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921.
Katharine A. Smith Elizabeth W. North Fengyan Shi Shih-Nan Chen Raleigh R. Hood Evamaria W. Koch Roger I. E. Newell 《Estuaries and Coasts》2009,32(4):748-757
Seagrass beds have declined in Chesapeake Bay, USA as well as worldwide over the past century. Increased seston concentrations,
which decrease light penetration, are likely one of the main causes of the decline in Chesapeake Bay. It has been hypothesized
that dense populations of suspension-feeding bivalves, such as eastern oysters (Crassostrea virginica), may filter sufficient seston from the water to reduce light attenuation and enhance seagrass growth. Furthermore, eastern
oyster populations can form large three-dimensional reef-like structures that may act like breakwaters by attenuating waves,
thus decreasing sediment resuspension. We developed a quasi-three-dimensional Seagrass-Waves-Oysters-Light-Seston (SWOLS)
model to investigate whether oyster reefs and breakwaters could improve seagrass growth by reducing seston concentrations.
Seagrass growth potential (SGP), a parameter controlled by resuspension-induced turbidity, was calculated in simulations in
which wave height, oyster abundance, and reef/breakwater configuration were varied. Wave height was the dominant factor influencing
SGP, with higher waves increasing sediment resuspension and decreasing SGP. Submerged breakwaters parallel with the shoreline
improved SGP in the presence of 0.2 and 0.4 m waves when sediment resuspension was dominated by wave action, while submerged
groins perpendicular to the shoreline improved SGP under lower wave heights (0.05 and 0.1 m) when resuspension was dominated
by along-shore tidal currents. Oyster-feeding activity did not affect SGP, due to the oysters’ distance from the seagrass
bed and reduced oyster filtration rates under either low or high sediment concentrations. Although the current implementation
of the SWOLS model has simplified geometry, the model does demonstrate that the interaction between oyster filtration and
along-shore circulation, and between man-made structures and wave heights, should be considered when managing seagrass habitats,
planning seagrass restoration projects, and choosing the most suitable methods to protect shorelines from erosion. 相似文献
922.
Residual Exchange Flows in Subtropical Estuaries 总被引:1,自引:0,他引:1
Arnoldo Valle-Levinson Guillermo Gutierrez de Velasco Armando Trasviña Alejandro J. Souza Reginaldo Durazo Ashish J. Mehta 《Estuaries and Coasts》2009,32(1):54-67
Observations of residual exchange flows at the entrance to four subtropical estuaries, two of them semiarid, indicate that
these flows are mainly tidally driven, as they compare favorably with theoretical patterns of tidal residual flows. In every
estuary examined, the tidal behavior was that of a standing or near-standing wave, i.e., tidal elevation and tidal currents
were nearly in quadrature. The pattern of exchange flow that persisted at every estuary exhibited inflow in the channel and
outflow over the shoals. Curiously, but also fortuitously, this pattern coincides with the exchange pattern driven by density
gradients in other estuaries. The tidal stresses and the residual elevation slopes should be the dominant mechanisms that
drive such tidal residual pattern because the Stokes transport mechanism is negligible for standing or near-standing waves.
Time series measurements from the semiarid estuaries showed fortnightly modulation of the residual flow by tidal forcing in
such a way that the strongest net exchange flows developed with the largest tidal distortions, i.e., during spring tides.
This modulation is opposite to the modulation that typically results in temperate estuaries, where the strongest net exchange
flows tend to develop during neap tides. The fortnightly modulation on tidal residual currents could be inferred from previous
theoretical results because residual currents arise from tidal distortions but is made explicit in this study. The findings
advanced herein should allow the drawing of generalities about exchange flow patterns in subtropical estuaries where residual
flows are mainly driven by tides. 相似文献
923.
