Sediment Size Characteristics Research Articles

Spatial variability of surface sediments in the Malacca strait and its implications for sedimentary environments

The Malacca Strait is not only a part of shelf deposition but also a crucial conduit connecting the continental shelf to the deep ocean. Here, we present the grain size characteristics of sediments in the Malacca Strait and regional hydrographic data obtained from global models to delineate its depositional configurations and further identify its sedimentary environments. The results reveal that the residual sediments are concentrated in the central region of the strait, whereas the modern terrigenous sediments are distributed along the coast of Malaysia and the broad northern region of the strait. The central region of MS, ProvinceⅠ, has the coarsest sediments with moderate sorting, indicating a residual sedimentary environment where the sands are residual paleo-river deposits. There are poorly sorted silty sands in ProvinceⅡ, which display finer mean grain size than that of ProvinceⅠ. It suggests a reworked sedimentary environment with variations of sediment sources between the southern and northern regions. It is a palimpsest of the paleo-deltaic deposits in the southern region, which have been intensely altered by modern high-energy hydrodynamic conditions. In terms of the northern part of ProvinceⅡ, it has modern reworked deposits shaped by wave, tide, and monsoon currents. The finer sediments, which correspond to the distribution of higher silt content, are supplied by the surrounding land in ProvinceⅢ indicating a modern environment primarily influenced by seasonally varying sedimentary dynamics, such as monsoon currents, well-developed eddies, and water masses. We thus propose that the depositional configurations in the Malacca Strait are confined by depositional dynamic environments, which restrict modern terrigenous sediments distributed along the coast and scarcely transported into the central region of the strait which exposes residual deposits. This study highlights the importance of both modern and relict sediments in the strait and our findings are important to better understanding modern strait sedimentary systems with high tidal-current-energy.

Non-linear response of sediment size characteristics and associated transport patterns to soil structural stability in sheet erosion under field rainfall simulation

Sediment transport patterns (suspension, saltation and rolling) not only depend on flow hydrodynamic characteristics, but also on sediment properties, both of which are largely influenced by soil structural stability. However, how sediment sizes and transport patterns response to original soils over a wide range of structural stability remains unclear. Herein, field plot rainfall simulation experiments on five soils along an increased gradient of structural stability were conducted at two rainfall intensities of 45 and 90 mm h−1 to investigate effective sediment size characteristics and associated transport patterns in sheet erosion. Differing from runoff and sediment yield, sediment sizes were most influenced by soil structural stability (F = 386.9) rather than rainfall intensity (F = 246.7, p < 0.001). Mean weight diameter of sediment (0.16–0.84 mm) and the proportion of coarse particles (>0.25 mm) exhibited a unimodal variation with increased soil structural stability. The effective sediment size distribution shifted from unimodal with the peak at < 0.10 mm for low and high stable soils to bimodal with two peaks at < 0.10 mm and 0.5–1 mm for medium stable soils. Correspondingly, suspension-saltation (>74 %) dominated sediment transport for low and high stable soils, which was determined by fine sized fragments from aggregate breakdown and small flow transport capacity, respectively; while both suspension-saltation (<0.10 mm) and rolling (>0.25 mm) jointly contributed to sediment transport for medium stable soils, which was controlled by the interaction between fragment size and flow transport capacity. Moreover, the positive effect of rainfall intensity on sediment sizes was more remarkable for the medium than for the low and high stable soils. A conceptual model of sediment transport mechanisms in response to soil structural stability is firstly proposed, which merits consideration in the future development of erosion modeling and soil conservation techniques.

Evolution of the groundwater flow system driven by the sedimentary environment since the Last Glacial Maximum in the central Yangtze River Basin

