Does the permeability of gravel river beds affect near‐bed hydrodynamics?

The permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near‐bed flow dynamics, and thus on fluid advection at the sediment–water interface. This study provides the first quantification of this effect for water‐worked gravel beds. Laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography. The turbulence over permeable beds is less intense, more organised and more efficient at momentum transfer because eddies are more coherent. Furthermore, turbulent kinetic energy is lower, meaning that less energy is extracted from the mean flow by this turbulence. Consequently, the double‐averaged velocity is higher and the bulk flow resistance is lower over permeable beds, and there is a difference in how momentum is conveyed from the overlying flow to the bed surface. The main implications of these results are three‐fold. First, local pressure gradients, and therefore rates of material transport, across the sediment–water interface are likely to differ between impermeable and permeable beds. Second, near‐bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary. Third, more sophisticated flow resistance models are required for coarse‐grained rivers that consider not only the bed surface but also the underlying permeable structure. Overall, our results suggest that the effects of bed permeability have critical implications for hyporheic exchange, fluvial sediment dynamics and benthic habitat availability. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Spatial variability in beach biogeomorphology in a tropical archipelago

Beaches of tropical island coasts exhibit high levels of diversity in composition and form in comparison with their continental counterparts. To investigate the nature and origin of this diversity, individual beach morphology and sedimentology was investigated in the British Virgin Islands (BVI), a Caribbean archipelago of > 60 high volcanic and low reef islands. The islands exhibit a diversity of orientations (some facing the Atlantic and some the Caribbean), elevation and gradient, rock type and wave energy. An examination of 100 beaches in the archipelago revealed a first-order division into sand (70 beaches) and coral rubble (30 beaches). These beaches occur in seven planform types (determined by the antecedent geological framework) and are further subdivided according to shoreface type (seagrass, sandy shoreface, or reef). Mainland-attached headland-embayment beaches are the most common form of sand beach while coral rubble beaches usually occur as barriers that enclose salt ponds and wetlands. Among sand beaches, carbonate content is greatest on Atlantic-facing beaches, and coral rubble beaches are more common on Caribbean-facing beaches. Grain size characteristics on sandy beaches are highly variable and range from fine to very coarse sands while coral rubble beaches range up to boulder-sized clasts. The local source material is a primary determinant of sediment composition. The local factors such as the underlying geology, source and availability of sediments are the primary determinants of beach form, composition and texture in the BVI. Oceanographic and climatic conditions such as the prevailing easterly trade winds and waves which seasonally range in direction from east-northeast to southeast as well as beach orientation to Atlantic- or Caribbean-facing waves also contribute to the variability, but in a secondary role. © 2019 John Wiley & Sons, Ltd.     

Wet season fine sediment dynamics on the inner shelf of the Great Barrier Reef

Fine sediment dynamics were recorded in February 2007 in coastal waters of the Great Barrier Reef during a moderate flood of the Tully River. An estuarine circulation prevailed on the inner continental shelf with a surface seaward velocity peaking at 0.1 m s⁻¹ and a near-bottom landward flow peaking at 0.05 m s⁻¹. Much of the riverine mud originating from eroded soils was exported onto a 10 km wide coastal strip during the rising stage of the river flood in the first flush. In coastal waters, suspended sediment concentration peaked at 0.2 kg m⁻³ near the surface and 0.4 kg m⁻³ at 10 m depth during calm weather, and 0.5 kg m⁻³ near the surface and 2 kg m⁻³ at 10 m depth during strong winds when bottom sediment was resuspended. Diurnal irradiance at 4 m depth was almost zero for 10 days. The sedimentation rate averaged 254 (±33) g m⁻² d⁻¹ over the 28-day study period, and concentrations of dissolved and particulate nutrients originating from the river were high. The observed low irradiance would have prevented coral photosynthesis, while the sedimentation rate would have been lethal to some juvenile corals. The mud may ultimately be minnowed out over long periods, however, flushing of the mud occurs at time scales much longer than the flood event and the mud is likely to affect coral physiology for significant periods after the flood has subsided. The data show the need to better control erosion on farmed land for the conservation of coral reefs on the inner shelf of the Great Barrier Reef.     

Long-term benthic responses to sustained disturbance by aggregate extraction in an area off the east coast of the United Kingdom

The temporal variability of benthic assemblages inhabiting offshore gravel deposits in the North Sea is poorly known, as purpose-collected long-term data sets have until recently been non-existent. It has therefore proved difficult to understand the stability and resilience of these benthic ecosystems after disturbance caused by the extraction of aggregates on an industrial scale. The present investigation examines an 8-year time series of data collected in and around an active commercial aggregate extraction site off the east coast of the United Kingdom. Both physical and biological data sets suggest a distinct yet localised effect after sustained gravel extraction, with impacted sediments generally appearing more physically homogeneous and faunistically impoverished than undisturbed sediments. Although inter-annual variability of selected assemblage metrics was reduced in disturbed sediments, differences in some assemblage metrics became significant between years. Despite such observations, significant impacts to the benthos in any given year were not sustained for long. However, the magnitude of impact in almost every year would be enough to merit remedial intervention based on an existing model of measuring acceptable levels of disturbance as a result of organic enrichment. Caution must be exercised in making any such recommendations, especially as there are presently no models specifically designed to assess the degree of acceptable disturbance from aggregate extraction. This study not only highlights the importance of and need for long-term data sets in order to better understand the difference between natural and human-induced variability in benthic assemblages, but also emphasises the need to develop more relevant monitoring tools to better manage the activities of the marine aggregate extraction industry.     

