Sediment Size Research Articles

WEPP interrill erodibility for clay soils in the crop lands of Northern NSW and Southern Queensland, Australia

Context Water Erosion Prediction Project (WEPP) soil erosion model parameters are rare for cultivated cropping soils in Australia. Aims Measure WEPP interrill erodibility (Ki) for cropping soils. Methods Plots were 50% side-slopes of row-crop furrows. Rainfall was applied for 30 min at 107 mm h−1 under a rainfall simulator, then at higher and lower rainfall intensities for 10 or 6 min. Several of these rainfall sequences were performed with drying between (events). Runoff, sediment concentrations and sediment sizes in runoff and the soil surface after rain and sediment settling velocities were measured. Soils were well-aggregated Vertosols and a Dermosol: clay 31–69%, silt ~20%. Settling velocities and undispersed particle size distributions for rainfall wet soil are provided for a range of soils in Supplemental Data, for use by WEPP users. Key results Runoff during 30 min and 54 mm of rain was 28–44 mm or 50–86% rainfall. Soil losses were 26–61 t ha−1 and sediment concentrations 67–127 g L−1. Infiltration, runoff, sediment concentration and soil losses were sometimes different for soils and events. Gatton and Emerald soils had lower soil losses and Cecilvale, Mywybilla and Narrabri soils had higher soil losses. Conclusions Mean Ki for Emerald and Gatton soils were significantly lower, 2 960 000 and 3 209 600 kg.s m−4, respectively. Ki values were not significantly different for the Cecilvale, Narrabri and Mywybilla soils, 3 900 000 kg.s m−4. Implications WEPP Ki values were like values found for USA cropping on clay soils. Sizes in the soil surface and sediment, and settling velocity distributions, were similar between soils but sediment sizes were finer.

Kinematic parameters of flow constrained by combined dams with through part of tetrahedra in compression region

Every major river in the world consists of mountainous, foothills, and flat areas characterized by different flow regimes. The foothill areas differ in slopes i = 0.001 ÷ 0.004, flow kinetics Fr > 0.15, and the size of sediments. The riverbed is unstable, and the banks are prone to erosion. The construction of coastal protection structures requires solving complex issues related to their design. The analysis showed that most of the studies, including ours, were carried out for the conditions of lowland rivers. This work aims to establish the flow features of the foothill sections of a combined dam with a through part of tetrahedra when the ratio of the through part ℓs to the total length of the dam ℓd is greater than or equal to 0.5, i.e., ℓs/ℓd. ≥ 0.5 The presence of a satellite flow behind the through part, a weakly perturbed core, and the presence of two zones of intense turbulent mixing was experimentally established, and the universality of the velocity distribution, which obeys the theoretical dependence of Schlichting-Abramovich, was confirmed. It is once again confirmed that the dependence is on the slope of the bottom, the Froude number, the degree of constraint, and the formation of "calm" and "critical" flow modes. The nature of the level changes along the length of the compression region in the core, and the satellite flow differ from each other, and the alignments occur in the vertical compression alignment. The problem is implemented for the "calm" mode using an integral relation characterizing the law of conservation of momentum in the flow, the equation of conservation of flow, and the differential equation of uneven motion recorded for the satellite flow behind the through part of the combined dam. The presence of a satellite flow, two zones of intense turbulent mixing, and the different nature of the leveled regime of the main and satellite flows are taken into account. A comparison of theoretical solutions with experimental ones shows their similarity.

Pilot study to explain runout distances of debris flow and immature debris flow considering depositing rate

In Japan, many debris flows and sediment-laden floods cause serious damage to human life and property. Effective measures require high-accuracy reproduction and prediction of runout distance (bed variation reach) via numerical simulations. One possible method to increase the accuracy of numerical simulation results involves reviewing the methods used to evaluate depositing rate for flows that contain many different sizes of sediments. This study performed experiments using an artificial channel and high-speed cameras to identify parameters that govern depositing rate. The channel implemented exhibited a change in slope point; thus, the slope of the upper reach differed from the slope of the lower reach. The experiments showed that the equation representing depositing rate is closely related to flow velocity. Additionally, the equation of depositing rate can be determined if other parameters are consistently considered.

