Particle size characteristics of suspended sediment in hillslope runoff and stream flow

This study examines the particle size characteristics of hillslope soils and fluvial suspended sediments in an agricultural catchment. Samples of surface runoff and stream flow were collected periodically and analysed for the size distributions of the effective (undispersed) sediment. This sediment was subsequently dispersed and the ultimate size distributions determined. The median effective particle size of stream suspended sediment was considerably coarser than the median ultimate particle size, indicating that most of the load included a substantial proportion of aggregates. Moreover, the proportion of fine material (i.e. silt and clay) increased, and the proportion of sand-sized material decreased, with increasing discharge. This decrease in sediment size with increased flow, which is contrary to the traditional assumption of a positive discharge/particle size relationship, is thought to reflect: (i) the influx of silt and clay, predominantly the former, originating on the catchment slopes and brought to the stream by overland flow along vehicle wheelings, roads and tracks; and (ii) erosion of fine material from the channel bed and banks. During large storms, however, the proportion of sand-sized sediment increased during the rising limb of the hydrograph, as a result of the entrainment of coarser source material from the valley floor during overbank flooding. The stream suspended sediment was finer than the catchment soils and considerably finer than material eroding from the catchment slopes during storms. The degree of clay and silt enrichment in the suspended sediments was largely the result of preferential deposition of the coarser fraction during the transport and delivery of sediment from its source to basin outlet. The data from this study confirm that a significant mode of sediment transport in fluvial systems is in the form of aggregates, and that the dispersed sediment size distribution is inappropriate for determining the transportability of sediment by flow. © 1997 by John Wiley & Sons, Ltd.     

Fractal dimensions of individual flocs and floc populations in streams

The fractal dimension of an individual floc is a measure of the complexity of its external shape. Fractal dimensions can also be used to characterize floc populations, in which case the fractal dimension indicates how the shape of the smaller flocs relates to that of the larger flocs. The objective of this study is to compare the fractal dimensions of floc populations with those of individual flocs, and to evaluate how well both indicate contributions of sediment sources and reflect the nature and extent of flocculation in streams. Suspended solids were collected prior to and during snowmelt at upstream and downstream sites in two southern Ontario streams with contrasting riparian zones. An image analysis system was used to determine area, longest axis and perimeter of flocs. The area–perimeter relationship was used to calculate the fractal dimension, D, that characterizes the floc population. For each sample, the fractal dimension, D_i , of the 28 to 30 largest individual flocs was determined from the perimeter–step‐length relationship. Prior to snowmelt, the mean value of D_i ranged from 1·19 (Cedar Creek, downstream) to 1·22 (Strawberry Creek, upstream and downstream). A comparison of the means using t‐tests indicates that most samples on this day had comparable mean values of D_i . During snowmelt, there was no significant change in the mean value of D_i at the Cedar Creek sites. In contrast, for Strawberry Creek the mean value of D_i at both sites increased significantly, from 1·22 prior to snowmelt to 1·34 during snowmelt. This increase reflects the contribution of sediment‐laden overland flow to the sediment load. At three of the sampling sites, the increase in fractal dimensions was accompanied by a decreases in effective particle size, which can be explained by an increase in bed shear stress. A comparison of fractal dimensions of individual flocs in a sample with the fractal dimensions of the floc populations indicates that both fractal dimensions provide similar information about the temporal changes in sediment source contributions, about the contrasting effectiveness of the riparian buffer zones in the two basins, and about the hydraulic conditions in the streams. Nevertheless, determining the individual fractal dimensions of a set of large flocs in a sample is very time consuming. Using fractal dimensions of floc populations is therefore the preferred method to characterize suspended matter. Copyright © 2000 John Wiley & Sons, Ltd.     

