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41.
Vegetation turnover in a braided river: frequency and effectiveness of floods of different magnitude 下载免费PDF全文
Nicola Surian Matteo Barban Luca Ziliani Giovanni Monegato Walter Bertoldi Francesco Comiti 《地球表面变化过程与地形》2015,40(4):542-558
This work addresses the temporal dynamics of riparian vegetation in large braided rivers, exploring the relationship between vegetation erosion and flood magnitude. In particular, it investigates the existence of a threshold discharge, or a range of discharges, above which erosion of vegetated patches within the channel occurs. The research was conducted on a 14 km long reach of the Tagliamento River, a braided river in north‐eastern Italy. Ten sets of aerial photographs were used to investigate vegetation dynamics in the period 1954–2011. By using different geographic information system (GIS) procedures, three aspects of geomorphic‐vegetation dynamics and interactions were addressed: (i) long‐term (1954–2011) channel evolution and vegetation dynamics; (ii) the relationship between vegetation erosion/establishment and flow regime; (iii) vegetation turnover, in the period 1986–2011. Results show that vegetation turnover is remarkably rapid in the study reach with 50% of in‐channel vegetation persisting for less than 5–6 years and only 10% of vegetation persisting for more than 18–19 years. The analysis shows that significant vegetation erosion is determined by relatively frequent floods, i.e. floods with a recurrence interval of c. 1–2.5 years, although some differences exist between sub‐reaches with different densities of vegetation cover. These findings suggest that the erosion of riparian vegetation in braided rivers may not be controlled solely by very large floods, as is the case for lower energy gravel‐bed rivers. Besides flow regime, other factors seem to play a significant role for in‐channel vegetation cover over long time spans. In particular, erosion of marginal vegetation, which supplies large wood elements to the channel, increased notably over the study period and was an important factor for in‐channel vegetation trends. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
42.
Matteo Vacchi Monica Montefalcone Chiara F. Schiaffino Valeriano Parravicini Carlo Nike Bianchi Carla Morri Marco Ferrari 《Marine pollution bulletin》2014
The upper portion of the meadows of the protected Mediterranean seagrass Posidonia oceanica occurs in the region of the seafloor mostly affected by surf-related effects. Evaluation of its status is part of monitoring programs, but proper conclusions are difficult to draw due to the lack of definite reference conditions. Comparing the position of the meadow upper limit with the beach morphodynamics (i.e. the distinctive type of beach produced by topography and wave climate) provided evidence that the natural landwards extension of meadows can be predicted. An innovative model was therefore developed in order to locate the region of the seafloor where the meadow upper limit should lie in natural conditions (i.e. those governed only by hydrodynamics, in absence of significant anthropogenic impact). This predictive model was validated in additional sites, which showed perfect agreement between predictions and observations. This makes the model a valuable tool for coastal management. 相似文献
43.
Lorenzo Iorio Herbert I. M. Lichtenegger Matteo Luca Ruggiero Christian Corda 《Astrophysics and Space Science》2011,331(2):351-395
Recent years have seen increasing efforts to directly measure some aspects of the general relativistic gravitomagnetic interaction
in several astronomical scenarios in the solar system. After briefly overviewing the concept of gravitomagnetism from a theoretical
point of view, we review the performed or proposed attempts to detect the Lense-Thirring effect affecting the orbital motions
of natural and artificial bodies in the gravitational fields of the Sun, Earth, Mars and Jupiter. In particular, we will focus
on the evaluation of the impact of several sources of systematic uncertainties of dynamical origin to realistically elucidate
the present and future perspectives in directly measuring such an elusive relativistic effect. 相似文献
44.
The aim of this paper is to study the effects of soil–structure interaction on the seismic response of coupled wall-frame structures on pile foundations designed according to modern seismic provisions. The analysis methodology based on the substructure method is recalled focusing on the modelling of pile group foundations. The nonlinear inertial interaction analysis is performed in the time domain by using a finite element model of the superstructure. Suitable lumped parameter models are implemented to reproduce the frequency-dependent compliance of the soil-foundation systems. The effects of soil–structure interaction are evaluated by considering a realistic case study consisting of a 6-storey 4-bay wall-frame structure founded on piles. Different two-layered soil deposits are investigated by varying the layer thicknesses and properties. Artificial earthquakes are employed to simulate the earthquake input. Comparisons of the results obtained considering compliant base and fixed base models are presented by addressing the effects of soil–structure interaction on displacements, base shears, and ductility demand. The evolution of dissipative mechanisms and the relevant redistribution of shear between the wall and the frame are investigated by considering earthquakes with increasing intensity. Effects on the foundations are also shown by pointing out the importance of both kinematic and inertial interaction. Finally, the response of the structure to some real near-fault records is studied. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
45.
