Changes in velocity profiles at roughness transitions in coarse grained channels   总被引：2，自引：0，他引：2  

ANDRE ROBERT  RE G. ROY†  BERNARD DE  SERRES† 《Sedimentology》1992,39(5):725-735

Heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness. Distinct spatial zones in terms of grain size have been reported, for example sand ribbons and bedload sheets. The transition from areas of finer to coarser grained surface sediment is often abrupt. However, the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels. This paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistancé on the structure of the turbulent boundary layer. Measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river. Immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface, boundary shear stress and roughness length increase considerably. Downstream from this transition, velocity profiles become concave upwards. Downstream and upstream sections show significant differences in terms of near bed velocities (deceleration downstream of the transition), velocity gradient and turbulence intensity of the streamwise velocity component. Comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity (or shear stress) associated with the pressure drag produced by large and isolated obstacles.  相似文献   

COMMENTS ON THE PAPER: RECALCULATION OF H-Lα SKY SURVEYS: NO NEED FOR ANISOTROPIC SOLAR WIND MASS OUTFLOWS?     

Bertaux  Jean-Loup  QuÉmerais  Eric  Lallement  Rosine  KyrÖlÄ  Erkki  Summanen  Tuula 《Solar physics》1997,170(2):365-370

In a recently published paper, Scherer and Fahr (1995) claimed that the departures of sky L emission measured by Prognoz 5 and 6 from an optically thin model can be attributed entirely to deficiencies of the optically thin approximation, and are not due to variations of solar wind ionization rate with latitude, as advocated since many years by our research group. They base their claim on the result of their new sophisticated model of L radiation transport.It is shown here that their new model, in the simple case of isotropic solar wind, predicts a distribution of intensity in a simple geometry which is completely contradicted by the observations: they find a minimum of intensity near the upwind direction, where a maximum has been observed consistently by all L instruments. Therefore, their conclusion based on an erroneous model must be rejected.  相似文献   

Transformation of Irradiance Measurements into Spectral Actinic Flux for Photolysis Rates Determination     

HÉLÈNE COTTE  CLAUDE DEVAUX  PATRICK CARLIER 《Journal of Atmospheric Chemistry》1997,26(1):1-28

The actinic flux is the only radiometric quantity suitable for photolysisfrequency determination. It is derived from solar spectral irradiancemeasurements performed by a portable spectroradiometer in the 300–850nm wavelength range. The spectral irradiance is first divided into a directpart and a diffuse part, according to the atmospheric conditions, and thenconverted into the corresponding actinic flux quantity. As an intermediary,the ratio of diffuse actinic flux to diffuseirradiance is calculated by the spherical harmonics radiative code withrespect to wavelength, solar zenith angle, surface albedo, and aerosolproperties.The results of extensive sensitivity studies of asa function of the main atmospheric parameters are discussed and lead to theconclusion that aerosol optical depth is the major critical value for aprecise determination. The global algorithm totransform spectral irradiance into actinic flux is finally applied forphotodissociation rate calculations by convolution of the obtained actinicflux spectra with the absorption cross-sections and quantum yields of themolecule studied. Photolysis rates of different atmospheric photooxidants havebeen measured with this spectroradiometric method during the summers of 1993and 1994 in Brittany and in Portugal. The ozone and nitrogen dioxidephotodissociation rates obtained present a good agreement with thej(O₃) and j(NO₂) actinometerresults, for the same experimental conditions in Brittany.  相似文献   

DEFORMATIONAL BEHAVIOURS OF ALLUVIAL UNITS DETECTED BY ADVANCED RADAR INTERFEROMETRY IN THE VEGA MEDIA OF THE SEGURA RIVER,SOUTHEAST SPAIN          下载免费PDF全文

CARMELO CONESA‐GARCÍA  ROBERTO TOMÁS  GERARDO HERRERA  FRANCISCO LÓPEZ‐BERMÚDEZ  MIGUEL CANO  FRANCISCA NAVARRO‐HERVÁS  PEDRO PÉREZ‐CUTILLAS 《Geografiska Annaler: Series A, Physical Geography》2016,98(1):15-38

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ON SOME NEOLITHIC (AND POSSIBLY PALAEOLITHIC) FINDS IN MONGOLIA,SINKIANG AND WEST CHINA     

德日進  楊鍾健 《地质学报》1933,(1)

In the course of the past two years,when they were in the field with theThird American (1930) and the Haardt-Citroen (1931) Central Asiatic Expedi-tions,the authors of the present note had opportunities for making a series ofarchaeological observations ov  相似文献   

ON AN ENIGMATIC PTEROPOD-LIKE FOSSIL FROM THE LOWER CAMBRIAN OF SOUTHERN SHANSI,BICONULITES GRABAUI,NOV.GEN.,NOV.SP.     

P.TEILHARD DE CHARDIN 《地质学报》1931,(1)

Ⅰ. GEOLOGY OF THE SITE The Lower Cambrian fossils here described were first found, insufficiently preserved, by Licent and Teilhard in 1926 at K(?)ntingshih (圪丁石), just along the border of the high limestone range, 50 lis N. of Cbishanhsien (S. Shansi).  相似文献   

ON THE RECENT MARINE BEDS,AND THE UNDERLYING FRESHWATER DEPOSITS,IN TIENTSIN By E.LICENT AND P.TEILHARD DE CHARDIN     

E.LICENT  P.TEILHAND DE CHAREIN 《地质学报》1927,6(2):131-132

Marine beds, containing numerous well preserved shells, are actuallyexposed in a broad pit recently open, one mile south from the Hoangho-PaihoMuseum (Race Course Road, Tientsin), in order to get the earth required forthe filling of some pools along the Race Course Road.  相似文献   

ON A PRESUMABLY PLEISTOCENE HUMAN TOOTH FROM THE SJARA-OSSO-GOL (SOUTHEASTERN ORDOS) DEPOSITS     

E.LICENT  TEILHARD DE CHARDIN  DAVIDSON BLACK 《地质学报》1926,(Z1)

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THEME 5 SEDIMENT PROBLEMS RELATED TO INTAKE STRUCTURES AND THE INFLUENCE OF INTAKES ON THE SEDIMENT REGIME OF RIVERS     

M. DE VRIES 《水文科学杂志》2013,58(4):543-544

ABSTRACT

The paper presents the observed effects on the streamflow of changing a tropical forest in the high rainfall belt of Zambia to agricultural use based on traditional farming methods. Hydrological observations were carried out on four small catchments under their natural conditions first, and later two of them under agricultural use with accompanying deforestation. Simple linear regression analysis of both monthly and annual runoff from the treated catchments on the monthly and annual runoff from undisturbed catchments showed that there was an increase in streamflow as a result of deforestation and subsistence agriculture. It is also shown that the shape of the flood hydrograph was changed as a result of changes in land use.  相似文献   

Interpretation of the behaviour of agrowell systems in Sri Lanka using radial flow models     

C. S. DE SILVA  K. R. RUSHTON 《水文科学杂志》2013,58(6):825-835

Abstract

This paper describes a study of the use of large diameter “agrowells” used for irrigation in Sri Lanka. Detailed field information for a single location is presented and a numerical radial flow model was used to interpret the field response. Initially the model was used to represent a pumping test in which the pumping and subsequent recovery were monitored. It was then used to investigate the operation of the well during a dry season. Satisfactory agreement could only be obtained when account was taken of the withdrawal of water from below the water table by trees. From the simulation it was possible to show that skilful farming practice could be used in steadily exploiting the resources for irrigation over the whole of the dry season.  相似文献