Sand dunes and valley fills from Preboreal glacial-lake outburst floods in south-eastern Norway – beyond the aeolian paradigm     

Louise Hansen  Georgios Tassis  Fredrik Høgaas 《Sedimentology》2020,67(2):810-848

A major glacial-lake outburst flood in the Glomma valley, south-eastern Norway, took place during the final decay of the Scandinavian Ice Sheet. A combined morphological, geophysical and sedimentological study provides new insight into the variety of processes and deposits of the flood. The studied succession, some tens of metres in thickness, comprises the fill of a major flood basin that developed during hydraulic ponding. Large-scale sand dunes and bars accumulated downstream of locations with expanding flow. Most notable are 10 m high, concentric dune ridges that accumulated downstream of a topographical constriction hosting a high-velocity flow. Flow expansion at the outlet generated intense turbulence and scouring. The sand-loaded eddies helped feeding the semi-stationary dune ridges that grew vertically and downflow under high aggradation rates. Internal structures vary but reflect an overall shift in sedimentation from prevailing supercritical flow to overall subcritical flow conditions at high flood levels. Loading by water and sediment caused large-scale, synsedimentary deformation, increasing local accommodation space. Fast-falling flood levels caused stronger flow locally, while mud and fine sand settled in stagnant pools. The fall caused a significant drop in hydrostatic pressure. This triggered a release of excess pore pressure causing massive dewatering and fluidisation. Water-escape structures include numerous (sub)vertical pipes. The present study shows that outburst flood-generated, large-scale dunes can develop in well-sorted, fine sand and are thereby easily confused with aeolian deposits. Several dune fields in south-eastern Norway are here reinterpreted as the product of at least two major flood events. Sandy dune fields with similar characteristics elsewhere in Scandinavia should likely also be reinterpreted, and the role of outburst floods during the final deglaciation of Scandinavia has seemingly been underestimated. The study emphasises the importance of ponding, flow expansion, sorting, rapidly changing pressure conditions and deformation for outburst flood-related sedimentation.  相似文献   

Solution of the linear diffusion equation for modelling erosion processes with a time varying diffusion coefficient     

Georgios Aim. Skianis  Dimitrios Vaiopoulos  Niki Evelpidou 《地球表面变化过程与地形》2008,33(10):1491-1501

In the present paper the differential equation of the temporal development of a landform (mountain) with a time dependent diffusion coefficient is solved. It is shown that the shape and dimensions of the landform at time t are independent of the specific variation of the diffusion coefficient with time; they only depend on the mean value of the diffusion coefficient in the time interval where the erosion process takes place. Studying the behaviour of the solution of the differential equation in the wave number domain, it is concluded that Fourier analysis may help in estimating, in quantitative terms, the initial dimensions, the age or, alternatively, the value of the diffusion coefficient of the landform. The theoretical predictions are tested on a hill of the southern part of the Ural mountainous region, in order to show how the results of the mathematical analysis can be used in describing, in quantitative terms, the morphological development of landforms due to erosion processes. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Food Waste Composting in a High Rate Continuous Feed Drum Reactor: Startup and Carbon Balance Analysis          下载免费PDF全文

Ioannis Zarkadas  Evangelia Angeli  Ioannis Sainis  Evangelos Voudrias  Georgios Pilidis 《洁净——土壤、空气、水》2018,46(2)

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Fish fauna recovery in a newly re-flooded Mediterranean coastal lagoon     

Emmanuil Koutrakis  Georgios Sylaios  Nikolaos Kamidis  Dimitrios Markou  Argyris Sapounidis 《Estuarine, Coastal and Shelf Science》2009,83(4):505-515

