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911.
Multiple intertidal bars are common features of wave‐dominated sandy beaches, yet their short‐term (<1 month) and small‐scale (<1 km) morphology and dynamics remain poorly understood. This study describes the morphodynamics of multiple intertidal bars in North Lincolnshire, England, during single and lunar tidal cycles under two contrasting conditions – first when significant wave height was <0·5 m and second when significant wave height frequently exceeded 1 m. The relative importance of swash, surf and shoaling processes in determining morphological change was examined using detailed field observations and a numerical model. The beach featured four intertidal bars and both cross‐shore and longshore bar morphology evolved during the field investigation, particularly under medium to high wave‐energy conditions. Numerical modelling suggests shoaling processes are most common on the seaward two bars under calm wave conditions (Hs < 0·5 m) and that surf zone processes become more common during neap tides and under more energetic (Hs < 0·5 m) conditions. Surf processes dominate the inner two bars, though swash influence increases in a landward direction. The numerical modelling results combined with low tide survey data and high‐resolution morphological measurements strongly suggest changes in the intertidal bar morphology are accomplished by surf zone processes rather than by shoaling wave or swash processes. This is because shoaling waves do not induce significant sediment transport to have any morphological effect, whereas swash action generally does not have enough scope to act as the swash zone is much narrower than the surf zone. It was found, however, that the absolute rate of morphological change under swash action and surfzone processes are of similar magnitudes and that swash action may induce a significant amount of local morphological change when the high tide mark is located on the upper bar, making this process important for bar morphodynamics. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
912.
Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States 总被引:2,自引:0,他引:2
Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United
States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin
and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites
are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are
interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the
mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation
was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg
ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and
polybaric crystal fractionation were all important processes in generating this Oligocene suite. In contrast, most of the
rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830–1,050°C), Sr-and Al-poor,
and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely
imposed by their derivation from parental basalt (with low fH2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation
of plagioclase and retarded crystallization of Fe–Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment.
Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation,
and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite
were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte
trend.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
913.
Streamflow scaling properties: investigating characteristic scales from different statistical approaches 总被引:1,自引:0,他引:1
This paper studies the links between scaling properties of river flow time series by comparing the results of three techniques applied to an extended data set of 34 French discharge gauging stations. The three approaches used are based on different mathematical tools and hypotheses: (1) shape analysis of flood hydrographs; (2) a multifractal framework through spectral and moment analyses, and (3) flood frequency analysis through the fitting of flood duration frequency curves (QdF). The general aim is to test the hypothesis of scaling invariance of river flow and the shape invariance of the hydrographs, in order to investigate the link between scaling properties and flow dynamics. In particular, the coherence between different approaches widely used in the literature to describe these characteristics is evaluated through the estimation of parameters defining the range of time‐scales on which the scaling properties are valid. The results show that most of these timescale parameters are linked to the flow dynamics and suggest that the approaches applied are interrelated. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
914.
Eric Deleersnijder Emmanuel Hanert Hans Burchard Henk A. Dijkstra 《Ocean Dynamics》2008,58(3-4):237-246
Occasionally, numerical simulations using local turbulence closure schemes to estimate vertical turbulent fluxes exhibit small-scale oscillations in space, causing the eddy coefficients to vary over several orders of magnitude on short distances. Theoretical developments suggest that these spurious oscillations are essentially due to the way the eddy coefficients depend on the vertical gradient of the model’s variables. An instability criterion is derived based on the assumptions that the artefacts under study are due to the development of small-amplitude, small time- and space-scale perturbations of a smooth solution. The relevance of this criterion is demonstrated by applying it to a series a closure schemes, ranging from the Pacanowski–Philander formulas to the Mellor–Yamada level 2.5 model. 相似文献
915.
The present experimental investigation focuses on the characteristics of near bed turbulence in a fully rough, uniform open-channel
flow over a gravel-type bed. Due to bed topography small scale heterogeneity, the flow is not uniform locally in the near
bed region and a double averaging methodology is applied over a length scale much larger than the gravel size. The double-averaged
Turbulent Kinetic Energy (TKE) budget derived in the context of the present flow over a gravel bed differs from the TKE budget
written for flow over a vegetation canopy. The non-constant shape of the roughness function measured in our gravel bed leads
to an additional bed-induced production term which is null for vertical roughness elements, such as simplified vegetation
elements.
The experimental estimation of the terms of the TKE budget reveals that the maximum turbulent activity takes place away from
the reference plane, near the roughness crests. However, within the interface sublayer the work of the bed induced velocity
fluctuations against the Reynolds stress is of the same magnitude as the main turbulence production term. Consequently, the
characteristics of the TKE budget have similarities with uniform flows over canopies and strongly differ from uniform flows
over smooth and transitionally rough flows over sedimentlike beds. 相似文献
916.
Mathijs van Ledden Geoffrey Vaughn Joost Lansen Frank Wiersma Mewburn Amsterdam 《Continental Shelf Research》2009
The objective of the research is to analyze in detail the causes and consequences of the unusual event at the coast of Guyana (South America) during October 16–19, 2005. Several sea defense structures were damaged and flooding of low-lying areas occurred. A data analysis of offshore wave and water level characteristics shows an abrupt change in wave direction from east to almost north on October 16, 2005 and a sudden increase in the offshore peak period up to extreme values. The offshore significant wave height was also relatively high, and these wave characteristics coincided with springtide conditions. The long-wave periods and the sharp transition in wave direction indicate that this event is associated with swell waves generated by a depression far away. An analysis of hurricanes and depressions reveals that a severe depression in the Northern Atlantic Ocean during October 11–15 was the origin of this swell event. Numerical computations with SWAN have been carried out to investigate the propagation of the offshore wave characteristics towards the shoreline. The SWAN model includes wave damping due to the presence of soft mud deposits. A calibration of the parameters has been carried out using joint offshore and onshore wave data from November 2006. The numerical simulations of the event in October 2005 clearly demonstrate that the mud banks damp the wave heights, but have almost no effect on the peak period. The resulting waves at the steep sea walls can be classified as surging waves causing severe runup and overtopping. The obtained insights are translated into practical recommendations for the Guyana Sea and River Defence Division in Guyana to build a sustainable management and maintenance of the sea defenses in the future. 相似文献
917.