Changing sediment supply during glacial-interglacial intervals in the North Atlantic revealed by particle size characterization and environmental magnetism

The Pliocene-Pleistocene transition is characterized by an abundance of Ice-Rafted Debris (IRD) in the North Atlantic basin. One of the regions affected by IRD during this period is the Gardar Drift, where the DSDP Leg 94 Hole 611A is located. This region received sediments from different sources during the glacial and interglacial intervals (e.g., Iceland and Greenland). We analyzed grain size and particle-size specific magnetic properties of sediments for their provenance characterization between ~2.64 and 2.52 Ma. Our results show that major proportion of bulk sediments during both glacial and interglacial periods were made up of basaltic-rich Icelandic sediments, whereas only during more intense glacial periods (Marine Isotope Stages 100 and 104), a small proportion of non-basaltic sand compositions were identified, possibly sourced from Greenland and other non-basaltic provenances. The non-basaltic sand fractions during the intense glacial periods were likely supplied as IRDs. In addition, a new level of coarse lithics (38 pcs. of >1 mm) composed of different rocks types (e.g., basalt, granite, granodiorite etc.) were identified in DSDP 611A Hole during the end of glacial MIS 104. The coarse lithic fragments showed distinctive magnetic properties than rest of the particle sizes and were classified as Iceberg-Rafted Debris (IBRD). Our results show that higher sand percentage was found during the intense glacial episodes, and their magnetic grain size analysis could help in distinguishing their provenance. We elaborate that particle size specific magnetic measurements of sand fractions could help in rapidly characterizing the glacial episodes in the subpolar North Atlantic.

A possible direct recycling of dredged sediments from the Usumacinta River (Mexico) into fired bricks

<abstract> <p>Reuse of dredged sediments is an effective approach to waste management. This study focuses on the reuse of Usumacinta River dredged sediments in fired bricks. Physico-chemical characteristics of sediments were investigated for their reuse. The grain size of sediments shows that Usumacinta sediments have a sandy texture with low organic matter. The presence of contaminants in these sediments is negligible. Suitability for bricks was observed with a clay workability chart, Winkler, and Augustinik diagram. Bricks were molded into cubic and prismatic brick specimens of size 20 × 20 × 20 mm<sup>3</sup> and 15 × 15 × 60 mm<sup>3</sup> for compressive and tensile strength. The molding moisture content of sediments was defined with the Sembenelli diagram. Bricks were dried at 60 ℃ and fired at a temperature of 700 to 1100 ℃. Linear shrinkage and density of Usumacinta sediments bricks increase with increasing temperature. Clayey sediments (T2 and J4) show higher shrinkage on drying. Testing of bricks shows their compressive strength varies between 0.10 to 19.38 MPa and the indirect tensile strength varies from 0.17 to 12.82 MPa. T2 sediment bricks have the lowest strength due higher percentage of sand. The compressive strength of bricks from T5 and J4 is comparatively higher and satisfies the strength requirements of bricks at a moderate temperature of 850 ℃.</p> </abstract>

EFFECT OF SEDIMENT SIZE ON SANDBAR MORPHOLOGY IN DIFFERENT COMPACTION CONDITIONS

EFFECT OF SEDIMENT SIZE ON SANDBAR MORPHOLOGY IN DIFFERENT COMPACTION CONDITIONS

Local Scour Patterns around a Bridge Pier with Cable-Wrapping

The performance of cable flow-altering bed scour countermeasures was experimentally evaluated based on the scour reduction, bed morphology, and the effects on the flow field. An unprotected 40 mm diameter pier was compared to piers protected with spiral cables (2, 4, 6, 8 and 10 mm diameters) wrapped at a 15-degree angle for two-bed sediment sizes with median grain sizes of 0.86 and 1.83 mm, for a cylinder Reynolds number of 7120. The scour depth was reduced by the cables by up to 52 percent compared to the unprotected pier case, a reduction that increased with increasing cable diameter for both sediment beds. Scour depth and sediment deposition varied by sediment size, where the scour hole was up to 45 percent deeper for the finer sediment bed than that of the coarser bed. Velocity and turbulence statistics showed that cables attenuated the flow within the scour hole by diminishing the downflow and horseshoe vortex, whereas in the case of finer sediment, spatially averaged turbulent kinetic energy and Reynolds shear stresses were respectively up to 1.4 and 1.8 times higher for the unprotected pier than the protected pier, resulting in scour depth reduction. The presence of the cable also reduced the vortex shedding frequency in the pier wake as indicated by a Strouhal number of around 0.175. The results demonstrate the potential of cable threading as a flow-altering scour countermeasure to reduce bridge pier scour.

