Prediction of sand transport rates by waves and currents in the coastal zone     

A. G. Davies  C. Villaret 《Continental Shelf Research》2002,22(18-19)

The predictions of a sand transport research model and Bijker's (J. Waterways, Harbours Coastal Eng. Div. ASCE 97 (WW4) (1971) 687) engineering model are compared with data obtained in wave-current conditions at three field sites. A key element in the present study is that the bed roughness at the three sites has been estimated from predictions of the sand ripple dimensions. The comparisons between suspended sand concentrations and transport rates show that a considerable amount of uncertainty (factor ±5 or more) arises when individual predictions are compared with the measurements. However, the overall bias in each set of comparisons is smaller than this, with overall agreement being within a factor of ±2 in most cases. While the results demonstrate that research models, adapted for field application, may be used to make practical sand transport computations with as much accuracy as engineering formulations, the true benefit of research models lies in the improved understanding of transport processes that they provide. This is illustrated with reference to the mechanism of grain size sorting caused by oblique incidence of waves on a current.  相似文献   

Paramètres statistiques de la houle au large de la côte sud-aquitaine     

Stéphane Abadie  Rémi Butel  Hélène Dupuis  Christophe Brière 《Comptes Rendus Geoscience》2005,337(8):769-776

Statistical parameters of waves on the south Aquitaine coast have been determined by comparison of two datasets: in situ local measurements on a short duration and numerical simulation outputs from the WAVEWATCH III model over a period of 6 years and 9 months. Data and simulation results are well correlated in the case of the significant wave height, whereas in the case of the peak periods $(T_{\mathrm{p}})$ and directions $(D_{\mathrm{p}})$, this correlation is lower. This is due respectively to the intrinsic unstable nature of $T_{\mathrm{p}}$ and the narrow distribution of $D_{\mathrm{p}}$. An adjustment of simulation data on measurement is then used to give an evaluation of the wave statistics for the southern fraction of the French Atlantic coast. Eventually, the relationships between the three statistical variables $(H_{\mathrm{s}},T_{\mathrm{p}},D_{\mathrm{p}})$ are analysed in order to determine representative sea states. The classification obtained shows the predominance of swell regimes, distributed on a very narrow angular sector over wind sea regimes. To cite this article: S. Abadie et al., C. R. Geoscience 337 (2005).  相似文献   

Grain destruction in interstellar shock waves     

B. T. Draine 《Astrophysics and Space Science》1995,233(1-2):111-123

Interstellar shock waves can erode and destroy grains present in the shocked gas, primarily as the result of sputtering and grain-grain collisions. Uncertainties in current estimates of sputtering yields are reviewed. Results are presented for the simple case of sputtering of fast grains being stopped in cold gas. An upper limit is derived for sputtering of refractory grains in C-type MHD shocks: shock speedsv _s 50 km s^–1 are required for return of more than 30% of the silicate to the gas phase. Sputtering can also be important for removing molecular ice mantles from grains in two-fluid MHD shock waves in molecular gas. Recent estimates of refractory grain lifetimes against destruction in shock waves are summarized, and the implications of these short lifetimes are discussed.  相似文献   

Simulations of the wardle instability of c-type shock waves     

G. Tóth 《Astrophysics and Space Science》1995,233(1-2):301-305

C-type shocks in the partially ionized ISM are modelled by numerical simulations. Under certain conditions the shocks are subject to the Wardle instability, which initially makes the shock front rippled, then in the non-linear stage can produce density variations in both the ion and neutral fluids. A systematic search in the numerically accessible parameter space is done to determine the wave vector k_max and the growth rates _max of the fastest growing modes. The neutral Alfvén number, and the angle _sbetween the shock normal and the upstream magnetic field determine the strength and obliqueness of the shock, as well as the dimensionless parameters of the fastest growing mode. The results confirm and extend Wardle's linear analysis.The non-linear evolution shows saturation of the instability and the formation of high density regions that detach from the shock front with the downstream flow. Numerical difficulties are partially solved by an implicit treatment of the ion-neutral friction terms, but strong shocks still can not be modelled efficiently. A fully implicit method for the ions and the magnetic field is used to model C-type shocks with low fractional ionization and high ion Alfvén speed.  相似文献   

A distribution function calculation of the H α profiles of SNR filaments     

A. J. Lim 《Astrophysics and Space Science》1995,233(1-2):273-280

In shocked media of high preshock ionisation, the lack of thermal contact between atoms in the neutral component of the gas can prevent the formation of a thermal equilibrium independent of the ionic component. The behaviour of the neutral gas in such a shock is dominated by the atomic processes driven by the postshock ionic component.A transport equation for the velocity distribution of the neutral gas is explicitly solved under the physical conditions of a 1D ionic shock transition. The resulting distributions are used to calculate predicted H line emission from such a system.  相似文献   

Mechanisms associated with the erosion of sand dune cliffs,Magilligan, Northern Ireland   总被引：1，自引：0，他引：1  

R. W. G. Carter  Gregory W. Stone 《地球表面变化过程与地形》1989,14(1):1-10

A fifteen year history of coastal dune recession at Magilligan, Northern Ireland has revealed both time and space contrasts in processes and morphology. Since 1968 ‘storm’ frequency has increased, while dune retreat reached a peak (5·4 m) in 1978–1979. Three types of dune eroding events are noted, resulting from particular wave/wind/tide combinations. The spatial variability in dune scarping is associated with vegetation and soil development, and slope failures range from granular avalanches to retrogressive rotational slumps.  相似文献   