Coral microatolls were examined from North Andaman and Little Andaman to understand the relative sea level change due to vertical
tectonic deformation above the subduction interface. The highest level of survival of coral microatoll before the 26 December,
2004 earthquake at eastern coast of North Andaman has been determined by Global Ocean Tide Model. The present position of
recently dead flat top microatoll with preserved internal structure at the eastern coast of North Andaman mainland indicates
31.21 cm uplift due to the 26 December, 2004 earthquake. Comparatively old cup shaped microatoll at the eastern fringe of
North Andaman group of islands and highly bioeroded fossil microatolls at the intertidal zone of Little Andaman bear the signature
of permanent vertical deformation in the past. 相似文献
924.
925.
Nicola Nocilla Aldo Evangelista Anna Scotto di Santolo 《Rock Mechanics and Rock Engineering》2009,42(5):815-833
In recent years, rock fall phenomena in Italy have received considerable attention for risk mitigation through in situ observations
and experimental data. This paper reports the study conducted at Camaldoli Hill, in the urban area of Naples, and at Monte
Pellegrino, Palermo, Italy. The rocks involved are volcanic Neapolitan yellow tuff (NYT) in the former area and dolomitic
limestone in the latter. Both rocks, even though with different strength characteristics, have shown a significant tendency
towards rock fragmentation during run out. This behavior was first investigated by comparing the volumes of removable blocks
on the cliff faces (V
0) and fallen blocks on the slopes (V
f). It was assumed that the ratio V
f/V
0 decreases with the distance (x
f) from the detachment area by an empirical law, which depends on a coefficient α, correlated with the geotechnical properties of the materials involved in the rock fall. Finally, this law was validated
by observation of well-documented natural rock falls (Palermo) and by in situ full-scale tests (Naples). From the engineering
perspective, consideration of fragmentation processes in rock fall modeling provides a means for designing low-cost mitigation
measures. 相似文献
926.
Understanding deformation mechanisms in Himalayan rocks is a challenging proposition due to the complex nature of the deformed
rocks and their genesis. Crustal deformation in the Himalayan thrust belt typically occurs in elastico-frictional (EF) or
quasi-plastic (QP) regimes at depths controlled mainly by regional strain-rate and geothermal gradient. However, material
property, grain-size and their progressive changes during deformation are also important controlling factors. We present evidence
of EF deformation from Gondwana rocks developed during the emplacement of one of the frontal horses (Jorthang horse) in the
Lesser Himalayan Duplex (LHD) structure associated with Lesser Himalayan rocks in the footwall of the Ramgarh thrust in the
Rangit window near Jorthang in the Sikkim Himalaya. The rocks in the horse exhibit systematic changes in microand meso-structures
from an undeformed protolith to cataclasite suggesting that it was emplaced under elastico-frictional conditions. Meso- to
micro-scale shear fractures are seen developed in Gondwana sandstone and slate while intercalated fine-grained shale-coal-carbonates
are deformed by cataclastic flow suggesting that material property and grain-size have played an important role in the deformation
of the Jorthang horse. In contrast, the hanging wall schists and quartzites of the Ramgarh thrust exhibit quasi-plastic deformation
structures. This suggests that the Jorthang horse was emplaced under shallower crustal conditions than the antiformally folded
Ramgarh thrust sheet even though the Ramgarh sheet presently overlies the Jorthang horse. 相似文献
927.
Justin I. Simon Jorge A. Vazquez Paul R. Renne Axel K. Schmitt Charles R. Bacon Mary R. Reid 《Contributions to Mineralogy and Petrology》2009,158(4):421-446
We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small
volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from
these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the
past 650 ka at Coso. In ≤230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite)
from Pleistocene predecessors. A significant subset of zircon in the ~85 ka rhyolites yielded ages between ~100 and 200 Ma,
requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are
found in an ~200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated
by significant changes in rhyolite composition over short time intervals (≤10’s to 100’s ka). In conjunction with radioisotopic
age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some
(and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent
separation from long-lived mushy magma bodies. 相似文献
928.
