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A numerical simulation of the generation and evolution of nonlinear internal waves across the Kara Strait
Qun LI, Huiding WU, Hongwei YANG, Zhanhai ZHANG
2019, 38(5): 1-9. doi: 10.1007/s13131-019-1437-z
Abstract(21) FullText HTML(9) FullText PDF(0)
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Nonlinear internal waves (NIWs) are ubiquitous around the Kara Sea, a part of the Arctic Ocean that is north of Siberia. Three hot spot sources for internal waves, one of which is the Kara Strait, have been identified based on Envisat ASAR. The generation and evolution of the NIWs through the interactions of the tide and topography across the strait is studied based on a nonhydrostatic numerical model. The model captures most wave characteristics shown by satellite data. A typical inter-packets distance on the Barents Sea side is about 25 km in summer, with a phase speed about 0.65 m/s. A northward background current may intensify the accumulation of energy during generation, but it has little influence on the other properties of the generated waves. The single internal solitary wave (ISW) structure is a special phenomenon that follows major wave trains, with a distance about 5–8 km. This wave is generated with the leading wave packets during the same tidal period. When a steady current toward the Kara Sea is included, the basic generation process is similar, but the waves toward the Kara Sea weaken and display an internal bore-like structure with smaller amplitude than in the control experiment. In winter, due to the growth of sea ice, stratification across the Kara Strait is mainly determined by the salinity, with an almost uniform temperature close to freezing. A pycnocline deepens near the middle of the water depth (Barents Sea side), and the NIWs process is not as important as the NIWs process in summer. There is no fission process during the simulation.
Analytical study of the effect of the geometrical parameters during the interaction of regular wave-horizontal plate-current
Smail NAASSE, Meriem ERRIFAIY, Chakib CHAHINE
2019, 38(5): 10-20. doi: 10.1007/s13131-019-1346-1
Abstract(23) FullText HTML(6) FullText PDF(1)
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The present work is an analytical study of the influence of geometrical parameters, such as length, thickness and immersion of the plate, on the reflection coefficient of a regular wave for an immersed horizontal plate in the presence of a uniform current with the same direction as the propagation of the incident regular wave. This study was performed using the linearized potential theory with the evanescent modes while searching for complex roots to the dispersion equation that are neither pure real nor pure imaginary. The results show that the effects of the immersion and the relative length on the reflection coefficient of the plate are accentuated by the presence of the current, whereas the plate thickness practically does not have an effect if it is relatively small.
Wave parameters retrieval for dual-polarization C-band synthetic aperture radar using a theoretical-based algorithm under cyclonic conditions
Yingying DING, Juncheng ZUO, Weizeng SHAO, Jian SHI, Xinzhe YUAN, Jian SUN, Jiachen HU, Xiaofeng LI
2019, 38(5): 21-31. doi: 10.1007/s13131-019-1438-y
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Theoretical-based ocean wave retrieval algorithms are applied by inverting a synthetic aperture radar (SAR) intensity spectrum into a wave spectrum, that has been developed based on a SAR wave mapping mechanism. In our previous studies, it was shown that the wave retrieval algorithm, named the parameterized first-guess spectrum method (PFSM), works for C-band and X-band SAR at low to moderate sea states. In this work, we investigate the performance of the PFSM algorithm when it is applied for dual-polarization c-band sentinel-1 (S-1) SAR acquired in extra wide-swath (EW) and interferometric wide-swath (IW) mode under cyclonic conditions. Strong winds are retrieved from six vertical-horizontal (VH) polarization S-1 SAR images using the c-band cross-polarization coupled-parameters ocean (C-3PO) model and then wave parameters are obtained from the image at the vertical-vertical (VV) polarization channel. significant wave height (SWH) and mean wave period (MWP) are compared with simulations from the WAVEWATCH-III (WW3) model. The validation shows a 0.69 m root mean square error (RMSE) of SWH with a –0.01 m bias and a 0.62 s RMSE of MWP with a –0.17 s bias. Although the PFSM algorithm relies on a good quality SAR spectrum, this study confirms the applicability for wave retrieval from an S-1 SAR image. Moreover, it is found that the retrieved results have less accuracy on the right sector of cyclone eyes where swell directly affects strong wind-sea, while the PFSM algorithm works well on the left and rear sectors of cyclone eyes where the interaction of wind-sea and swell is relatively poor.