Drana Lagoon, located at the NW site of Evros River Delta, was drained in 1987 and re-flooded in 2004 within the framework of an integrated wetland restoration project. This study presents the results of a monitoring program of the lagoon's oceanographic, water quality and fish fauna characteristics, during the pre- and post-restoration period. Results depict the presence of high salinity water (up to 41) due to seawater intrusion, strong evaporation in its interior and inadequate freshwater inflows. Overall, nutrient levels were low depicting local changes. Tidal variability at the mouth was approximately 0.2 m, producing high velocity tidal currents (up to 0.75 m/s). Eleven fish fauna species were collected; seven species were caught in both the inlet channel and the lagoon during the pre-restoration period and nine species in the post-restoration period. Atherina boyeri (37.6%) and Pomatoschistus marmoratus (31.7%) dominated the lagoon during the post-restoration period. Most of the A. boyeri specimens (88.5%) were caught inside the lagoon, while P. marmoratus had an almost equal distribution in the inlet channel and the lagoon (56.3% and 43.7% respectively). The presence of species of the Mugilidae family (5.2% total average catches after lagoon re-flooding) was mainly in the inlet channel (12.6% of the average catches) and not inside the lagoon (only 1.3% of the average catches). The small number of fish species inhabiting the lagoon might be the result of the recent restoration or it could be related with the increased water flow observed at the lagoon mouth during the flood and ebb tidal phases, and also in the presence of a smooth bank in the concrete waterspout that connects the entrance channel with the lagoon. The limited presence of the Mugilidae juveniles inside the lagoon could be related to the prevailing tidal inlet dynamics (i.e. strong ebb flow at lagoon inlet), thus preventing the species to enter the lagoon. In order to restore the lagoon environment, careful and gradual steps should be undertaken under the basis of continuous monitoring of hydrologic, environmental and fisheries system's status.  相似文献   

Calcareous nannofossil and planktonic foraminiferal distributional patterns during deposition of sapropels S6, S5 and S1 in the Libyan Sea (Eastern Mediterranean)   总被引：1，自引：1，他引：0  

Maria V. Triantaphyllou  Assimina Antonarakou  Margarita Dimiza  Christos Anagnostou 《Geo-Marine Letters》2010,30(1):1-13

In core ADE3-23 collected in the Libyan Sea, the nannofossil species Coccolithus pelagicus, Coronosphaera spp., Helicosphaera spp., Syracosphaera spp., Calcidiscus spp., small Gephyrocapsa spp., and the planktonic foraminifers Globigerina bulloides, Neogloboquadrina pachyderma, Globorotalia scitula, Turborotalita quinqueloba and Neogloboquadrina dutertrei prevail in sapropel S6 (midpoint at 172 ka b.p.), indicative of cold and highly productive surface conditions. Warm and highly stratified water-column conditions are recorded by the characteristic assemblage of Globigerinoides ruber, Globoturborotalita rubescens, Florisphaera profunda, Rhabdosphaera spp. during the sapropel S5 depositional interval (midpoint at 124 ka b.p.). Compared with S5, Globigerinita glutinata, Globorotalia inflata, Globigerinella siphonifera, Globorotalia truncatulinoides and the calcareous nannofossil Emiliania huxleyi characterise less stratified conditions within sapropel S1 (midpoint at 8.5 ka b.p.). Multivariate statistical analyses of calcareous nannofossil and planktonic foraminifers in core ADE3-23 identify planktonic assemblages which typify sapropels S6, S5 and S1 in the Libyan Sea. A warmer interval is recognised in the middle part of the cold S6, and can be associated with an influx of less saline waters and the occurrence of a faint, temporary deep chlorophyll maximum. Evidence for enhanced surface productivity and breakdown of stratification is observed in the middle–upper part of the warm S5, associated with climatic deterioration. Moreover, an increase in surface productivity in the upper S1 implies weak stratification. Our combined calcareous nannofossil and planktonic foraminiferal data add to the evidence that climate variability was more pronounced than commonly considered to date for all the three studied Eastern Mediterranean sapropel depositional intervals.  相似文献   

Space-time prisms on a sphere with applications to long-distance movement     

Bart Kuijpers  Georgios Technitis 《International journal of geographical information science》2020,34(10):1980-2003

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Indirect dating of an olive tree planting event using luminescence of the sediments lying beneath the roots of the tree: a pilot study in the south-western part of Anatolia,Turkey     

