Influence of Headland Breakwaters on Morphological Processes at Longfengtou Beach in Haitan Bay,Facing the Taiwan Strait     

Cuiping Kuang  Yue Ma  Binyu Wang  Jueyi Sui  Jie Gu  Jianhui Liu  Gang Lei 《中国海洋大学学报(英文版)》2018,17(6):1290-1300

Breakwaters can be used as artificial headlands in beach nourishment to mitigate coastal erosion. Longfengtou Beach, located on the southwest coast of Haitan Bay facing the Taiwan Strait on the northeast, suffers severe erosion, where the monsoon causes significant season hydrodynamic variations. Headland breakwaters are intended to be employed to mitigate coastal erosion. A 2D sediment transport model is established using MIKE21 based on current-wave coupling and calibrated by measured data. Summer and winter wave conditions are chosen as characteristic weak and strong waves respectively. The numerical results of suspended sediment concentration and seasonal morphological evolution are compared under the conditions with and without the headland breakwaters. Sediment transport in summer can be regarded as mainly determined by current field, while in winter wave effect is strong enough to change current field. The northern breakwater, nearly perpendicular to the ebb current direction, obstructs the currents and decreases velocity of the ebb currents, and confines the water carrying sediments within the protection area during the flood period. The southern breakwater also breaks the waves in advance and significantly reduces the hydrodynamic effects during the flood period and maintains high suspended sediment concentration in the protection area. In general, two headland breakwaters decrease the erosion near the beach and enlarge the deposition area, which play a significant role in prevention of sediment loss in nearshore area and mitigate coastal erosion.  相似文献   

Prevention of Shield-Induced Desynchronosis in Invertebrates by a Variable Magnetic Field of Extremely Low Frequency     

N. A. Temuryants  K. N. Tumanyants  A. S. Kostyuk  N. S. Yarmolyuk  E. N. Tumanyants 《Izvestiya Atmospheric and Oceanic Physics》2018,54(7):661-666

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Multi-angle backscatter classification and sub-bottom profiling for improved seafloor characterization     

Evangelos Alevizos  Mirjam Snellen  Dick Simons  Kerstin Siemes  Jens Greinert 《Marine Geophysical Researches》2018,39(1-2):289-306

This study applies three classification methods exploiting the angular dependence of acoustic seafloor backscatter along with high resolution sub-bottom profiling for seafloor sediment characterization in the Eckernförde Bay, Baltic Sea Germany. This area is well suited for acoustic backscatter studies due to its shallowness, its smooth bathymetry and the presence of a wide range of sediment types. Backscatter data were acquired using a Seabeam1180 (180 kHz) multibeam echosounder and sub-bottom profiler data were recorded using a SES-2000 parametric sonar transmitting 6 and 12 kHz. The high density of seafloor soundings allowed extracting backscatter layers for five beam angles over a large part of the surveyed area. A Bayesian probability method was employed for sediment classification based on the backscatter variability at a single incidence angle, whereas Maximum Likelihood Classification (MLC) and Principal Components Analysis (PCA) were applied to the multi-angle layers. The Bayesian approach was used for identifying the optimum number of acoustic classes because cluster validation is carried out prior to class assignment and class outputs are ordinal categorical values. The method is based on the principle that backscatter values from a single incidence angle express a normal distribution for a particular sediment type. The resulting Bayesian classes were well correlated to median grain sizes and the percentage of coarse material. The MLC method uses angular response information from five layers of training areas extracted from the Bayesian classification map. The subsequent PCA analysis is based on the transformation of these five layers into two principal components that comprise most of the data variability. These principal components were clustered in five classes after running an external cluster validation test. In general both methods MLC and PCA, separated the various sediment types effectively, showing good agreement (kappa >0.7) with the Bayesian approach which also correlates well with ground truth data (r²?>?0.7). In addition, sub-bottom data were used in conjunction with the Bayesian classification results to characterize acoustic classes with respect to their geological and stratigraphic interpretation. The joined interpretation of seafloor and sub-seafloor data sets proved to be an efficient approach for a better understanding of seafloor backscatter patchiness and to discriminate acoustically similar classes in different geological/bathymetric settings.  相似文献   