Upper ocean high resolution regional modeling of the Arabian Sea and Bay of Bengal
Dwivedi Suneet, Mishra Alok Kumar, Srivastava Atul
2019, 38(5): 32-50. doi: 10.1007/s13131-019-1439-x
Abstract(38) FullText HTML(44) FullText PDF(8)
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In this paper, effort is made to demonstrate the quality of high-resolution regional ocean circulation model in realistically simulating the circulation and variability properties of the northern Indian Ocean (10°S–25°N, 45°–100°E) covering the Arabian Sea (AS) and Bay of Bengal (BoB). The model run using the open boundary conditions is carried out at 10 km horizontal resolution and highest vertical resolution of 2 m in the upper ocean. The surface and sub-surface structure of hydrographic variables (temperature and salinity) and currents is compared against the observations during 1998–2014 (17 years). In particular, the seasonal variability of the sea surface temperature, sea surface salinity, and surface currents over the model domain is studied. The high-resolution model’s ability in correct estimation of the spatio-temporal mixed layer depth (MLD) variability of the AS and BoB is also shown. The lowest MLD values are observed during spring (March-April-May) and highest during winter (December-January-February) seasons. The maximum MLD in the AS (BoB) during December to February reaches 150 m (67 m). On the other hand, the minimum MLD in these regions during March-April-May becomes as low as 11–12 m. The influence of wind stress, net heat flux and freshwater flux on the seasonal variability of the MLD is discussed. The physical processes controlling the seasonal cycle of sea surface temperature are investigated by carrying out mixed layer heat budget analysis. It is found that air-sea fluxes play a dominant role in the seasonal evolution of sea surface temperature of the northern Indian Ocean and the contribution of horizontal advection, vertical entrainment and diffusion processes is small. The upper ocean zonal and meridional volume transport across different sections in the AS and BoB is also computed. The seasonal variability of the transports is studied in the context of monsoonal currents.
Spatial and temporal evolution of landfast ice near Zhongshan Station, East Antarctica, over an annual cycle in 2011/2012
Jiechen ZHAO, Qinghua YANG, Bin CHENG, Matti LEPPÄRANTA, Fengming HUI, Surui XIE, Meng CHEN, Yining YU, Zhongxiang TIAN, Ming LI, Lin ZHANG
2019, 38(5): 51-61. doi: 10.1007/s13131-018-1339-5
Abstract(30) FullText HTML(9) FullText PDF(2)
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Annual observations of first-year ice (FYI) and second-year ice (SYI) near Zhongshan Station, East Antarctica, were conducted for the first time from December 2011 to December 2012. Melt ponds appeared from early December 2011. Landfast ice partly broke in late January, 2012 after a strong cyclone. Open water was refrozen to form new ice cover in mid-February, and then FYI and SYI co-existed in March with a growth rate of 0.8 cm/d for FYI and a melting rate of 2.7 cm/d for SYI. This difference was due to the oceanic heat flux and the thickness of ice, with weaker heat flux through thicker ice. From May onward, FYI and SYI showed a similar growth by 0.5 cm/d. Their maximum thickness reached 160.5 cm and 167.0 cm, respectively, in late October. Drillings showed variations of FYI thickness to be generally less than 1.0 cm, but variations were up to 33.0 cm for SYI in March, suggesting that the SYI bottom was particularly uneven. Snow distribution was strongly affected by wind and surface roughness, leading to large thickness differences in the different sites. Snow and ice thickness in Nella Fjord had a similar " east thicker, west thinner” spatial distribution. Easterly prevailing wind and local topography led to this snow pattern. Superimposed ice induced by snow cover melting in summer thickened multi-year ice, causing it to be thicker than the snow-free SYI. The estimated monthly oceanic heat flux was ~30.0 W/m2 in March–May, reducing to ~10.0 W/m2 during July–October, and increasing to ~15.0 W/m2 in November. The seasonal change and mean value of 15.6 W/m2 was similar to the findings of previous research. The results can be used to further our understanding of landfast ice for climate change study and Chinese Antarctic Expedition services.
