Numerical modelling of horizontal sediment-laden jets

Sediment-laden turbulent flows are commonly encountered in natural and engineered environments. It is well known that turbulence generates fluctuations to the particle motion, resulting in modulation of the particle settling velocity. A novel stochastic particle tracking model is developed to predict the particle settling out and deposition from a sediment-laden jet. Particle velocity fluctuations in the jet flow are modelled from a Lagrangian velocity autocorrelation function that incorporates the physical mechanism leading to a reduction of settling velocity. The model is first applied to study the settling velocity modulation in a homogeneous turbulence field. Consistent with basic experiments using grid-generated turbulence and computational fluid dynamics (CFD) calculations, the model predicts that the apparent settling velocity can be reduced by as much as 30 % of the stillwater settling velocity. Using analytical solution for the jet mean flow and semi-empirical RMS turbulent velocity fluctuation and dissipation rate profiles derived from CFD predictions, model predictions of the sediment deposition and cross-sectional concentration profiles of horizontal sediment-laden jets are in excellent agreement with data. Unlike CFD calculations of sediment fall out and deposition from a jet flow, the present method does not require any a priori adjustment of particle settling velocity.     

Sounds associated with the reproductive behavior of the black grouper (Mycteroperca bonaci)

Passive acoustic and synchronous video recordings were made at two spawning aggregation sites to study the sounds associated with reproductive behaviors of Mycteroperca bonaci. A characteristic sound was produced during courtship displays involving behaviors commonly observed for groupers of this genus at aggregations. The sound has a short pulsing section followed by a longer tonal portion with a mean peak frequency below 100 Hz. Courtship-associated sounds were quantified over one spawning season at Mona Island, Puerto Rico. Most of the daily sound production occurred during a period of 2 h prior to sunset. The highest rates of sound production lasted for a period of 10 days with lunar periodicity over three consecutive months coincident with the reported season of reproduction. Passive acoustics provide a tool to measure the variability of the reproductive activity of M. bonaci over time and may provide a method to evaluate current strategies designed to protect multi-species spawning aggregations that are critical for the recovery of threatened groupers.     

Waves and turbulence in katabatic winds

The measurements taken during the Vertical Transport and Mixing Experiment (VTMX, October, 2000) on a northeastern slope of Salt Lake Valley, Utah, were used to calculate the statistics of velocity fluctuations in a katabatic gravity current in the absence of synoptic forcing. The data from ultrasonic anemometer-thermometers placed at elevations 4.5 and 13.9 m were used. The contributions of small-scale turbulence and waves were isolated by applying a high-pass digital (Elliptical) filter, whereupon the filtered quantities were identified as small-scale turbulence and the rest as internal gravity waves. Internal waves were found to play a role not only at canonical large gradient Richardson numbers $(\overline{\hbox {Ri}_\mathrm{g} } >1)$ , but sometimes at smaller values $(0.1 < \overline{\hbox {Ri}_\mathrm{g}}<1)$ , in contrast to typical observations in flat-terrain stable boundary layers. This may be attributed, at least partly, to (critical) internal waves on the slope, identified by Princevac et al. [1], which degenerate into turbulence and help maintain an active internal wave field. The applicability of both Monin-Obukhov (MO) similarity theory and local scaling to filtered and unfiltered data was tested by analyzing rms velocity fluctuations as a function of the stability parameter z/L, where L is the Obukhov length and z the height above the ground. For weaker stabilities, $\hbox {z/L}<1$ , the MO similarity and local scaling were valid for both filtered and unfiltered data. Conversely, when $\hbox {z/L}>1$ , the use of both scaling types is questionable, although filtered data showed a tendency to follow local scaling. A relationship between z/L and $\overline{\hbox {Ri}_\mathrm{g} }$ was identified. Eddy diffusivities of momentum $\hbox {K}_\mathrm{M}$ and heat $\hbox {K}_\mathrm{H}$ were dependent on wave activities, notably when $\overline{\hbox {Ri}_\mathrm{g} } > 1$ . The ratio $\hbox {K}_{\mathrm{H}}/\hbox {K}_{\mathrm{M}}$ dropped well below unity at high $\overline{\hbox {Ri}_\mathrm{g} }$ , in consonance with previous laboratory stratified shear layer measurements as well as other field observations.     

Food allocation rules vary with age and experience in a cooperatively breeding parrot

Although it is known that parents can differ in their optimal resource allocation to offspring in size-structured broods, the mechanisms determining differences in allocation rules of carers are not yet clarified. In cooperatively breeding species, breeders and non-reproductive helpers often differ in their fitness payoffs of providing care and in their breeding experience. Cooperative breeders thus provide an appropriate system to examine two hypotheses originally proposed to explain differences in food allocation among parents: (i) food allocation between carers differs because of the distinct cost-benefit ratio of selective feeding (i.e. breeders and helpers are expected to differ in food allocation) and (ii) carers differ in their ability to feed selectively (i.e. differences in food allocation are expected between experienced adults and inexperienced yearlings). We compared feeding rates with which breeders, old helpers and yearling helpers provisioned nestlings of different hatching rank. The influence of experience upon food allocation was further assessed by comparing food allocation of yearlings early and late during nesting. We show that allocation rules differ between age classes because breeders and old helpers fed the youngest chicks most, whereas yearlings showed the opposite pattern. The role of experience was supported by the fact that yearlings adjusted food allocation to that observed in experienced adults during the breeding season. We thus suggest that food allocation in El Oro parakeets depends either on differential skills of adults to transfer food to the youngest chick or on their ability to recognize nestling needs.     

Spatio-temporal log-Gaussian Cox processes for modelling wildfire occurrence: the case of Catalonia, 1994–2008

Wildfires have become one of the principal environmental problems in the Mediterranean basin. While fire plays an important role in most terrestrial plant ecosystems, the potential hazard that it represents for human lives and property has led to the application of fire exclusion policies that, in the long term, have caused severe damage, mainly due to the increase of fuel loadings in forested areas, in some forest systems. The lack of an easy solution to forest fire management highlights the importance of preventive tasks. The observed spatio-temporal pattern of wildfire occurrences may be idealized as a realization of some stochastic process. In particular, we may use a space–time point pattern approach for the analysis and inference process. We studied wildfires in Catalonia, a region in the north-east of the Iberian Peninsula, and we analyzed the spatio-temporal patterns produced by those wildfire incidences by considering the influence of covariates on trends in the intensity of wildfire locations. A total of 3,166 wildfires from 1994–2008 have been recorded. We specified spatio-temporal log-Gaussian Cox process models. Models were estimated using Bayesian inference for Gaussian Markov Random Field through the integrated nested Laplace approximation algorithm. The results of our analysis have provided statistical evidence that areas closer to humans have more human induced wildfires, areas farther have more naturally occurring wildfires. We believe the methods presented in this paper may contribute to the prevention and management of those wildfires which are not random in space or time.