Factors influencing biodeposit production by the New Zealand freshwater mussel Echyridella menziesii     

Kevin J. Collier  Susan J. Clearwater  Paula H. M. W. Neijenhuis  Susanna A. Wood 《新西兰海洋与淡水研究杂志》2018,52(1):1-15

Laboratory experiments on the New Zealand freshwater mussel Echyridella menziesii were used to investigate the short-term effects (7–8 days) of food type on rates of biodeposition and benthic substrate respiration. Post-feeding biodeposition rates ranged from 0.34 to 1.52?mg?g^?1?h^?1 (mean?=?0.50?mg g^?1?h^?1) and were unaffected by the addition of toxin-producing Microcystis. Addition of suspended sediment (30?mg?L^?1) visibly altered substrate composition, and increased total and inorganic biodeposit production rates by 24–33% compared to mussels fed commercial phytoplankton stock. Biodeposition rates of mussels in lake bed substrates were 38% higher than those in silica sand for identical feeding regimes, suggesting that a significant proportion of material produced in this experiment could have been derived from feeding on organic matter in the lake bed sediments. Respiration rates were higher in treatments with Microcystis but were unaffected by the presence of mussels. This laboratory study suggests that biodeposition by E. menziesii is resilient to short-term exposure to Microcystis, and highlights the ability of mussels to alter benthic substrate composition by incorporating suspended sediment into substrates.  相似文献   

Census survey approach to quantifying īnanga spawning habitat for conservation and management     

Shane Orchard  Michael J. Hickford 《新西兰海洋与淡水研究杂志》2018,52(2):284-294

Here, we describe a methodology for quantifying the spawning habitat of īnanga (Galaxias maculatus), a protected native fish species. Our approach is demonstrated with a survey of the Heathcote/ōpāwaho following the Canterbury earthquakes that produced unexpected findings. Spawning habitat was detected over a 2.5?km reach and the area occupied by spawning sites (75m²) was much larger than in previous records (ca.?21m²). Sites dominated by the invasive Phalaris arundinaceae were found to support high egg numbers. Spawning has not previously been recorded on this species and it is identified in the literature as a threat to spawning habitat. Considerable spatio-temporal variation was also detected in the location of spawning sites and pattern of egg production. Together, these aspects illustrate the need for a comprehensive survey methodology to reliably quantify spawning habitat. The Heathcote/ōpāwaho example shows the utility of our census approach for achieving this, and supporting habitat conservation objectives.  相似文献   

Numerical Simulation of the Vortex-Induced Vibration of A Curved Flexible Riser in Shear Flow     

Hong-jun Zhu  Peng-zhi Lin 《中国海洋工程》2018,32(3):301-311

A series of fully three-dimensional(3 D) numerical simulations of flow past a free-to-oscillate curved flexible riser in shear flow were conducted at Reynolds number of 185–1015. The numerical results obtained by the two-way fluid–structure interaction(FSI) simulations are in good agreement with the experimental results reported in the earlier study. It is further found that the frequency transition is out of phase not only in the inline(IL) and crossflow(CF) directions but also along the span direction. The mode competition leads to the non-zero nodes of the rootmean-square(RMS) amplitude and the relatively chaotic trajectories. The fluid–structure interaction is to some extent reflected by the transverse velocity of the ambient fluid, which reaches the maximum value when the riser reaches the equilibrium position. Moreover, the local maximum transverse velocities occur at the peak CF amplitudes, and the values are relatively large when the vibration is in the resonance regions. The 3 D vortex columns are shed nearly parallel to the axis of the curved flexible riser. As the local Reynolds number increases from 0 at the bottom of the riser to the maximum value at the top, the wake undergoes a transition from a two-dimensional structure to a 3 D one. More irregular small-scale vortices appeared at the wake region of the riser, undergoing large amplitude responses.  相似文献   