Unusual coastal ocean cooling in the northern South China Sea by a katabatic cold jet associated with Typhoon Mujigea (2015)
Yuxin SHI, Lingling XIE, Quanan ZHENG, Shuwen ZHANG, Mingming LI, Junyi LI
2019, 38(5): 62-75. doi: 10.1007/s13131-019-1440-4
Abstract(24) FullText HTML(7) FullText PDF(1)
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This study deals with a unusual cooling event after Typhoon Mujigea passed over the northern South China Sea (SCS) in October 2015. We analyze the satellite sea surface temperature (SST) time series from October 3 to 18, 2015 and find that the cooling process in the coastal ocean had two different stages. The first stage occurred immediately after typhoon passage on October 3, and reached a maximum SST drop of –2°C on October 7 as the usual cold wake after typhoon. The second stage or the unusual extended cooling event occurred after 7 d of the typhoon passage, and lasted for 5 d from October 10 to 15. The maximum SST cooling was –4°C and occurred after 12 d of typhoon passage. The mechanism analysis results indicate that after landing and moving northwestward to the Yunnan-Guizhou Plateau (YGP), Typhoon Mujigea (2015) met the westerly wind front on October 5. The low-pressure and positive-vorticity disturbances to the front triggered meridional air flow and low-pressure trough, thus induced a katabatic cold jet downward from the Qinghai-Tibet Plateau (QTP) passing through the YGP to the northwestern SCS. The second cooling reached the maximum SST drop 4 d later after the maximum air temperature drop of –9°C on October 11. The simultaneous air temperature and SST observations at three coastal stations reveal that it is this katabatic cold jet intrusion to lead the unusual SST cooling event.
Asymmetric response of the eastern tropical Indian SST to climate warming and cooling
Shan XU, Yiyong LUO, Fukai LIU
2019, 38(5): 76-85. doi: 10.1007/s13131-019-1441-3
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The response of the eastern tropical Indian Ocean (ETIO) to heat fluxes of equal amplitude but opposite sign is investigated using the Community Earth System Model (CESM). A significant positive asymmetry in sea surface temperature (SST) is found over the ETIO—the warming responses to the positive forcing exceeds the cooling to the negative forcing. A mixed layer heat budget analysis is carried out to identify the mechanisms responsible for the SST asymmetry. Results show that it is mainly ascribed to the ocean dynamical processes, including vertical advections and diffusion. The net surface heat flux, on the contrary, works to reduce the asymmetry through its shortwave radiation and latent heat flux components. The former is due to the nonlinear relationship between SST and cloud, while the latter is resulted mainly from Newtonian damping and air-sea stability effects. Changes in the SST skewness are also evaluated, with more enhanced negative SST skewness over the ETIO found for the cooling than heating scenarios due to the asymmetric thermocline-SST feedback.
High-resolution sequence architecture and depositional evolution of the Quaternary in the northeastern shelf margin of the South China Sea
Hanyao LIU, Changsong LIN, Zhongtao ZHANG, Bo ZHANG, Jing JIANG, Hongxun TIAN, Huan LIU
2019, 38(5): 86-98. doi: 10.1007/s13131-019-1442-2
Abstract(23) FullText HTML(9) FullText PDF(0)
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The northeastern shelf margin of the South China Sea (SCS) is characterized by the development of large scale foresets complexes since Quaternary. Based on integral analysis of the seismic, well logging and paleontological data, successions since ~3.0 Ma can be defined as one composite sequence, consist of a set of regional transgressive to regressive sequences. They can be further divided into six 3rd order sequences (SQ0–SQ5) based on the Exxon sequence stratigraphic model. Since ~1.6 Ma, five sets of deltaic systems characterized by development of wedge-shaped foresets complexes or clinoforms had been identified. High-resolution seismic data and the thick foresets allowed further divided of sub-depositional sequences (4th order) of regression to transgression, which is basically consistent with published stacked benthic foram O-isotope records. Depositional systems identified in the study area include deltaic deposits (inner-shelf deltas and shelf-edge deltas), incised valleys, and slope slumping massive deposits. Since ~1.6 Ma, clinoforms prograded from the southern Panyu Lower Uplift toward the northern Baiyun Depression, shelf slope break migrated seaward, whereas the shelf edge of SQ0 migrated landward. The development of incised valleys in the continental shelf increased upward, especially intensive on the SB3 and SB2. The slumping massive deposits increased abruptly since SB2, which corresponds to the development of incised valleys. The evolution of depositional systems of continental slope mainly controlled by the combined influence of sea level changes, tectonic movements, sediment supply and climate changes. Since ~3.0 Ma, relative sea level of the northern SCS had been experienced transgression (~3.0 Ma BP) to regression (~1.6 Ma BP). The regional regression and maximum transgressions of the composite sequences were apparently enhanced by uplift or subsidence related to tectono-thermal events. In addition, climatic variations including monsoon intensification and the mid-Pleistocene transition may have enhanced sediment supply by increasing erosion rate and have an indispensable influence on the development of the incised valleys and 5 sets of deltaic systems since ~1.6 Ma.