Hydraulic Performance of Inverted Siphons for Irrigation Water Supply Using Physical Modeling

In this paper, we investigated the performance of the Rambatan siphonsin IndramayuRegency, Indonesia, via physical modeling. The model was built in the laboratory with a scale of 1:10, consisting of two siphons, each with a diameter of 0.3 m and a length of 14 m for the model scale. Several scenarios were considered for the physical modeling, of which the focus was on analyzing the discharge capacity, sediment flushing, flow pattern, and pressure. Using the (prototype) design discharge of 6.36 m3/s and some variations of sediment size for the original configuration of the model, we observed the insufficient hydraulic capacity of the siphons. Several modifications were thus applied to the original design, namely by modifying the upstreamchannel and extending the upper wall of the downstreamchannel. The result showed that the hydraulic performance increased after applying such modification scenarios, where the siphons were able to discharge 6.36 m3/s. In addition, we observed that the flushing pipe diameter of 0.5 m was faster to flush the sediment inside the siphons. The results of this study are useful for the related stakeholders to increase water supply productivity.

Procedures for diatom analyses and hydrodynamic separation of extreme wave paleo-deposits from tropical sediment environments

Diatoms are an important proxy in tsunami and storm research studies to determine potential sediment source and changes in environments pre- and post-event. However, the use of this proxy has proven challenging when studying extreme wave paleo events in sediments from the humid tropics because of the scarcity of diatoms in sand beds, and in clay or silt beds above and below. This problem has been associated with (1) dissolution of diatom valves in the humid tropics, (2) rate of burial and poor preservation of tsunami deposits, and (3) sediment and diatom size. Here we propose a practical solution to this problem – using hydrodynamic separation in the analysis of diatoms of extreme wave paleo-deposits. We compare diatom identification results from selected sand beds and units above and below, collected in a humid tropical environment (Guerrero, Mexico) using a standard procedure and the proposed fast hydrodynamic separation procedure to concentrate diatom-sized fraction. Thanks to the newly proposed methodology, the results showed a significant improvement in the number of observed diatom data in the studied sand units. We discuss two different techniques to achieve this aim and suggest how improvements on the quantitative research of diatoms from extreme wave deposits, particularly tsunamis, from the humid tropics.

Distribution and controlling factors of microplastics in surface sediments of typical deep-sea geomorphological units in the northern South China Sea

Marine microplastics are widely distributed in deep-sea sedimentary environments and are altering sediment compositions and ecological conditions on the seafloor. However, the relation between the distribution of microplastics in deep-sea sediments and the sedimentary dynamic conditions is poorly understood. In this study, we collected surface sediments from some typical geomorphological units (sand dune, sediment drift, and submarine canyon channel/levee) in the northern South China Sea to study composition and distribution of the deep-sea microplastics and their controlling factors. The results show that the microplastic abundance in surface sediments ranges from 19 to 347 p·kg–1, and the identified microplastics consist of 10 types, including dominant polycarbonate (29%), polyethylene (27%), polyester fiber (16%), polyvinyl chloride (13%), and polypropylene (7%), and minor polyethylene terephthalate resin, acrylonitrile-butadiene-styrene, epoxy resin, hydrocarbon resin, and acrylic. The source analysis shows that the deep-sea microplastics may be influenced by riverine inputs from Taiwan and South China. In addition, the microplastic spatial distribution shows that the sand dune and canyon channel contain the highest abundances (136–347 p·kg–1) and more types (4–6 types) of microplastics, which are dominated by relatively high-density polycarbonate or polyvinyl chloride. The canyon levee contains the lowest abundances (19–132 p·kg–1) and less types (1–3 types) of microplastics, which are dominated by relatively low-density polyester fiber or polyethylene. Nevertheless, the microplastic composition of the sediment drift is between those of the canyon channel and the canyon levee. The abundance and polymer type (density) of microplastics all increase with the increased mean grain size of detrital sediments, which represents the progressively enhanced intensity of sedimentary dynamic conditions. We therefore infer that the sedimentary dynamic conditions control the composition and distribution of microplastics in the deep-sea sediments. This study highlights that some deep-sea environments with stronger sedimentary dynamic conditions may accumulate more microplastics, which is of great significance for evaluating the storage and ecological damage of deep-sea microplastics.