• Fluctuations in the Yangtze River and sedimentary environment control GFS evolution. • Sedimentary evolution caused unconformable distribution of groundwater age in a GFS. • Continuous uplift of discharge base level favors preservation of ancient groundwater. • Holocene fine-grained sedimentary strata led to the stagnation of regional GFS. Understanding the circulation and evolution of groundwater and its control mechanism in the Jianghan Plain, which exhibits the complex sedimentary evolution and a progressively deteriorating groundwater environment, is highly imperative and challenging to promote the practical application of groundwater flow system (GFS) and protect the groundwater resources in the region. The aim of this work was to elucidate the evolution pattern of the GFS in the Jianghan Plain and its underlying mechanism. To this end, the grain size characteristics of sediments, geochronological information, stable isotopes in clay porewater, paleoclimate indicators, and existing groundwater age were analyzed to reveal the sedimentary environment of aquifer systems at the basin scale in the study area. It was found that the sedimentary environment changed from one of deep downcutting during the Last Glacial Maximum (LGM) to a fluvial facies that was rapidly infilled with coarse-grained sediments during the last deglaciation period (LDP), and then to a stable lacustrine facies with fine-grained sediments during the Holocene warm period (HWP). These changes were closely linked to the fluctuations of the Yangtze River. Consequently, the existing GFS pattern in the basin presents an unconformable distribution of groundwater age, indicating that it results from the temporal superposition of groundwater flow controlled by the historical sedimentary environment. Dramatic sea level fluctuations since the LGM have resulted in significant changes in the water level of the Yangtze River. This process has impacted the driving force of groundwater, resulting in the evolution of the GFS from a pattern of a fully developed regional GFS during the LGM to the present pattern of a multi-hierarchy nested GFS.

Particle size characteristics of sediment by sheet erosion and their responses to related parameters on a Loess hillslope: a plot-scale study

Abstract Particle size distribution (PSD) of sediment by sheet erosion not only indicates erosion behaviors, but also provides a basis for potential nutrient losses. PSDs are significantly influenced by topographic, rainfall, and associate runoff characteristics, such as rainfall intensity (I), rainfall kinetic energy (KE), slope gradient (S), runoff velocity (v), and stream power (Ω), while their effects have not been fully understood so far. To fill this research gap, in this study, simulated rainfall experiments were conducted to investigate the characteristics of effective and ultimate PSDs of sediments, as well as their responses to related parameters, under five levels of I and S. The results showed that (1) for effective PSDs, there was a significant enrichment of clay-sized and silt-sized fractions (P&amp;lt;2μm and P2–50μm) in sediment (percentage range: 12.11–20.64% and 57.09–65.94%), whereas there was a depletion of sand-sized fractions (P50–250μm and P250–2000μm). The ultimate PSDs of sediment were, however, similar to the soil matrix under experimental conditions. (2) With an increase in I and S, P&amp;lt;2μm and P2–50μm decreased exponentially or logarithmically, and P50–250μm and P&amp;gt;250μm increased correspondingly. The contribution rates (PCs) of the effect of I on clay-, silt-, fine-sand-, and coarse-sand-sized fractions in sediment were 23.44, 35.45, 29.09, and 56.82%, respectively, whereas the PCs of that of S were 56.54, 24.42, 59.89, and 31.47%, respectively, indicating that S plays a more pivotal role in influencing P&amp;lt;2μm and P50–250μm, whereas P2–50μm and P&amp;gt;250μm were more sensitive to I than to S. (3) The aggregation ratio (AR) for the fractions of different sizes indicated that clay-sized particles tend to be transported as aggregates, while silt-sized particles tend to be transported as primary particles. A higher I or steeper S leads to a larger proportion of particles being transported as aggregates. (4) KE, Ω, and unit stream power (U) were the most correlated parameters influencing effective PSDs. Equations were finally established using KE, Ω, and U to predict the effective PSDs of sediment by sheet erosion. This study can further the understanding of the sheet erosion process and provide a scientific basis for the establishment of a sheet erosion model.

Grain size characteristics of surface sediment and its response to the dynamic sedimentary environment in Qiantang Estuary, China