Acoustic backscattering measurement from sandy seafloor at 6–24 kHz in the South Yellow Sea
Guangming KAN, Baohua LIU, Zhiguo YANG, Shengqi YU, Lehua QI, Kaiben YU, Yanliang PEI
2019, 38(5): 99-108. doi: 10.1007/s13131-019-1388-4
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The acoustic bottom backscattering strength was measured at the frequency range of 6–24 kHz on a typical sandy bottom in the South Yellow Sea by using omnidirectional sources and omnidirectional receiving hydrophones. In the experiment, by avoiding disturbances due to scattering off the sea surface and satisfying the far-field condition, we obtained values of acoustic bottom backscattering strength ranging from –41.1 to –24.4 dB within a grazing angle range of 18°–80°. In the effective range of grazing angles, the acoustic scattering strength generally increases with an increase in the grazing angles, but trends of the variation were distinct in different ranges of frequency, which reflect different scattering mechanisms. The frequency dependence of bottom backscattering strength is generally characterized by a positive correlation in the entire frequency range of 6–24 kHz at the grazing angles of 20°, 40° and 60° with the linear regression slopes of 0.222 9 dB/kHz, 0.513 0 dB/kHzand 0.174 6 dB/kHz, respectively. At the largest grazing angle of 80°, the acoustic backscattering strength exhibits no evident frequency dependence.
Effects of igneous bodies on modification of modern slope morphology: Insights from the continental slope offshore Dongsha Islands, South China Sea
Chao LIANG, Xinong XIE, Hua WANG, Guangjian ZHONG, Entao LIU, Ming SUN, Hai YI, Chunyu QIN, Haiyang CAO, Jie HE, Yanpu ZHAO
2019, 38(5): 109-117. doi: 10.1007/s13131-019-1354-y
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A statistical analysis for the morphological parameters extracted from numerous seismic profiles, and a high-resolution seismic study of the southeastern slope of the Dongsha Islands (South China Sea) with water depth between approximately 500 and 3 100 m, has revealed the variation of morphological features due to the intrusion of igneous bodies and associated sedimentary processes. Three types of the continental slope are distinguished: (1) a rough and steep slope with multiple igneous bodies (Type 1), (2) a relatively smooth and gentle slope with the single igneous body (Type 2), and (3) a smooth and gentle slope without igneous bodies (Type 3). These igneous bodies, formed in the post-seafloor spreading of the South China Sea, are often characterized by high positive seismic amplitudes, and chaotic reflections with complex shapes. The igneous bodies in Type 1 separated the slope into two or more upper sub-sags and a lower main-sag, in which the sub-sags and main-sag could be filled with sediments transported by alongslope bottom currents at the same time. Whereas, the igneous body in Type 2 just separated the slope into an upper sub-sag and a lower main-sag, in which the sediments could be transported into the lower main-sag only after the upper sub-sag has been filled up. Type 3 represents a normal slope with common clinoform progradation. The modern slope morphologies in the study area are the results of adjustments of the continental slope due to the intrusion of igneous bodies and associated sedimentary processes. The distinctions among three types of modern slope morphologies indicate different depositional conditions and adjustments of slope morphologies.