The particle size characteristics of fine‐grained channel deposits in the River Exe Basin,Devon, UK

Deposition and storage of fine‐grained (<62·5 μm) sediment in the hyporheic zone of gravel bed rivers frequently represents an important cause of aquatic habitat degradation. The particle size characteristics of such fine‐grained bed sediment (FGBS) exert an important control on its hydrodynamic properties and environmental impact. Traditionally, particle size analysis of FGBS in gravel bed rivers has focused on the absolute size distribution of the chemically dispersed mineral fraction. However, recent work has indicated that in common with fluvial suspended sediment, significant differences may exist between the absolute and the in situ, or effective, particle size composition of FGBS, as a result of the existence of aggregates, or composite particles. In the investigation reported in this paper, sealable bed traps that could be remotely opened to sample sediment deposited during specific storm runoff events and a laser back‐scatter probe were used to quantify the temporal and spatial variability of both the absolute and effective particle size composition of FGBS, and the associated suspended sediment from four gravel bed rivers in the Exe Basin, Devon, UK. The absolute particle size distributions of both the FGBS and suspended sediment evidenced c. >95%<62·5 μm sized primary particles and displayed a seasonal winter–summer fining, while the opposite trend was displayed by the effective particle size distribution of the FGBS and suspended sediment. The effective particle size distributions of both were typically highly aggregated, comprising up to 68%>62·5 μm sized particles. Spatial variation in the effective particle size and aggregation parameters was of secondary importance relative to temporal variation. The effective particle size distribution of the FGBS was consistently coarser and more aggregated than the associated suspended sediment and there was evidence of aggregate break‐up in samples of resuspended bed sediment. The implications of these findings for sediment transport modelling are considered. Copyright © 1999 John Wiley & Sons, Ltd.     

Suspended sediment monitoring in alluvial gullies: A laboratory and field evaluation of available measurement techniques

Gully erosion is a significant source of fine suspended sediment (<63 μm) and associated nutrient pollution to freshwater and marine waterways. Researchers, government agencies, and monitoring groups are currently using monitoring methods designed for streams and rivers (e.g., autosamplers, rising stage samplers, and turbidity loggers) to evaluate suspended sediment in gullies. This is potentially problematic because gullies have several hydrological features and monitoring operational challenges that differ to those of continually flowing streams and rivers (e.g., short and intense flows, high suspended sediment concentrations, and rapid scouring and aggradation). Here we present a laboratory and field-based assessment of the performance of common suspended sediment monitoring techniques applied to gullies. We also evaluate a recently-described method; the pumped active suspended sediment (PASS) sampler, which has been modified for monitoring suspended sediment in gully systems. Discrete autosampling provided data at high temporal resolution, however, it had poor collection efficiency (25 ± 10%) of coarser sediment particles (i.e., sand). Rising stage sampling, while robust and cost-effective, suffered from large amounts of condensation under field conditions (25–35% of sampler volume), due to harsh climatic conditions creating large diurnal temperature differences at the field site, thereby diluting sample concentrations and introducing additional measurement uncertainty. The turbidity logger exhibited a highly variable response when calibrated at each site with physically collected suspended sediment samples (R² = 0.17–0.83), highlighting that this approach should be used with caution. The modified PASS sampler proved to be a reliable and representative measurement method for gully sediment water quality, however, the time-integrated nature of the method limits its temporal resolution compared to the other monitoring methods. We recommend monitoring suspended sediment in alluvial gully systems using a combination of complementary techniques (e.g., PASS and RS samplers) to account for the limitations associated with individual methods.     

Composite suspended sediment particles in river systems: their incidence,dynamics and physical characteristics

