Tom Clarke

Acoustic measurements of suspended sand concentration in the C2S2 experiment at Stanhope Lane, Prince Edward Island

Abstract A 3 MHz acoustic concentration meter (ACM) was deployed with electromagnetic flowmeters in the nearshore region in October 1984 at Stanhope Lane, Prince Edward Island, Canada, as a component of the Canadian Coastal Sediment Study. The ACM measured the backscatter of acoustic energy in a vertical beam pattern within approximately 1 m of the seabed. Laboratory calibration experiments were conducted to verify a method for converting acoustic backscatter into suspended sand concentration profiles. The ACM functioned well when there were no air bubbles in the water. Both the suspended sand concentration profiles and the bottom location were measured with an approximately 1 s time resolution and a 1 cm vertical spatial resolution. The suspension of sand is related to the wave and current forcing, and is dominated by events of short duration.

SARS: what have we learned?

Its less than four months since the World Health Organization issued global warnings about a mysterious and deadly form of pneumonia. Natures reporters pose key questions about the outbreak, and assess our preparedness to deal with future viral threats.Its less than four months since the World Health Organization issued global warnings about a mysterious and deadly form of pneumonia. Natures reporters pose key questions about the outbreak, and assess our preparedness to deal with future viral threats.

Sediment resuspension by surface-wave action: An examination of possible mechanisms

Abstract Detailed spectral and coherence analyses of both nephelometer data and near-bottom acoustic concentration profiles support the conclusion of Lesht et al. (1980) that surfacewave activity is the most important input to the sediment resuspension process. The sand-sized fraction is suspended primarily by bursts of turbulence which are related to peak values of the envelope of surface waves. Two mechanisms for the resuspension of fine sediment are considered. When the bed is bimodal sand and silt, pore-water motion induced by wave-pressure fluctuations may carry fine particles into the boundary layer where they are suspended. Motion of the sand matrix as bedload will also expose fine particles to the flow so that they become suspended. The combination of these two mechanisms accounts for the observed quasi-linear relation between wave orbital velocity and suspended sediment concentration for the Long Island data. The second mechanism is also consistent with observations of near-bottom fluid velocity and suspended sediment concentration in Norton Sound, Alaska.

Sand ridges on the inner Atlantic shelf of North America: Morphometric comparisons with Huthnance stability model

Sand ridge fields on the inner shelf of the Middle Atlantic Bight are generally thought to have formed in response to northeasterly storm flows as the shoreface underwent erosional retreat with postglacial sea-level rise. However, the hydrodynamic mechanism is poorly unerstood. Coastal boundary models see the ridges as responses of the seafloor to distortions in the flow induced by the coastal boundary. Stability models propose that an irregular initial topography will evolve toward an ordered array of bedforms in response to repeated flow events. The two classes of models are not mutually exclusive, nor are members within each class mutually exclusive. Results of measurements of ridge spacing on the inner Atlantic shelf of North America agree with the predictions of stability models.

Generalization of neural networks to the complex plane

A complex-valued generalization of neural networks is presented. The dynamics of complex neural networks have parallels in discrete complex dynamics which give rise to the Mandelbrot set and other fractals. The continuation to the complex plane of common activation functions and the resulting neural dynamics are discussed. An activation function with more desirable characteristics in the complex plane is proposed. The dynamics of this activation function include the possibility of self oscillation. Possible applications in signal processing and neurobiological modeling are discussed

Dispersal Pathways for Particle-Associated Pollutants

Particle-associated pollutants (totaling 107 metric tons per year) are introduced into the New York Bight by ocean dumping, estuarine discharge, sewage outfalls, eolian transport, and shipping waste and spillage. Oceanic and estuarine circulation processes dilute and transport the particles by a natural dispersal system that also tends to be highly distributive; particle-associated pollutants apparently seek the same sinks in the Hudson River shelf valley and intracoastal weltlands, regardless of their point of introduction.

Piezometer Probes for Assessing Effective Stress and Stability in Submarine Sediments

Multisensor piezometer probes were deployed at four different sites in the Mississippi Delta in water depths ranging from 13.5 to 43.6 m with sensor penetration depths of up to 15.6 meters. Absolute and differential pressure sensors were used to measure pore water pressure and excess pressures, respectively. The free water column pressure was measured with absolute pressure sensors. Pore pressures induced by probe insertion were determined as well as ambient excess pore pressures following the time-dependent decay of induced pressures. Significant differences in the pore pressures and related geotechnical properties were found between East Bay and Main Pass sediments. Generally higher probe insertion pressures and lower ambient excess pore pressures were characteristic of Main Pass compared to East Bay. Probe insertion pressures (Ui) were found to correlate well with the undrained shear strength (Su) of the sediments, indicating reasonably good agreement with the predicted relation: Ui = 6Su as suggested by an earlier study42. Using this relationship undrained shear strengths were calculated and compared with measured values.

Possible bottom current response to surface winds in the Hudson Shelf Channel

Current measurements made in the Hudson Shelf Channel during the summer of 1973 show essentially channel axial bottom current even though the channel aspect ratio is small in the area of measurement. Although the current record is of short duration, correlation of water movement with surface winds is suggested by the data. The sense of summertime nontidal bottom flow in the channel (up or down channel) would appear to be controlled by the surface wind direction (offshore or onshore). These results would suggest the likelihood of net down-channel flow during the summer months.

A continuum hard-sphere model of protein adsorption

Protein adsorption plays a significant role in biological phenomena such as cell-surface interactions and the coagulation of blood. Two-dimensional random sequential adsorption (RSA) models are widely used to model the adsorption of proteins on solid surfaces. Continuum equations have been developed so that the results of RSA simulations can be used to predict the kinetics of adsorption. Recently, Brownian dynamics simulations have become popular for modeling protein adsorption. In this work a continuum model was developed to allow the results from a Brownian dynamics simulation to be used as the boundary condition in a computational fluid dynamics (CFD) simulation. Brownian dynamics simulations were used to model the diffusive transport of hard-sphere particles in a liquid and the adsorption of the particles onto a solid surface. The configuration of the adsorbed particles was analyzed to quantify the chemical potential near the surface, which was found to be a function of the distance from the surface and the fractional surface coverage. The near-surface chemical potential was used to derive a continuum model of adsorption that incorporates the results from the Brownian dynamics simulations. The equations of the continuum model were discretized and coupled to a CFD simulation of diffusive transport to the surface. The kinetics of adsorption predicted by the continuum model closely matched the results from the Brownian dynamics simulation. This new model allows the results from mesoscale simulations to be incorporated into micro- or macro-scale CFD transport simulations of protein adsorption in practical devices.

Introducing age-based parameters into simulations of crowd dymanics

Very few crowds consist of individuals who are exactly the same. Defining variables, such as age, and how they affect an individual¿s movement, could increase realism in simulations of crowd movement. In this paper, we present and discuss how age variations of individuals can be included in crowd simulations. Starting with the Helbing, Molnar, Farkas, and Vicsek model (HMFV), we modeled age differences by modifying the strength of the existing social forces. We created simulation scenarios with the varied strengths and used multiple approaches for validation, including experts¿ subjective validation and experimental validation via comparison of model predictions with observed crowd movements. The results indicated that individual characteristics such as age can be modeled by social forces. Future extensions of our work would be to include individuals, small subgroups, and/or large groups of people to model multicultural crowd behavior.