Patterns of gas hydrate accumulation in mass transport deposits related to canyon activity: Example from Shenhu drilling area in the South China Sea
Chao FU, Shengli LI, Xinghe YU, Jinqiang LIANG, Zenggui KUANG, Yulin HE, Lina JIN
2019, 38(5): 118-128. doi: 10.1007/s13131-019-1443-1
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Since 2017, a plenty of gas hydrates have been drilled in a new area of Shenhu, and good heterogeneity has been found throughout the spatial distribution of the reservoir. After distinguishing different sedimentary sequence types and matching their formation with slope deposition settings, this study proposes three mass transport deposit (MTD) patterns related to canyon activity that occurred contemporaneously or epigenetically with it: well preserved MTDs, MTDs eroded by canyon migration, and MTDs dislocated by contemporaneous faults. Based on seismic reflection characteristics, this study proposed methods of quantitatively analyzing sedimentary factors, such as measuring the turbidities flow rate in the canyon, and results are interpreted with respect to canyon activity. Combining the above parameters and their relationship with gas hydrate accumulation, fine-grained seals overlapping coarse MTDs reservoirs are found to be indispensable to gas hydrate accumulation, as they prevent the release of free gas. Based on grain size data of hydrate samples from drilling wells, multi-layered gas hydrate reservoirs capped by fine-grained sediments and overlapping mud show favorable hydrate-bearing prospects. The release of gas hydrates, however, is mostly caused by the lack of mud sealing in relation to canyon activity, such as turbidities flow erosion and contemporaneous fault breaking. Canyon migration with respect to MTDs may be the actual cause of erosion of overlapping syn-sedimentary layers, and high bottom flows may contribute to an increase in the release of free gas. It is believed that contemporaneous faults caused by unstable canyon walls may break the muddy over layers and decrease the accumulation pressure of gas hydrate bearing. Thus, according to the sedimentary characteristics of MTDs and the hydrate accumulation process, three responding accumulation or releasing patterns are proposed, which respond to the different types of MTDs distinguished above: a well-preserved MTD accumulation pattern; a canyon migration eroded MTD release pattern; and a micro-contemporaneous fault dislocated MTD release pattern.
Multibeam water column data research in the Taixinan Basin: Implications for the potential occurrence of natural gas hydrate
Yilan CHEN, Jisheng DING, Haiquan ZHANG, Qiuhua TANG, Xinghua ZHOU, Xiaoyu LIU
2019, 38(5): 129-133. doi: 10.1007/s13131-019-1444-0
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A multi beam sonar survey is carried out in the continental slope of the Taixinan Basin to obtain submarine topographic and water column data. The data are processed to obtain water column images. Anomalous water column images, displaying plume characteristics, are found in gas hydrate enriched areas in the Taixinan Basin. This indicates the presence of natural gas resources in the Taixinan Basin. The multibeam sonar system is shown to provide an accurate and effective approach for detecting sub-sea gas hydrate.
A new automatic oceanic mesoscale eddy detection method using satellite altimeter data based on density clustering
Jitao LI, Yongquan LIANG, Jie ZHANG, Jungang YANG, Pingjian SONG, Wei CUI
2019, 38(5): 134-141. doi: 10.1007/s13131-019-1447-x
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The mesoscale eddy is a typical mesoscale oceanic phenomenon that transfers ocean energy. The detection and extraction of mesoscale eddies is an important aspect of physical oceanography, and automatic mesoscale eddy detection algorithms are the most fundamental tools for detecting and analyzing mesoscale eddies. The main data used in mesoscale eddy detection are sea level anomaly (SLA) data merged by multi-satellite altimeters’ data. These data objectively describe the state of the sea surface height. The mesoscale eddy can be represented by a local equivalent region surrounded by an SLA closed contour, and the detection process requires the extraction of a stable closed contour structure from SLA maps. In consideration of the characteristics of mesoscale eddy detection based on SLA data, this paper proposes a new automatic mesoscale eddy detection algorithm based on clustering. The mesoscale eddy structure can be extracted by separating and filtering SLA data sets to separate a mesoscale eddy region and non-eddy region and then establishing relationships among eddy regions and mapping them on SLA maps. This paper overcomes the problem of the sensitivity of parameter setting that affects the traditional detection algorithm and does not require a sensitivity test. The proposed algorithm is thus more adaptable. An eddy discrimination mechanism is added to the algorithm to ensure the stability of the detected eddy structure and to improve the detection accuracy. On this basis, the paper selects the Northwest Pacific Ocean and the South China Sea to carry out a mesoscale eddy detection experiment. Experimental results show that the proposed algorithm is more efficient than the traditional algorithm and the results of the algorithm remain stable. The proposed algorithm detects not only stable single-core eddies but also stable multi-core eddy structures.