Most of the existing data on the effective particle size characteristics of fluvial suspended sediment derive from instantaneous sampling methods that may not be representative of the overall suspended sediment loads. This presents difficulties when there is a need to incorporate effective particle size data into numerical models of floodplain sedimentation and sediment‐associated contaminant transfer. We have used a field‐based water elutriation apparatus (WEA) to assemble a large (36 flood) database on the time‐integrated nature of the effective and absolute particle size characteristics of suspended sediment in four subcatchments of the River Exe basin of southwest England. These catchments encompass a wide range of terrains and fluvial environments that are broadly representative of much of the UK and temperate, low relief northwest Europe. The WEA provides important data on the physical characteristics of composite particles that are not attainable using other methods. This dataset has allowed, for the first time, detailed interbasin comparisons of the time‐integrated particle size characteristics of suspended sediment and reliable estimates of the contribution of five effective size classes to the mean annual suspended sediment load of the study catchments. The suspended sediment load of each river is dominated by composite rather than primary particles, with, for example, almost 60% (by mass) of the sediment load of the River Exe at Thorverton transported as composite particles > 16 µm in size. All the effective size classes contain significant clay components. A key outcome of this study is the recognition that each catchment has a distinctive time‐integrated effective particle size signature. In addition, the time‐integrated effective particle size characteristics of the suspended loads in each of the catchments display much greater spatial variability than the equivalent absolute particle size distributions. This indicates that the processes producing composite particles vary significantly between these catchments, and this has important implications for our understanding of the dynamics of suspended sediment properties. Copyright © 2007 John Wiley & Sons, Ltd.     

Response of soil erosion and sediment sorting to the transport mechanism on a steep rocky slope

A deeper understanding of the sediment characteristics associated with rock fragment content can improve our knowledge of the erosional processes and transport mechanisms of sediments on steep rocky slopes. This research used simulated rainfall experiments lasting for 1 h at a rate of 90 mm h⁻¹ and employed 5 × 1 × 0.4 m parallel troughs filled with purple soils with different rock fragment volumetric contents (0, 5, 10, 20, 30 and 40%) on a 15° slope gradient. For each simulated event, runoff and sediment were sampled at 1- and 3-min intervals, respectively, to study, in detail, the temporal changes in the size distributions of the eroded sediments. The results show that sediment concentrations, soil erosion rates and soil loss ratios significantly decreased as rock fragment content increased for rock fragment contents from 0 to 40% in purple soils. During the transportation process, clay particles often formed aggregates and were then transported as larger particles. Silt particles were more likely to be transported as primary particles with a low degree of sediment aggregation. Sand-sized particles, which constituted a greater proportion of the original soil than the eroded sediments, were formed from other fine particles and transported as aggregates rather than as primary particles. Suspension-saltation, which mainly transports fine particles of 0.02–0.05 mm and coarse particles larger than 0.5 mm in size, was the most important transport mechanism on steep rocky slopes. The results of this study can help to explain the inherent laws of erosional processes on steep rocky slopes and can provide a foundation for improving physical models of soil erosion. © 2019 John Wiley & Sons, Ltd.     

Pumped rainfall simulators: the impact of rain pulses on sediment concentration and size

Many pumped rainfall simulators used in soil erosion studies use pulsed rain to control the rainfall intensity. We examined the effect of the rain pulsing on sediment concentration and size using three different pulse cycles with the same rainfall intensity. There was considerable variation in sediment concentration through the pulse cycle: the highest concentration was up to four times that of the lowest concentration. Furthermore, the particle size distribution also varied: the peak median particle size was double the lowest median particle size. The magnitude of differences in sediment concentration and particle size were greater the longer the pulse cycle and these dynamics will vary between rainfall simulators and studies. We suggest the impact of the pulsing on sediment is significant and should be investigated prior to experimentation so that sampling periods are designed to avoid bias introduced by fine temporal scale sediment dynamics. Copyright © 2009 John Wiley & Sons, Ltd.     

