Longshore drift estimation using fluorescent tracers: New insights from an experiment at Comporta Beach, Portugal   总被引：1，自引：0，他引：1  

Ana Silva  Rui Taborda  Aurora Rodrigues  Joo Duarte  Joo Cascalho 《Marine Geology》2007,240(1-4):137-150

A fluorescent sand-tracer experiment was performed at Comporta Beach (Portugal) with the aim of acquiring longshore sediment transport data on a reflective beach, the optimization of field and laboratory tracer procedures and the improvement of the conceptual model used to support tracer data interpretation.

The field experiment was performed on a mesotidal reflective beach face in low energetic conditions (significant wave height between 0.4 and 0.5 m). Two different colour tracers (orange and blue) were injected at low tide and sampled in the two subsequent low tides using a high resolution 3D grid extending 450 m alongshore and 30 m cross-shore. Marked sand was detected using an automatic digital image processing system developed in the scope of the present experiment.

Results for the two colour tracers show a remarkable coherence, with high recovery rates attesting data validity. Sand tracer displayed a high advection velocity, but with distinct vertical distribution patterns in the two tides: in the first tide there was a clear decrease in tracer advection velocity with depth while in the second tide, the tracer exhibited an almost uniform vertical velocity distribution. This differing behaviour suggests that, in the first tide, the tracer had not reached equilibrium within the transport system, pointing to a considerable time lag between injection and complete mixing. This issue has important implications for the interpretation of tracer data, indicating that short term tracer experiments tend to overestimate transport rates. In this work, therefore, longshore estimates were based on tracer results obtained during the second tide.

The estimated total longshore transport rate at Comporta Beach was 2 × 10^− 3 m³/s, more than four times larger than predicted using standard empirical longshore formulas. This discrepancy, which results from the unusually large active moving layer observed during the experiment, confirms the idea that most common longshore transport equations under-estimate total sediment transport in plunging/surging waves.  相似文献   

Contribution of artificial reefs to the diet of the white sea bream (Diplodus sargus)   总被引：1，自引：0，他引：1  

Leitao  Francisco; Santos  Miguel N.; Monteiro  Carlos C. 《ICES Journal of Marine Science》2007,64(3):473-478

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Modelling linear wave transformation induced by dissipative structures—Random waves     

R. Silva  M.A. Losada  P. Salles 《Ocean Engineering》2006,33(16):2174-2194

The relevant theory is presented and numerical results are compared with the analytical solution for the interaction of non-breaking waves with an array of vertical porous circular cylinders on a horizontal bed. The extension to the cases of unidirectional and multidirectional waves is obtained by means of a transfer function. The influence of the mechanical properties of porous structures and wave irregularity on wave transformation is analysed. Results for unidirectional and multidirectional wave spectra are compared to those obtained for regular waves. The model presented reproduces well the analytical results and provides a tool for analysing several engineering problems.  相似文献   

The Effect of Human Exclusion on the Population Structure of Key-Hole Limpets Fissurella crassa and F. limbata on the Coast of Central Chile     

Doris  Oliva Juan Carlos  Castilla 《Marine Ecology》1986,7(3):201-217

Abstract. We describe the basic ecology of two key-hole limpets, Fissurella crassa and Fissurella limbata , which are heavily harvested by coastal shellfishermen (mariscadores) at the rocky intertidal of central Chile. The ecological role of human predation on these species was assessed. Anthropic exclusion from a stretch of the rocky intertidal at Las Cruces (non-harvested area) for about two and a half years resulted in changes of the densities and size distributions of these fissurelid species as compared to populations in control zones (harvested area). Differential human predation generates, on key-hole limpets, responses of different intensities related to the accessibility of man to the mesohabitats (sheltered platforms, exposed platforms and vertical walls) and fringes of the intertidal (mid-low intertidal and Lessonia nigrescens fringe). Thus, size distributions and mean densities of both fissurelids differ widely between the harvested and non-harvested areas on both platform habitats but not on vertical walls. In the mid-low intertidal fringe, differences are greater than in the less accessible L. nigrescens intertidal fringe. Moreover, we discuss the way in which other species, directly harvested by mariscadores (i. e., the mollusc Concholepas concholepas) can affect the keyhole limpet populations of central Chile rocky shores. Finally, we highlight the ecological role of these limpets on the dynamics of intertidal ecosystems and how they can modify community landscape.  相似文献   

Estimates of heat flow and heat production and a thermal model of the São Francisco craton     

Carlos H. Alexandrino  Valiya M. Hamza 《International Journal of Earth Sciences》2008,97(2):289-306

An updated analysis of geothermal data from the highland area of eastern Brazil has been carried out and the characteristics of regional variations in geothermal gradients and heat flow examined. The database employed includes results of geothermal measurements at 45 localities. The results indicate that the Salvador craton and the adjacent metamorphic fold belts northeastern parts of the study area are characterized by geothermal gradients in the range of 6–17°C/km. The estimated heat flow values fall in the range of 28–53 mW/m², with low values in the cratonic area relative to the fold belts. On the other hand, the São Francisco craton and the intracratonic São Francisco sedimentary basin in the southwestern parts are characterized by relatively higher gradient values, in the range of 14–42°C/km, with the corresponding heat flow values falling in the range of 36–89 mW/m². Maps of regional variations indicate that high heat flow anomaly in the São Francisco craton is limited to areas of sedimentary cover, to the west of the Espinhaço mountain belt. Crustal thermal models have been developed to examine the implications of the observed intracratonic variations in heat flow. The thermal models take into consideration variation of thermal conductivity with temperature as well as change of radiogenic heat generation with depth. Vertical distributions of seismic velocities were used in obtaining estimates of radiogenic heat production in crustal layers. Crustal temperatures are calculated based on a procedure that makes simultaneous use of the Kirchoff and Generalized Integral Transforms, providing thereby analytical solutions in 2D and 3D geometry. The results point to temperature variations of up to 300°C at the Moho depth, between the northern Salvador and southern São Francisco cratons. There are indications that differences in rheological properties, related to thermal field, are responsible for the contrasting styles of deformation patterns in the adjacent metamorphic fold belts.  相似文献   

