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131.
Monique M. Villatoro Carl L. Amos Georg Umgiesser Christian Ferrarin Luca Zaggia Charlotte E.L. Thompson Daniele Are 《Continental Shelf Research》2010
This paper presents results of recent measurements of sand transport made in Chioggia inlet as part of an extensive monitoring programme in the Venetian inlets. Measurements were made in order: (1) to define a relationship between sand transport magnitude and tidal flow; (2) to derive the thresholds for sand transport; (3) to identify the dominant modes of transport; (4) to evaluate the concentration profiles of sand within the benthic boundary layer; (5) to compare bedload transport observations with model predictions using existent bedload formulae; and (6) to produce yearly estimates of bedload transport across the inlet. The vertical distribution of sand in the water column was sampled using modified Helley–Smith bedload samplers at three sites. Transport was found to vary according to the flow and bed grain size, with considerable temporal and spatial variability. A difference of up to three orders of magnitude in transport was observed through the inlet, with higher transport rates measured on the seaward part. The dominant mode of transport in the central inlet was suspension, while bedload was dominant in the mouths. The measured profiles of sand concentration varied with the tidal stage and seabed grain size according to the Rouse parameter (R). R was high at the inlet mouths (1<R<2), indicative of a well-developed bedload layer. The inverse movability number (Ws/U*) was also higher at these sites and appeared to be grain size dependant. Formulae for bedload transport were tested against field data; stochastic methods such as Einstein–Brown, Engelund–Hansen and Van Rijn produce the best fits. The coupled model SHYFEM-Sedtrans05 appears to simulate well observed transport for most conditions of flow. Long-term bedload predictions indicate a dominant export of sand, with a yearly average of 4500 m3. 相似文献
132.
133.
Pore-scale dispersion (PSD), aquifer heterogeneity, sampling volume, and source size influence solute concentrations of conservative tracers transported in heterogeneous porous formations. In this work, we developed a new set of analytical solutions for the concentration ensemble mean, variance, and coefficient of variation (CV), which consider the effects of all these factors. We developed these models as generalizations of the first-order solutions in the log-conductivity variance of point concentration proposed by [Fiori A, Dagan G. Concentration fluctuations in aquifer transport: a rigorous first-order solution and applications. J Contam Hydrol 2000;45(1–2):139–163]. Our first-order solutions compare well with numerical simulations for small and moderate formation heterogeneity and from small to large sampling and source volumes. However, their performance deteriorates for highly heterogeneous formations. Successively, we used our models to study the interplay among sampler size, source volume, and PSD. Our analysis shows a complex and important interaction among these factors. Additionally, we show that the relative importance of these factors is also a function of plume age, of aquifer heterogeneity, and of the measurement location with respect to the mean plume center of gravity. We found that the concentration moments are chiefly controlled by the sampling volume with pore-scale dispersion playing a minor role at short times and for small source volumes. However, the effect of the source volume cannot be neglected when it is larger than the sampling volume. A different behavior occurs for long periods, which may be relevant for old contaminations, or for small injection volumes. In these cases, PSD causes a significant dilution, which is reflected in the concentration statistics. Additionally, at the center of the mean plume, where high concentrations are most likely to occur, we found that sampling volume and PSD are attenuating mechanisms for both concentration ensemble mean and coefficient of variation, except at very large source and sampler sizes, where the coefficient of variation increases with sampler size and PSD. Formation heterogeneity causes a faster reduction of the ensemble mean concentrations and a larger uncertainty at the center of the mean plume. Therefore, our results highlight the importance of considering the combined effect of formation heterogeneity, exposure volume, PSD, source size, and measurement location in performing risk assessment. 相似文献
134.
Gennaro Budetta Daniele Carbone & Filippo Greco 《Geophysical Journal International》1999,138(1):77-88
Gravity changes are presented from a series of field microgravity surveys conducted at Mt Etna between August 1994 and November 1996, a period including the 1995–1996 explosive summit activity. Data were collected along a microgravity network of 69 stations at a monthly to annual sampling rate, depending on each subarray of the network.
Results show that seasonal changes in water level within the volcano may induce gravity changes of up to 20 μgal on Etna's southern slope, and indicate that significant magma movement occurred within and below Etna's edifice between 1994 and 1996. In particular, between September 1994 and October 1995, a mass increase of 2 × 101 0 kg occurred 2000 m beneath the summit craters. Between October 1995 and July 1996 this mass was lost, while another 2 × 101 0 kg was injected at about 1000 m a.s.l. into the 1989 fracture system. From the gravity data alone, it is not possible to distinguish whether the first shallow intrusion (1994–1995) was then injected laterally into the 1989 fracture, or summit activity was fed by the first shallow intrusion, while new magma entered the 1989 fracture system.
While magma was being redistributed within the volcanic edifice, measurements along an E–W-trending profile on the southern slope of the volcano detected some 1.5 × 101 1 kg of magma accumulating 2–3 km below sea level between October 1995 and November 1996. 相似文献
Results show that seasonal changes in water level within the volcano may induce gravity changes of up to 20 μgal on Etna's southern slope, and indicate that significant magma movement occurred within and below Etna's edifice between 1994 and 1996. In particular, between September 1994 and October 1995, a mass increase of 2 × 10
While magma was being redistributed within the volcanic edifice, measurements along an E–W-trending profile on the southern slope of the volcano detected some 1.5 × 10
135.
