Gregory H. Sambrook Smith

The effect of environmentally relevant conditions on PVP stabilised gold nanoparticles

Nanoparticles are a major product from the nanotechnology industry and have been shown to have a potentially large environmental exposure and hazard. In this study, sterically stabilised polyvinyl pyrrolidone (PVP) 7 nm gold nanoparticles (NPs) were produced and characterised as prepared by surface plasmon resonance (SPR), size and aggregation, morphology and surface charge. Changes in these properties with changes in environmentally relevant conditions (pH, ionic strength, Ca concentration and fulvic acid presence) were quantified. These sterically stabilised NPs showed no aggregation with changes in pH or inorganic ions, even under high (0.1 M) Ca concentrations. In addition, the presence of fulvic acid resulted in no observable changes in SPR, size, aggregation or surface chemistry, suggesting limited interaction between the PVP stabilised nanoparticles and fulvic acid. Due to the lack of aggregation and interaction, these NPs are expected to be highly mobile and potentially bioavailable in the environment.

Can we distinguish flood frequency and magnitude in the sedimentological record of rivers

Consideration of the origin of alluvial deposits and their paleoenvironmental interpretation has traditionally involved two schools of thought: that they are either the result of processes that, on average, have acted uniformly through time, or that they are related to exceptional events that occur infrequently. Despite the long-running debate of gradualism versus catastrophism within the Earth Sciences, there are surprisingly few quantitative data to assess the magnitude of events that produce alluvial sedimentary successions. This paper reports on a unique ‘natural experiment’ where surface (digital elevation model, DEM) and subsurface (ground penetrating radar, GPR) data were taken immediately prior to, and after, a large (1-in-40 yr) flood event on the sandy, braided, South Saskatchewan River, Canada. Results show that although this high-magnitude flood reworked the entire braidplain, the scale of scour and style of deposition was similar to that associated with lower-magnitude, annual, floods. The absence of a distinct imprint of this large flood within the deposits is related to the fact that as river discharge rises, and begins to flow overbank, channel width increases at a much faster rate than flow depth, and thus the rate of increase in channel bed shear stress declines. Hence, rather than being a product of either frequent or rare events, alluvial deposits will likely be created by a range of different magnitude floods, but discriminating between these different scale events in the rock record may be extremely difficult.

The use and application of GPR in sandy fluvial environments: methodological considerations

Abstract Ground penetrating radar (GPR) is a popular technique for imaging and interpreting sedimentary architecture. However, current literature shows a wide range in the quality of information provided on the GPR methodology and processing technique. It is therefore difficult to judge the validity of the GPR interpretations and this produces inherent difficulties for comparison between surveys. This paper describes the key steps required to collect, process and interpret GPR surveys in sandy fluvial sediments. GPR data from the South Saskatchewan River, Canada, are used to illustrate each stage of data collection and processing. Particular attention is given to the appropriate set-up conditions for the GPR software and hardware, the selection of data-processing techniques and velocity analysis. Methods for the interpretation of GPR reflectors are also investigated using ground-truth control provided by a cut-face exposure. This paper presents recommendations for a systematic and rigorous methodology for the collection, processing and interpretation of GPR data in sandy fluvial environments. The paper suggests that all data-collection parameters and processing steps should be recorded or tabulated in any GPR publication to facilitate comparisons between surveys.

Effect of bed permeability and hyporheic flow on turbulent flow over bed forms.

This paper uses particle imaging velocimetry to provide the first measurements detailing the flow field over a porous bed in the presence of bed forms. The results demonstrate that flow downstream of coarse-grained bed forms on permeable beds is fundamentally different to that over impermeable beds. Most significantly, the leeside flow separation cell is greatly modified by jets of fluid emerging from the subsurface, such that reattachment of the separated flow does not occur and the Reynolds stresses bounding the separation zone are substantially lessened. These results shed new light on the underlying flow physics and advance our understanding of both ecological and geomorphological processes associated with permeable bed forms. Water fluxes at the bed interface are critically important for biogeochemical cycling in all rivers, yet mass and momentum exchanges across the bed interface are not routinely incorporated into flow models. Our observations suggest that ignoring such exchange processes in coarse-grained rivers may overlook important implications. These new results also provide insight to explain the distinctive morphology of coarse-grained bed forms, the production of openwork textures in gravels, and the absence of ripples in coarse sands, all of which have implications for modeling and prediction of sediment entrainment and flow resistance.

