AbstractEstuaries worldwide have experienced modifications including channel deepening and intertidal reclamation over several centuries, resulting in altered fine sediment routing. Estuaries respond differently based on preexisting geometries, freshwater and sediment supplies, and extents and types of modification. The Coos Bay Estuary in Oregon is a relatively small estuary with complex geometry that has been extensively modified since 1865. A sediment transport model calibrated to modern conditions is used to assess the corresponding changes in sediment dynamics. Over ∼150 years, channel deepening (from ∼6.7 to 11 m), a 12% increase in area, and a 21% increase in volume have led to greater tidal amplitudes, salinity intrusion, and estuarine exchange flow. These changes have reduced current magnitudes, reduced bed stresses, and increased stratification, especially during rainy periods. Historically, fluvially derived sediment was dispersed across broad, deltaic‐style flats and through small tidal channels. Now, river water and sediments are diverted into a dredged navigation channel where an estuarine turbidity maximum (ETM) forms, with modeled concentrations >50 mg/L and measured concentrations >100 mg/L during discharge events. This “new” ETM supplies sediment to proximal embayments in the middle estuary and the shallow flats. Overall, sediment trapping during winter (and high river discharges) has increased more than two‐fold, owing to increased accommodation space, altered pathways of supply, and altered bed stresses and tidal asymmetries. In contrast to funnel‐shaped estuaries with simpler geometries and river‐channel transitions, these results highlight the importance of channel routing together with dredging in enhancing sediment retention and shifting pathways of sediment delivery.

Analytical model for lateral depth-averaged velocity distributions in curved channels

The distribution of depth-averaged velocity is crucial to many engineering problems. This paper presents an analytical solution for the lateral depth-averaged velocity distribution in a 120° curved channel with asymmetric trapezoidal cross-sections in which the curved flow is not fully developed. The proposed analytical model was derived from the streamwise depth-integrated Reynolds-averaged Navier–Stokes equation, which considers the bed stress, Reynolds stress, secondary flow and local flow acceleration. In addition, practical boundary conditions are proposed to consider the trapezoidal cross-sections with linearly varying side beds and the flow separation at the inner bank near the bend entrance. The analytical solutions using the proposed model were found to be in good agreement with measured data. The main contributions of this study are the development of a conventional analytical model for sophisticated circumstances and the proposal of practical boundary conditions that consider flow separation.

Insights from a global snapshot of the change in elective colorectal practice due to the COVID-19 pandemic.

There is a need to understand the impact of COVID-19 on colorectal cancer care globally and determine drivers of variation. To evaluate COVID-19 impact on colorectal cancer services globally and identify predictors for behaviour change. An online survey of colorectal cancer service change globally in May and June 2020. Attending or consultant surgeons involved in the care of patients with colorectal cancer. Changes in the delivery of diagnostics (diagnostic endoscopy), imaging for staging, therapeutics and surgical technique in the management of colorectal cancer. Predictors of change included increased hospital bed stress, critical care bed stress, mortality and world region. 191 responses were included from surgeons in 159 centers across 46 countries, demonstrating widespread service reduction with global variation. Diagnostic endoscopy was reduced in 93% of responses, even with low hospital stress and mortality; whilst rising critical care bed stress triggered complete cessation (p = 0.02). Availability of CT and MRI fell by 40-41%, with MRI significantly reduced with high hospital stress. Neoadjuvant therapy use in rectal cancer changed in 48% of responses, where centers which had ceased surgery increased its use (62 vs 30%, p = 0.04) as did those with extended delays to surgery (p<0.001). High hospital and critical care bed stresses were associated with surgeons forming more stomas (p<0.04), using more experienced operators (p<0.003) and decreased laparoscopy use (critical care bed stress only, p<0.001). Patients were also more actively prioritized for resection, with increased importance of co-morbidities and ICU need. The COVID-19 pandemic was associated with severe restrictions in the availability of colorectal cancer services on a global scale, with significant variation in behaviours which cannot be fully accounted for by hospital burden or mortality.

Study on the influence of different carrier gases on the fluidization properties of glass bead and FCC powders

The powder flow was influenced by carrier gas in the unit operation, such as pneumatic conveying and hopper discharge, in the pressurized entrained flow gasification technology. Carrier gases play a vital role in fluidization/ defluidization of powders. In this paper, we investigate the fluidization/ defluidization of absorbent Fluid Catalytic Cracking (FCC) and non-absorbent glass bead with two different carrier gases, N2 and CO2. The flowability of aerated powders with different carrier gases were obtained in the fixed bed region whose superficial gas velocity was lower than minimum fluidization gas velocity and fluidization region. The powder flowability was characterized by basic flow energy, flowability rate index and aeration energy with the help of FT4 powder Rheometer. The influence of different carrier gases on fluidization/ defluidization of powder bed was further discussed. Transition line of A/C powders with different carrier gases was revealed. Effect of gas type and interparticle force were calculated, compared with the flowability of aerated powder bed. In addition, combined with bed stress analysis, different bed fluidization mechanism with different gas velocities were revealed. Our findings might help people understand the influence of carrier gases on the fluidization/ defluidization of powders in the relative chemical unit operations.

The role of initial speed in projectile impacts into light granular media

Projectile impact into a light granular material composed of expanded polypropylene (EPP) particles is investigated systematically with various impact velocities. Experimentally, the trajectory of an intruder moving inside the granular material is monitored with a recently developed non-invasive microwave radar system. Numerically, discrete element simulations together with coarse-graining techniques are employed to address both dynamics of the intruder and response of the granular bed. Our experimental and numerical results of the intruder dynamics agree with each other quantitatively and are in congruent with existing phenomenological model on granular drag. Stepping further, we explore the ‘microscopic’ origin of granular drag through characterizing the response of granular bed, including density, velocity and kinetic stress fields at the mean-field level. In addition, we find that the dynamics of cavity collapse behind the intruder changes significantly when increasing the initial speed . Moreover, the kinetic pressure ahead of the intruder decays exponentially in the co-moving system of the intruder. Its scaling gives rise to a characteristic length scale, which is in the order of intruder size. This finding is in perfect agreement with the long-scale inertial dissipation type that we find in all cases.