Monitoring Beach Shape Development and Sediment Dynamics on a Sandy Beach with Low Anthropogenic Influence

This study examined the evolution of the beach profile and sedimentary balance in Playa del Coco, Mexico, during one year (September 2013–September 2014) to monitor these conditions at a site with little or no anthropogenic influence. The type of beach profile was determined according to the energy profile of the geomorphology, resulting in three types of profiles: low, medium, and high energy. In addition, sediment volumes were quantified, and erosion or accumulation at each study site was described. The results showed that the medium-energy profile was characteristic of the beach most of the time. High-energy conditions were recorded only once due to the influence of a high-category hurricane (Odile, III) during the hurricane season. The dominant sediment size was medium, moderately well-classified to well-classified, coinciding with the medium-energy profile. Coarse, well-sorted sand was recorded at the end of the study, coinciding with the highest loss of sediment on the beach. Playa del Coco showed a state of quasi-equilibrium before the end of the annual cycle, recovering the characteristics of the beach at the beginning of the study. After that time, there was a great loss of sedimentary material influenced by Hurricane Odile (III), modifying the beach’s state of recovery. Therefore, the stability of this beach could be cyclical and respond to the self-organization principle rather than to seasonal parameters. However, the duration of the cycles that influence Playa del Coco seems to be determined by the hurricane season, especially the combination of two main factors: the proximity to the coast of the hurricane and the amount of rainfall associated with the hurricane.

Application of infrared thermal imaging technique in in-situ temperature field measurement of hydrate-bearing sediment under thermal stimulation

Natural gas hydrate is a green energy with great development prospects. The exploitation of natural gas hydrate has been attracting much attention. The temperature field distribution of hydrate-bearing sediment is a crucial indication on the heat or mass transfer characteristics and thermal properties of hydrate-bearing sediment during hydrate exploitation. However, to date, the acquisition means of temperatures of hydrate-bearing sediment have been limited to the discrete temperature measurement by using the contact-type temperature sensors inserted into hydrate sediment. The continuous evolution of temperature field of hydrate-bearing sediment can be hardly achieved. Thus, in this study, a new method applying the non-contact and non-destructive infrared thermal imager was developed and applied in the in-situ measurement of the continuous evolution of temperature field of hydrate-bearing sediment. The results confirm the feasibility of application of the infrared thermal imaging technique in the hydrate-related research. The complete continuous temperature fields of hydrate-bearing sediment and non-hydrate-bearing sediment under thermal stimulation were imaged and compared. The results indicate that the temperature field distribution of hydrate sediment is predominately controlled by the effective thermal conductivity of hydrate-bearing sediment. In addition, the effects of the type of hydrate-bearing sediment, heating power and particle size of sediment were investigated.

Formation of Turbidity Maximum in the Modaomen Estuary of the Pearl River, China: The Roles of Mouth Bar

AbstractAn estuarine turbidity maximum (ETM) functions as a filter and plays a key role in transporting suspended sediment. The features of ETM and their formation mechanisms vary with different types of estuaries. Their responses to morphological factors require further study. In this study, the Modaomen Estuary of the Pearl River was selected to examine the formation mechanisms of the ETM and roles of the mouth bar. Hydrodynamics, suspended sediment concentration, and sediment size distribution were measured via cruise shipboard in the mouth bar zone during the dry season of 2020, which indicated that the ETM occurred in the central and western bars. Furthermore, numerical modeling of an idealized estuary based on Delft3D was performed to explore the effects of the existence of a mouth bar on the dynamic structures causing the ETM, including saltwater intrusion, residual longitudinal circulation, and bed sediment resuspension. The results indicated that the existence of a mouth bar contributed to the occurrence of double residual circulations and enhanced bed shear stress on the mouth bar, which favor the ETM formation. In general, two different dynamic regimes were responsible for the ETM formation, that is, sediment trapping by saltwater intrusion and sediment resuspension induced by tidal currents. The mouth bar played a key role in these two mechanisms, including altering the estuarine circulation and bed shear stress. The results contribute to a better elucidation of the ETM formation in estuaries and provide scientific guidelines for estuarine management and engineering.

