共查询到20条相似文献，搜索用时 312 毫秒
1.
The Adriatic Sea general circulation model coupled to a third generation wave model SWAN and a sediment transport model was implemented in the Adriatic Sea to study the dynamics of the sediment transport and resuspension in the northern Adriatic Sea (NAS) during the Bora event in January 2001. The bottom boundary layer (BBL) was resolved by the coupled model with high vertical resolution, and the mechanism of the wave–current interaction in the BBL was also represented in the model. The study found that, during the Bora event of 13–17 January 2001, large waves with significant wave height 2 m and period of 5 s were generated by strong winds in the northwestern shelf of the Adriatic where the direction of wave propagation was orthogonal to the current. The combined motion of the wave and current in the BBL increased the bottom stress over the western Adriatic shelf, resulting in stronger sediment resuspension there. Combining stronger bottom resuspension and strong upward vertical flux of resuspended sediments due to turbulent mixing, the model predicted that sediment concentration near the Po River was much higher than that predicted by the model run without wave forcing. The study also shows that wave–current interaction in the BBL reduced the western Adriatic Coastal Currents (WACCs) in the shallower north. It is concluded that wave forcing significantly changed the sediment distributions and increased the total horizontal fluxes over the western shelf. These results signified wave effect on sediment flux and distribution in the NAS, and suggested that waves cannot be neglected in the study of dynamics of sediment transport and resuspension in the shallow coastal seas. By including the tidal forcing in the coupled model, we also examined the effect of tides on the sediment transport dynamics in the NAS. 相似文献
2.
Sediment transport processes from the topset to the foreset of a crenulated clinoform (Adriatic Sea)
《Continental Shelf Research》2007,27(3-4):452-474
Crenulated clinoforms of complex and uncertain origin characterize large portions of the Late-Holocene prograding mud wedge in the western Adriatic continental shelf. Sediment failure was originally postulated as the most plausible mechanism for the formation of the crenulations. Subsequent work has shown that, although the origin of the crenulations may have been related to deformation processes, their maintenance through time seems to be better explained by different sediment accumulation rates in the flat and steep flanks. In order to establish relationships between active sediment dynamics, across-shelf transport and sediment accumulation in these crenulated clinoforms, two tripods and a mooring were deployed off the Pescara River during autumn and winter 2002–2003 as part of the EuroSTRATAFORM program, and in combination with the Po and Apennine Sediment Transport and Accumulation (PASTA) study. The tripods were placed on the shallow topset region and close to the clinoform roll-over point (i.e., offlap break), at 12 and 20-m water depth, respectively, and the mooring was located at 50-m depth, in the crenulated foreset region. Several sediment-resuspension events were recorded, mainly related to Bora and Sirocco storms, during which wave–orbital and current velocities increased considerably. Sediment transport in the topset region was predominantly towards the SE, following the direction of the coastal current and the bathymetry, but showing a significant offshore component at the roll-over point that was intensified during storm events. Currents at the foreset region were also directed to the SE. In mid-waters they were clearly aligned with the local bathymetry, whereas near the bottom they had an important and persistent offshore component. This current behavior seems to be associated with an intense bottom Ekman transport that causes the near-bottom current to be deflected to the left (i.e., offshore) with respect to the direction of the surface current. This mechanism enhances the suspended-sediment transport from the topset down the foreset region along the Adriatic prograding mud wedge, contributing to the basinward clinoform progradation and controlling the depth of the clinoform roll-over point. In addition, activity of near-inertial internal waves was also recorded by the near-bottom instrument deployed in the foreset region. During periods characterized by a strong near-inertial signal, increases of the water turbidity clearly coincided with an intensified offshore velocity component, which suggest that this mechanism also contributes to the transport of suspended sediment across the clinoform. Both the bottom Ekman transport and the internal waves are mechanisms that could be responsible for the formation/maintenance of the Adriatic seafloor crenulations until present-day, although several arguments suggest that the latter likely plays the major role. 相似文献
3.