Flume‐ and field‐based evaluation of a time‐integrated suspended sediment sampler for the analysis of sediment properties

Suspended sediment has been identified as a vector for nutrient and contaminant transport in the fluvial environment. A time‐integrated sampler (the Phillips sampler), which emerged over a decade ago as a cost‐effective tool for in situ suspended sediment collection, is increasingly being used to collect samples for the analysis of sediment properties such as particle size composition, and nutrient and contaminant concentrations. This study evaluates the sampler under both flume and field conditions for efficiency in the mass and grain size of the suspended sediment collected. The sampler was tested in a flume using both kaolinite and sediment samples (sieved to < 180 µm) collected from the Quesnel River, British Columbia, Canada. In the kaolinite trails, the sampler preferentially collected coarser grain sizes compared to the original sediment, probably due to finer sediment remaining in suspension and therefore passing through the sampler, and also possibly due to flocculation of the kaolinite upon introduction to the flume. Conversely, the sampler collected river sediment that was finer than the original sediment, probably due to some settling of coarser sediment observed at the bottom of the flume. Once allowance was made for these operational issues associated with the flume, maximum sediment mass efficiency for kaolinite and river sediment was 43% and 87%, respectively. Sediment collected by the time‐integrated sampler during field deployment and adjacent channel bed sediment were also compared. The sampler collected sediment with a representative grain size distribution. However, there were differences in the geochemical (arsenic and selenium) concentrations of channel bed sediment and sediment collected by the Phillips sampler which may be a function of differences in the behavior of geochemical elements associated with the two types of sediment. This work suggests that further research is needed to evaluate the role of the Phillips sampler in collecting sediment for contaminant and nutrient analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Laboratory and theoretical evaluation of impact of packing density,particle shape,and uniformity coefficient on erodibility of coarse-grained soil particles

Sedimentation – including erosion, transport, and deposition of coarse-grained particles – is a primary and growing environmental, engineering, and agricultural issue around the world. Soil erosion occurs when the hydrodynamic force induced by flowing water exceeds the geotechnical resistance of soils, as measured by critical shear stress for initiation of soil-particle motion. Even though various quantitative methods have been suggested with respect to different types of soil, the most widely accepted formula to estimate critical shear stress for coarse-grained soil is a direct function of the median grain size of the soil particles; however, the erosion resistance of soils also varies with other geotechnical properties, such as packing density, particle shape, and uniformity coefficient. Thus, in this study, a combined rolling–lift model for particle detachment was derived based on theoretical analysis. A series of experimental flume tests were conducted with specimens prepared with standard soil types, as well as laboratory-prepared mixtures of coarse-grained soil to validate the theoretical model and determine the effect of other geotechnical properties on the erosion characteristics of coarse grains, coupled with the effect of median particle size. The results indicated that the median grain size is the primary variable determining the resistance of coarse grains, but the critical shear stress also varies with the packing density of the soil matrix. In addition, angular particles show more erosion resistance than rounded particles, and the erosion potential of a soil decreased when the grain is well graded (higher value of uniformity coefficient). Additionally, regression analysis was performed to quantify the effect of each parameter on the critical shear stress of coarse grains. © 2020 John Wiley & Sons, Ltd.     

Predictability of river flow and suspended sediment transport in the Mississippi River basin: a non‐linear deterministic approach

As the Mississippi River plays a major role in fulfilling various water demands in North America, accurate prediction of river flow and sediment transport in the basin is crucial for undertaking both short‐term emergency measures and long‐term management efforts. To this effect, the present study investigates the predictability of river flow and suspended sediment transport in the basin. As most of the existing approaches that link water discharge, suspended sediment concentration and suspended sediment load possess certain limitations (absence of consensus on linkages), this study employs an approach that presents predictions of a variable based on history of the variable alone. The approach, based on non‐linear determinism, involves: (1) reconstruction of single‐dimensional series in multi‐dimensional phase‐space for representing the underlying dynamics; and (2) use of the local approximation technique for prediction. For implementation, river flow and suspended sediment transport variables observed at the St. Louis (Missouri) station are studied. Specifically, daily water discharge, suspended sediment concentration and suspended sediment load data are analysed for their predictability and range, by making predictions from one day to ten days ahead. The results lead to the following conclusions: (1) extremely good one‐day ahead predictions are possible for all the series; (2) prediction accuracy decreases with increasing lead time for all the series, but the decrease is much more significant for suspended sediment concentration and suspended sediment load; and (3) the number of mechanisms dominantly governing the dynamics is three for each of the series. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of the undecomposed layer and semi-decomposed layer of Quercus variabilis litter on the soil erosion process and the eroded sediment particle size distribution

