160,539 research outputs found
Nonlinear dynamics of sand banks and sand waves
No full textSand banks and sand waves are two types of sand structures that are commonly observed on an off-shore sea bed. We describe the formation of these features using the equations of the fluid motion coupled with the mass conservation law for the sediment transport. The bottom features are a result of an instability due to tide–bottom interactions. There are at least two mechanisms responsible for the growth of sand banks and sand waves. One is linear instability, and the other is nonlinear coupling between long sand banks and short sand waves. One novel feature of this work is the suggestion that the latter is more important for the generation of sand banks. We derive nonlinear amplitude equations governing the coupled dynamics of sand waves and sand banks. Based on these equations, we estimate characteristic features for sand banks and find that the estimates are consistent with measurements
Linear instability mechanisms for sand wave formation
No full textA height- and flow-dependent model for turbulent viscosity is employed to explain the generation of sand waves in tidal seas. This new model resolves the problem of excitation of very long waves in sand wave formation, because it leads to damping of the long waves and gives a finite separation between the most excited mode and the zero mode. For parameters within their physically realistic ranges, a linear analysis of the resulting system yields a first excited mode whose wavelength is similar to the characteristic wavelength of sand waves observed in nature. The physical mechanism of sand wave formation as predicted by the new model is explained in detail. The dispersion relation obtained can be the starting point for a weakly nonlinear analysis of the system
Sand Creek Watershed Project / Sand Creek Watershed Management Plan
No full textThe overall goal of the Sand Creek Watershed Project is to improve and protect the designated uses of the watershed. In order to achieve this overall goal and attain compliance with the TMDL established in Sand Creek, four goals have been established and prioritized. The primary goal of the Sand Creek Watershed Project is to restore or improve the cold water fishery. The secondary goal of the project is to protect and improve the habitats of native aquatic life and wildlife. Both goals can be achieved by reducing the amount of known pollutants affecting both of these uses. Pollutant reduction can be achieved through proper storm water management that would also serve to address harmful changes in the stream’s flow regime. The third goal of the project is to improve and protect partial body contact recreational uses, such as wading and fishing, by reducing pathogen concentrations, hydrocarbons, toxic substances, and trash. These four known and suspected pollutants also affect total body contact recreation uses, such as swimming. The fourth goal of the Sand Creek Watershed Project is to improve and protect total body contact. Structural and vegetative BMPs, policy and management BMPs, and Information and Education (I&E) activities will be needed to reduce known pollutants affecting these impaired and threatened uses
Sand transport in multiphase pipelines
No full textOver the life of an oil and gas reservoir, it is likely to encounter sand production. In offshore production fields, as there are lack of processing facilities nearby, gas, liquid and sand are often transported together in long distance pipelines. The existence of sand could accumulate in the pipelines under inappropriate operation condition and eventually will lead to a blockage. Thus, to design such systems requires knowledge on how sand is transported, when and where it will accumulate.
This thesis summarizes the experimental work undertaken using the 2 inch, 3 inch and 4 inch multiphase facilities. Generally, the main objectives of the experiments were to i) observe and enhance the understanding of sand transport characteristics in water and air-water flows; ii) investigate sand concentration effect and pipe diameter effect on sand minimum transport condition (MTC); iii) investigate the effect of pipeline orientation (0, +5, +10 and +20 degrees) and viscosity effect (Carboxy Methyl Cellulose (CMC) solution with viscosity of 7, 20cP; Oil with viscosity of 105, 250 and 340cP) on sand MTC; iv) validate the equivalent pressure drop concept for predicting sand MTC in sand-air-water flow and v) extend current MTC prediction model for sand-water flow to account for different sand concentrations .
Similar sand behaviour was observed in horizontal sand-water flow in all pipe sizes tested. At minimum transport velocity, sand particles were observed transporting in form of sand streaks. For horizontal sand-air-water flow, sand transport characteristics and MTC were strongly dependent on the air-water flow regime. Sand was found to be transported more efficiently within slug or roll wave body, where turbulence is generated intensively.
Parametric studies were conducted to investigate the factors affecting sand MTC in water and air-water flows in pipeline. It was found that the MTC will increase as sand concentration and pipe diameter increase. Pipeline orientation was found having little effect on sand behaviours and MTC in upwardly inclined water flow. However, in upwardly inclined air-water flow, although sand particles were observed sometime moving backward with the liquid film, the superficial gas and liquid velocities required to transport sand were less than those in the horizontal pipeline due to the fact that slug flow regime was found more prevailing in inclined pipeline. In addition, the liquid viscosity effect on sand MTC in single phase liquid flow was investigated due to the increase of concerns relating to solids transport in high viscosity crudes. It appeared that, in turbulent flow, sand MTC increased slightly as the fluid viscosity increased. However, when the bulk flow became laminar, the MTC decreased as the fluid viscosity increased.