The composition and grain size characteristics of sediment in estuarine and coastal environments provide important information on the material source, hydrodynamic environment, environmental events, etc. However, few studies have focused on the sediment characteristics of the entire Qiantang Estuary, particularly, on the correlation between hydrodynamics and sediment properties. Through systematic sampling of the surface sediment in a large area of the Qiantang Estuary, the spatial distribution characteristics of the surface sediment of the Qiantang Estuary, such as the grain size composition and particle size parameters, in different sections of the estuary area are analyzed. By comparing the particle size data of different estuaries, the similarities and differences of the particle size parameters of the Qiantang Estuary and other estuaries are compared and analyzed. According to the differences in the sediment composition, the distribution status of the particle size frequency curves and the morphological characteristics of the probability curves in the entire study area, the Qiantang Estuary is divided into 7 dynamic depositional areas: Fuchunjiang Hydropower Station–Fuyang, Fuyang–Qibao, Qibao–Cao'e River, Cao'e River–Zhapu, North Shore of Hangzhou Bay, middle part of Hangzhou Bay, and South Shore of Hangzhou Bay, which are different from the previous estuary segmentation system. The study area is divided into 13 dynamic depositional sub-regions using the Pejrup M triangle diagram. Combining the calculation results of the mathematical model and the Pejrup M triangle diagram, the relation between the dynamic deposition environment and the bottom shear stress is discussed. It is found that the spatial distribution of the bottom shear stress in the study area is in good agreement with the zoning of the dynamic sedimentary environment.

Runoff Characteristics and Soil Loss Mechanism in the Weathered Granite Area under Simulated Rainfall

Soils developed from the parent materials of highly weathered granite are particularly susceptible to soil erosion. Therefore, it is of great significance to conduct in-depth research on runoff characteristics and soil loss mechanisms in weathered granite areas. Using the weathered granite area in the hilly region of southeastern China as the research object, we conducted indoor artificial rainfall simulation experiments involving three slope steepnesses (SSs), 8°, 15°, and 25°, and five rainfall intensities (RIs), 0.5, 1.0, 1.5, 2.0, and 2.5 mm/min. The results showed that sediment load (SL) has positively linear relationships with mean runoff velocity (V), Reynolds number (Re), Froude number (Fr), shear stress (τ), and stream power (w). The eroded sediment was principally composed of silt and clay that accounted for 65.41–73.41% of the total SL. There was a boundary point at 0.02 mm for the particle size distribution (PSD) of the eroded sediment. The enrichment ratio (Er) of sand-grained particles (0.02–2 mm) ranged from approximately 0.45 to 0.65, while the Er of fine-grained particles (&lt;0.02 mm) ranged from approximately 1.37 to 1.80. These results increase our understanding of the relationships among RI, SS, runoff, and soil losses from weathered granite hillslopes, particularly the relationships between different hydraulic parameters and sediment size characteristics.

Grain Size Characteristics of Sediments Found in Typical Landscapes in the Playa of Ebinur Lake, Arid Central Asia

A playa usually refers to a salt desert landscape mainly composed of loose and fine lacustrine sediments. Severe wind erosion on a playa causes the playa to become a source of dust and salt dust and poses a threat to vast areas downwind. Currently, little is known about the impact of wind erosion on the particle size distribution of sediments in different landscapes in the playa. In the present study, six dominant different landscapes in a natural state with the same sedimentary environment in the playa of Ebinur Lake were selected to provide insights into the different characteristics of particle size distribution under the effect of long-term wind erosion. The results reveal that the grain-size composition clearly differed among different landscapes. All samples had a common dominant size group consisting of very fine sand and sand. The very fine sand and sand content of Haloxylon ammodendron desert zone (LS5) was the lowest, while the clay and silt content was the highest at both depths among the six landscapes. The lowest clay and silt fraction and highest sand fraction appeared in the herbal desert zone (LS3) at both depths. Almost all of the sediment samples were of a bimodal distribution mode, with significant differences. The cumulative curve showed a similar S-shape, while the probability cumulative curve showed an inverted S-shape with three subpopulations of granularity characteristics. The smallest mean particle diameter appeared in LS5. The majority of the sediments were moderately to poorly sorted. The mean particle size of the sediments from the six landscapes was significantly different (p &lt; 0.05), while no significant difference was observed among the other three parameters. Generally, it can be inferred that LS5 can reduce wind speed effectively, probably due to the smaller leaves and dense branches of Haloxylon ammodendron, which results in a high level of coverage. The results of the present study will have some implications for the grain size characteristics for changes in intensity in regional wind erosion environment and will also have some basis for wind erosion prevention and control in the playa of Ebinur Lake.