A wave tank experiment on the erosional mechanism at a cliff base     

Tsuguo Sunamura 《地球表面变化过程与地形》1982,7(4):333-343

A laboratory experiment using a two-dimensional wave tank was designed to investigate the mechanism of erosion at a cliff base by waves armed with rock fragments. The experiment was performed under constant wave conditions by systematically changing the amount of beach sand at the foot of steep model cliffs of the same slope and strength. Cliff erosion occurred when the beach material at the cliff/beach junction was moved by waves. Turbulence created by bores rushing up on the beach mobilized the sand and exerted a mechanical shearing force on the cliff face using the sand as an abrasive. The analysis of results indicated that the effect of the abrasive doubled when the cliff/beach junction was located above Still Water Level (SWL) as compared to when it was below SWL. The assailing force of the sediment-laden water masses was proportional to the square of the bore speed immediately in front of the cliff face. The factor of proportionality is related to the quantity of beach sand entrapped in the turbulent fluid.  相似文献   

Energy mode distribution: An analysis of the ratio of anti-Stokes to Stokes amplitudes generated by a pair of counterpropagating Langmuir waves     

F.J.R. Simes Júnior  M.V. Alves  F.B. Rizzato 《Journal of Atmospheric and Solar》2005,67(17-18):1680

Results from plasma wave experiments in spacecrafts give support to nonlinear interactions involving Langmuir, electromagnetic, and ion-acoustic waves in association with type III solar radio bursts. Starting from a general form of Zakharov equation (Zakharov, V.E., 1985. Collapse and self-focusing of Langmuir waves. Hand-book of Plasma Physics Cap.2, 81–121) the equations for electric fields and density fluctuations (density gratings) induced by a pair of counterpropagating Langmuir waves are obtained. We consider the coupling of four triplets. Each two triplets have in common the Langmuir pump wave (forward or backward wave) and a pair of independent density gratings. We numerically solve the dispersion relation for the system, extending the work of (Alves, M.V., Chian, A.C.L., Moraes, M.A.E., Abalde, J.R., Rizzato, F.B., 2002. A theory of the fundamental plasma emission of type- III solar radio bursts. Astronomy and Astrophysics 390, 351–357). The ratio of anti-Stokes (AS) (ω₀+ω) to Stokes (S) (ω₀-ω^*) electromagnetic mode amplitudes is obtained as a function of the pump wave frequency, wave number, and energy. We notice that the simultaneous excitation of AS and S distinguishable modes, i.e., with Re{ω}=ω_r≠0, only occurs when the ratio between the pump wave amplitudes, r is ≠1 and the pump wave vector k₀ is , W₀ being the forward pump wave energy. We also observe that the S mode always receives more energy.  相似文献   

Turbulence Characteristics Of The Stable Boundary Layer Over A Mid-Latitude Glacier. Part Ii: Pure Katabatic Forcing Conditions     

C. J. P. P. Smeets  P. G. Duynkerke  H. F. Vugts 《Boundary-Layer Meteorology》2000,97(1):73-107

Observations obtained over a glacier surface in a predominantlykatabatic flow and with a distinctwind maximum below 13-m height are presented. The data werecollected using a 13-m high profilemast and two sonic anemometers (at about 2.5-m and 10-m heights).The spectra at frequencies belowthat of the turbulence range appear to deviate considerably fromthe curves obtained by Kaimal andco-workers during the 1968 Kansas experiment. The characteristicsof these deviations are compared tothe observations of others in surface-layers disturbed by anykind of large-scale outer-layer (orinactive) turbulence. In our case the disturbances arelikely to be induced by the highmountain ridges that surround the glacier. Moreover, the deviationsobserved in the cospectra seemto result from an, as yet, unspecified interaction between theinactive outer-layer turbulenceand the local surface-layer turbulence. Near the distinctwind maximum turbulence production ceasedwhile turbulence itself did not, probably the result ofturbulence transport from other levels. Consequently, we studied thelocal similarity relations using _w instead of u_* as an alternative velocity scale. Wellbelow the wind maximum, and for relatively low stability(0< Ri_g <0.2), the flow behaves accordingto well established local-scaling similarity relationshipsin the stable boundary layer. For higherstability (Ri_g > 0.2), and near or above the wind maximum, the boundary-layer structure conforms tothat of z-less stratification suggesting that the eddy sizeis restricted by the local stability ofthe flow. In line with this we observed that the sensibleheat fluxes relate remarkably well to thelocal flow parameters.  相似文献   

ST radar observations of atmospheric waves over mountainous areas: a review     

J. Röttger 《Annales Geophysicae》2000,18(7):750-765

Lee and mountain waves are dominant dynamic processes in the atmosphere above mountain areas. ST VHF radars had been intensively used to investigate these wave processes. These studies are summarized in this work. After discussing features of long-period quasi-stationary lee waves, attention is drawn to the frequent occurrence of freely propagating waves of shorter periods, which seem to be more common and characteristic for wave processes generated over mountainous areas. Characteristics of these waves such as their relation to the topography and background winds, the possibility of trapping by and breaking in the tropopause region and their propagation into the stratosphere is investigated. These orographically produced waves transport energy and momentum into the troposphere and stratosphere, which is considered an important contribution to the kinetic energy of the lower atmosphere. The occurrence of inertia-gravity waves in the stratosphere had been confused with lee waves, which is discussed in conclusion. Finally further questions on mountain and lee waves are drawn up, which remain to be solved and where investigations with ST radars could play a fundamental role.  相似文献