Simulated rainfall experiments were performed on bare, undecomposed litter layer and semi-decomposed litter layer slopes with litter biomasses of 0, 50, 100 and 150 g m⁻², respectively, to evaluate the effect of the undecomposed layer and semi-decomposed layer of Quercus variabilis litter on the soil erosion process and the particle size distribution of eroded sediment. The undecomposed layer and semi-decomposed layer of litter reduced the runoff rate by 10.91–27.04% and 12.91–36.05%, respectively, and the erosion rate by 13.35–40.98% and 17.16–59.46%, respectively. The percentage of smaller particles (clay and fine silt particles) decreased and the percentage of larger particles (coarse silt and sand particles) increased with an increased rainfall duration on all treated slopes, while the extent of the eroded sediment particle content varied among the treated slopes with the rainfall duration, with bare slopes exhibiting the largest variability, followed by undecomposed litter layer slopes and finally semi-decomposed litter layer slopes. The clay and sand particles were transported as aggregates, and fine silt and coarse silt particles were transported as primary particles. Compared with the original soil, sediment eroded from all treated slopes was mainly enriched in smaller particles. Furthermore, the loss of the smaller particles from the undecomposed litter layer slopes was lower than that from the semi-decomposed litter layer slopes, indicating that the undecomposed litter layer alleviated soil coarsening to some extent. The findings from this study improve our understanding of how litter regulates slope erosion and provide a reference for effectively controlling soil erosion.     

浮游绿藻对沉水植物苦草生长的抑制作用

浮游植物大量生长引起的遮光通常被认为是导致富营养湖泊中沉水植物的衰退的主要原因,但是藻类对沉水植物的竞争抑制作用还包括化感作用和无机碳竞争等重要方面,它们可能产生很大影响.为了验证这一观点,本文测定了除遮光以外浮游植物对沉水植物的抑制作用.在相似遮光下(10%-33%),对比研究了不同遮光方式(黑塑料网的物理遮光/浮游绿藻的生物遮光)对苦草(Vallisneria natans)生长和光合作用的影响.结果表明在遮阳网作用下苦草的生长指标(叶数、叶长、叶片叶绿素含量和生物量)和净光合速率都显著高于浮游绿藻作用下苦草,且后一种处理下苦草叶片上端有腐烂现象.由此可见,浮游绿藻对沉水植物生长除了通过遮光抑制生长外,还有较遮光更加严重的抑制作用.这可能是由于浮游绿藻通过化感作用和对可利用光合有效辐射光能和溶解无机碳的竞争优势抑制了苦草的光合作用.     

Crust strength: a wind tunnel study of the effect of impact by saltating particles on cohesive soil surfaces

A wind tunnel study examined the effect of distributions of saltating particles on sediment surfaces which were characterized by distributions of their tensile strength. The sediments consisted of varying proportions of large sand‐sized particles with a fine particle cement. The energies of the impacting particles and the surface strengths were compared with the mass of material lost from the surface. It is important to consider distributions of parameters rather than mean values only, since abrasion and erosion may occur from surfaces not predicted from average strength and saltation velocities. At the impact velocities used in this study (mean velocity 4·4 m s^?1, with standard deviation of 0·51), surfaces containing less than 12 per cent fine material were easily eroded, but insignificant erosion occurred when the fine particle content exceeded 60 per cent. Small amounts of cementing material were easily ruptured, allowing the large sand grains to be moved (largely in creep) by the bombarding particles. A significant amount of energy was lost to the bed. As the percentage of fine material increased, the surface became more difficult to break up and less energy was lost to the bed. The probability that erosion will occur for known energy distributions of impacting particles and surface strength can be calculated and the mass loss increases exponentially with a decrease in the percentage of fine cementing particles. Copyright © 2001 John Wiley & Sons, Ltd.     