Deep learning classification of coastal wetland hyperspectral image combined spectra and texture features: A case study of Huanghe (Yellow) River Estuary wetland
Yabin HU, Jie ZHANG, Yi Ma, Xiaomin LI, Qinpei SUN, Jubai AN
2019, 38(5): 142-150. doi: 10.1007/s13131-019-1445-z
Abstract(42) FullText HTML(7) FullText PDF(0)
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This paper develops a deep learning classification method with fully-connected 8-layers characteristics to classification of coastal wetland based on CHRIS hyperspectral image. The method combined spectral feature and multi-spatial texture feature information has been applied in the Huanghe (Yellow) River Estuary coastal wetland. The results show that: (1) Based on testing samples, the DCNN model combined spectral feature and texture feature after K-L transformation appear high classification accuracy, which is up to 99%. (2) The accuracy by using spectral feature with all the texture feature is lower than that using spectral only and combing spectral and texture feature after K-L transformation. The DCNN classification accuracy using spectral feature and texture feature after K-L transformation was up to 99.38%, and the outperformed that of all the texture feature by 4.15%. (3) The classification accuracy of the DCNN method achieves better performance than other methods based on the whole validation image, with an overall accuracy of 84.64% and the Kappa coefficient of 0.80. (4) The developed DCNN model classification algorithm ensured the accuracy of all types is more balanced, and it also greatly improved the accuracy of tidal flat and farmland, while kept the classification accuracy of main types almost invariant compared to the shallow algorithms. The classification accuracy of tidal flat and farmland is up to 79.26% and 56.72% respectively based on the DCNN model. And it improves by about 2.51% and 10.6% compared with that of the other shallow classification methods.
Research Note
An interpretation of wave refraction and its influence on foreshore sediment distribution
Vincent Jayaraj Joevivek, Nainarpandian Chandrasekar, Ramakrishnan Jayangondaperumal, Vikram Chandra Thakur, Krishnan Shree Purniema
2019, 38(5): 151-160. doi: 10.1007/s13131-019-1446-y
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To analyze the grain size and depositional environment of the foreshore sediments, a study was undertaken on wave refraction along the wide sandy beaches of central Tamil Nadu coast. The nearshore waves approach the coast at 45° during the northeast (NE) monsoon, at 135° during the southwest (SW) monsoon and at 90° during the non-monsoon or fair-weather period with a predominant wave period of 8 and 10 s. A computer based wave refraction pattern is constructed to evaluate the trajectories of shoreward propagating waves along the coast in different seasons. The convergent wave rays during NE monsoon, leads to high energy wave condition which conveys a continuous erosion at foreshore region while divergent and inept condition of rays during the SW and non-monsoon, leads to moderate and less energy waves that clearly demarcates the rebuilt beach sediments through littoral sediment transport. The role of wave refraction in foreshore deposits was understood by grain size and depositional environment analysis. The presence of fine grains with the mixed population, during the NE monsoon reveals that the high energy wave condition and sediments were derived from beach and river environment. Conversely, the presence of medium grains with uniform population, during SW and non-monsoon attested less turbulence and sediments were derived from prolong propagation of onshore-offshore wave process. These upshots are apparently correlated with the in situ beach condition. On the whole, from this study it is understood that beaches underwent erosion during the NE monsoon and restored its original condition during the SW and non-monsoon seasons that exposed the stability of the beach and nearshore condition.
Perspective
Invasion of the biosphere by synthetic polymers: What our current knowledge may mean for our future
Charles J. Moore
2019, 38(5): 161-164. doi: 10.1007/s13131-019-1424-4
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