Anthropogenic sediment traps and network dislocation in a lowland UK river

AbstractFarm ponds, reservoirs and in‐stream weirs exist in most lowland UK river catchments and often dominate natural features such as lakes, wetlands, floodplains and debris dams. Artificial structures have served multiple purposes, including provision of power for historic flour milling and iron ore crushing and provision of water for medieval fishponds, canals, crop irrigation and potable supply. Although unintentional, they can significantly affect longitudinal connectivity, including sediment delivery pathways, through river catchments.We report results from three spatially nested case studies that were undertaken in the Rother catchment, ranging in scale from small farm ponds of a few square metres in area, to larger in‐stream weirs and reservoirs (locally called ponds). Reservoirs typically trap sediment, decreasing sediment availability downstream, while inducing valley sediment accumulation upstream. We focus on the quantity and particle size characteristics of sediment trapped behind these structures compared to catchment soils and to sediments that are transported through, and deposited in, ‘natural’ gravel‐bed reaches.At all scales our results demonstrate that sediment trapping and release are particle size specific. Fine to coarse sands (125 μm to 2 mm diameter) and coarser sediments are retained behind structures at all scales, while silts and clays (&lt;63 μm diameter) and organic matter are generally depleted in the stored sediment. Even though 75% of the surveyed reservoirs have very low estimated trap efficiencies (&lt;5%), they slowly fill over time with sediment.An important management question relates to the likely benefits of impoundment, structure or sediment removal, and whether fine (here defined as &lt;63 μm) or coarser (&gt;63 μm) sediment is a priority for management.

Size Characteristics of Sediments Eroded under Different Masson Pine Litter Covers in South China

The particle size distribution characteristics of runoff sediments are vital for understanding the effect of the mechanism of soil erosion on slopes. The objective of this study was to investigate the particle-size distribution of sediments eroded from slopes covered by different litter coverage masses under artificial rainfall simulation. Litter was spread on the surface of a soil tank according to different biomasses (0 g·m−2, 100 g·m−2, 200 g·m−2 and 400 g·m−2). The mean weight diameter (MWD), fractal dimension (D) and enrichment ratio (ER) are characteristic parameters of sediment particle size. The MWD and D were more sensitive to soil erosion and had a significant negative correlation with the slope angle and rainfall intensity. The performance of the MWD on the slope (5°) was less than the MWD on the slope (10°). The relationship between eroded sediment distribution characteristic parameters and the litter coverage mass had a significant influence on the content of coarse particles. The content of fine particles accelerated, decreased and then stabilized, whereas coarse particles increased first and then stabilized. The litter diameter and surface area were the main parameters that affected the MWD and D. Under different rain intensities and slopes, the ER varied inconsistently with litter coverage mass. Coarse particles were eroded easily and selectively, and soil erosion had no sorting effect on fine particles. These findings support the quantitative study of the relationship between the amount of litter coverage mass and the particle size of soil sediments.

Effects of erosion-induced land degradation on effective sediment size characteristics in sheet erosion

Land degradation by erosion alters soil properties to a varying extent and definitely influences the subsequent soil erosion. However, little is known about the impact of erosion-induced land degradation on the erosion process from the aspect of sediment size characteristics, which may not only indicate aggregate breakdown and sediment transport mechanism, but also influence sediment deposition. Here, the effective sediment size distribution (undispersed) and its dynamics during sheet erosion on different eroded lands (non, moderately and very-severely eroded) were studied under field rainfall simulation on five types of soils (Calcic Luvisols, Ferric Luvisols, Plinthic Alisols, Plinthic Acrisols and Acric Ferralsols) formed separately on loess deposits, quaternary red clays and basalt from temperate to tropical climate. The rainfalls at the intensities of 45 and 90 mm h−1 were simulated on pre-wetted bare fallow plots (3 × 0.8 m2, 10° slope). The effect of erosion degree on the effective sediment size showed a unimodal trend from temperate Luvisols to tropic Ferralsols, coupled with the influence of rainfall intensity to a varying extent. Under the low-intensity rainfall, except for Ferralsol, the moderate erosion degree increased the sediment size by −49% to 72% relative to non-eroded lands, in contrast to a decrease of 6–64% for the very-severe erosion degree. The high-intensity rainfall was shown to lessen the erosion degree effect for Luvisols, magnify it for Alisol, and change it to positive for Acrisol. Sediment transport was dominated by suspension-saltation of <0.10 mm size fraction for Luvisols regardless of erosion degree due to the weak soil structure, and shifted from both rolling (0.5–2 mm) and suspension-saltation on the non/moderately eroded lands to only suspension-saltation on the very-severely eroded lands for the other soil types. Rainfall intensity, pH and bulk density contributed jointly to sediment size (Adj-R2 = 0.76, p < 0.001), and exchangeable sodium showed a positive effect on its temporal variation (Adj-R2 = 0.36, p < 0.001). The varying effects of land degradation by erosion on sediment size and transport mechanism at the soil pedon and regional scales merit consideration in land management and rehabilitation as well as erosion modeling.

