A transferable method for the automated grain sizing of river gravels

The spatial and temporal resolution of surface grain-size characterization is constrained by the limitations of traditional measurement techniques. In this paper we present an extremely rapid image-processing-based procedure for the measurement of exposed fluvial gravels and other coarsegrained sediments, defining the steps required to minimize the errors in the derived grain-size distribution. This procedure differs significantly from those used previously. It is based around a robust object-detection algorithm that produces excellent results on images exhibiting a wide range of sedimentary conditions, crucially, without any user intervention or site-specific parameterization. The procedure is tested using a dataset comprising 39 images from three rivers with contrasting grain lithology, shape, roundness and packing configuration and representing a very wide range of textures. It is shown to perform more consistently than the best existing automated method, achieving a precision equivalent to that obtainable by Wolman sampling, but taking between one sixth and one twentieth of the time. The error in area-by-number grain-size distribution percentiles is typically less than 0.05 Ï

Short-term fluctuations in heavy metal concentrations during flood events through abandoned metal mines, with implications for aquatic ecology and mine water treatment

The variability in heavy metal concentrations and physico-chemical parameters during rain-fed river floods that pass through abandoned metal mines is poorly understood due to the difficulties of sampling these events. Such information is essential for the characterisation of contaminant dynamics and for investigations of contaminant/ecosystem relations and the effectiveness of remediation. This study investigates the role of flood flows in contaminant mobilisation and temporary increases in toxicity at an abandoned metal mine in central Wales, UK. Flood events substantially increase the potential toxicity of river water. The principal contaminants are dissolved Pb, mobilized by increased acidity resulting from the dissolution and flushing of efflourescent salts accumulated on the surface of mine spoil. The implications of flood runoff and contaminant mobilisation for aquatic ecology and mine water treatment are discussed

Sensitivity of interfacial hydraulics to the microtopographic roughness of water-lain gravels

Flow within the interfacial layer of gravel-bed rivers is poorly understood, but this zone is important because the hydraulics here transport sediment, generate flow structures and interact with benthic organisms. We hypothesized that different gravel-bed microtopographies generate measurable differences in hydraulic characteristics within the interfacial layer. This was tested using a high density of spatially and vertically distributed, velocity time series measured in the interfacial layers above three surfaces of contrasting microtopography. These surfaces had natural water-worked textures, captured in the field using a casting procedure. Analysis was repeated for three discharges, with Reynolds numbers between 165000 and 287000, to evaluate whether discharge affected the impact of microtopography on interfacial flows. Relative submergence varied over a small range (3.5 to 8.1) characteristic of upland gravel-bed rivers. Between-surface differences in the median and variance of several time-averaged and turbulent flow parameters were tested using non-parametric statistics. Across all discharges, microtopographic differences did not affect spatially averaged (median) values of streamwise velocity, but were associated with significant differences in its spatial variance, and did affect spatially averaged (median) turbulent kinetic energy. Sweep and ejection events dominated the interfacial region above all surfaces at all flows, but there was a microtopographic effect, with Q2 and Q4 events less dominant and structures less persistent above the surface with the widest relief distribution, especially at the highest Reynolds number flow. Results are broadly consistent with earlier work, although this analysis is unique because of the focus on interfacial hydraulics, spatially averaged 'patch scale' metrics and a statistical approach to data analysis. An important implication is that observable differences in microtopography do not necessarily produce differences in interfacial hydraulics. An important observation is that appropriate roughness parameterizations for gravel-bed rivers remain elusive, partly because the relative contributions to flow resistance of different aspects of bed microtopography are poorly constrained

Increase in coarse sediment transport associated with disturbance of gravel river beds by signal crayfish (Pacifastacus leniusculus)

There is growing acknowledgement of the interaction between animals and the river bed on which they live and the implications of biological activity for geomorphic processes. It has been observed that signal crayfish (Pacifastacus leniusculus) disturb gravel substrates, potentially promoting sediment transport and impacting ecological communities. However, the mechanisms involved and the extent of their impact remain poorly understood, especially in relation to other processes that affect grain mobility in gravel-bed rivers. A series of flume experiments, using loose and water-worked gravel beds of narrowly graded grain sizes that were exposed to 6 h of crayfish activity under low-velocity flows, showed a substantial increase in the number of grains entrained by subsequent higher-velocity flows when compared with control runs in which crayfish were never introduced. Crayfish alter the topography of their substrate by constructing pits and mounds, which affect grain protrusion. When walking and foraging, they also alter gravel fabric by reorienting and changing the friction angle of surface grains. In water-worked surfaces, this fabric rearrangement is shown to lead to a statistically significant, partial reversal of the structuring that had been achieved by antecedent flow. For these previously water-worked surfaces, the increase in entrainment arising from disturbance by crayfish was statistically significant, with grain transport nearly twice as great. This suggests that signal crayfish, an increasingly widespread invasive species in temperate latitudes beyond their native NW North America, have the potential to enhance coarse-grained bedload flux by altering the surface structure of gravel river beds and reducing the stability of surface grains. This study illustrates further the importance of acknowledging the impact of mobile organisms in conditioning the river bed when assessing sediment entrainment mechanics in the context of predicting bedload flux