A Dam Construction Event Recorded by High-Resolution Sedimentary Grain Size in an Outflow-Controlled Lake (Hulun Lake, China)

The distribution of sediment grain size can record past environmental conditions and human activity. In this study, radioisotope of 210Pb and 137Cs and a grain size of a 41 cm core in Hulun Lake were applied to reconstruct the high-resolution sedimentation history. The profiles of the grain size of the lake sediments show that silt (4–63 µm) was the largest contribution with an average content of 84.05%, and the second largest contribution was sand (>63 µm) with an average content of 15.68%. The median grain size and the mean grain size in the whole sediment core was 22.39 μm and 36.85 μm, respectively. Correlations of the sedimentological variables with instrumental measurements were also analyzed. The peak–trough value of the mean grain size of the sediments in Hulun Lake can reflect the magnitude of rainfall intensity and river discharge. The clay and silt contents at a depth of approximately 32–38 cm was different from other depths throughout the core, which showed continuous maxima with an average content of 0.35% and 94.08%. These changes in grain size correspond to the period of dam construction in 1963–1970. Therefore, the sediment grain size of Hulun Lake effectively recorded the dam-building activity.

Structure and Flocculation of Ion Associates of Carrageenan and Poly(diallyldimethylammonium chloride) Depending on the Component Ratio.

Carrageenan is a polysaccharide of a plant origin, commonly used as a thickening and gelling agent in the food, pharmaceutical, and cosmetic industries. Due to the negative charges of its sulfate groups, carrageenan macromolecules strongly interact with oppositely charged polyions. The ionic complexes of carrageenan with poly(diallyldimethylammonium chloride) were obtained at the molar ratios 4:1, 2;1, 1:1, 1:2, and 1:4. The structure and characteristics of the polyanion-polycation associates were studied by XRD, IR, optical microscopy, and via sedimentation and particle size measurements. It was found that the suspended particles flocculate and settle fastest when the molar ratio of the polyions is near 1:1. Turbidimetric titration experiments enabled us to measure the molar ratio of cationic to anionic groups at the onset of flocculation, and the value in question was found to be 1:1.32. In other words, a mass of 511 mg carrageenan corresponds to one millimole of ester sulfate (monobasic) groups. The measurement of the onset of flocculation has been employed for the accurate determination of carrageenan in real samples of food products. The color and turbidity of the sample do not interfere with the determination results.

Environmental controls on the distribution of metals in porewater and their diffusion fluxes at the sediment-water interface of the western Pacific

The intermountain basin of the western Pacific is rich in polymetallic nodules with commercial exploitation potentialities. Regarding the potential impacts of metals during deep sea mining operations, the regeneration process of metals with environmental controls and their diffusion fluxes at the sediment water interface (SWI) are of great importance. Our results demonstrated that the deep-sea environment in the western Pacific seafloor was characterized as organic carbon-starved sediments, with a total organic carbon content (TOC) between 0.2%–0.5%. Sedimentary Eh (SEH) ranged from 400 mV to 570 mV, indicating suboxidized to oxidized conditions. The vertical profiles of dissolved metals in porewater showed an enrichment of nodule-related metals and conservative metals (Mo, Li, Rb, Cs) in the uppermost suboxidized zone. They were related to desorption with labile particles, as well as remobilization by the reduction of Mn- and Fe-oxides under suboxidized condition. The diffusion fluxes of those nodule-related metals (Co, Ni, Mn, Pb, Zn) to the overlying water were low and fluctuated within the study area. The relatively high concentrations in the overlying water also confirmed a continuum with equivalent contributions to nodule formation from the bottom water as well as from the pore water. The principal components analysis (PCA) results further indicated that environmental gradients in Eh, TOC, δ13Corg, and mean grain size of sediment as well as spatial gradients in distance from the nearest sea mountain, latitude, and water depth posed a significant impact on the diffusion fluxes of different metals. Despite many uncertainties in sediment disturbance during nodule mining activities, this baseline information supports further estimation of the potential release of metals under sediment disturbance.

Flow-driven soil erosion processes in a calcareous semiarid soil: Rill length and flow rate impacts

