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311.
On the displacement of marked pebbles on two coarse-clastic beaches during short fair-weather periods (Marina di Pisa and Portonovo, Italy) 总被引:1,自引:1,他引:0
Duccio Bertoni Edoardo Grottoli Paolo Ciavola Giovanni Sarti Giuliano Benelli Alessandro Pozzebon 《Geo-Marine Letters》2013,33(6):463-476
The aim of the investigation was to define the mechanisms of sediment transport in the swash zone of microtidal coarse-clastic beaches in the very short term by evaluating the displacement rates of marked pebbles under low-energy wave conditions. Tests were performed at two sites (Marina di Pisa, Ligurian Sea, and Portonovo, central Adriatic Sea) to check the consistency of the data over a range of different grain sizes. Two recovery campaigns were carried out at both sites, one 6 h and the other 24 h after the injection. During the experiments wave action was at a minimum (wave heights never exceeded 0.3 m). The results show that 20% of pebbles ranging in diameter from 30–90 mm moved significantly (more than 0.5 m) already 6 h after the injection, with some tracers being lost (3%). After 24 h, 40% of the pebbles were significantly displaced and 10% were lost. The preferential downslope movement of tracers, which suggests that coarse sediment movement under low-energy conditions is mainly controlled by gravity processes enhanced by steep beachface slopes, represents the novelty of the results reported here. It would appear that swash processes on low-energy beaches cause a significant rate of pebble displacement through the destabilization induced by wave uprush and backwash. Despite the microtidal range, the position of the mean water level plays a major role in changing the beach level at which swash processes can actually trigger pebble movement. The results of this study show that considerable, and mostly seaward-directed, coarse sediment transport takes place even during short fair-weather periods. 相似文献
312.
Paolo Perona Carlo Camporeale Eliana Perucca Maurizio Savina Peter Molnar Paolo Burlando Luca Ridolfi 《Aquatic Sciences - Research Across Boundaries》2009,71(3):266-278
We discuss the importance of modelling riparian vegetation and river flow interactions under differing hydrologic regimes.
Modelling tools have notable implications with regard to the understanding of riverine ecosystem functioning and to promote
sustainable management of water resources. We present both deterministic and stochastic approaches with different levels of
simplification, and discuss their use in relation to river and vegetation dynamics at the related scale of interest. We apply
such models to both meandering and braided rivers, in particular focusing on the floodplain dynamics of an alpine braided
river affected by water impoundment. For this specific case we show what the expected changes in riparian vegetation may be
in a ‘controlled release’ scenario for the postdam river Maggia, Switzerland. Finally, the use of these models is discussed
in the context of current research efforts devoted to river restoration practice. 相似文献
313.
Zhe Jiang Paolo Perona Robert Francis Peter Molnar Paolo Burlando 《Aquatic Sciences - Research Across Boundaries》2009,71(3):350-355
In this technical note we compare silica gel grains and quartz sand as sediment media for vegetation root growth in laboratory
experiments for ecohydrology and ecohydraulics. Silica gel grains become quite transparent when saturated with water. This
would be useful in order to non-invasively observe the rate of growth of plant roots and plan parallel laboratory experiments
made in more typical sand sediments. In this work, we compare the results of preliminary tests conducted using quartz sand
with the same grain size distribution of silica gel grains. We show that the complex microstructure of silica gel grains seems
to influence the evaporation and, in turn, plant growth dynamics. The potential and limitations of the use of silica grains
are accordingly discussed in light of more detailed experiments. 相似文献
314.
Francesca Zuccarello Lidia Contarino Paolo Romano Viviana Battiato Salvo L. Guglielmino 《Acta Geophysica》2009,57(1):24-30
In this paper we report some results obtained from multi-wavelength observations carried out to study the mechanisms operating
in flares and filament eruptions. Most of these studies have given indication of the presence of phenomena that might be considered
signatures of magnetic reconnection, while others have pointed out the important role played by magnetic helicity transport
in corona before the eruptive phase. 相似文献
315.
The search for the optimal spatial scale for observing landforms to understand physical processes is a fundamental issue in geomorphology. Topographic attributes derived from Digital Terrain Models (DTMs) such as slope, curvature and drainage area provide a basis for topographic analyses. The slope–area relationship has been used to distinguish diffusive (hillslope) from linear (valley) processes, and to infer dominant sediment transport processes. In addition, curvature is also useful in distinguishing the dominant landform process. Recent topographic survey techniques such as LiDAR have permitted detailed topographic analysis by providing high-quality DTMs. This study uses LiDAR-derived DTMs with a spatial scale between 1 and 30 m in order to find the optimal scale for observation of dominant landform processes in a headwater basin in the eastern Italian Alps where shallow landsliding and debris flows are dominant. The analysis considered the scaling regimes of local slope versus drainage area, the spatial distribution of curvature, and field observations of channel head locations. The results indicate that: i) hillslope-to-valley transitions in slope–area diagrams become clearer as the DTM grid size decreases due to the better representation of hillslope morphology, and the topographic signature of valley incision by debris flows and landslides is also best displayed with finer DTMs; ii) regarding the channel head distribution in the slope–area diagrams, the scaling regimes of local slope versus drainage area obtained with grid sizes of 1, 3, and 5 m are more consistent with field data; and iii) the use of thresholds of standard deviation of curvature, particularly at the finest grid size, were proven as a useful and objective methodology for recognizing hollows and related channel heads. 相似文献
316.