《Continental Shelf Research》2007,27(3-4):375-399
A mooring and tripod array was deployed from the fall of 2002 through the spring of 2003 on the Po prodelta to measure sediment transport processes associated with sediment delivered from the Po River. Observations on the prodelta revealed wave-supported gravity flows of high concentration mud suspensions that are dynamically and kinematically similar to those observed on the Eel shelf [Traykovski, P., Geyer, W.R., Irish, J.D., Lynch, J.F., 2000. The role of wave-induced density-driven fluid mud flows for cross-shelf transport on the Eel River continental shelf. Continental Shelf Research 20, 2113–2140]. Due to the dynamic similarity between the two sites, a simple one-dimensional (1D) across-shelf model with the appropriate bottom boundary condition was used to examine fluxes associated with this transport mechanism at both locations. To calculate the sediment concentrations associated with the wave-dominated and wave-current resuspension, a bottom boundary condition using a reference concentration was combined with an “active layer” formulation to limit the amount of sediment in suspension. Whereas the wave-supported gravity flow mechanism dominated the transport on the Eel shelf, on the Po prodelta flux due to this mechanism is equal in magnitude to transport due to wave resuspension and wind-forced mean currents in the cross-shore direction. Southward transport due to wave resuspension and wind forced mean currents move an order of magnitude more sediment along-shore than the down-slope flux associated wave-supported gravity flows. 相似文献
4.
《Continental Shelf Research》2007,27(3-4):526-541
Sea level changes can reorganize sediment pathways on continental shelves in ways that can alter sediment supply and the resulting sedimentary deposits. The Adriatic Sea is one place where changing sediment pathways and along-strike currents have a major impact on sequence architecture. The Adriatic Sea, the marine portion of the Apenninic and Dinaric–Hellenic foreland basins, is being filled longitudinally, similar to other active forelands, and sediment transport patterns dramatically reorganized during Quaternary sea level cycles. We investigate the dynamics of sequence formation in the central Adriatic near the Gargano Promontory and the Mid-Adriatic Deep (MAD), where four depositional sequences each recording 100-kyr glacio-eustatic cycles have been mapped. These sequences are composed primarily of progradational units separated by regional unconformities. The geometry of the units is such that the constitutive clinoforms flatten out at their seaward termination into relatively planar strata, particularly in the upper parts of each sequence. Attempts to numerically simulate the sequences using the modeling software Sequence4 were frustrated by the difficulty of flattening the clinoforms seaward of the rollover (or depositional shelf break). The clinoform flattening observed in the Adriatic sequences contrasts with clinoform and depositional shelf break development that is characteristic of both conceptual and numerical models of sequences, including the one used here. We, therefore, modified the numerical model to account for the changes in sediment pathways that occur in the Adriatic Sea. During times with high sea level, such as the present, sediment from the Po and smaller Apennine Rivers is transported southwards along the coast by marine coastal currents and storms. At times of low sea level, fluvial transport of an enlarged Po River, in which the Apennine Rivers are likely captured as tributaries, discharges directly into the MAD basin. This produces a reciprocal sediment supply pattern where the coastal dispersal at high sea level produces prograding clinoforms, but cuts off as the exposure of the northern shelf at low sea level switches supply to the fluvial system, which discharges into the 260-m deep MAD. When the model is adapted for the shift in supply, the clinoforms flatten as nearshore sediment supply decreases and is replaced by deposition in the MAD during sea level lowstands. Including these sediment supply changes as a function of sea level enabled us to obtain a good fit to the overall stratigraphic architecture, supporting conceptual depositional models based on seismic stratigraphy. Similar dramatic shifts in sediment supply and sequence architecture may also occur at other settings, such as where canyons capture fluvial systems and bypass the terrigenous sediment supply to the deep basin. The paucity of observations of the unusual geometry seen at the Adriatic margin suggests that only rarely does the shore and fluvial sediment discharge overreach the shelf edge and cut off along-strike sediment transport to continental margins. 相似文献
5.