Pratima M. Kessarkar V. Purnachandra Rao R. Shynu Ishfaq Mir Ahmad Prakash Mehra G. S. Michael D. Sundar 《Journal of Earth System Science》2009,118(4):369-377
Systematic studies on the suspended particulate matter (SPM) measured on a seasonal cycle in the Mandovi Estuary, Goa indicate
that the average concentrations of SPM at the regular station are ∼20mg/l, 5mg/l, 19mg/l and 5mg/l for June–September, October–January,
February–April and May, respectively. SPM exhibits low-to-moderate correlation with rainfall indicating that SPM is also influenced
by other processes. Transect stations reveal that the SPM at sea-end stations of the estuary are at least two orders of magnitude
greater than those at the river-end during the monsoon. Estuarine turbidity maximum (ETM) of nearly similar magnitude occurs
at the same location in two periods, interrupted by a period with very low SPM concentrations. The ETM occurring in June–September
is associated with low salinities; its formation is attributed to the interactions between strong southwesterly winds (5.1–5.6ms−1) and wind-induced waves and tidal currents and, dominant easterly river flow at the mouth of the estuary. The ETM occurring
in February–April is associated with high salinity and is conspicuous. The strong NW and SW winds (3.2–3.7ms−1) and wind-driven waves and currents seem to have acted effectively at the mouth of the estuary in developing turbidity maximum.
The impact of sea breeze appears nearly same as that of trade winds and cannot be underestimated in sediment resuspension
and deposition 相似文献
929.
Satish J. Patel Bhawanisingh G. Desai 《Journal of the Geological Society of India》2009,74(2):233-259
Animal-sediment relationships of two benthic communities (Crustaceans and Polychaetes) were studied around Mandvi coast in
the Gulf of Kachchh, Western India. This coast consists of many micro-geomorphic landforms in which benthic communities are
inhabited and select their niches and produce endemic biogenic structures. Five intertidal subfacies have been described and
four types of grounds are identified, based on substrate consistency. 18 species of crustaceans, 15 species of polychaetes
and unsegmented worm nemertea have been identified. Crustacean behavioural activities were observed in dunes, beaches and
ridge-runnel in the form of burrowing, pellet making, feeding and crawling traces. Pelleted wall lining burrows of the suspension
feeder stomatopodean species of Oratosquilla striata are also abundant in runnels. Motile, deposit feeder polychaetes are abundant on the ridges and are occasionally found on
the lower reaches of the beaches, while suspension and filter feeders are found in the runnels. Lagoons consist of mainly
grouped funnel branched burrows of Oniphus eremita which is identical to ichnogenus Balanoglossites. Nemertea, which are opportunistic algal grazers, have exploited restricted niches for dwelling-feeding purposes and constructed
vertical burrow with pentamerous conical mound. The shore platform consists of cemented, calcareous tubes of filter feeder
Serpula along with symbiotic encrusters like Ostrea and barnacles. Ichnocoenoses are discussed and three-dimensional ichno-sedimentologic models are reconstructed for Beach,
Ridge, Runnel and Lagoon of the Mandvi intertidal zone. 相似文献
930.
Laura Carrillo Emilio Palacios-Hernández Mario Yescas Ana María Ramírez-Manguilar 《Estuaries and Coasts》2009,32(5):906-916
Salinity profiles and meteorological data were analyzed during February, May, and September 2006 in Chetumal Bay, a large,
shallow estuary of the Western Caribbean. Local meteorological conditions revealed three seasons: (1) a dry season (March–May);
(2) a wet season (June–October); and (3) the nortes season, with northerly wind events (October–February). During the nortes
and wet seasons, salinity ranged between 13 and 16 psu, and salinity was highest in the dry season, ranging between 18 and
22 psu over most of the area; a strong stratification and a significant contribution of salty water characterized this season.
Strong horizontal gradients were observed near Rio Hondo during the three seasons. Deep and narrow peculiar bathymetric features
called the pozas showed a strong stratification and a relatively high salinity. The northern part of Chetumal Bay and probably
the entire system are far from being homogeneous. 相似文献