Physical infrastructure interdependency and regional resilience index after the 2011 Tohoku Earthquake in Japan 下载免费PDF全文
A resilience index is used to quantify preventive measures, emergency measures, and restoration measures of complex systems, such as physical infrastructures, when they are subjected to natural disasters like earthquakes, hurricanes, floods, etc. Interdependencies among these systems can generate cascading failures or amplification effects, which can also affect the restoration measures right after an extreme event and generate a reduction of the resilience index. In this article, a method is proposed to evaluate the physical infrastructure resilience of a region affected by a disaster considering infrastructure interdependency. It is illustrated using available restoration curves from the March 11 2011 Tohoku Earthquake in Japan. The weights assigned to each infrastructure, which are used to determine resilience, are evaluated using the degree of interdependency indices which are obtain by time series analysis. Results show that the weight coefficients thus obtained do not influence the resilience index significantly; however, the methodology proposed is unbiased from subjective judgment and is able to identify the critical lifelines. Furthermore, the results of the case study presented here suggest that to obtain meaningful estimation of the weight coefficients, it is necessary to consider the period range between two perturbations (e.g., main shock and aftershock). Future infrastructure disruption data (from this and other earthquakes) would be needed to generalize this finding that will allow also to quantify the changes in the restoration curves caused by the magnitude and distance of the shocks from the epicenter, as well as the intrinsic properties of the physical infrastructures. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
317.
Paolo Perona Benoît Crouzy Stuart McLelland Peter Molnar Carlo Camporeale 《地球表面变化过程与地形》2014,39(12):1651-1662
Many rivers worldwide show converging sections where a characteristic limiting front for vegetation establishment on gravel bars is observed. An important conceptual model was advanced in 2006 by Gurnell and Petts, who demonstrated that for the convergent section of the Tagliamento River the downstream front of vegetation establishment can be explained by unit stream power. We introduce a theoretical framework based on 1D ecomorphodynamic equations modified to account for the biological dynamics of vegetation. We obtain the first analytical result explaining the position and river width where vegetation density is expected to vanish in relation to a characteristic streamflow magnitude and both hydraulic and biological parameters. We apply our model to a controlled experiment within a convergent flume channel with growing seedlings perturbed by periodic floods. For a range of timescales where hydrological and biological processes interact, we observe the formation of a front in the convergent section beyond which vegetation cannot survive, the location of which is explained by flow magnitude. This experiment confirms that the timescales of the involved processes and the unit stream power determine the existence and the position of the front within convergent river reaches, respectively. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
318.
Paolo Paronuzzi 《Rock Mechanics and Rock Engineering》2009,42(5):783-813
This paper reports the field evidence and the kinematical study of the motion of two blocks (A and B) mobilised by a rockfall
in Lavone (Valtrompia, northern Italy) on 14th February 1987. The two sequences of impact marks left by the blocks on the
ground surface were measured and the lithostratigraphical features of the debris slope were surveyed. On the basis of the
field-collected input data, several computer simulations were carried out to calculate the coefficients of restitution (E) satisfying the trajectory conditions. The computed output values, obtained by running a specific automatic program for rockfall
modelling, show that rebound trajectories require high coefficients of restitution (0.8 ≤ E ≤ 0.9). Back-calculated impact velocities range from 9.2 to 19.8 m/s. Trajectory heights vary from 0 to 2.4 m above the slope
surface. Block trajectories differ considerably according to the circumstances of initial air projection, i.e. to the initial
rebound angle (α
r). The calculated values of α
r denote a considerable range (36°), emphasising the high variability and the random nature of this parameter. The described
case history shows that rockfall computer analyses can be an effective tool to describe the bouncing propagation of single
blocks, but care must be taken in choosing the restitution coefficient E and the geometrical parameters of initial air projections. 相似文献
319.
Thermal behaviour and kinetics of dehydration of gypsum in air have been investigated using in situ real-time laboratory parallel-beam
X-ray powder diffraction data evaluated by the Rietveld method. Thermal expansion has been analysed from 298 to 373 K. The
high-temperature limits for the cell edges and for the cell volume, calculated using the Einstein equation, are 4.29 × 10−6, 4.94 × 10−5, 2.97 × 10−5, and 8.21 × 10−5. Thermal expansion of gypsum is strongly anisotropic being larger along the b axis mainly due to the weakening of hydrogen bond. Dehydration of gypsum has been investigated in isothermal conditions within the 348–403 K range with a temperature
increase of 5 K. Dehydration proceeds through the CaSO4·2H2O → CaSO4·0.5H2O → γ-CaSO4 steps. Experimental data have been fitted with the Avrami equation to calculate the empirical activation energy of the process.
No change in transformation mechanism has been observed within the analysed temperature range and the corresponding E
a is 109(12) kJ/mol. 相似文献
320.
Thermal behaviour of γ-anhydrite (γ-CaSO4, soluble anhydrite) has been investigated in situ real-time using laboratory parallel-beam X-ray powder diffraction data.
Thermal expansion has been analysed from 303 to 569 K with temperature steps of 4 K. Lattice parameters and volume were fitted
with a second-order polynomial to calculate thermal expansion coefficients. Thermal expansion of γ-anhydrite is anisotropic
being larger along the c axis. Within the 343–383 K thermal range, γ-anhydrite has been found to partially re-hydrate to bassanite CaSO4·0.5H2O. At 455 K the transformation γ-CaSO4 → β-CaSO4, insoluble anhydrite, starts reaching completion at 653 K. 相似文献