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71.
Mathias J. Collins Noah P. Snyder Graham Boardman William S.L. Banks Mary Andrews Matthew E. Baker Maricate Conlon Allen Gellis Serena McClain Andrew Miller Peter Wilcock 《地球表面变化过程与地形》2017,42(11):1636-1651
Dam removals with unmanaged sediment releases are good opportunities to learn about channel response to abruptly increased bed material supply. Understanding these events is important because they affect aquatic habitats and human uses of floodplains. A longstanding paradigm in geomorphology holds that response rates to landscape disturbance exponentially decay through time. However, a previous study of the Merrimack Village Dam (MVD) removal on the Souhegan River in New Hampshire, USA, showed that an exponential function poorly described the early geomorphic response. Erosion of impounded sediments there was two‐phased. We had an opportunity to quantitatively test the two‐phase response model proposed for MVD by extending the record there and comparing it with data from the Simkins Dam removal on the Patapsco River in Maryland, USA. The watershed sizes are the same order of magnitude (102 km2), and at both sites low‐head dams were removed (~3–4 m) and ~65 000 m3 of sand‐sized sediments were discharged to low‐gradient reaches. Analyzing four years of repeat morphometry and sediment surveys at the Simkins site, as well as continuous discharge and turbidity data, we observed the two‐phase erosion response described for MVD. In the early phase, approximately 50% of the impounded sediment at Simkins was eroded rapidly during modest flows. After incision to base level and widening, a second phase began when further erosion depended on floods large enough to go over bank and access impounded sediments more distant from the newly‐formed channel. Fitting functional forms to the data for both sites, we found that two‐phase exponential models with changing decay constants fit the erosion data better than single‐phase models. Valley width influences the two‐phase erosion responses upstream, but downstream responses appear more closely related to local gradient, sediment re‐supply from the upstream impoundments, and base flows. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
72.
Walter Mangini Alberto Viglione Julia Hall Yeshewatesfa Hundecha Serena Ceola Alberto Montanari 《水文科学杂志》2018,63(4):493-512
This study analyses the differences in significant trends in magnitude and frequency of floods detected in annual maximum flood (AMF) and peak over threshold (POT) flood peak series, for the period 1965–2005. Flood peaks are identified from European daily discharge data using a baseflow-based algorithm and significant trends in the AMF series are compared with those in the POT series, derived for six different exceedence thresholds. The results show that more trends in flood magnitude are detected in the AMF than in the POT series and for the POT series more significant trends are detected in flood frequency than in flood magnitude. Spatially coherent patterns of significant trends are detected, which are further investigated by stratifying the results into five regions based on catchment and hydro-climatic characteristics. All data and tools used in this study are open-access and the results are fully reproducible. 相似文献
73.
Tamara Cibic Oriana Blasutto Nicoletta Burba Serena Fonda Umani 《Estuarine, Coastal and Shelf Science》2008
From January 2003 to December 2004 microphytobenthic primary production was estimated both from in situ (MPPs) and in the laboratory (MPPp) 14C-incubation of slurries collected in a coastal site of the Gulf of Trieste (northern Adriatic Sea). MPPs values varied from −7.54 ± 3.12 to 34.59 ± 7.66 mg C m−2 h−1 over the whole period. The lowest MPPs were observed in November 2003 and August 2004, while the highest MPPs in July 2003 and May 2004, in correspondence with high PAR at the bottom. Significant correlations between MPPs and the microphytobenthic biomass (BIOM) (r = 0.75, p < 0.001), between MPPs and PAR at the bottom (r = 0.54, p < 0.01) and between MPPs and OXY (r = 0.50, p < 0.05) were revealed. MPPp values were higher than MPPs ones in 15 out of 23 observations, with the highest MPPp recorded in July 2003. At 17 m depth a seasonal pattern of sampling months was revealed by the cluster analysis. The role of abiotic parameters in determining this seasonal pattern was highlighted by the PCA, with the first axis correlated with MPPs and PAR, and the second one with temperature. Applying the fuzzy sets it resulted that spring months showed a higher degree of membership with MPPs, summer months with temperature and autumn–winter months with OXY. The microphytobenthic community did not seem to be photosynthetically active throughout the study period. From August–September to December low or negative MPPs values were recorded. We infer that during these months a shift from the autotrophic to heterotrophic metabolism of the benthic microalgae occurred in correspondence with low PAR and/or high temperature at the bottom. Despite the progressive lowering of the trophy of the study area occurred during the last 20 years, we found higher primary production values than those estimated two decades earlier. 相似文献
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75.