Dramatic typhoon-induced variability in the grain size characteristics of sediments at a meso-macrotidal beach

Knowledge of sediment grain size characteristics is important for a better understanding of the morphodynamic behavior of beaches worldwide. In this paper, dramatic changes in the grain size of sediments at Yintan Beach, South China in response to Typhoon Rammasun (tropical storm no. 1409) were analyzed on the basis of the collected sediments, beach elevation, and hydrological data from June 1 to July 31. Variations in beach geomorphology were interpreted using the digital elevation taken by LiDAR technology, while changes in sediment grain size were detected by sediment parameters and associated sediment components. The results show that the sediments of Yintan Beach were nearly pure sand before Rammasun and became coarser and slightly better sorted with a higher negative skew after Rammasun. The sediment grain size pattern shifted from a high content (39%) of very fine sand in the lower intertidal zone pre-Rammasun to a high content (76%) of fine sand in the swash zone post-Rammasun. During Rammasun, wave action was responsible for the main transition of grain size pattern in the lower intertidal and mid-tidal zones, and tidal level and beach morphology could affect the grain size distribution of the sandbar and trough in the swash zone. In addition, the increase in the coarse-grained fraction in the upper beach and berm was primarily due to wind action.

A palaeoenvironmental study of particle size‐specific connectivity—New insights and implications from the West Sussex Rother Catchment, United Kingdom

AbstractConnectivity has become an important conceptual and practical framework for understanding and managing sediment transfers across hillslopes, between hillslopes and rivers, and between rivers and other compartments along the river corridor (e.g., reservoirs, channel substrate, and floodplain). Conventionally, connectivity focuses on the quantity of sediment transferred but here, we also consider the size of the finer sediment (typically particles &lt;500 μm diameter). We examine the role of small rapidly silting reservoirs in the river Rother on storing sediment and disrupting downstream sediment transfers. Spatial and temporal changes in the particle size characteristics of sediment deposited in one of the ponds is explored in detail. Downstream of this pond, we collected sediment from the river on nine occasions over 17 months using two sampling methods at two locations; first, immediately downstream of the pond and a second ~700 m further downstream but upstream of the confluence with the Rother. Results showed a significant depletion in sand‐sized particles immediately downstream of the pond but the sand had been recovered from an in‐channel source before the river reached the downstream sampling point.

Relationship between grain size distribution and intertidal faunal abundance in selected stations of Someshwara, Panambur and Bengre beaches

The knowledge of sediment texture is important in the studies of distribution of living and non-living resources, identification of source of sediments, sediment patterns and nutrients. It is known that the macro-benthos in the aquatic environment are influenced by the texture of the sediment in which they establish themselves and live. Sediment quality guidelines are important tool for assessment of contamination in marine and estuarine sediments with biological signi?cance of intertidal beaches. The grain size characteristics of sediments from aggrading and receding in beaches have been analyzed by several investigators, only very limited research has been done on the relationship between the intertidal faunal abundance, distribution and the sediment grain size distributions from eroding and accreting beaches of Mangaluru. Sediment variations mainly affect the species distribution, pattern of diversity, breeding, survival and other activities. The present study was carried out to determine the grain size distribution and species abundance in coastal Waters of Someshwara, Panambur and Bengre beaches along the Mangaluru, south west coast of India. The sampling of coastal water and sediment was carried out from August 2016 to July 2017 along the Mangaluru coast was studied. Spatial and temporal variations of the grain size distribution like granules, coarse sand, fine sand, and very fine sand textural components like sand, silt, clay and organic carbon percentages analyzed and correlated with intertidal species distribution, abundance in the selected stations.