Galimberti Matteo Marinoni Nicoletta Della Porta Giovanna Marchi Maurizio Dapiaggi Monica 《Mineralogy and Petrology》2017,111(5):793-806
Mineralogy and Petrology - Limestone represents the main raw material for ordinary Portland cement clinker production. In this study eight natural limestones from different geological environments... 相似文献
46.
In this paper the kinematic seismic interaction of single piles embedded in soil deposits is evaluated by focusing the attention on the bending moments induced by the transient motion. The analysis is performed by modeling the pile like an Euler–Bernoulli beam embedded in a layered Winkler-type medium. The excitation motion is obtained by means of a one-D propagation analysis. A comprehensive parametric analysis is carried out by varying the main parameters governing the dynamic response of piles like the soil properties, the bedrock location, the diameter and embedment in the bedrock of piles. On the basis of the parametric analysis, a new design formula for predicting the kinematic bending moments for both the cross-sections at the deposit–bedrock interface and at the pile head is proposed. 相似文献
47.
We study pairwise interactions of elliptical quasi-geostrophic (QG) vortices as the limiting case of vanishingly thin uniform potential vorticity ellipsoids. In this limit, the product of the vertical extent of the ellipsoid and the potential vorticity within it is held fixed to a finite non-zero constant. Such elliptical “lenses” inherit the property that, in isolation, they steadily rotate without changing shape. Here, we use this property to extend both standard moment models and Hamiltonian ellipsoidal models to approximate the dynamical interaction of such elliptical lenses. By neglecting non-elliptical deformations, the simplified models reduce the dynamics to just four degrees of freedom per vortex. For simplicity, we focus on pairwise interactions between identical elliptical vortices initially separated in both the horizontal and vertical directions. The dynamics of the simplified models are compared with the full QG dynamics of the system, and show good agreement as expected for sufficiently distant lenses. The results reveal the existence of families of steadily rotating equilibria in the initial horizontal and vertical separation parameter space. For sufficiently large vertical separations, equilibria with varying shape exist for all horizontal separations. Below a critical vertical separation (stretched by the constant ratio of buoyancy to Coriolis frequencies N / f), comparable to the mean radius of either vortex, a gap opens in horizontal separation where no equilibria are possible. Solutions near the edge of this gap are unstable. In the full QG system, equilibria at the edge of the gap exhibit corners (infinite curvature) along their boundaries. Comparisons of the model results with the full nonlinear QG evolution show that the early stages of the instability are captured by the Hamiltonian elliptical model but not by the moment model that inaccurately estimates shorter-range interactions. 相似文献
48.
The effects of low-temperature on the crystal structure of a natural epidote [Ca1.925Fe0.745Al2.265Ti0.004Si3.037O12(OH), a = 8.8924(7), b = 5.6214(3), c = 10.1547(6)? and β = 115.396(8)° at room conditions, Sp. Gr. P21
/m] have been investigated with a series of structure refinements down to 100 K on the basis of X-ray single-crystal diffraction
data. The reflection conditions confirm that the space group is maintained within the T-range investigated. Structural refinements at all temperatures show the presence of Fe3+ at the octahedral M(3) site only [%Fe(M3) = 70.6(4)% at 295 K]. Only one independent proton site was located and two possible
H-bonds occur, with O(10) as donor and O(4) and O(2) as acceptors. The H-bonding scheme is maintained down to 100 K and is
supported by single crystal room-T polarised FTIR data. FTIR Spectra over the region 4,000–2,500 cm−1 are dominated by the presence of a strongly pleochroic absorption feature which can be assigned to protonation of O(10)–O(4).
Previously unobserved splitting of this absorption features is consistent with a NNN influence due to the presence of Al and
Fe3+ on the nearby M(3) site. An additional relatively minor absorption feature in FTIR spectra can be tentatively assigned to
protonation of O(10)–O(2). Low-T does not affect significantly the tetrahedral and octahedral bond distances and angles, even when distances are corrected
for “rigid body motions”. A more significant effect is observed for the bond distances of the distorted Ca(1)- and Ca(2)-polyhedra,
especially when corrected for “non-correlated motion”. The main low-T effect is observed on the vibrational regime of the atomic sites, and in particular for the two Ca-sites. A significant reduction
of the magnitude of the thermal displacement ellipsoids, with a variation of U
eq
(defined as one-third of the trace of the orthogonalised U
ij
tensor) by ~40% is observed for the Ca-sites between 295 and 100 K. Within the same T-range, the U
eq
of the octahedral and oxygen sites decrease similarly by ~35%, whereas those of the tetrahedral cations by ~22%. 相似文献
49.