The DEEP2 galaxy redshift survey: evolution of the colour–density relation at 0.4 < z < 1.35

Using a sample of 19 464 galaxies drawn from the DEEP2 Galaxy Redshift Survey, we study the relationship between galaxy colour and environment at  0.4 < z < 1.35  . We find that the fraction of galaxies on the red sequence depends strongly on local environment out to   z > 1  , being larger in regions of greater galaxy density. At all epochs probed, we also find a small population of red, morphologically early-type galaxies residing in regions of low measured overdensity. The observed correlations between the red fraction and local overdensity are highly significant, with the trend at   z > 1  detected at a greater than 5σ level. Over the entire redshift regime studied, we find that the colour–density relation evolves continuously, with red galaxies more strongly favouring overdense regions at low z relative to their red-sequence counterparts at high redshift. At   z ≳ 1.3  , the red fraction only weakly correlates with overdensity, implying that any colour dependence to the clustering of  ∼ L *  galaxies at that epoch must be small. Our findings add weight to existing evidence that the build-up of galaxies on the red sequence has occurred preferentially in overdense environments (i.e. galaxy groups) at   z ≲ 1.5  . Furthermore, we identify the epoch  ( z ∼ 2)  at which typical  ∼ L *  galaxies began quenching and moved on to the red sequence in significant number. The strength of the observed evolutionary trends at  0 < z < 1.35  suggests that the correlations observed locally, such as the morphology–density and colour–density relations, are the result of environment-driven mechanisms (i.e. 'nurture') and do not appear to have been imprinted (by 'nature') upon the galaxy population during their epoch of formation.     

X-ray probes of magnetospheric interactions with Jupiter's auroral zones, the Galilean satellites, and the Io plasma torus

Remote observations with the Chandra X-ray Observatory and the XMM-Newton Observatory have shown that the jovian system is a source of X-rays with a rich and complicated structure. The planet's polar auroral zones and its disk are both powerful sources of X-ray emission. Chandra observations revealed X-ray emission from the Io plasma torus and from the Galilean moons Io, Europa, and possibly Ganymede. The emission from the moons is due to bombardment of their surfaces by highly energetic magnetospheric protons, and oxygen and sulfur ions. These ions excite atoms in their surfaces leading to fluorescent X-ray emission lines. These lines are produced against an intense background continuum, including bremsstrahlung radiation from surface interactions of primary magnetospheric and secondary electrons. Although the X-ray emission from the Galilean moons is faint when observed from Earth orbit, an imaging X-ray spectrometer in orbit around one or more of these moons, operating from 200 eV to 8 keV with 150 eV energy resolution, would provide a detailed mapping of the elemental composition in their surfaces. Surface resolution of 40 m for small features could be achieved in a 100-km orbit around one moon while also remotely imaging surfaces of other moons and Jupiter's upper atmosphere at maximum regional resolutions of hundreds of kilometers. Due to its relatively more benign magnetospheric radiation environment, its intrinsic interest as the largest moon in the Solar System, and its mini-magnetosphere, Ganymede would be the ideal orbital location for long-term observational studies of the jovian system. Here we describe the physical processes leading to X-ray emission from the surfaces of Jupiter's moons and the properties required for the technique of imaging X-ray spectroscopy to map the elemental composition of their surfaces, as well as studies of the X-ray emission from the planet's aurora and disk and from the Io plasma torus.     

Neural network time series prediction of environmental variables in a small upland headwater in NE Scotland

Short‐term prediction of environmental variables such as stream flow rate is useful to members of the general public and environmental scientists alike, providing the ability to predict environmental disasters or scientifically interesting events. Here, a neural‐network based method is presented, which is capable of providing advance flood warnings or the prediction of high stream flow events for research purposes in a small upland headwater in NE Scotland. This method relies on training from past time series data acquired in the field, and provides the ability to predict a range of hydrological and meteorological variables up to 24 h ahead using feedback of predicted values at time t as new inputs for the next time step t + 1. The system is rapid and effective, relies on standard neural network training methods, and has the potential to be implemented in a web‐based monitoring and prediction package. The model design could be implemented at any study site where time series data has been gathered, and is sufficiently flexible to accept whatever data is available. Copyright © 2007 John Wiley & Sons, Ltd.     

Subjectively Weighted Development Scenarios for Urban Allocation: A Case Study of South East Queensland

This article presents a GIS‐based methodology to integrate a measure of geographic attractiveness of localities in the process of allocating potential dwellings in the context of a large urban region. The methodology was developed for a study area in Brisbane‐South East Queensland (SEQ), known as the Sunbelt Region, a rapidly growing region and a popular tourist destination in Australia. In this article, we have used a multivariate technique to develop a set of parameterised linear equations to define underlying dimensions that drive residential location decision choices. Aesthetic and accessibility factors were identified in the factor analysis from data collected via a survey of Quality of Life. Spatial measures were based on a combination of network distance and kernel density estimation to calculate ‘aesthetic’ and ‘accessibility’ scenarios, which were then overlaid and multiplied by their subjective weights to create an ‘overall attractiveness scenario’. These development scenarios were integrated as a set of criteria to control the allocation of potential dwelling capacity over the next 25 years at a grid cell level.