Rainfall-induced Soil Erosion and Sediment Sizes of a Residual Soil under 1D and 2D Rainfall Experiments

Raindrop impact and surface flow trigger the downstream movement of soil particles by the processes of rainfall-induced soil erosion. A set of laboratory simulated rainfall experiments was carried out to study soil loss and size characteristics of discharged sediments of a soil under a rainfall intensity of 70mm/h, controlled initial soil suctions and moistures. The rainfall simulation was instrumented with tensiometers and moisture sensors. A new device capable of deriving impact force, velocity, and kinetic energy of a falling waterdrop was developed. Sediment sizes in runoff were characterized by a laser particle size analyzer in order to correlate with the properties of rainfall. 1D simulated rainfall experiments were also employed to study soil detachment and soil susceptibility to rainfall under both saturated and unsaturated soil conditions. The processes of soil erosion and outflow size characteristics of sediments relating to effect of suction were discussed. The proposed set of experiments will be a viable tool for measuring soil loss, sediment runoff, and sediment sizes discharged from a farmland pertaining to properties of rain, soil and flow.

Size characteristics of sediments eroded from three soils in China under natural rainfall

Purpose The particle-size distribution of runoff sediment is important in understanding, characterizing and modeling the transport behavior of sediment and sediment-associated chemicals. The objective of this study was to investigate the particle-size distribution of sediments eroded from three soils in China under natural rainfall.

Beyond competition: the stress‐gradient hypothesis tested in plant–herbivore interactions

The stress-gradient hypothesis predicts that interactions among plants are context dependent, shifting from facilitation to competition as environmental stress decreases. Although restricted to facilitation/competition, the mechanistic model behind the hypothesis is easily modified to include other negative interactions that are as important as competition in structuring natural communities, e.g., herbivory. To evaluate this hypothesis we experimentally tested if the balance between the facilitative and trophic effect of an intertidal, burrowing, herbivorous crab in marsh plants is context dependent and shifts from positive to negative as stress decreases. By sampling salt marshes differing in sediment size characteristics, we show that sites with larger sediment particle size had less stressful oxygen levels than sites with fine sediment particles, and that the level of stress was reduced by the presence of crab burrows. We then conducted a factorial experiment manipulating sediment size and crab presence. Results show that, by decreasing soil anoxic stress, crabs increase plant growth in stressful zones, but their ecological importance as herbivores increases in more benign zones. Our findings suggest that the balance between positive and negative interactions along stress gradients is more important than previously perceived and also applies to facilitation and herbivory between animals and plants.

Short‐term morphological change and sediment dynamics in the intertidal zone of a macrotidal beach

AbstractThis paper describes the morphological and sedimentological evolution of a macrotidal beach over a 20 day period under varying hydrodynamic conditions (significant breaker heights of 0·3–2 m and tidal ranges of 2–5 m). During the field campaign, an intertidal bar developed around the mid‐tide level, migrated onshore, welded to the upper beach and was then flattened under energetic wave conditions. The bar had a wave breakpoint origin and its formation was triggered by a reduction in tidal range, causing more stationary water‐level conditions, rather than an increase in wave height. Most of the onshore bar migration took place while the bar was positioned in the inner to mid‐surf zone position, such that the bar moved away from the breakpoint and exhibited ‘divergent’ behaviour. The depth of disturbance over individual tidal cycles was 10–20% of the breaker height. Such values are more typical of steep reflective beaches, than gently sloping, dissipative beaches, and are considered to reflect the maximum height of wave‐generated ripples. The grain size distribution of surficial sediments did not vary consistently across the beach profile and temporal changes in the sedimentology were mostly unrelated to the morphological response. The lack of clear links between beach morphology and sedimentology may be in part due to shortcomings in the sampling methodology, which ignored the vertical variability in the sediment size characteristics across the active layer.