Essential ancillary data requirements for the validation of surrogate measurements of bedload: non-invasive bed material grain size and definitive measurements of bedload flux

Achieving a significant advance in understanding the sedimentary dynamics of rivers, especially those with coarse-grained beds, depends upon the acquisition of data that adequately reflect sediment flux. In a similar vein, the successful development of a functional relation between bedload flux and contemporary hydraulics that is transferable from river to river requires an understanding of relations between the immediate source of bedload – the river bed – and the flow. This would benefit from deployment of a quick but efficient method of assessing the grain-size distribution of river-bed material, since this is one of several determinants of bedload flux and is a property that can be readily established in a previously ungauged channel. This paper reminds hydraulic engineers and geomorphologists of the need to deploy a method of data capture that allows the performance of surrogate measures of bedload to be assessed adequately. In particular, it highlights the performance and short-comings of the Birkbeck Sampler. This is an automatic slot sampler that provides a continuous, direct and sensitive measure of bedload flux that is as definitive as is feasible in the complex confusion of a river in spate. It has been deployed in a wide range of river environments and has proved to be both reliable and durable, having provided bedload records on ephemeral channels for more than a decade in one case. The paper also indicates a need for the use of a method that facilitates rapid and frequent surveys of river-bed materials in order to understand bedload dynamics that are measured by any means, not least those that detect bedload surrogates. The Digital Gravelometer is described, along with its advantages and limitations. Time-savings alone make this a valuable addition to the river scientist’s tool-kit

Automated sizing of coarse-grained sediments : image-processing procedures

This is the first in a pair of papers in which we present image-processing based procedures for the measurement of fluvial gravels. The spatial and temporal resolution of surface grain-size characterization is constrained by the time-consuming and costly nature of traditional measurement techniques. Several groups have developed image-processing based procedures, but none have demonstrated the transferability of these techniques between sites with different lithological, clast form and textural characteristics. Here we focus on imageprocessing procedures for identifying and measuring image objects (i.e. grains); the second paper examines the application of such procedures to the measurement of fluvially-deposited gravels. Four image-segmentation procedures are developed, each having several internal parameters, giving a total of 416 permutations. These are executed on 39 images from three field sites at which the clasts have contrasting physical properties. The performance of each procedure is evaluated against a sample of manually digitized grains in the same images, by comparing three derived statistics. The results demonstrate that it is relatively straightforward to develop procedures that satisfactorily identify objects in any single image or a set of images with similar sedimentary characteristics. However, the optimal procedure is that which gives consistently good results across sites with dissimilar sedimentary characteristics. We show that neighborhood-based operations are the most powerful, and a morphological bottom-hat transform with a double threshold is optimal. It is demonstrated that its performance approaches that of the procedures giving the best results for individual sites. Overall, it out-performs previously published, or improvements to previously published, methods

Sediment geochemistry of streams draining abandoned lead/zinc mines in central Wales: the Afon Twymyn