Understanding the dynamic mechanisms of rill erosion is vital for accurately modeling of soil erosion. In this research, rill erosion was experimentally studied using different rill lengths. A flume, 11 m long and 0.2 m wide adjusted to 3% slope, was used for constructing rectangular rills of 0.05 m width and six different lengths (1 to 12 m). The experiments were carried out on a clayey calcareous soil from a semiarid region under two flow rates of QL = 0.025 and QH = 0.045 L s−1. Temporal sediment concentrations (C) were measured at the rill outlet for 40 min and the sediment size distribution (PSD) was recorded twice during each test. The common decreasing trend of C was affected by both flow rate and rill length. The formation of head-cuts along the bed of long rills was an important erosion process affecting C and PSD. The compiled data were used to determine the exponential models' parameters of both C and soil detachment rate based on the steady, unsteady, and event-average values. While, the equilibrium sediment transport occurred by increasing rill length beyond 7 m under QL, the tested rills under QH were not sufficiently long for the flow to reach its maximum transport capacity at the gentle slope used and for the calcareous soil tested. Based on the sediment PSD, the selective transport of coarse particles was evidenced. Selectivity transport was affected by rill length reached its maximum at the rill length of 7 m.The findings provides better understanding of rill erosion processes for calcareous soils on gentle slopes in semiarid regions. Further research is needed for better understanding of the rill erosion processes on short gentle slopes to provide experimentally supports for the existing theoretical concepts of rill erosion models.

Transport equation for nonuniform suspended sediment

Theoretical research on nonuniform sediment suspension is limited due to difficulties of elucidating nonuniform particle interaction. A new transport equation is generalized for nonuniform suspended sediment based on multi-dispersed two-phase flow theory, to account for nonuniform particle interaction and its effect on sediment suspension. Instead of implementing numerical discretization, constitutive relation of drift velocity is derived by solving the momentum equations with the aid of the Sherman–Morrison–Woodbury formula and the perturbation asymptotic technique. Applied to open channel flows, concentration profiles for each grain size of sediment are derived and a satisfactory consistency is obtained in comparison with both experimental and field data. Nonuniform sediment suspension is attributed to fluid turbulence, gravity, particle inertia and nonuniform particle interaction. The nonuniform effect plays an important role in the vicinity of the riverbed and is positively correlated to sediment concentration and grain size ratios, which helps bring insight into the suspension mechanics.

Roles of raindrop impact in detachment and transport processes of interrill soil erosion

To date the roles of raindrop impact in sediment entrainment and transport processes of interrill soil erosion are not yet fully quantified. The objectives are to 1) evaluate the effects of raindrop impact on sediment entrainment, 2) systematically quantify the relative importance of raindrop-driven and flow-driven transport, and 3) characterize sediment size distributions in different sediment transport regimes. A loam soil with 48.4% sand and 23% clay was packed into flumes (L × W × H: 1.8 × 0.5 × 0.1 m) and subjected to intensities of 60, 90, 120 mm h−1 at gradients of 5, 10, and 16°. Air filter and tarp covers were used to vary impact energy and flow length. Results show that sediment is largely entrained by raindrop impact and transported by raindrop-impacted flow. Interrill erosion consists of two composite processes: raindrop-induced detachment/entrainment and raindrop-impacted flow transport. The former includes direct detachment by raindrop impact and ‘flow detachment’. The latter includes raindrop-driven and flow-driven transports. The proportions between the two transports vary with slope steepness, slope length, flow depth (or flow discharge), rainfall intensity, and sediment characteristics. Raindrop-driven transport is competent but inefficient, while the opposite is true for flow-driven transport. Because raindrop impact plays dual roles in detaching soil (and/or entraining sediment) and enhancing sediment transport, a drop energy factor must be included in interrill erosion models to adequately simulate the dual roles. More studies are required to derive a drop energy function that takes into consideration the rainfall energy dissipation by canopies of various plants under natural conditions.

Experimental study of sediment wash-off process over urban road and its dependence on particle size distribution.

Sediment originating from the urban road runoff is a main contributor to water pollution in urban areas. The size of the road sediment varies significantly, but its influence on sediment wash-off process has not been well investigated. In this study, sediments with different particle size distributions have been used in rainfall-runoff experiments over idealized urban road surface. The results show that, under the same experimental conditions, the capacity factor CF increases with the decrease of the median particle diameter D50, which is the dominant influencing factor on CF. The wash-off coefficient k is affected by both D50 and the grading of sediment. During the wash-off process, D50 of the sediment collected at the outlet increases with time. Such a grain coarsening phenomenon is particularly apparent when the road is originally covered with very fine sediments. Furthermore, the presence of coarse grains slows down the transport of fine sediment whose size is smaller than 14 μm. This shielding effect significantly affects the sediment wash-off process in the early stage of a rainfall event, while later on the interaction between particles of different sizes becomes unimportant. This study advances the understanding of sediment wash-off mechanism on urban road surface.