浮游绿藻对沉水植物苦草生长的抑制作用

水体中氧化还原电位的变化会对藻类生长和竞争产生直接或间接的影响.本文采用单种培养和混合培养的方式,研究了在铜绿微囊藻(Microcystis aeruginosa)与斜生栅藻(Scenedesmus obliquus)竞争生长过程中氧化还原电位降低对铜绿微囊藻优势形成的影响,同时测定了铜绿微囊藻生理和形态的变化.结果表明:在单种培养条件下,铜绿微囊藻的生长速率明显高于栅藻,降低氧化还原电位对两种藻的生长速率没有影响;在混合培养条件下,两种藻的生长均受到了抑制,但降低氧化还原电位却明显提高了铜绿微囊藻的生长速率,而降低了斜生栅藻的生长速率,说明铜绿微囊藻的竞争能力得到了加强,斜生栅藻的竞争能力有所削弱;同时试验也发现在竞争生长的条件下,培养基氧化还原电位的降低诱导了铜绿微囊藻细胞体积变大,酯酶活性增强以及叶绿素荧光强度增加,这些生理参数的改变可能是铜绿微囊藻在环境中氧化还原电位降低时竞争能力得以增强的重要原因.     

From manned to unmanned aircraft: Adapting airborne particle size mapping methodologies to the characteristics of sUAS and SfM

Subaerial particle size data holds a wealth of valuable information for fluvial, coastal, glacial and other sedimentological applications. Recently, we have gained the opportunity to map and quantify surface particle sizes at the mesoscale using data derived from small unmanned aerial system (sUAS) imagery processed using structure from motion (SfM) photogrammetry. Typically, these sUAS‐SfM approaches have been based on calibrating orthoimage texture or point cloud roughness with particle size. Variable levels of success are reported and a single, robust method capable of producing consistently accurate and precise results in a range of settings has remained elusive. In this paper, we develop an original method for mapping surface particle size with the specific constraints of sUAS and SfM in mind. This method uses the texture of single sUAS images, rather than orthoimages, calibrated with particle sizes normalised by individual image scale. We compare results against existing orthoimage texture and roughness approaches, and provide a quantitative investigation into the implications of the use of sUAS camera gimbals. Our results indicate that our novel single image method delivers an optimised particle size mapping performance for our study site, outperforming both other methods and delivering residual mean errors of 0.02 mm (accuracy), standard deviation of residual errors of 6.90 mm (precision) and maximum residual errors of 16.50 mm. Accuracy values are more than two orders of magnitude worse when imagery is collected by a similar drone which is not equipped with a camera gimbal, demonstrating the importance of mechanical image stabilisation for particle size mapping using measures of image texture. Copyright © 2017 John Wiley & Sons, Ltd.     

Use of a single-bowl continuous-flow centrifuge for dewatering suspended sediments: Effect on sediment physical and chemical characteristics

The use of a single-bowl continuous-flow centrifuge (CFC, Sharples-Pennwalt Model AS-12) for dewatering suspended sediment from large volumes of river water is evaluated. Sediment-recovery efficiency of 86-91 per cent is comparable to that of other types of CFC units. The recovery efficiency is limited by the particle-size distribution of the feed water and by the limiting particle diameter that is retained in the centrifuge bowl. The limiting particle diameter, using the parameters for this study (bowl radius = 10.5cm; bowl length = 71.1 cm; rotational velocity = 16000 r min^?1; flow rate = 2 L min^?1, and an assumed hydrated particle density = 1.7 gm cm^?3), is 370 nm. There seems to be no particle-size fractionation within the centrifuge bowl—the median particle size was the same at the top as at the bottom. Particle electrophoretic mobility plays some role in fractionation of particles within the centrifuge. The mobility ranged from ?1.19 to ?2.01 × 10^?8 m² V^?1 s^?1, which is typical of clays coated with organic matter, the charge of which is partially neutralized by divalent cations and iron. Contamination by trace metals and organics is minimized by coating all surfaces that come in contact with the sample with either FEP or PFA Teflon and using a removable FEP Teflon liner in the centrifuge bowl. Because of the physical and chemical factors affecting particle fractionation within the centrifuge, care must be exercised in interpreting the environmental consequences of particles collected by continuous-flow centrifugation.     