Matteo Alvaro Fernando Cámara M. Chiara Domeneghetti Fabrizio Nestola Vittorio Tazzoli 《Contributions to Mineralogy and Petrology》2011,162(3):599-613
A natural Ca-poor pigeonite (Wo6En76Fs18) from the ureilite meteorite sample PCA82506-3, free of exsolved augite, was studied by in situ high-temperature single-crystal
X-ray diffraction. The sample, monoclinic P21/c, was annealed up to 1,093°C to induce a phase transition from P21/c to C2/c symmetry. The variation with increasing temperature of the lattice parameters and of the intensity of the b-type reflections (h + k = 2n + 1, present only in the P21/c phase) showed a displacive phase transition P21/c to C2/c at a transition temperature T
Tr = 944°C, first order in character. The Fe–Mg exchange kinetics was studied by ex situ single-crystal X-ray diffraction in
a range of temperatures between the closure temperature of the Fe–Mg exchange reaction and the transition temperature. Isothermal
disordering annealing experiments, using the IW buffer, were performed on three crystals at 790, 840 and 865°C. Linear regression
of ln k
D versus 1/T yielded the following equation:
ln k\textD = - 3717( ±416)/T(K) + 1.290( ±0.378); (R2 = 0.988) \ln \,k_{\text{D}} = - 3717( \pm 416)/T(K) + 1.290( \pm 0.378);\quad (R^{2} = 0.988) . The closure temperature (T
c) calculated using this equation was ∼740(±30)°C. Analysis of the kinetic data carried out taking into account the e.s.d.'s
of the atomic fractions used to define the Fe–Mg degree of order, performed according to Mueller’s model, allowed us to retrieve
the disordering rate constants C
0
K
dis+ for all three temperatures yielding the following Arrhenius relation:
ln( C0 K\textdis + ) = ln K0 - Q/(RT) = 20.99( ±3.74) - 26406( ±4165)/T(K); (R2 = 0.988) \ln \left( {C_{0} K_{\text{dis}}^{ + } } \right) = \ln \,K_{0} - Q/(RT) = 20.99( \pm 3.74) - 26406( \pm 4165)/T(K);\quad (R^{2} = 0.988) . An activation energy of 52.5(±4) kcal/mol for the Fe–Mg exchange process was obtained. The above relation was used to calculate
the following Arrhenius relation modified as a function of X
Fe (in the range of X
Fe = 0.20–0.50):
ln( C0 K\textdis + ) = (21.185 - 1.47X\textFe ) - \frac(27267 - 4170X\textFe )T(K) \ln \left( {C_{0} K_{\text{dis}}^{ + } } \right) = (21.185 - 1.47X_{\text{Fe}} ) - {\frac{{(27267 - 4170X_{\text{Fe}} )}}{T(K)}} . The cooling time constant, η = 6 × 10−1 K−1 year−1 calculated on the PCA82506-3 sample, provided a cooling rate of the order of 1°C/min consistent with the extremely fast late
cooling history of the ureilite parent body after impact excavation. 相似文献
50.
A. Merloni T. Di Matteo † A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2000,318(1):L15-L19
The simultaneous presence of a strong quasi-periodic oscillation, of period ∼10 s, in the optical and X-ray light curves of the X-ray transient XTE J1118+480 suggests that a significant fraction of the optical flux originates from the inner part of the accretion flow, where most of the X-rays are produced. We present a model of magnetic flares in an accretion disc corona where thermal cyclo-synchrotron emission contributes significantly to the optical emission, while the X-rays are produced by inverse Compton scattering of the soft photons produced by dissipation in the underlying disc and by the synchrotron process itself. Given the observational constraints, we estimate the values for the coronal temperature, optical depth and magnetic field intensity, as well as the accretion rate for the source. Within our model we predict a correlation between optical and hard X-ray variability and an anticorrelation between optical and soft X-rays. We also expect optical variability on flaring time-scales (∼tens of ms), with a power-density spectrum similar to that observed in the X-ray band. Finally, we use both the available optical/extreme-ultraviolet/X-ray spectral energy distribution and the low-frequency time variability to discuss limits on the inner radius of the optically thick disc. 相似文献