After visually obtained the sand MTC in air-water flow, the measured pressure gradients were compared between MTC condition for sand-water flow for different sand concentrations, the results indicate that the equivalent pressure gradients concept is a valid approach to extend the sand MTC prediction from water flow to air-water flow conditions for the purpose of pipeline design.
Two concentration correction correlations (dual range and single range) were proposed. The modified model could account for a wider range of sand concentration (from 0.000005 to 0.3 volume fraction) in water flow. The predicted MTC velocities showed good agreement with the experimental results
Regeneration of sand waves after dredging
No full textSand waves are large bed waves on the seabed, being a few metres high and lying hundreds of metres apart. In some cases, these sand waves occur in navigation channels. If these sand waves reduce the water depth to an unacceptable level and hinder navigation, they need to be dredged. It has been observed in the Bisanseto Channel in Japan that the sand waves tend to regain their shape after dredging. In this paper, we address modelling of this regeneration of sand waves, aiming to predict this process. For this purpose, we combine a very simple, yet effective, amplitude-evolution model based on the Landau equation, with measurements in the Bisanseto Channel. The model parameters are tuned to the measured data using a genetic algorithm, a stochastic optimization routine. The results are good. The tuned model accurately reproduces the measured growth of the sand waves. The differences between the measured weave heights and the model results are smaller than the measurement noise. Furthermore, the resulting parameters are surprisingly consistent, given the large variations in the sediment characteristics, the water depth and the flow field. This approach was tested on its predictive capacity using a synthetic test case. The model was tuned based on constructed predredging data and the amplitude evolution as measured for over 2 years. After tuning, the predictions were accurate for about 10 years. Thus, it is shown that the approach could be a useful tool in the optimization of dredging strategies in case of dredging of sand waves
Booming Sand Dunes
No full text"Booming" sand dunes are able to produce low-frequency sound that resembles a pure note from a music instrument. The sound has a dominant audible frequency (70-105 Hz) and several higher harmonics and may be heard from far distances away. A natural or induced avalanche from a slip face of the booming dune triggers the emission that may last for several minutes. There are various references in travel literature to the phenomenon, but to date no scientific explanation covered all field observations.
This thesis introduces a new physical model that describes the phenomenon of booming dunes. The waveguide model explains the selection of the booming frequency and the amplification of the sound in terms of constructive interference in a confined geometry. The frequency of the booming is a direct function of the dimensions and velocities in the waveguide. The higher harmonics are related to the higher modes of propagation in the waveguide.
The experimental validation includes quantitative field research at the booming dunes of the Mojave Desert and Death Valley National Park. Microphone and geophone recordings of the acoustic and seismic emission show a variation of booming frequency in space and time. The analysis of the sensor data quantifies wave propagation characteristics such as speed, dispersion, and nonlinear effects and allows the distinction between the source mechanism of the booming and the booming itself.
The migration of sand dunes results from a complicated interplay between dune building, wind regime, and precipitation. The morphological and morphodynamical characteristics of two field locations are analyzed with various geophysical techniques. Ground-penetrating radar images the subsurface structure of the dunes and reveal a natural, internal layering that is directly related to the history of dune migration. The seismic velocity increases abruptly with depth and gradually increases with downhill position due to compaction. Sand sampling shows local cementation of sand grains within the discrete layers that explains the increase in velocity and decrease in porosity. The subsurface layering may influence the speed of dune migration and therefore have important consequences on desertification.