On determining the geometric and kinematic characteristics of coherent flow structures over a gravel bed: a new approach using combined PLIF‐PIV

In gravel‐bed rivers, the microtopography of the bed is known to exert a significant influence on the generation of turbulent flow structures that owe their origin to fluid shear generated near the bed. Although field and laboratory measurements have indicated that flows over gravel beds contain a range of coherent flow structures, the kinematic and dynamic properties of these structures are still poorly understood. This paper describes a new experimental methodology to quantify simultaneously both the kinematic and dynamic characteristics of coherent flow structures based upon combined planar laser‐induced fluorescence and particle imaging velocimetry (PLIF‐PIV). The results confirm that the primary generative mechanism of coherent flow structures is at the bed, where merging hairpin vortices form around bed clasts and generate larger‐scale fluid motions that advect downstream. Copyright © 2010 John Wiley & Sons, Ltd.     

Predicting gravel bed river response to environmental change: the strengths and limitations of a regime‐based approach

Rivers respond to environmental changes such as climate shifts, land use changes and the construction of hydro‐power dams in a variety of ways. Often there are multiple potential responses to any given change. Traditionally, potential stream channel response has been assessed using simple, qualitative frameworks based largely on professional judgement and field experience, or using some form of regime theory. Regime theory represents an attempt to use a physically based approach to predict the configuration of stable channels that can transport the imposed sediment supply with the available discharge. We review the development of regime theory, and then present a specific regime model that we have created as a stand‐alone computer program, called the UBC Regime Model (UBCRM). UBCRM differs from other regime models in that it constrains its predictions using a bank stability criterion, as well as a pattern stability criterion; it predicts both the stable channel cross‐sectional dimensions as well as the number of anabranches that the stream must have in order to establish a stable channel pattern. UBCRM also differs from other models in that it can be used in a stochastic modelling mode that translates uncertainty in the input variables into uncertainty in the predicted channel characteristics. However, since regime models are fundamentally based on the concept of grade, there are circumstances in which the model does not perform well. We explore the strengths and weaknesses of the UBCRM in this paper, and we attempt to illustrate how the UBCRM can be used to augment the existing qualitative frameworks, and to help guide professionals in their assessments. Copyright © 2016 John Wiley & Sons, Ltd.     

Using a pore‐scale model to quantify the effect of particle re‐arrangement on pore structure and hydraulic properties

A pore‐scale model based on measured particle size distributions has been used to quantify the changes in pore space geometry of packed soil columns resulting from a dilution in electrolyte concentration from 500 to 1 mmol l^?1 NaCl during leaching. This was applied to examine the effects of particle release and re‐deposition on pore structure and hydraulic properties. Two different soils, an agricultural soil and a mining residue, were investigated with respect to the change in hydraulic properties. The mining residue was much more affected by this process with the water saturated hydraulic conductivity decreasing to 0·4% of the initial value and the air‐entry value changing from 20 to 50 cm. For agricultural soil, there was little detectable shift in the water retention curve but the saturated hydraulic conductivity decreased to 8·5% of the initial value. This was attributed to localized pore clogging (similar to a surface seal) affecting hydraulic conductivity, but not the microscopically measured pore‐size distribution or water retention. We modelled the soil structure at the pore scale to explain the different responses of the two soils to the experimental conditions. The size of the pores was determined as a function of deposited clay particles. The modal pore size of the agricultural soil as indicated by the constant water retention curve was 45 µm and was not affected by the leaching process. In the case of the mining residue, the mode changed from 75 to 45 µm. This reduction of pore size corresponds to an increase of capillary forces that is related to the measured shift of the water retention curve. Copyright © 2007 John Wiley & Sons, Ltd.