Fluvial form in modern continental sedimentary basins: Distributive fluvial systems: COMMENT

[Weissmann et al. (2010)][1] discuss what they term “distributary fluvial systems” (DFS) to address a key question in the earth sciences: what modern systems are appropriate analogues to the rock record? They conclude “Since DFSs dominate depositional patterns in all continental sedimentary

Deposits of the sandy braided South Saskatchewan River: Implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization

Estimation of the dimensions of fluvial geobodies from core data is a notoriously difficult problem in reservoir modeling. To try and improve such estimates and, hence, reduce uncertainty in geomodels, data on dunes, unit bars, cross-bar channels, and compound bars and their associated deposits are presented herein from the sand-bed braided South Saskatchewan River, Canada. These data are used to test models that relate the scale of the formative bed forms to the dimensions of the preserved deposits and, therefore, provide an insight as to how such deposits may be preserved over geologic time. The preservation of bed-form geometry is quantified by comparing the alluvial architecture above and below the maximum erosion depth of the modern channel deposits. This comparison shows that there is no significant difference in the mean set thickness of dune cross-strata above and below the basal erosion surface of the contemporary channel, thus suggesting that dimensional relationships between dune deposits and the formative bed-form dimensions are likely to be valid from both recent and older deposits. The data show that estimates of mean bankfull flow depth derived from dune, unit bar, and cross-bar channel deposits are all very similar. Thus, the use of all these metrics together can provide a useful check that all components and scales of the alluvial architecture have been identified correctly when building reservoir models. The data also highlight several practical issues with identifying and applying data relating to cross-strata. For example, the deposits of unit bars were found to be severely truncated in length and width, with only approximately 10% of the mean bar-form length remaining, and thus making identification in section difficult. For similar reasons, the deposits of compound bars were found to be especially difficult to recognize, and hence, estimates of channel depth based on this method may be problematic. Where only core data are available (i.e., no outcrop data exist), formative flow depths are suggested to be best reconstructed using cross-strata formed by dunes. However, theoretical relationships between the distribution of set thicknesses and formative dune height are found to result in slight overestimates of the latter and, hence, mean bankfull flow depths derived from these measurements. This article illustrates that the preservation of fluvial cross-strata and, thus, the paleohydraulic inferences that can be drawn from them, are a function of the ratio of the size and migration rate of bed forms and the time scale of aggradation and channel migration. These factors must thus be considered when deciding on appropriate length:thickness ratios for the purposes of object-based modeling in reservoir characterization.

The role of discharge variability in determining alluvial stratigraphy

We illustrate the potential for using physics-based modeling to link alluvial stratigraphy to large river morphology and dynamics. Model simulations, validated using ground penetrating radar data from the Rio Parana, Argentina, demonstrate a strong relationship between bar-scale set thickness and channel depth, which applies across a wide range of river patterns and bar types. We show that hydrologic regime, indexed by discharge variability and flood duration, exerts a first-order influence on morphodynamics and hence bar set thickness, and that planform morphology alone may be a misleading variable for interpreting deposits. Indeed, our results illustrate that rivers evolving under contrasting hydrologic regimes may have very similar morphology, yet be characterized by marked differences in stratigraphy. This realization represents an important limitation on the application of established theory that links river topography to alluvial deposits, and highlights the need to obtain field evidence of discharge variability when developing paleoenvironmental reconstructions. Model simulations demonstrate the potential for deriving such evidence using metrics of paleocurrent variance.

Pliocene femur of Theropithecus from the Luangwa Valley, Zambia.

The Luangwa Valley in eastern Zambia has the potential to be an important source of PlioPleistocene fauna and archaeology, and may represent a dispersal corridor for fauna, including hominins, between eastern and southern Africa. This is highlighted by the recent discovery of a mineralised and nearly complete primate right femur, described in this article and attributed to Theropithecus cf. darti, that was found with stone flakes in the middle reaches of the Luangwa River Valley.

A numerical investigation into the importance of bed permeability on determining flow structures over river dunes

Although permeable sediments dominate the majority of natural environments past work concerning bedform dynamics has considered the bed to be impermeable, and has generally neglected flow between the hyporheic zone and boundary layer. Herein, we present results detailing numerically modelled flow which allow the effects of bed permeability on bedform dynamics to be assessed. Simulation of an isolated impermeable bedform over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside, in the form of returning jets that generate horseshoe-shaped vortices. The returning flow significantly influences the leeside flow, modifying the separation zone, lifting the shear layer adjoining the separation zone away from the bed. Simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside. With two dunes placed in series the flow over the downstream dune is influenced by the developing boundary layer on the leeside of the upstream dune. For the permeable bed case the upwelling flow lifts the separated flow from the bed, modifies the shear layer through the coalescence with vortices generated, and causes the shear layer to undulate rather than be parallel to the bed. These results demonstrate the significant effect that bed permeability has on the flow over bedforms that may be critical in affecting the flux of water and nutrients.