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11.
An extensive previously published (Hughes et al. Mar Geol 355, 88–97, 2014) field data set representing the full range of micro-tidal beach states (reflective, intermediate and dissipative) is used to investigate swash saturation. Two models that predict the behavior of saturated swash are tested: one driven by standing waves and the other driven by bores. Despite being based on entirely different premises, they predict similar trends in the limiting (saturated) swash height with respect to dependency on frequency and beach gradient. For a given frequency and beach gradient, however, the bore-driven model predicts a larger saturated swash height by a factor 2.5. Both models broadly predict the general behavior of swash saturation evident in the data, but neither model is accurate in detail. While swash saturation in the short-wave frequency band is common on some beach types, it does not always occur across all beach types. Further work is required on wave reflection/breaking and the role of wave-wave and wave-swash interactions to determine limiting swash heights on natural beaches. 相似文献
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In this study, the decadal evolution of a washover fan on the west coast of Denmark is examined from its initial generation in 1990 until 2015. Since its inception, the bare and flat washover fan surface has recovered and accreted slowly due to re-activation by overwash during surges and due to aeolian activity and dune formation, stimulated by vegetation growth. The volume of sand on the washover has increased steadily at an average rate of about 23 m3/yr per unit length of shoreline, and a total of 175,000 m3 of sand is now deposited on the fan, while at the same time the shoreline has receded by some 250 m. The evolution can be divided into three stages: 1) An initiation phase when storm surge levels and energetic wave conditions caused a breach in the foredunes and overwash processes formed a washover fan with a relatively low elevation above mean sea level; 2) An initial recovery phase during which waves supplied sand to the fan during frequent overwash activity and winds transported this sand into marginal dunes surrounding the fan; and 3) A later recovery phase when the surface of the fan had accreted to a level where vegetation could survive and trap sediment into new foredune growth across the fan. The rate of accretion has been overall linear but scales with neither annual overwash frequency, nor with aeolian transport potential. Instead, the linear accretion is more closely related to the steady onshore migration of nearshore bars that weld to the beach and provide a sand supply for transfer to the fan. The fan evolution demonstrates the importance of washover fans in preserving barrier resilience during transgressional phases caused by increasing mean sea level. © 2019 John Wiley & Sons, Ltd. 相似文献
13.
Ilya V. Veksler Yana M. Fedorchuk Troels F. D. Nielsen 《Contributions to Mineralogy and Petrology》1998,131(4):347-363
The evolution of nephelinitic melts in equilibrium with mica-bearing liquidus assemblages and melting relations have been
studied on two silica-undersaturated joins of the KAlSiO4– Mg2SiO4– Ca2SiO4– SiO2– F system at atmospheric pressure by quench runs in sealed platinum capsules. Fluorine has been added to the batch compositions
by the direct exchange of fluorine for oxygen (2F− = O2−). The first join is the pseudo-ternary Forsterite – Diopside – KAlSiO3F2 system. Forsterite, diopside, F-phlogopite and leucite crystallisation fields and a fluoride-silicate liquid immiscibility
solvus are present on the liquidus surface of the join. Sub-liquidus and sub-solidus phases include akermanite, cuspidine,
spinel, fluorite and some other minor fluorine phases. The second system is the pseudo-binary Akermanite – F-phlogopite join
that intersects the Forsterite – Diopside – KAlSiO3F2 join. Akermanite, forsterite, diopside, F-phlogopite, leucite and cuspidine are found to crystallise on the join. Forsterite
(fo) and leucite (lc) are related to F-phlogopite (phl) by a reaction with the fluorine-bearing liquid: fo + lc + l = phl,
and the reaction proceeds until forsterite or leucite are completely consumed. The reaction temperature and resulting phase
association depend on batch composition. Thus, leucite is not stable in the sub-solidus of the Akermanite – F-phlogopite join,
but is preserved in a part of the Forsterite – Diopside – KAlSiO3F2 system where forsterite reacts out, or does not crystallise at all. The phlogopite-in reaction has an important effect on
the composition of the coexisting liquid. The liquids initially saturated in forsterite evolve to extremely Ca rich, larnite-normative
residuals. The experimental data show that larnite-normative melilitolites can crystallise from evolved melilititic melts
generated from “normal” melanephelinitic parental magmas with no normative larnite. The evolution towards melilitites requires fractionation of phlogopite-bearing assemblages under volatile pressure.