Wave-induced Maintenance of Suspended Sediment Concentration during Slack in a Tidal Channel on a Sheltered Macro-tidal Flat,Gangwha Island,Korea     

Guan-hong Lee  KiRyong Kang 《Ocean Science Journal》2018,53(3):583-594

A field campaign was conducted to better understand the influence of wave action, in terms of turbulence and bed shear stress, on sediment resuspension and transport processes on a protected tidal flat. An H-frame was deployed in a tidal channel south of Gangwha Island for 6 tidal cycles during November 2006 with instrumentation including an Acoustic Doppler Velocimeter, an Acoustic Backscatter System, and an Optical Backscatter Sensor. During calm conditions, the current-induced shear was dominant and responsible for suspending sediments during the accelerating phases of flood and ebb. During the high-tide slack, both bed shear stress and suspended sediment concentration were reduced. The sediment flux was directed landward due to the scour-lag effect over a tidal cycle. On the other hand, when waves were stronger, the wave-induced turbulence appeared to keep sediments in suspension even during the high-tide slack, while the current-induced shear remained dominant during the accelerating phases of flood and ebb. The sediment flux under strong waves was directed offshore due to the sustained high suspended sediment concentration during the high-tide slack. Although strong waves can induce offshore sediment flux, infrequent events with strong waves are unlikely to alter the long-term accretion of the protected southern Gangwha tidal flats.  相似文献   

Flux Rope Formation Due to Shearing and Zipper Reconnection     

J. Threlfall  A. W. Hood  E. R. Priest 《Solar physics》2018,293(6):98

Zipper reconnection has been proposed as a mechanism for creating most of the twist in the flux tubes that are present prior to eruptive flares and coronal mass ejections. We have conducted a first numerical experiment on this new regime of reconnection, where two initially untwisted parallel flux tubes are sheared and reconnected to form a large flux rope. We describe the properties of this experiment, including the linkage of magnetic flux between concentrated flux sources at the base of the simulation, the twist of the newly formed flux rope, and the conversion of mutual magnetic helicity in the sheared pre-reconnection state into the self-helicity of the newly formed flux rope.  相似文献   

On the Possibility of Deflecting an Asteroid from Collision with the Earth Using the Kinetic Method     

L. L. Sokolov  N. A. Petrov  A. A. Vasil’ev  G. A. Kuteeva  A. S. Shmyrov  B. B. Eskin 《Solar System Research》2018,52(4):338-346

The possibilities of deflecting an asteroid from its collision course with the Earth by changing its velocity with an impact are considered. Using the asteroid Apophis as an example, the time dependence of the positions and sizes of the keyholes leading to collision is studied. It has been found that the possibility of deflecting this asteroid usually exists, and the impact can be accomplished in principle, given the capabilities of modern space technology. A change in the velocity should be performed before the encounter of 2029 in order to use the gravitational maneuver effect. The possible accuracy of determining Apophis’ orbit and the keyholes that lead to collision and are associated with the resonance returns are considered.  相似文献   

Variational and symplectic integrators for satellite relative orbit propagation including drag     

Leonel Palacios  Pini Gurfil 《Celestial Mechanics and Dynamical Astronomy》2018,130(4):31

Orbit propagation algorithms for satellite relative motion relying on Runge–Kutta integrators are non-symplectic—a situation that leads to incorrect global behavior and degraded accuracy. Thus, attempts have been made to apply symplectic methods to integrate satellite relative motion. However, so far all these symplectic propagation schemes have not taken into account the effect of atmospheric drag. In this paper, drag-generalized symplectic and variational algorithms for satellite relative orbit propagation are developed in different reference frames, and numerical simulations with and without the effect of atmospheric drag are presented. It is also shown that high-order versions of the newly-developed variational and symplectic propagators are more accurate and are significantly faster than Runge–Kutta-based integrators, even in the presence of atmospheric drag.  相似文献