《Continental Shelf Research》2007,27(3-4):296-308
This paper focuses on the delivery of water and sediment to the northern Adriatic to better understand the short-term evolution of continental margin sedimentation under natural and human impact. For that reason, the Po and six Apennine rivers (Metauro, Musone, Potenza, Tronto, Chienti and Pescara) are investigated. The climate-driven hydrological model HydroTrend is used to simulate discharge and sediment loads where observational data are limited. The northern Apennine hinterland has a significant impact on the sediment flux leaving the Po River, contributing 56% of the sediment it delivers to the Adriatic Sea. The Po River experienced a strong decrease in its sediment load (17.2–6.4 Mt/yr) across 1933–1987, in contrast to a small increase in its water discharge. The rivers draining the southern Apennine hinterland contribute more than 50% of the sediment load entering the Adriatic Sea, and this is in spite of human modification of their discharge through numerous small reservoirs that invariably reduce a river's sediment load. As a result, hyperpycnal flows, which historically carried 20–40% of the sediment flux from these Apennine rivers, become rare. Sediment load reduction is also reflected by retreat of the Apennine coastline. Based on the ART model (used in HydroTrend), the total sediment load to the northern Adriatic is 43 MT/yr where the northern Alpine rivers contribute 8 MT/yr, the Po River 13 MT/yr and the Apennine rivers contribute 22 MT/yr. 相似文献
6.
The residual flow in the inlets of Venice lagoon subject to Bora and Sirocco winds has been studied. Current velocities have been monitored since 2001 using Acoustic Doppler Current Profilers (ADCP) installed on the beds of the inlets that connect the lagoon to the Adriatic Sea; these inlets are Lido, Malamocco and Chioggia. Wind velocity data have also been continuously measured at an oceanographic platform 14.8 km offshore from the lagoon; these data were subsequently decomposed into Principal Components, which are associated with Bora and Sirocco wind directions. Analyses show that the inflow in Lido inlet is strongly related to the Bora wind. The outflow in Chioggia inlet occurs during Bora events but shows a slightly weaker correlation with the wind speed, while Malamocco inlet shows little or no influence of Bora winds on flow patterns. A net residual inflow through Lido and Malamocco inlet was found, while outflow prevails in Chioggia inlet. During Bora events, the average residual inflow increased three-fold in Lido inlet, whereas the outflow in Chioggia inlet doubled. The current velocities in Lido and Chioggia inlets are best described by an exponential function of wind velocity with exponents of −0.1187 and −0.0924, respectively. The response to Sirocco events was evident mainly in Chioggia inlet. Specifically, there was a slow down of the outflow in linear proportion to wind speed. In excess of 10 m/s a complete current reversal was observed. Lido and Malamocco inlets showed little or no response to Sirocco winds, except during rare cases when wind speeds exceeded 15 m/s. 相似文献
7.
Dispersal of Mississippi and Atchafalaya sediment on the Texas-Louisiana shelf: Model estimates for the year 1993 总被引：1，自引：0，他引：1
Kehui Xu Courtney K. HarrisRobert D. Hetland James M. Kaihatu 《Continental Shelf Research》2011,31(15):1558-1575
A three-dimensional coupled hydrodynamic-sediment transport model for the Texas-Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow-water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed.At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20-40 km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner-most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters (<10 m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the “Dead Zone” (low-oxygen area) within a seasonal-annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment-transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and “stepwise-like” compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave-current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf. 相似文献
8.
Peter T. Harris Michael G. Hughes Elaine K. Baker Robert W. Dalrymple Jock B. Keene 《Continental Shelf Research》2004,24(19):2431-2454
Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea. Net bedload transport vectors and the occurrence of elongate tidal bars indicate that mutually evasive ebb- and flood-dominant transport zones occur in each of the distributary channels. Suspended sediment experiments at two locations show a phase relationship between tidal velocity and sediment concentration such that the net suspended sediment flux is directed seaward. Processes that control the export of fluid muds with concentrations up to 10 g l−1 from the distributary channels across the delta front and onto the pro-delta are assessed in relation to the available data. Peak spring tidal current speeds (measured at 100 cm above the bed) drop off from around 100 cm s−1 within the distributary channels to <50 cm s−1 on the delta front. Gravity-driven, 2-m thick, fluid mud layers generated in the distributary channels are estimated to require at least 35 h to traverse the 20-km-wide, low-gradient (2×10−3 degrees) delta front. The velocities of such currents are well below those required for autosuspension. A 1-month time series of suspended sediment concentration and current velocity from the delta front indicates that tidal currents alone are unable to cause significant cross-delta mud transport. Wave-induced resuspension together with tides, storm surge and barotropic return-flow may play a role in maintaining the transport of fine sediment across the delta front, but insufficient data are available at present to make any reliable estimates. 相似文献
9.