Eren Şahiner  Georgios S. Polymeris  M. Altay Atlihan  Selcuk Aktürk  Niyazi Meriç 《第四纪科学杂志》2020,35(5):706-715

The aim of the present study is to attempt assessing the age of a monumental olive tree located between the Antique Cities of Militus (Didim-Aydın region) and Iasos (Milas-Muğla region). Wood from the trunk of an olive tree is not appropriate for conventional dating approaches such as dendrochronology or ¹⁴C. The sediments closely located surrounding and beneath the roots of the olive tree are considered indicative of the age of the planting event; therefore these sediments were dated using both quartz and feldspar luminescence signal protocols. Methodological aspects including the preheating plateaus, equivalent dose statistical approaches and dose rate using gamma spectrometry are also discussed, as dating of the associated palaeochannel sediments of the area are presented for the first time in the dating literature. The optically stimulated luminescence and/or infrared stimulated luminescence ages are extrapolated to date the event of the tree planting; it is the first time in the literature that an age is reported for an olive tree in the eastern Mediterranean region. The present study stands as the first experimental evidence that olive trees have been cultivated in the area since the Iron Age.  相似文献   

The Effect of the Stress Path on Squeezing Behavior in Tunneling   总被引：1，自引：0，他引：1  

Linard Cantieni  Georgios Anagnostou 《Rock Mechanics and Rock Engineering》2009,42(2):289-318

Summary  The interplay between support systems and the rock when tunneling under squeezing conditions is normally studied by means of two-dimensional analyses. The present paper shows that the underlying plane strain assumption involved in a two-dimensional analysis may lead, under certain conditions, to ground pressure and deformation values that are considerably lower than those produced by stress analyses that take into account spatial effects in the vicinity of the tunnel face. The differences are due to the stress path dependency in the elasto-plastic behavior of the ground and, more specifically, to the inability of the plane strain model to map the actual radial stress history, which involves a complete radial unloading (and, later, a re-loading) of the tunnel boundary over the unsupported span. This inherent weakness of any plane strain analysis is relevant from the design standpoint, particularly for heavily squeezing conditions that require a yielding support. For the majority of tunneling conditions and methods, however, involving as they do the completion of a stiff support within a few meters of the face, the errors introduced by the plane strain assumption are not important from a practical point of view.  相似文献   

Finite strain elastoplastic solutions for the undrained ground response curve in tunnelling          下载免费PDF全文

Apostolos Vrakas  Georgios Anagnostou 《国际地质力学数值与分析法杂志》2015,39(7):738-761

The instantaneous response of saturated low permeability grounds to tunnel excavation is important for deformations and stability close to the tunnel face. It is characterized by zero volume change in combination with the development of excess pore pressures. In tunnelling through poor quality ground under great depth of cover and high in situ pore pressure, heavily squeezing conditions (characterized by very large convergences) may occur soon after excavation. This paper presents exact finite strain analytical solutions for the undrained ground response around cylindrical and spherical openings that are unloaded from uniform and isotropic initial stress states, on the basis of the Modified Cam Clay (MCC) model and the Mohr–Coulomb (MC) model. The solution for a Drucker–Prager material is also given as it requires only a very small modification to the MC solution. The so‐called ground response curve, that is, the relationship between the support pressure and the cavity wall displacement, is derived in closed form for the MC model. The solution for the MCC problem is semi‐analytical in that it uses the trapezium rule for the computation of a definite integral. The influence of the significant parameters of the problem on the predicted deformation behaviour is shown by means of dimensionless charts. Finally, the practical usefulness of the solutions presented is illustrated by applying them to the breccia zones of the planned Gibraltar Strait tunnel – an extreme case of weak, low permeability ground under high pore pressure. The solutions can serve as a trustworthy benchmark for numerical procedures that incorporate material and geometric nonlinearities. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Storm outage modeling for an electric distribution network in Northeastern USA     

D. W. Wanik  E. N. Anagnostou  B. M. Hartman  M. E. B. Frediani  M. Astitha 《Natural Hazards》2015,79(2):1359-1384

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