Soil detachment and transport on field‐ and laboratory‐scale interrill areas: erosion processes and the size‐selectivity of eroded sediment

AbstractField‐ and laboratory‐scale rainfall simulation experiments were carried out in an investigation of the temporal variability of erosion processes on interrill areas, and the effects of such variation upon sediment size characteristics. Poorly aggregated sandy soils from the semi‐arid environment of Senegal, West Africa, were used on both a 40 m2 field plot and a 0·25 m2 laboratory plot; rainfall intensity for all experiments was 70 mm h−1 with a duration of 1 to 2 hours. Time‐series measurements were made of the quantity and the size distribution of eroded material: these permitted an estimate of the changing temporal balance between the main erosion processes (splash and wash). Results from both spatial scales showed a similar temporal pattern of runoff generation and sediment concentration. For both spatial scales, the dominant erosional process was detachment by raindrops; this resulted in a dynamic evolution of the soil surface under raindrop impact, with the rapid formation of a sieving crust followed by an erosion crust. However, a clear difference was observed between the two scales regarding the size of particles detached by both splash and wash. While all measured values were lower than the mean weight diameter (MWD) value of the original soil (mean 0·32 mm), demonstrating the size‐selective nature of wash and splash processes, the MWD values of washed and splashed particles at the field scale ranged from 0·08 to 0·16 mm and from 0·12 to 0·30 mm respectively, whereas the MWD values of washed and splashed particles at the laboratory scale ranged from 0·13 to 0·29 mm and from 0·21 to 0·32 mm respectively. Thus only at the field scale were the soil particles detached by splash notably coarser than those transported by wash. This suggests a transport‐limited erosion process at the field scale. Differences were also observed between the dynamics of the soil loss by wash at the two scales, since results showed wider scatter in the field compared to the laboratory experiments. This scatter is probably related to the change in soil surface characteristics due to the size‐selectivity of the erosion processes at this spatial scale. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Chemical and physical characteristics of iron oxides in riverine and glacial meltwater sediments

The speciation and grain size distribution of iron have been examined in several riverine and glacial sediments, in order to evaluate controls on the concentrations and modes of occurrence of iron oxides in weathering-derived particulates. Three operationally defined fractions have been recognised, relating to the reactivity of iron towards dissolved sulphide; (1) highly reactive iron (Fe HR), consisting of ferric oxides, (2) poorly reactive iron (Fe PR), consisting of magnetite and some sheet silicate iron, and (3) unreactive residual silicate iron (Fe U). The finer fractions of each riverine and glacial suite are enriched in Fe HR, and usually also have higher Fe HR/FeT ratios. However, coarser fractions may also have elevated Fe HR contents, and relationships of Fe HR to both surface area and Al indicate that iron oxides dominantly occur in association with the surfaces of aluminosilicate minerals, which are common in finer grained fractions but may also be relatively enriched in coarser fractions. A similar range of Fe HR surface loadings is evident in the riverine and glacial meltwater suites, from 0.6 to 1.8 mg Fe m −2 for the riverine sediments, and 0.5 to 1.5 mg Fe m −2 for the glacial meltwater sediments, despite wide variations in catchment/bedrock lithology and climatic conditions. Significantly lower iron oxide loadings (0.1 mg Fe m 2) were found in subglacial sediments. Most Fe HR is closely associated with organic C, although the latter occurs as blebs or smears rather than continuous coatings (e.g. Ransom et al., 1997 [Ransom, B., Bennett, R.H., Baerwald, R., Shea, K., 1997. TEM study of in situ organic matter on continental margins: occurrence and the ‘monolayer’ hypothesis. Mar. Geol., 138, 1–9.]). Electron microscope observations show that iron oxides may occur as nanoparticulate (10–20 nm) spheres at the edges of sheet aluminosilicate minerals, as crystalline oxides infilling surface pits in coarser-grained aluminosilicate minerals, and occasionally as discrete crystalline oxides. The similar occurrences of iron oxides in both riverine and glacial environments indicates that the delivery of reactive iron oxides to the oceans during glacial–interglacial cycles will be affected by the masses and grain size characteristics of sediments delivered by glacial and riverine fluxes, rather than by differences in iron mineralogy. These delivery rates will be modified by behaviour of the oxide nanoparticles during transport from the continents.

Sediment size characteristics of urban drains in Malaysian cities

The existing conventional drainage systems in cities throughout Malaysia have been built mainly to cater for the increase in surface runoff due to rapid development that occurs in the city borders. The drainage system is normally made up of open rigid concrete drain channels susceptible to maintenance problems such as sediment deposition, litter and utility pipes crossing. This paper highlights the results of surveys carried out to determine the sediment size characteristics in urban drains of five cities in Malaysia. A preliminary result from a sediment profile observation at Sungai Raja drainage system, Alor Setar is also described to establish the effect of drain size and slope on deposition trend.