《Continental Shelf Research》2006,26(17-18):2050-2072
A 5-yr data set of near-bed current and suspended-sediment concentration measured within 2 m of the seabed in 60-m water depth has been analyzed to evaluate the interannual variability of physical processes and sediment transport events on the Eel River continental shelf, northern California. This data set encompasses a wide range of shelf conditions with winter events characterized as: Major Flood (1996/97), strong El Niño (1997/98), strong La Niña (1998/99), and Major Storm (1999/00). Data were collected at a site located 25 km north of the Eel River mouth, on the landward edge of the mid-shelf mud deposit. During the winter months sediment resuspension is forced primarily by near-bed oscillatory flows, and sediment transport occurs both as suspended load and as gravity-driven (fluid-mud) flows. Winter conditions that caused periods of increased sediment transport existed on average for 142 d yr−1 over the total record, ranging between 89 d in the Major Flood year (1996/97) and 171 d in the La Niña year (1998/99). Hourly averaged values of significant wave height varied between 0.5 and 10.7 m and hourly averaged values of near-bed orbital velocities ranged between 0 and 125 cm s−1. During the five winters, sediment threshold conditions were exceeded an average of 35% of the time, ranging from 19% in the Major Flood year (1996/97) to 52% in the La Niña year (1998/99). Mean concentration of suspended sediment, measured at 30 cmab, ranged from values close to 0–8 g l−1. Among winters, major sediment flux events exhibited different patterns due to varying combinations of physical processes including river floods, waves, and shelf circulation. Within winters, the major period of sediment flux varied from a 3-d fluid mud event (Major Flood winter) to a 50-d period of persistent southerlies (El Niño winter) and a winter of continuous storm cycles (La Niña winter). Winter-averaged suspended-sediment concentration appeared to vary in response to river discharge, while total sediment flux responded to storm intensity. The net sediment flux appeared to depend on timing of river discharge and shelf conditions. On the Eel River shelf, the mid-shelf mud deposit apparently is not emplaced by deposition from the river plume, but by secondary processes from the inner shelf including off-shelf transport of sediment suspensions and gravity-driven fluid-mud flows. Thus, these inner-shelf processes redistribute sediment supplied by the Eel River (a point source) making the inner shelf a line source of sediment that forms and nourishes the mid-shelf deposit. Large-scale shelf circulation patterns and interannual variability of the physical forcing are also important in determining the locus of the mid-shelf deposit, and both are influenced by climate variations. Post-depositional alteration of the deposit also depends on the subsequent shelf conditions following major floods. 相似文献
10.
《Continental Shelf Research》2007,27(3-4):400-416
During February and June of 2003, selected physical and biological sediment properties were measured at nine mud-bottom sites (11–22 m) along the western margin of the Adriatic Sea. Seabed properties were compared with shipboard measurements of sediment erodibility made at the same sites to gain insight into the physical and biological controls on sediment erodibility operating in the western Adriatic. In addition, spatial patterns of erodibility were compared with long-term records of sediment accumulation in the region to determine if patterns of erodibility were responsible for the discrepancy between sediment sources and sinks on the shelf. Results indicate that sediment erodibility along the western Adriatic Sea varied both in time and space. In the wintertime, sediment in the vicinity of the Po delta (in the northern region) was less easily eroded than to the south. In the summertime, the pattern reversed. The physical characteristics of the seabed including porosity and grain-size were important factors controlling erodibility during the winter, although in a manner opposite to expectations (i.e., higher porosity sediment was less erodible). No relationship between the physical characteristics of the seabed and erodibility was observed in the summer, when it was likely that erodibility was influenced by benthic organisms, especially microphytobenthos. Results of this study suggest long-term sediment accumulation patterns along the western Adriatic Sea are unlikely to be controlled by patterns of erodibility. However, due to several complicating factors, especially along-margin variability in wave energy, the impact of sediment erodibility on long-term patterns of accumulation remains unclear. Further studies that resolve both finer and larger-scale spatial and temporal variability in sediment erodibility and that incorporate year-to-year forcing of the coastal ocean are needed to more accurately resolve the relationship between erodibility and sediment accumulation in the western Adriatic, as well as other coastal environments. 相似文献
11.