Purpose Despite the decline of metal mining in the UK during the early 20th century, a substantial legacy of heavy metal contamination persists in river channel and floodplain sediments. Poor sediment quality is likely to impede the achievement of ‘good’ chemical and ecological status for surface waters under the European Union Water Framework Directive. This paper examines the environmental legacy of the Dylife lead/zinc mine in the central Wales mining district. Leachable heavy metal concentrations in the bed sediments of the Afon Twymyn are established and the geochemical partitioning, potential mobility and bioavailability of sediment-associated heavy metals are established. Materials and methods Sediment samples were collected from the river bed and dry-sieved into two size fractions (<63 and 64–2,000 µm). The fractionated samples were then subjected to a sequential extraction procedure to isolate heavy metals (Pb, Zn, Cu, Cd, Fe, Mn) in three different geochemical phases. Sediment samples were then analysed for heavy metals using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). Results and discussion The bed sediment of the Afon Twymyn is grossly polluted with heavy metals. Within the vicinity of the former mine, Pb concentrations are up to 100 times greater than levels reported to have deleterious impacts on aquatic ecology. Most heavy metals exist in the most mobile easily exchangeable and carbonate-bound geochemical phases, potentially posing serious threats to ecological integrity and constituting a significant, secondary, diffuse source of pollution. Metal concentrations decrease sharply downstream of the former mine, although there is a gradual increase in the proportion of readily extractable Zn and Cd. Conclusions Implementation of sediment quality guidelines is important in order to achieve the aims of the Water Framework Directive. Assessments of sediment quality should include measurements of background metal concentrations, river water physico-chemistry and, most importantly, metal mobility and potential bioavailability. Uniformity of sediment guidelines throughout Europe and flexibility of targets with regard to the most heavily contaminated mine sites are recommended

Stabilization of fine gravels by net-spinning caddisfly larvae

We examined the impact of Hydropsychidae caddisfly larvae on the incipient motion of two sizes of narrowly graded fine-gravel (4–6 and 6–8 mm). This impact was assessed relative to the collective impact of other abiotic and biotic processes that are potentially important conditioning agents of fine-gravels. Trays of gravel were placed in the River Soar, Leicestershire, UK, where they were colonized to natural densities by caddisfly larvae. Identical trays that were surrounded by a 250 μm mesh were also deployed, preventing colonization but allowing field conditioning of sediments, including minor reworking of grains and biofilm development. After 21 days in the river, trays were removed to a laboratory flume where grain entrainment stresses were established. In addition to the colonized and conditioned treatments, critical shear stresses were measured for identical sediments that were not placed in the river (laboratory gravels). Gravels that were colonized by Hydropsychidae required significantly greater shear stresses for entrainment than conditioned trays (p ≤ 0·002), however, there was no significant difference between conditioned and laboratory gravels. This implies that the presence of caddisfly can be a more important influence on fine-gravel stability than some conditioning processes. Shields parameter was compared across treatments and across the two gravel size-fractions using two-way ANOVA. No significant differences or interactions were observed, indicating that 4–6 mm gravel was stabilized to a similar degree as 6–8 mm gravel by conditioning and colonization processes. Our results extend earlier studies in two important ways: (1) entrainment stresses were established for fine gravels that were colonized at natural densities, under natural stream conditions; and (2) the caddisfly effect was measured relative to both field-conditioned and unconditioned laboratory controls. The temporal and spatial distribution of silk-spinning caddisfly larvae suggests that they have the potential to influence fine-sediment mobility in many rivers, worldwide

Movements of a macroinvertebrate species across a gravel-bed substrate: effects of local hydraulics and micro-topography under increasing discharge

Flow refugia provide a mechanism that can explain the persistence of macroinvertebrate communities in flood-prone, gravel-bed rivers. The movement behaviour of macroinvertebrates is a key element of the flow refugia hypothesis but surprisingly little is known about it. In particular, little is known about how local near-bed hydraulics and bed microtopography affect macroinvertebrate movements. We used a novel casting technique to reproduce a natural gravel-bed substrate in a large flume where we were able to observe the movement behaviour of the cased caddisfly Potamophylax latipennis at different discharges. The crawling paths and drift events of animals were analysed from video recordings and used to classify sites on the substrate according to the type of insect movement. We used Acoustic Doppler Velocimeter (ADV) measurements close to the boundary to characterise the hydraulic conditions at different sites and a detailed Digital Elevation Model (DEM) to characterise sites topographically. Animals made shorter more disjointed crawling journeys as discharge increased, although they tended to follow consistent paths across the substrate. As we hypothesised, crawling behaviour was locally associated with low elevations, low flow velocities and low turbulent kinetic energies, while sites that insects avoided were characterised by higher elevations, velocities and turbulence. Discrimination was greater at higher discharges. We suppose that these relations reflect the need of animals to reduce the risk of entrainment and minimise energy expenditure by avoiding areas of high fluid drag. As discharge increased there was a general upward shift in the frequency distributions of local velocities and turbulent kinetic energies. The animals responded to these shifts and it is clear that their different activities were not limited to fixed ranges of velocity and turbulence. We assume that the absolute hydraulic forces would become a limiting factor at some higher discharge. At the discharges examined here, which are below those required to generate framework particle entrainment, patterns of animal movement appear to be associated with the animals’ experiences of relative velocities rather than absolute hydraulic forces