The segmentation procedure as a tool for discrete modeling of hydrometeorological regimes     

P. Hubert 《Stochastic Environmental Research and Risk Assessment (SERRA)》2000,14(4):297-304

A possible cause of nonstationarity in time series is the existence of some abrupt modification of their statistical parameters, and especially of a sudden change of the mean. Series with such a change exhibit a strong temporal persistence, with high values of the Hurst coefficient, but with poor possibilities to fit any autoregressive model. Some classical tests (Pettitt, 1979; Buishand, 1982) enable to find a possible change point of the mean and then to split the original nonstationary series into two stationary sub-series. The Bayesian procedure defined by Lee and Heghinian (1977) supposes the “a-priori” existence of a change of the mean somewhere in the series and yields at each time step an “a-posteriori” probability of mean change. But these classical tests and procedures consider only one change point in the original series. To go further and to explore the theoretical multiple singularity models defined by Klemeš (1974) and Potter (1976), a segmentation procedure of time series has been designed. This procedure yields an optimal partition (from a least squares point of view) of the original series into as many subseries as possible, all differences between two contiguous means remaining simultaneously significant. This last requirement is ensured using the Scheffe test of contrasts. The main problem has been to master the combinatory explosion while exploring the tree of all possible segmentations of a series. Some applications of the procedure to hydrometeorological time series are reviewed and some possible improvements are presented.  相似文献   

Simulating Magnetized Jets     

Rony Keppens  Hubert Baty  Jeroen Bergmans  Fabien Casse 《Astrophysics and Space Science》2004,293(1-2):217-224

A suitable model for the macroscopic behavior of accretion disk-jet systems is provided by the equations of MagnetoHydroDynamics (MHD). These equations allow us to perform scale-encompassing numerical simulations of multidimensional nonlinear magnetized plasma flows. For that purpose, we continue the development and exploitation of the Versatile Advection Code (VAC) along with its recent extension which employs dynamically controlled grid adaptation. In the adaptive mesh refinement AMRVAC code, modules for simulating any-dimensional special relativistic hydro- and magnetohydrodynamic problems are currently operational. Here, we review recent 3D MHD simulations of fundamental plasma instabilities, relevant when dealing with cospatial shear flow and twisted magnetic fields. Such magnetized jet flows can be susceptible to a wide variety of hydro (e.g. Kelvin-Helmholtz) or magnetohydrodynamic (e.g. current driven kink) instabilities. Recent MHD computations of 3D jet flows have revealed how such mutually interacting instabilities can in fact aid in maintaining jet coherency. Another breakthrough from computational magnetofluid modeling is the demonstration of continuous, collimated, transmagnetosonic jet launching from magnetized accretion disks. Summarizing, MHD simulations are rapidly gaining realism and significantly advance our understanding of nonlinear astrophysical magnetofluid dynamics.  相似文献   

Turbulent mixing in a small estuary: Detailed measurements     

Mark Trevethan  Hubert Chanson 《Estuarine, Coastal and Shelf Science》2009

Between 2003 and 2007, a series of field studies were performed in a typical small coastal plain type estuary (Eprapah Creek) located on the Southeast coast of Australia. The aim of these field studies was to investigate the turbulence and turbulent mixing properties in the estuarine zone. During these studies, high frequency turbulence and physio-chemistry data were collected continuously over a relatively long duration (up to 50 h). This article provides a summary of the key outcomes of these studies, highlighting the implications that these findings have on the modelling of small estuaries. The studies showed that the response of the turbulence and water quality properties were distinct under spring and neap tidal forcing and behaved differently in the middle and upper estuarine zones. The behaviour of turbulence properties to spring tidal forcing differed from that observed in larger estuaries and seemed unique to small estuarine systems. An investigation of several key turbulence parameters used in the modelling of estuarine mixing showed that many assumptions used in larger estuaries must be applied with caution or are simply untrue in small estuaries. For example the assumption that the mixing coefficient parameters are constant over the tidal cycle in a small estuary is simply untrue. These distinctions between the turbulence and mixing properties in small and large estuarine systems highlight the need for the continued study of small estuaries, so this type of system can be properly understood.  相似文献   

The application of two‐dimensional hydrodynamic models for underwater archaeological finds from the Vistula River in Warsaw,Poland          下载免费PDF全文

Hubert Kowalski  Artur Magnuszewski  Artur Radecki‐Pawlik 《水文研究》2018,32(18):2888-2900