《Continental Shelf Research》2007,27(3-4):475-488
Across a limited depth range (5–10 m) on many continental shelves, the dominant sediment size changes from sand to mud. This important boundary, called the sand–mud transition (SMT), separates distinct benthic habitats, causes a significant change in acoustic backscatter, represents a key facies change, and delimits more surface-reactive mud from less surface-reactive sand. With the goal of improving dynamical understanding of the SMT, surficial sediments were characterized across two SMTs on the Adriatic continental shelf of Italy. Geometric mean diameter, specific surface area (SSA), mud fraction (<63 μm) and heavy metal concentrations were all measured. The SMT related to the Tronto River is identified between 15 and 20 m water depth while the SMT associated with the Pescara River varies between 15 and 25 m water depth. The sediment properties correlate with a new, process-based sedimentological parameter that quantifies the fraction of the sediment in the seabed that was delivered as flocs. These correlations suggest that floc dynamics exert strong influence over sediment textural properties and metal concentrations. Relative constancy in the depth of the SMT along this portion of the margin and its lack of evolution over a period during which sediment input to the margin has dramatically decreased suggest that on the Adriatic continental shelf energy is the dominant control on the depth of the SMT. 相似文献
12.
N. P. Smith 《Ocean Dynamics》2004,54(3-4):435-440
Current meter and temperature data were collected over a 402-day period from an outer shelf and a tidal channel study site in the Exuma Cays, Bahamas. The shelf width is less than 2 km, and floods and ebbs through a nearby tidal channel extend across the entire shelf and reduce coherence of wind forcing and along-shelf flow. The data are used in perturbation analyses to investigate the across-shelf turbulent transport of heat and momentum over seasonal time scales. Data show a net landward transport of both heat and momentum over the course of the study, but the perturbation products contain distinct seasonal cycles. In fall and winter months, across-shelf heat and momentum fluxes are landward, while during spring and summer months fluxes are seaward. Comparison of shelf-water temperature with the temperature of bank water leaving on the ebb suggests that seasonal cycles of across-shelf heat and momentum in shelf waters are influenced by the seasonal export of relatively warm and cool water from Great Bahama Bank.Responsible Editor: Iris Grabemann 相似文献
13.
The source and transport mechanisms of land-derived Okinawa Trough sediments were studied using the field data of temperature, salinity and turbidity in the East China Seas. The results suggest that there are two primary sediments sources from the Chinese Mainland to the Okinawa Trough: one is the Old Huanghe River submarine delta, and the other is the Changjiang River sediments, which are distributed at the Changjiang River estuary and the off-coast of Zhejiang and Fujian provinces. It is difficult for the Huanghe River suspended sediments to arrive in the Okinawa Trough via the new estuary. Although the Taiwan warm current blocks the seaward terrigenous transportation to a certain extent, part of the coastal suspended sediments are transported to the outer shelf. Suspended particulate matter is unable to get through the barrier of the Kuroshio Current under normal conditions. However, episodic events, such as winter storms, internal-tidal waves and turbidity flows, are capable of transporting suspended particulate matter into the Okinawa Trough. The super typhoon “Ewiniar” induced strong waves and influenced the thermocline depth and suspended sediment concentration of the East China Seas. The typhoon-induced waves pushed the thermocline depth down to around 40 m and caused the resuspension of large volumes of sediments in its path. In the other East China Seas regions, the typhoon-induced swells deepened the thermocline depth by about 5 m and increased suspended sediment concentrations. The typhoon effect on suspended sediment concentration of the East China Seas disappeared within 2 weeks. 相似文献
14.