Distinguishing volcanic debris avalanche deposits from their reworked products: the Perrier sequence (French Massif Central) 总被引:1,自引:1,他引:0
Benjamin Bernard Benjamin van Wyk de Vries Hervé Leyrit 《Bulletin of Volcanology》2009,71(9):1041-1056
Debris avalanches associated with volcanic sector collapse are usually high-volume high-mobility phenomena. Debris avalanche
deposit remobilisation by cohesive debris flows and landslides is common, so they can share textural characteristics such
as hummocks and jigsaw cracks. Distinguishing original deposits from reworked products is critical for geological understanding
and hazard assessment because of their different origin, frequency and environmental impact. We present a methodology based
on field evidence to differentiate such epiclastic breccias. Basal contact mapping constrained by accurate altitude and location
data allows the reconstruction of deposit stratigraphy and geometry. Lithological analysis helps to distinguish the different
units. Incorporation structures, kinematic indicators and component mingling textures are used to characterise erosion and
transport mechanisms. We apply this method to the enigmatic sequence at Perrier (French Massif Central), where four units
(U1–U4) have been interpreted either as debris flow or debris avalanche deposits. The sequence results from activity on the
Monts Dore Volcano about 2 Ma ago. The epiclastic units are matrix supported with an almost flat top. U2 and U3 have clear
debris flow deposit affinities such as rounded clasts and intact blocks (no jigsaw cracks). U1 and U4 have jigsaw cracked
blocks with matrix injection and stretched sediment blocks. U1 lacks large blocks (>10 m wide) and has a homogenous matrix
with an upward increase of trapped air vesicle content and size. This unit is interpreted as a cohesive debris flow deposit
spawned from a debris avalanche upstream. In contrast, U4 has large mega-blocks (up to 40 m wide), sharp contacts between
mixed facies zones with different colours and numerous jigsaw fit blocks (open jigsaw cracks filled by monogenic intra-clast
matrix). Mega-blocks are concentrated near the deposit base and are spatially associated with major substratum erosion. This
deposit has a debris avalanche distal facies with local debris flow affinities due to partial water saturation. We also identify
two landslide deposits (L1 and L2) resulting from recent reworking that has produced a similar facies to U1 and U4. These
are distinguishable from the original deposits, as they contain blocks of mixed U1/U4 facies, a distinctly less consolidated
and more porous matrix and a fresh hummocky topography. This work shows how to differentiate epiclastic deposits with similar
characteristics, but different origins. In doing so, we improve understanding of present and past instability of the Monts
Dore and identify present landslide hazards at Perrier. 相似文献
918.
临界转换的早期预警信号 总被引:1,自引:0,他引:1
Marten Scheffer George Sugihara Jordi Bascompte Victor Brovkin Vasilis Dakos Hermann Held Max Rietkerk 李迎春 闫伟 蒋长胜 《国际地震动态》2009,(9):1-14
从生态系统到金融市场和气候在内的很多复杂动力系统,都会有临界点,在这样的点上系统可能会发生突变,从而演变到一个对立的动力模式上。在这样的临界点到达之前对其进行预测极为困难,但是现在,不同科学领域的研究工作表明,普遍性的早期预警信号有迹可循。对一系列不同类型的系统来说,这些信号会指示是否正在趋近一个临界阈值。 相似文献
919.
920.
An unstructured grid storm surge model of the west coast of Britain, incorporating a high-resolution representation of the
Mersey estuary is used to examine storm surge dynamics in the region. The focus of the study is the major surge that occurred
during the period 11–14 November 1977, which has been investigated previously using uniform grid finite difference models
and a finite element model of the west coast of Britain. However, none of these models included the Mersey estuary. Comparison
of solutions in the eastern Irish Sea with those computed with these earlier models showed that, away from the Liverpool Bay
region, the inclusion of the Mersey estuary had little effect. However, at the entrance to the Mersey, its inclusion did influence
the solution. By including a detailed representation of the Mersey estuary within the model, it was possible to conduct a
detailed study of storm surge propagation in the Mersey, which had never previously been performed. This detailed study showed
for the first time that the surge’s temporal variability within the estuary is influenced by surge elevation at its entrance.
This varies with time as a function of spatial and temporal variations of wind stress over the west coast of Britain. Within
the Mersey, calculations show that the spatial variability is mainly determined by changes in bottom topography, which had
not been included in earlier finite difference models. However, since water depth is influenced by variations in tidal elevation,
this, together with tide surge interaction through bottom friction and momentum advection, influences the surge. The ability
of the finite element model to vary the mesh in near-shore regions to such an extent that it can resolve the Mersey and hence
the impact of the Mersey estuary upon the Liverpool Bay circulation shows that it has distinct advantages over earlier finite
difference models. In the absence of detailed measurements within the Mersey at the time of the surge, it was not possible
to validate predicted surge elevations within the Mersey. However, significant insight into physical processes influencing
the surge propagation down the estuary, its reflection and spatial/temporal variability could be gained. 相似文献