Low water levels occurring on the Warsaw section of the Vistula River during droughts in 2012 and 2015 stirred considerable interest by exposing parts of the riverbed. The river's low flow discharge created favourable conditions for archaeologists who, from the river's bottom, managed to salvage sculptures, architectural artefacts, and military accessories dating back to the middle of the 17th century. Literature shows that archaeological finds at the bottom of a major lowland river are unique and are attributed to very specific hydrological and geological conditions. Embankments and works meant to regulate the channel of the Vistula River in Warsaw narrowed the channel and accelerated the erosion of the river's bottom. A comparison of rating curves created between 1919 and 2015 shows that the river's bottom had lowered by 225 cm. The diameter of sediment particles that could be moved at maximum shear stress has been calculated using the Meyer–Peter and Muller formula. According to the calculated shear stress corresponding to the highest historical flood of 1844 particles smaller than 0.05 m were transported down the river, and it is true that artefacts of that size had not been collected at the site. Taking into consideration the very specific geological and hydrological conditions, we believe that finding so many 17th century artefacts with a well‐documented history at a site at the bottom of major lowland river with intensive channel processes is indeed unique. Additionally, it is the first time that results of hydrodynamic modelling have been successfully used for archaeological survey to define the critical shear stress and the smallest diameter of artefacts, which could remain stable on a river's bottom.  相似文献   

Turbulence and suspended sediment processes in the Garonne River tidal bore in November 2016     

David Reungoat  Xinqian Leng  Hubert Chanson 《国际泥沙研究》2019,34(5):496-508

A tidal bore is a water discontinuity at the leading edge of a ood tide wave in estuaries with a large tidal range and funneling topography. New measurements were done in the Garonne River tidal bore on 14 15 November 2016, at a site previously investigated between 2010 and 2015. The data focused on long, continuous, high-frequency records of instantaneous velocity and suspended sediment con- centration (SSC) estimate for several hours during the late ebb, tidal bore passage and ood tide. The bore passage drastically modi ed the ow eld, with very intense turbulent and sediment mixing. This was evidenced with large and rapid uctuations of both velocity and Reynolds stress, as well as large SSCs during the ood tide. Granulometry data indicated larger grain sizes of suspended sediment in water samples compared to sediment bed material, with a broader distribution, shortly after the tidal bore. The tidal bore induced a sudden suspended sediment ux reversal and a large increase in suspended sedi- ment ux magnitude. The time-variations of turbulent velocity and suspended sediment properties indicated large uctuations throughout the entire data set. The ratio of integral time scales of SSC to velocity in the x-direction was on average TE,SSC/TE,x 0.16 during the late ebb tide, compared to TE,SSC/ TE,x 0.09 during the late ood tide. The results imply different time scales between turbulent velocities and suspended sediment concentrations.  相似文献   

Progressive change of tidal wave characteristics from the eastern Yellow Sea to the Asan Bay,a strongly convergent bay in the west coast of Korea     

Moon-Jin Park  Hubert H. G. Savenije  Huayang Cai  Eui Kyu Jee  Nam Hoon Kim 《Ocean Dynamics》2017,67(9):1137-1150

Although there have been studies on the tide in convergent bay (or estuary), the tide change in terms of phase speed, amplitude, and phase difference between elevation and tidal current from a coastal ocean to a convergent bay has not been clearly shown so far. This study systematically examines the change of tidal wave characteristics from the eastern Yellow Sea to the Asan Bay, a strongly convergent bay on the west coast of Korea, using observations and an analytical model. As the tidal wave propagates from the eastern Yellow Sea into the Asan Bay, the phase speed, amplitude, and phase difference between elevation and tidal current increase along the channel. Such a phenomenon represents a unique example of tide change from a coastal ocean to a convergent bay, indicating dominance of convergence over friction in the Asan Bay. Both analytically computed tidal amplitude and travelling time compare well with observations. In the Asan Bay, the influence of the reflected wave is only felt in the upper one fifth of the bay and is almost unperceivable in the rest of the bay. The analytical analyses presented in this paper are particularly useful for understanding the relative importance of channel convergence, bottom friction, and reflected wave on the tidal characteristics change along the channel and the proposed method could be applicable to other estuaries.  相似文献   

Recent transfer of coastal sediments to the Laurentian Channel,Lower St. Lawrence Estuary (Eastern Canada), through submarine canyon and fan systems     

Hubert Gagné  Patrick Lajeunesse  Guillaume St-Onge  Andrée Bolduc 《Geo-Marine Letters》2009,29(3):191-200