《Continental Shelf Research》2007,27(3-4):338-358
Thirty-three surface sediment samples from cross-shelf transects on the northern Adriatic shelf were collected in December 2000, soon after a 100-yr flood of the Po River, in order to determine the distribution of organic carbon (OC) along the main sediment dispersal system. To evaluate the temporal variability, stations were re-occupied eight times at seasonal intervals until June 2003. Downcore sediment profiles from two sites characterized by high flood deposit thicknesses were also examined to assess the OC variability within the flood layer. In December 2000, the highest contents of OC (up to 1.24 wt%) were measured in front of the main distributary mouths (Pila, Tolle and Gnocca-Goro) where the greatest thicknesses of the flood deposit were recorded. However, the influence of the Po di Gnocca-Goro sediment supply on the OC surface distribution declined after ∼1.5 years from the fall-2000 river flood, probably because these mouths are less active when the water discharge is lower. The δ13C of organic matter was used to trace the dispersal of fluvial OC on the continental shelf. The δ13C values ranged from −25.9‰ to −23.1‰. The fraction of fluvially derived organic particles decreased with increasing water depth according to a radial dispersal pattern around the Po River delta. This pattern persisted in all cruises. δ13C values increased progressively until April 2002, suggesting an increasing marine contribution to the OC content but decreased again following a second minor flood event in November 2002. The molar C/N ratio was on average 10.0±1.6, with slightly lower values in southern and central areas.Assuming contributions from three OC end-members (terrestrial, riverine and marine), a mixing model based on δ13C and the ratio of N to C (statistically more robust than C/N; Goñi, M.A., Teixeir, M.J., Perkley, D.W., 2003. Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA). Estuarine, Coastal and Shelf Science 57, 1023–1048) was applied in order to quantitatively assess the OC sources for Po shelf sediments. δ13C is significantly and positively correlated with the marine OC fraction. The terrestrial fraction is inversely correlated with N/C, while the riverine fraction is positively correlated with N/C. The terrestrial OC source was the most abundant end-member (>70%) showing only little temporal variability regardless of the Po River water discharge. Temporal and spatial changes in OC composition suggest that: (a) the Po River prodelta is always dominated from terrestrial OC input; (b) the Po della Pila supplies most terrestrial OC, whilst other tributaries (e.g., Po di Gnocca-Goro) are secondary sources. However, these mouths are as important as the Po della Pila in affecting the riverine OC signature; (c) offshore, biological primary production raises the marine OC contribution.At two sites on the Po River prodelta, the 2000-flood deposit shows slight but consistent compositional changes of organic matter (N/C and δ13C) which can help to recognize other flood events in the sedimentary record. The OC budget for the 2000-flood deposit accounts for a terrestrial+riverine OC supply of 68–162 Gg (109 g) against an OC deposition of 106–162 Gg (excluding the marine fraction), which implies a rapid and efficient sedimentation of the flood material, and scarce or negligible export out the study area. Flood events may thus enhance terrestrial carbon burial, whereas marine carbon arrives more slowly and may be largely mineralized at the sediment–water interface. 相似文献
15.
《Continental Shelf Research》2007,27(3-4):359-374
In this paper, we discuss what controls the occurrences of shallow gas off the Western Adriatic coast. This region of the Western Adriatic, including the Po delta and the late-Holocene mud wedge, was sampled with the R/V Seward Johnson II within the framework of the EuroSTRATAFORM project. As a part of the program, we were evaluating the causes of anomalous sub-surface and seafloor features, and specifically, quantifying shallow gas and identifying its impact on geophysical properties. The highest gas concentration off the Po delta region yielded 4.1×104 ppm, where as offshore Tronto River, the highest methane value was 7.9×104 ppm. Headspace gas analyses match well with acoustic anomalies along the Western Adriatic coast. Shallow biogenic gas off the Po delta (NW Adriatic) is spatially correlated with organic matter deposition driven by rapid and thick flood deposition. In contrast to the Po Delta, where sub-surface gas is associated with the flood deposition of organic matter, shallow gas along the Central Adriatic occurs deeper in cores, and appears to be related to methanogenesis of organic matter present in deeper deposits. 相似文献
16.
Data analysis of continental shelf currents and coastal sea level, together with the application of a semi-analytical model, are used to estimate the importance of remote wind forcing on the subinertial variability of the current in the central and northern areas of the South Brazil Bight. Results from both the data analysis and from the semi-analytical model are robust in showing subinertial variability that propagates along-shelf leaving the coast to the left in accordance with theoretical studies of Continental Shelf Waves (CSW). Both the subinertial variability observed in along-shelf currents and sea level oscillations present different propagation speeds for the narrow northern part of the SBB (~?6–7 m/s) and the wide central SBB region (~?11 m/s), those estimates being in agreement with the modeled CSW propagation speed. On the inner and middle shelf, observed along-shelf subinertial currents show higher correlation coefficients with the winds located southward and earlier in time than with the local wind at the current meter mooring position and at the time of measurement. The inclusion of the remote (located southwestward) wind forcing improves the prediction of the subinertial currents when compared to the currents forced only by the local wind, since the along-shelf-modeled currents present correlation coefficients with observed along-shelf currents up to 20% higher on the inner and middle shelf when the remote wind is included. For most of the outer shelf, on the other hand, this is not observed since usually, the correlation between the currents and the synoptic winds is not statistically significant. 相似文献
17.