The positive qualitative and quantitative correlation between the subsurface layering in the dune and the manifestation of the booming sound implies a close relation between environmental factors and the booming emission. In this thesis, the frequency of booming is correlated with the depth of the waveguide and the seismic velocities. The variability on location and season suggests that the waveguide theory successfully unravels the phenomenon
of booming sand dunes.</p
Initial Plant Growth in Sand Mine Spoil Amended with Peat Moss and Fertilizer Under Greenhouse Conditions: Potential Species for Use in Reclamation
No full textThe Great Lakes Basin exhibits the largest collection of freshwater sand dunes in the world. Sand dunes are ecologically important and support a unique assemblage of flora and fauna. Sand dunes are also economically valuable. However, when sand dunes are mined, soil quality is drastically reduced. Therefore, soil quality improvements followed by revegetation maybe necessary for successful reclamation. This study evaluates the germination and initial growth of 2 legume species, sundial lupine (Lupinus perennis) and Illinois bundleflower (Desmanthus illinoensis), and 2 warm-season grass species, Indian grass (Sorghastrum nutans) and little bluestem (Schizachyrium scoparium), in the presence of 2 soil amendments (inorganic fertilizer and sphagnum peat moss) added to spoil from a local sand mine. We sowed species in pots and propagated them under greenhouse conditions. Results indicate that sundial lupine and Illinois bundleflower exhibited the greatest germination and growth among species. Peat moss had the greatest overall impact on germination and growth while the addition of fertilizer positively affected initial growth. Based on these results, sundial lupine is recognized as a primary candidate for sand mine reclamation, while Illinois bundleflower is also recommended as an appropriate species for revegetation efforts. We recommend using soil amendments that are functionally equivalent to peat in increasing soil water holding capacity. We further suggest that fertilization may be accomplished by including legumes in plant species mixes used for revegetation. Results presented here may help to identify appropriate species and soil amendments for the reclamation of former sand mines or restoration of freshwater sand dunes
MODELLING OF PILED RAFT FOUNDATIONS IN SAND
No full textA piled raft is a composite foundation in which the piles are used as
settlement reducers and they share, with the raft, the load from the
superstructure. The applied load is transferred from the raft to the
shallow soil and to the pile heads, and from the piles it is diffused
through the shaft and the base to deeper soil. The pile–raft and pile–pile
interactions represent the distinctive aspect of the piled raft foundations
since they modify the load–bearing behaviour of each foundation
component, compared to an analogous isolated element, thus determining
the overall foundation behaviour.
The main aim of this thesis is to highlight the effects of the raft–soil–pile
interactions on the resistance and stiffness of axially loaded piled raft
foundations in sand.
A series of centrifuge tests on models of rigid circular piled rafts in loose
saturated sand has been performed to this end, employing both non
displacement and displacement piles. The raft settlement and the load
transmitted to the pile heads and bases were monitored during the tests,
which also included unpiled raft and isolated pile tests.
The test results have been analysed in terms of bearing capacity and
stiffness; the former according to a load efficiency method, the latter by
comparing the values obtained from centrifuge tests with those evaluated
through a simplified analytical method.
ii Modelling of Piled Raft Foundations in Sand D. Giretti
In order to clarify the effect of the pressure transmitted by the raft to the
soil on the behaviour of a single capped pile, some of the centrifuge tests
were simulated via finite element numerical analyses, using an elasto–plastic strain hardening constitutive model for the sand. The geometry
and the dimensions of the numerical models corresponded to those of the
physical ones and the simulations were carried out applying an
accelerated gravitational field to the mesh.
The influence of a granular layer, interposed between the raft and the pile
heads, on the load transfer mechanism has also been analysed, through an
additional series of centrifuge tests which was performed on square rigid
raft models on displacement piles in dry dense sand
The role of leishmania proteophosphoglycans in sand fly transmission and infection of the Mammalian host.
No full textLeishmania are transmitted by the bite of their sand fly vector and this has a significant influence on the virulence of the resulting infection. From our studies into the interaction between parasite, vector, and host we have uncovered an important missing ingredient during Leishmania transmission. Leishmania actively adapt their sand fly hosts into efficient vectors by secreting Promastigote Secretory Gel (PSG), a proteophosphoglycan (PPG)-rich, mucin-like gel which accumulates in sand fly gut and mouthparts. This has the effect of blocking the fly, such that during bloodfeeding both parasites and gel are co-transmitted in an act of regurgitation. We are discovering that this has further implications for the mammalian infection, again, in favor of the parasite. Experimentally, PSG exacerbates cutaneous and visceral leishmaniasis and can promote the chronicity of Leishmania infection, even in mouse strains normally capable of controlling leishmaniasis. The underlying mechanism of PSG's action is a major focus of our ongoing work. This review aims to synthesize what is known about the role and action of PSG and its constituent proteophosphoglycans, for parasite colonization of the sand fly, transmission, and mammalian infection. Lastly, we discuss potential exploitation of this important vector-transmitted product and future avenues of research
Analysis of the Shaft Resistance of Nondisplacement Piles in Sand
No full textThe paper examines, using numerical modelling, the problem of the limit shaft resistance of non-displacement piles installed in sands. The modelling makes use of an advanced, two-surface-plasticity constitutive model. The constitutive model predicts the soil response in both the small- and the large-strain range, while taking into account the effects of the intermediate principal effective stress and of the inherent anisotropy of the sand. Finite element analyses of shearing along the pile shaft are performed in order to examine the development of limit unit shaft resistance and the changes in stress state around the shaft upon axial loading of the pile. Special focus is placed on the operative value of the lateral earth pressure coefficient when limit shaft resistance is reached. The analyses offer useful insights regarding the factors controlling the value of unit shaft resistance in sands. The simulations predict a significant build-up of horizontal effective stress for dense sands. Based on these simulations, we propose a relationship between the lateral earth pressure coefficient for use in the calculation of the limit shaft resistance of the pile and the initial density and stress state of the sand
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