Multibeam sonar data, acoustic sub-bottom profiles and box cores were used to study the activity of submarine canyons and fans near the city of Les Escoumins, on the North Shore of the Lower St. Lawrence Estuary (Eastern Canada). The multibeam data were used to generate a high-resolution digital terrain model that reveals the presence of a large number of canyons and fans along the northern slopes of the Laurentian Channel. This paper focuses on two of the larger canyons, and their associated submarine fans. The sub-bottom profiles on the fans reveal high-amplitude reflections at the sediment/water interface and near the seafloor surface, indicating the occurrence of layers of coarse material. A turbidite was observed in a box core sampled in one of the fans, confirming the nature of the coarse layer. Geophysical and sedimentological data indicate that the canyons and fans play an important role in transferring coastal sandy sediments to the deeper marine environments by longshore drift-initiated turbidity flows, and thereby contribute to the negative sediment budget along the coast. The morphology of the canyons indicates that they were produced by a combination of erosive turbidity flows and retrogressive failures. The two box cores sampled on the fans reveal a recent (~last 60 years) quasi-exponential increase in sand content near the surface of the cores, possibly reflecting recent deforestation and/or increased coastal erosion.  相似文献   

Forest–water dynamics within a mountainous catchment in Austria     

Elisabeth Pötzelsberger  Hubert Hasenauer 《Natural Hazards》2015,77(2):625-644

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Evidence for pre-orogenic,Early Devonian rifting in the Variscan belt: stratigraphy and structure of the Palaeozoic cover of the Mauges Unit (Upper Allochthon,Armorican massif,France)     

Céline Ducassou  Michel Ballèvre  Hubert Lardeux  Cécile Robin 《International Journal of Earth Sciences》2011,100(7):1451-1475

The Palaeozoic sediments from the Mauges Unit (Armorican Massif, France) are the best-preserved pre-orogenic sequences belonging to the Upper Allochthon. Two coherent sequences are identified. The southern unit (Chateaupanne Unit) represents the cover of the Proterozoic basement and consists of Ordovician sediments unconformably overlain by Emsian carbonates followed by Emsian to earliest Eifelian immature sandstones. The northern unit (Tombeau Leclerc Unit) consists of an Hirnantian to Emsian condensed sequence, in reverse position, that has been thrust over the southern unit. The Devonian unconformity is interpreted as evidence for an Early Devonian extension, recorded by normal faults affecting both the Early Devonian limestones and the underlying Ordovician series. This crustal extension, recorded here for the first time, is possibly related to the opening of a back-arc basin (Saint-Georges-sur-Loire) associated with the subduction of an ocean located further south (Galicia-Brittany-Massif Central Ocean).  相似文献   

Changes in North Sea macrofauna communities and species distribution between 1986 and 2000     

Ingrid KrönckeHenning Reiss  Jacqueline D. EggletonJohn Aldridge  Magda J.N. BergmanSabine Cochrane  Johan A. CraeymeerschSteven Degraer  Nicolas DesroyJean-Marie Dewarumez  Gerard C.A. DuineveldKarel Essink  Hans HillewaertMarc S.S. Lavaleye  Andreas MollStefan Nehring  Richard NewellEivind Oug  Thomas PohlmannEike Rachor  Mike RobertsonHeye Rumohr  Michaela SchratzbergerRebecca Smith  Edward Vanden Berghe  Jan van DalfsenGert van Hoey  Magda VincxWouter Willems  Hubert L. Rees 《Estuarine, Coastal and Shelf Science》2011,94(1):1-15

The North Sea Benthos Project 2000 was initiated as a follow-up to the 1986 ICES North Sea Benthos Survey with the major aim to identify changes in the macrofauna species distribution and community structure in the North Sea and their likely causes.The results showed that the large-scale spatial distribution of macrofauna communities in the North Sea hardly changed between 1986 and 2000, with the main divisions at the 50 m and 100 m depth contours. Water temperature and salinity as well as wave exposure, tidal stress and primary production were influential environmental factors on a large (North Sea-wide) spatial scale.The increase in abundance and regional changes in distribution of various species with a southern distribution in the North Sea in 2000 were largely associated with an increase in sea surface temperature, primary production and, thus, food supply. This can be most likely related to the North Sea hydro-climate change in the late 1980s influenced by the variability in the North Atlantic Oscillation (NAO). Only one cold-temperate species decreased in abundance in 2000 at most of the stations. Indications for newly established populations of offshore non-native species were not found.Differences in macrofauna community structure on localised spatial scales were predominantly found north of the 50 m depth contour off the British coast along the Flamborough Head Front towards the Dogger Bank, off the coast of Jutland and at the Frisian Front. These changes were most likely attributed to stronger frontal systems in 2000 caused by the increased inflow of Atlantic water masses in relation to the hydro-climate change in the late 1980s.  相似文献