Extension of the solution of Kepler's equation to high eccentricities     

Sandro Da Silva Fernandes 《Celestial Mechanics and Dynamical Astronomy》1994,58(3):297-308

The classic Lagrange's expansion of the solutionE(e, M) of Kepler's equation in powers of eccentricity is extended to highly eccentric orbits, 0.6627 ... <e<1. The solutionE(e, M) is developed in powers of (e–e*), wheree* is a fixed value of the eccentricity. The coefficients of the expansion are given in terms of the derivatives of the Bessel functionsJ _n (ne). The expansion is convergent for values of the eccentricity such that |e–e*|<(e*), where the radius of convergence (e*) is a positive real number, which is calculated numerically.  相似文献   

Tunneling and interference of Alfvén waves     

Y. D. Žugžda  V. Locāns 《Solar physics》1982,76(1):77-108

The propagation and interference of Alfvén waves in magnetic regions is studied. A multilayer approximation of the standard models of the solar atmosphere is used. In each layer, there is a linear law of temperature variation and a power law of Alfvén velocity variation. The analytical solutions of a wave equation are stitched at the layer boundaries. The low-frequency Alfvén waves (P > 1 s) are able to transfer the energy from sunspots into the corona by tunneling only. The chromosphere is not a resonance filter for the Alfvén waves. The interference and resonance of Alfvén waves are found to be important to wave propagation through the magnetic coronal arches. The transmission coefficient of Alfvén waves into the corona increases sharply on the resonance frequences. To take into account the wave absorption in the corona, a method of equivalent schemes is developed. The heating of a coronal arch by Alfvén waves is discussed.  相似文献   

Eta carinae nebulosities in 1963     

Carlos Donzelli  J. L. Sérsic  Adela Abraham 《Astrophysics and Space Science》1990,174(2):285-289

The purpose of this note is to give further information regarding the positions of some condensations around Carinae in 1963, an epoch not yet considered in the literature. The new information is unable to confirm the deceleration hypothesis of theeir motion.  相似文献   

The behavior of trace elements during schwertmannite precipitation and subsequent transformation into goethite and jarosite   总被引：5，自引：0，他引：5  

Patricia Acero  Carlos Ayora  José-Miguel Nieto 《Geochimica et cosmochimica acta》2006,70(16):4130-4139

Schwertmannite is a ubiquitous mineral formed from acid rock drainage (ARD), and plays a major role in controlling the water chemistry of many acid streams. The formation of schwertmannite was investigated in the acid discharge of the Monte Romero abandoned mine (Iberian Pyrite Belt, SW, Spain). Schwertmannite precipitated from supersaturated solutions mainly owing to the oxidation of Fe(II) to Fe(III) and transformed with time into goethite and jarosite. In a few hours, schwertmannite precipitation removed more than half of the arsenic load from solution, whereas the concentration of divalent trace metals (Zn, Cu, Pb, Cd, Ni, and Co) remained almost unchanged. In the laboratory, natural schwertmannite was kept in contact with its coexisting acid water in a flask with a solid-liquid mass ratio of 1:5 for 353 days. During this time, the pH of the solution dropped from 3.07 to 1.74 and the concentrations of sulfate and Fe increased. During the first 164 days, schwertmannite transformed into goethite plus H₃O-jarosite but, subsequently, goethite was the only mineral to form. Some of the trace elements, such as Al, Cu, Pb, and As were depleted in solution during the first stage as schwertmannite transformed into goethite plus H₃O-jarosite. On the contrary, the transformation of schwertmannite to goethite (with no jarosite) during the second stage released Al, Cu, and As to the solution. Despite the variation in their concentrations in solution, approximately 80% of the total Al and Cu inventories and more than 99% As and Pb remained in the solid phase throughout the entire aging process.  相似文献   

Holocene record of productivity in the NW Iberian continental shelf     

V. Alves Martins  C. Patinha  E. Ferreira da Silva  J.-M. Jouanneau  O. Weber  F. Rocha   《Journal of Geochemical Exploration》2006,88(1-3):408

A multiproxy approach (textural characteristics, heavy metal concentrations and benthic foraminifera data) was adopted to study the OMEX core KSGX 40, collected at the Galicia Mud Deposit (NW Iberian outer continental shelf) and recording the last ca. 4.8 ka cal BP. Geochemical profiles of Fe, Mn, Zn, Co, Cr, Cu, Ni, Pb, Al and Ca show significant temporal variations. Benthic foraminifera productivity proxy suggests that the recorded muddy events were contemporaneous of a higher C_org flux to the marine benthic environment. The higher flux of organic matter, following a long-term intensification of the upwelling and coinciding with a finer sedimentation, led to depressed levels of oxygen beneath the water-sediments interface and to early diagenetic processes of several redox-sensitive elements.  相似文献