This study provides a single-point position estimation technique for interplanetary missions by observing visible planets using star trackers. Closed-form least-squares solution is obtained by minimizing the sum of the expected object-space squared distance errors. A weighted least-squares solution is provided by an iterative procedure. The weights are evaluated using the distances to the planets estimated by the least-squares solution. It is shown that the weighted approach only requires one iteration to converge and results in significant accuracy gains compared to simple least squares approach. The light-time correction is taken into account while the star-light aberration cannot be implemented in single-point estimation as it requires knowledge of the observer velocity. The proposed method is numerically validated through a statistical scenario as follows. A three-dimensional grid of test cases is generated: two dimensions sweep through the ecliptic plane and the third dimension sweeps through time from January 1, 2018 to January 1, 2043 in 5-year increments. The observer position is estimated at each test case and the estimate error is recorded. The results obtained show that a large majority of positions are well suited to position estimation by using star trackers pointing to visible planets, and reliable and accurate single-point position estimations can be provided in interplanetary missions. The proposed approach is suitable to be used to initiate a filtering technique to increase the estimation accuracy. 相似文献
136.
Gianni Cortecci 《Marine Geology》1975,19(5):M69-M74
The sulfur-34 isotope profile of sulfate in a red-clay sediment gravity core, 120 cm long, from the South Pacific indicates a very weak, if any, bacterial sulfate-reducing activity. This activity is not shown by the oxygen-18 distribution. The sulfur and oxygen isotopic values are close to those of the overlying seawater sulfate and to the average oceanic values.Of all the parameters mainly able to affect the bacterial activity, such as temperature, molecular oxygen and organic matter, the very small concentration of organic matter seems to be responsible for the almost total lack of sulfate reduction. 相似文献
137.
Gianni Matteucci 《Climate Dynamics》1991,6(2):67-81
A global energy balance model employing the stochastic resonance mechanism, previously used to explain the climatic variability of the late Pleistocene, has now been extended to account for the climatic variations over the full Pleistocene. The possibility that extremely long-term changes (of the order of millions of years) in the boundary conditions of the climate system have altered the response of the Pleistocene climate to the external orbital forcing has been investigated. It is shown that, by slowly changing the only free parameter of the model, the system can undergo a pitchfork bifurcation. The bifurcation point separates a linear regime (identified with the early Pleistocene climate) from a strongly nonlinear regime (the late Pleistocene) where the stochastic resonance mechanism produces rapid and symmetric transitions between the two stable steady states of the system. The main differences in the dynamic features of the two regimes are the change in amplitude of the oscillations, the relative importance of the stochastic forcing, the change in shape of the probability distribution, and the corresponding change in the power centered around the 100000 year cycle: in qualitative agreement with the observed geological record. With the introduction of the external orbital forcing, now spectrally complete and included without requiring any additional hypothesis, the model reproduces the previous results, namely the good correlation with the isotopic record, the appearance of the dominant spectral peaks, as well as the redness of the power spectrum. In particular, it is shown that the orbital forcing in eccentricity acts as a pacemaker of the major glacial cycles of the late Pleistocene through the mechanism of stochastic resonance. A stochastic sensitivity analysis is then applied to validate the significance of the results and to investigate the predictability of the climate system over the time-scales of the orbital cycles. 相似文献
138.
Daniele Galli 《Astrophysics and Space Science》1993,204(1):111-127
In this paper we compute spherical figures of equilibrium resulting from the balance of toroidal magnetic field and rotation. These two distortion effects are assumed to act on rotating magneto-polytropic constant-mass sequences, that is, sequences with each of their members having mass constant and equal to that of the corresponding nondistorted model. 相似文献
139.
Daniele?Borghesi Davide?Vione Valter?Maurino Claudio?MineroEmail author 《Journal of Atmospheric Chemistry》2005,52(3):259-281
This paper reports the results of a study on the transformation of benzene in the presence of solid nitrate salts (NaNO3, NH4NO3) under irradiation in a gas-solid photoreactor. Sodium and ammonium nitrate have been chosen as representative of the composition
of atmospheric particulate, benzene as a model aromatic molecule. The purpose is to simulate the transformations that aromatic
compounds undergo on the surface of dispersed particles in the atmosphere. Irradiation of sodium nitrate causes hydroxylation
and nitration of benzene, yielding phenol and nitrobenzene. This is most likely due to the generation of •OH and •NO2 radicals upon nitrate photolysis, with •OH + O2 leading to the formation of phenol and •OH + •NO2 yielding nitrobenzene. The percentage of oxygen in the reaction environment influences the transformation pathways, with
phenol formation being favoured and nitrobenzene formation depressed by high O2 concentration. In the presence of hematite (α-Fe2O3, another component of atmospheric particulate) very relevant formation of nitrobenzene takes place even with 21% oxygen (simulated
air), indicating that the interaction between hematite and nitrate can lead to the formation of aromatic nitroderivatives
on the surface of atmospheric particulate. The effect of hematite is possibly due to protonation of peroxynitrite, formed
upon nitrate photoisomerisation, to peroxynitrous acid, a powerful nitrating agent. A similar effect leads to relevant formation
of nitrobenzene under atmospheric conditions upon irradiation of the acid salt ammonium nitrate. 相似文献
140.
Fulvio?TononEmail author Armando?Mammino Daniele?Vanni Pier?Giorgio?Borghi Alessandro?Bertero 《Geotechnical and Geological Engineering》2005,23(5):487-518
The Pretunnel technology allows the final or preliminary lining to be built ahead of a tunnel face. The paper shows how this technology has been used for mechanizing the full-face excavation of large tunnels in difficult ground conditions with minimum settlements. Analytical methods are presented for the analysis of the lining both in deep and shallow applications. Fields of applicability are established using results of parametric studies based on the methods proposed. 相似文献