Experimental results from an array of moored current meters and an HF ocean surface radar support the idea that line broadening on the radar spectra is caused by the velocity distribution within the radar target cell. The experiment was done in the wake of a small island where the velocity variations were severe. An estimate is made of the line broadening which can be expected. In a turbulent flow with dissipation rate of the orderepsilon sim 10^{-10}m^{2}s^{-3}and target cell size 1 3000 m, the line broadening isDeltaf sim 10^{-3}Hz. This would be resolved with a radar time series ofsim 20min and indicates that the HF ocean surface radar technique has potential in the observation of surface velocity distributions. 相似文献
The sedimentary record of 130 km of microtidal (0.9 m tidal range) high wave energy (1.5 m average wave height) barrier island shoreline of the Cape Lookout cuspate foreland has been evaluated through examination of 3136 m of subsurface samples from closely spaced drill holes. Holocene sedimentation and coastal evolution has been a function of five major depositional processes: (1) eustatic sea-level rise and barrier-shoreline transgression; (2) lateral tidal inlet migration and reworking of barrier island deposits; (3) shoreface sedimentation and local barrier progradation; (4) storm washover deposition with infilling of shallow lagoons; and (5) flood-tidal delta sedimentation in back-barrier environments.
Twenty-five radiocarbon dates of subsurface peat and shell material from the Cape Lookout area are the basis for a late Holocene sea-level curve. From 9000 to 4000 B.P. eustatic sea level rose rapidly, resulting in landward migration of both barrier limbs of the cuspate foreland. A decline in the rate of sea-level rise since 4000 B.P. resulted in relative shoreline stabilization and deposition of contrasting coastal sedimentary sequences. The higher energy, storm-dominated northeast barrier limb (Core and Portsmouth Banks) has migrated landward producing a transgressive sequence of coarse-grained, horizontally bedded washover sands overlying burrowed to laminated back-barrier and lagoonal silty sands. Locally, ephemeral tidal inlets have reworked the transgressive barrier sequence depositing fining-upward spit platform and channel-fill sequences of cross-bedded, pebble gravel to fine sand and shell. Shoreface sedimentation along a portion of the lower energy, northwest barrier limb (Bogue Banks) has resulted in shoreline progradation and deposition of a coarsening-up sequence of burrowed to cross-bedded and laminated, fine-grained shoreface and foreshore sands. In contrast, the adjacent barrier island (Shackleford Banks) consists almost totally of inlet-fill sediments deposited by lateral tidal inlet migration. Holocene sediments in the shallow lagoons behind the barriers are 5–8 m thick fining-up sequences of interbedded burrowed, rooted and laminated flood-tidal delta, salt marsh, and washover sands, silts and clays.
While barrier island sequences are generally 10 m in thickness, inlet-fill sequences may be as much as 25 m thick and comprise an average of 35% of the Holocene sedimentary deposits. Tidal inlet-fill, back-barrier (including flood-tidal delta) and shoreface deposits are the most highly preservable facies in the wave-dominated barrier-shoreline setting. In the Cape Lookout cuspate foreland, these three facies account for over 80% of the sedimentary deposits preserved beneath the barriers. Foreshore, spit platform and overwash facies account for the remaining 20%. 相似文献
Grassfire plumes are weakly ionized gas. The ionization in the fire plume is due to thermal and chemi-ionization of incumbent species, which may include graphitic carbon, alkalis and thermally excited radicals, e.g., methyl. The presence of alkalis (e.g., potassium and sodium) in the fires makes thermal ionization a predominant electron producing mechanism in the combustion zone. Alkalis have low dissociation and ionization potentials and therefore require little energy to thermally decompose and give electrons. Assuming a Maxwellian velocity distribution of flame particles and electron-neutral collision frequency much higher than plasma frequency, the propagation of radio waves through a grassfire is predicted to have attenuation and phase shift. Radio wave propagation measurements were performed in a moderate intensity (554 kW m?1) controlled grassfire at 30- and 151-MHz frequencies on a 44 m path using a radio wave interferometer. The maximum temperature measured in the controlled burn was 1071 K and the observed fire depth was 0.9 m. The radio wave interferometer measured attenuation coefficients of 0.033 and 0.054 dB m?1 for 30- and 151-MHz, respectively. At collision frequency of 1.0 × 1011 s?1, maximum electron density was determined to be 5.061 × 1015 m?3. 相似文献
This paper introduces a generic framework for multi-risk modelling developed in the project ‘Regional RiskScape’ by the Research Organizations GNS Science and the National Institute of Water and Atmospheric Research Ltd. (NIWA) in New Zealand. Our goal was to develop a generic technology for modelling risks from different natural hazards and for various elements at risk. The technical framework is not dependent on the specific nature of the individual hazard nor the vulnerability and the type of the individual assets. Based on this generic framework, a software prototype has been developed, which is capable of ‘plugging in’ various natural hazards and assets without reconfiguring or adapting the generic software framework. To achieve that, we developed a set of standards for treating the fundamental components of a risk model: hazards, assets (elements at risk) and vulnerability models (or fragility functions). Thus, the developed prototype system is able to accommodate any hazard, asset or fragility model, which is provided to the system according to that standard. The software prototype was tested by modelling earthquake, volcanic ashfall, flood, wind, and tsunami risks for several urban centres and small communities in New Zealand. 相似文献
Tidal flushing of animal burrows in mangrove swamps provides an effective and important mechanism for transport of salt and other soluble substances. The burrows have complex morphologies consisting of multiple loops. Using established computational modelling techniques, burrow geometries were simulated from characteristic burrow dimensions and the effect of multiple loops on flushing is studied. The computational models show that flushing is enhanced in multiple-loop burrows as upper loops can be completely evacuated, increasing the volume of burrow water removed. The models indicate that there may be a depth limit to which flushing occurs in complex burrow structures. Periscope loops extend further than one characteristic loop depth and are shown to affect the flushing of burrows if the surface-water plume penetrates to their lower depth. Periscope loops with surface openings on the downstream side of the burrow, relative to the tidal inundation, most frequently experience this condition and thus have a great impact on burrow flushing. Field measurements of salinity agree with the hypothesis that there is a depth limit of flushing for complex burrows that is independent of burrow salinity, and suggest a value of 30–35 cm. The measurements also support the suggestion that significant flushing of burrows occurs within a single tidal event. An experiment considered the movement of animals across the interface created by low-salinity surface water and high-salinity burrow water after a partial flushing event. The motion enhances the effective diffusion coefficient of salt by a factor of at least 102, as compared with free diffusion of salt in water, resulting in a more even distribution of dissolved salt in the burrow water between tidal inundations. This distribution may lead to an increase in the depth to which flushing occurs during subsequent inundations. 相似文献