Received: 3 June 1997 / Accepted: 5 January 1998 相似文献
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This paper examines the processes responsible for the morphodynamics of an intertidal swash bar at Skallingen, Denmark, during seven successive storms (one with a large surge of +3·02 m DNN). During this period a subtidal bar migrated landward onto the foreshore and continued to migrate across the intertidal zone as a swash bar. The onshore migration of the inner subtidal bar resulted from the erosion of sediment from the upper foreshore and dune ramp during the large storm surge that was transported seaward, causing the landward displacement of the bar through accretion on the landward slope. The magnitude and direction of suspended sediment transport within the intertidal zone, and more specifically at and close to the crest of the swash bar, varied with the ratio of both the significant (Hs) and average (Havg) wave heights to the water depth (hcr) at the swash bar crest (the local depth minimum). The transition between onshore and offshore suspended sediment transport was associated with the average wave of the incident distribution breaking on the swash bar crest (Havgh ≈ 0·33). While the onshore‐directed transport was largest at infragravity frequencies, sediment resuspension was best explained by the skewed accelerations under the surf bores. Offshore transport was dominated by the cross‐shore mean currents (undertow) that developed when the significant wave of the distribution broke on the swash bar crest (Hsh ≈ 0·33) and weakened as the average wave of the distribution started to break at the crest (Havgh ≈ 0·33) and the surf zone approached saturation. In contrast to subtidal bars, the swash bar at Skallingen exhibited a divergent behaviour with respect to the cross‐shore position of the breaker zone, migrating onshore when the average wave broke seaward of the crest and migrating offshore when the average wave broke landward of the crest. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
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Amir Safi Troels N. Vilhelmsen Ibrahim Alameddine Majdi Abou Najm Mutasem El-Fadel 《Ground water》2019,57(4):612-631
Groundwater model predictions are often uncertain due to inherent uncertainties in model input data. Monitored field data are commonly used to assess the performance of a model and reduce its prediction uncertainty. Given the high cost of data collection, it is imperative to identify the minimum number of required observation wells and to define the optimal locations of sampling points in space and depth. This study proposes a design methodology to optimize the number and location of additional observation wells that will effectively measure multiple hydrogeological parameters at different depths. For this purpose, we incorporated Bayesian model averaging and genetic algorithms into a linear data-worth analysis in order to conduct a three-dimensional location search for new sampling locations. We evaluated the methodology by applying it along a heterogeneous coastal aquifer with limited hydrogeological data that is experiencing salt water intrusion (SWI). The aim of the model was to identify the best locations for sampling head and salinity data, while reducing uncertainty when predicting multiple variables of SWI. The resulting optimal locations for new observation wells varied with the defined design constraints. The optimal design (OD) depended on the ratio of the start-up cost of the monitoring program and the installation cost of the first observation well. The proposed methodology can contribute toward reducing the uncertainties associated with predicting multiple variables in a groundwater system. 相似文献
18.
Christian Christiansen Troels Aagaard Jesper Bartholdy Merete Christiansen Jørgen Nielsen Niels Nielsen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):107-126
Average sea-level rise in the northern part of the Danish Wadden Sea has been 1.3 mm a?1 during the last 100 years but during the last 25 years a rise of 4.2 mm a?1 was observed. Concurrent with the recent sea-level rise the Skallingen barrier spit has migrated landward by 3–5 m a?1. Long term sediment budgets for each of the morphological units involved in the migration are reviewed (e.g. onshore in the shore face +90.000 m3 a?1, longshore -641.000 m3 a?1, foredunes +65,000 m3 a?1, overwash fan including shorenormal dunes +11,000 m3 a?1, spit terminus -96,000 m3 a?1, tidal flat + 10,000 m3 a?1 and backbarrier salt marsh +33,000 m3 a?1) and used to establish the relative importance of sediment transport processes involved in barrier migration. Strong interannual variations exist in the long term budget making evaluation of barrier behaviour based on short term measurements doubtful. In a cross shore sense the barrier spit is accreting in spite of the sea level rise. This is specially pronounced at an active overwash fan. However, due to substantial sediment losses to longshore transport the barrier, gets narrower and shorter during its transgressive behaviour. This may indicate that preservation of barriers in the geological record during rapid sea level rise is promoted when sediment loss to longshore transport is insignificant. 相似文献
19.
Measurements of near-bed shear stress were undertaken in the shallow subtidal zone at Durras Beach, NSW, Australia using a sideways-looking acoustic velocity meter installed within the wave boundary layer. The wave climate was swell-dominated and wave conditions comprised shoaling and breaking waves as well as surf bores. The sediment at the field site was medium-grained sand, and observations of bedform geometry were conducted using a pencilbeam-sonar system. Using frequency-filtering techniques, the measured stresses were partitioned into terms representing turbulent (Reynolds) stress, stresses due to gravity and infragravity-scale oscillatory motions, and wave-turbulence-mean current cross-terms. Gravity wave-orbital scale motions contributed the largest fraction of the stresses, comprising 24% on average, followed by long-wave advection of vertical orbital motion (16%). The presence of wave orbital-scale motions near or at the water/sediment interface was likely due to the porous nature of the seabed, facilitating interfacial flow. Shear stresses did not scale with bed roughness but exhibited a linear relationship with the relative wave height. This indicates that for the experimental conditions, surf zone processes overwhelmed bed roughness effects on shear stress and friction. Calculations of the wave friction factor, fw, showed that in a natural surf zone, this was a factor 3–4 larger than conventional predictions. © 2020 John Wiley & Sons, Ltd. 相似文献
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Jakob K. Jakobsen Christian Tegner C. Kent Brooks Adam J. R. Kent Charles E. Lesher Troels F. D. Nielsen Michael Wiedenbeck 《Contributions to Mineralogy and Petrology》2010,159(1):61-79
Troctolite blocks with compositions akin to the Hidden Zone are exposed in a tholeiitic dyke cutting across the Skaergaard
intrusion, East Greenland. Plagioclase in these blocks contains finely crystallised melt inclusions that we have homogenised
to constrain the parental magma to 47.4–49.0 wt.% SiO2, 13.4–14.9 wt.% Al2O3 and 10.7–14.1 wt.% FeOT. These compositions are lower in FeOT and higher in SiO2 than previous estimates and have distinct La/SmN and Dy/YbN ratios that link them to the lowermost volcanic succession (Milne Land Formation) of the regional East Greenland flood basalt
province. New major- and trace element compositions for the FG-1 dyke swarm, previously taken to represent Skaergaard magmas,
overlap with the entire range of the regional flood basalt succession and do not form a coherent suite of Skaergaard like
melts. These dykes are therefore re-interpreted as feeder dykes throughout the main phase of flood basalt volcanism. 相似文献