This paper reports a detailed characterization of an antigorite-bearing serpentinite, deformed at seismic slip-rate (1.1 m/s) in a high-velocity friction apparatus. Micro/nanostructural investigation of the slip zone (200 μm thick) revealed a zonal arrangement, with a close juxtaposition of horizons with significantly different strength, respectively consisting of amorphous to poorly-crystalline phases (with bulk anhydrous composition close to starting antigorite) and of highly-crystalline assemblages of forsterite and disordered enstatite (200 nm in size and in polygonal-like nanotextures). The slip zone also hosts micro/nanometre sized Cr-magnetite grains, aligned at low angle with respect to the slipping surface and inherited from the host serpentinite.Overall observations suggest that frictional heating at asperities on the slipping surface induced a temperature increase up to 820–1200 °C (in agreement with flash temperature theory), responsible for serpentine complete dehydration and amorphization, followed by crystallization of forsterite and enstatite (under post-deformation, static conditions). The results of this study may provide important keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of serpentinite-hosted faults through the seismic cycle. 相似文献
76.
S. Viti H. R. A. Jones M. J. Richter R. J. Barber J. Tennyson J. H. Lacy 《Monthly notices of the Royal Astronomical Society》2008,388(3):1305-1313
We present high-resolution ( R = 90 000) mid-infrared spectra of M dwarfs. The mid-infrared region of the spectra of cool low-mass stars contains pure rotational water vapour transitions that may provide us with a new methodology in the determination of the effective temperatures for low-mass stars. We identify and assign water transitions in these spectra and determine how sensitive each pure rotational water transition is to small (25 K) changes in effective temperature. We find that, of the 36 confirmed and assigned pure rotational water transitions, at least 10 should be sensitive enough to be used as temperature indicators. 相似文献
77.
Izaskun Jiménez-Serra Jesús Martín-Pintado Arturo Rodríguez-Franco Paola Caselli Serena Viti Tom Hartquist 《Astrophysics and Space Science》2008,313(1-3):159-163
We present the first C-shock and radiative transfer model that calculates the evolution of the line profiles of neutral and
ion species like SiO, H13CO+ and HN13C for different flow times along the propagation of the shock through the unperturbed gas. We find that the line profiles
of SiO characteristic of the magnetic precursor stage have very narrow linewidths and are centered at velocities close to
the ambient cloud velocity, as observed toward the young shocks in the L1448-mm outflow. Consistently with previous works,
our model also reproduces the broad SiO emission detected in the high velocity gas in this outflow, for the downstream postshock
gas in the shock. This implies that the different velocity components observed in L1448-mm are due to the coexistence of different
shocks at different evolutionary stages. 相似文献
78.
We argue that seismotectonic activity in the Central Mediterranean area and the Aegean–Balkan zone is driven by the NNE-ward motion of Africa and westward motion of Anatolia with respect to Eurasia. These boundary conditions can plausibly and coherently account for E–W shortening and roughly S–N extension in the Aegean domain, thrusting and uplift at the boundary between the Aegean–Balkan system and the Adriatic/Ionian domain (Hellenic trench, Cephalonia fault, Epirus, Albanides and Southern Dinarides), the kinematics of the Adria plate (a large block encompassing the Adriatic continental domain, the northern Ionian zone and Hyblean-Adventure block) and consequently, the complex pattern of deformation recognized at its boundaries. Furthermore, the fact that in our scheme Adria moves almost in connection with Africa is consistent with the lack of an active decoupling zone between Adria and Africa, an evidence that can hardly be reconciled with the kinematics so far proposed for these two plates. The reasons why we adopt an Africa–Eurasia relative motion different from that implied by the popular NUVEL-1 global solution are discussed in detail. Finally, we make some considerations about the possible implications of the presently available geodetic data on the long-term plate kinematics. 相似文献
79.
80.
Serena Viti 《Astrophysics and Space Science》2003,287(1-4):213-216
There is now compelling evidence that dark molecular clouds are clumpy. Much of the clumpiness is unresolved by single-dish telescopes but is apparent in the data from array telescopes. Molecular clumps may also be observed close to Herbig-Haro (HH) objects. These clumps are easily observable because they are `illuminated' due to the UV radiation from the shock front of the HH jet. A detailed observational and theoretical study of one HH clump has been performed and it indicates that this clump must be transient and has a similar density and temperature to those clumps detected in the cloud interior. Thus, HH clumps may be used as an independent method of determining physical parameters of the clumpiness of molecular clouds. 相似文献