E. Tragou V. Zervakis E. Papageorgiou S. Stavrakakis V.Lykousis 《Continental Shelf Research》2005,25(19-20):2315
This work constitutes an attempt to assess the relative importance of natural versus anthropogenic forcing for sediment resuspension on the shelf of the Thermaikos Gulf (NW Aegean) through a 2-year experiment in the framework of the E.U. project INTERPOL. Four periods of different hydrographic characteristics were identified, including two periods of stronger vertical homogenisation and two of stratified conditions. The former periods were characterized by stronger near-bed currents, while relatively weak internal wave motions dominated the periods of stratification. The near-bed currents showed strong coherence with the wind during the period of full homogenisation, whereas during stratification periods the wind provided indirect forcing evident mostly in the internal-wave bands. The site was too deep for the surface waves to cause any sediment resuspension. In conclusion, the observed near-bed currents at the site of interest did not appear to produce significant local resuspension of sediment; notably, the current-turbidity correlation suggested a shoreward transport of suspended material. The large-scale turbidity variability was rather related to the onset of the trawling period in mid-October 2001 and the increased riverine discharge in late summer/autumn 2002. 相似文献
18.
Sediment transport and the potential for erosion or deposition have been investigated on the Palos Verdes (PV) and San Pedro shelves in southern California to help assess the fate of an effluent-affected deposit contaminated with DDT and PCBs. Bottom boundary layer measurements at two 60-m sites in spring 2004 were used to set model parameters and evaluate a one-dimensional (vertical) model of local, steady-state resuspension, and suspended-sediment transport. The model demonstrated skill (Brier scores up to 0.75) reproducing the magnitudes of bottom shear stress, current speeds, and suspended-sediment concentrations measured during an April transport event, but the model tended to underpredict observed rotation in the bottom-boundary layer, possibly because the model did not account for the effects of temperature–salinity stratification. The model was run with wave input estimated from a nearby buoy and current input from four to six years of measurements at thirteen sites on the 35- and 65-m isobaths on the PV and San Pedro shelves. Sediment characteristics and erodibility were based on gentle wet-sieve analysis and erosion-chamber measurements. Modeled flow and sediment transport were mostly alongshelf toward the northwest on the PV shelf with a significant offshore component. The 95th percentile of bottom shear stresses ranged from 0.09 to 0.16 Pa at the 65-m sites, and the lowest values were in the middle of the PV shelf, near the Whites Point sewage outfalls where the effluent-affected layer is thickest. Long-term mean transport rates varied from 0.9 to 4.8 metric tons m−1 yr−1 along the 65-m isobaths on the PV shelf, and were much higher at the 35-m sites. Gradients in modeled alongshore transport rates suggest that, in the absence of a supply of sediment from the outfalls or PV coast, erosion at rates of ∼0.2 mm yr−1 might occur in the region southeast of the outfalls. These rates are small compared to some estimates of background natural sedimentation rates (∼5 mm yr−1), but do not preclude higher localized rates near abrupt transitions in sediment characteristics. However, low particle settling velocities and strong currents result in transport length-scales that are long relative to the narrow width of the PV shelf, which combined with the significant offshore component in transport, means that transport of resuspended sediment towards deep water is as likely as transport along the axis of the effluent-affected deposit. 相似文献
19.
The peculiarities of the hydrological regime of the delta and near-shore zone of the Po River are discussed. The intrusion of salt seawater into the delta is described. The history of the Po Delta formation has been restored on the basis of the analysis of historical, archeological, and cartographic data. As shown, the peculiarities of hydrological and morphological processes in the Po River mouth are associated with natural and specifically with human-induced variations in sediment runoff of the river, with levee construction along branches, subsidence of deltaic deposits, and eustatic rise of the sea level. 相似文献
20.
Marcello G. Magaldi Tamay M. Özgökmen Annalisa Griffa Michel Rixen 《Ocean Dynamics》2010,60(1):93-122
This numerical study focuses on the response of the Western Adriatic Current to wind forcing. The turbulent buoyant surface
current is induced by the Po river outflow in the Adriatic Sea. Idealized and realistic wind conditions are considered by
retaining the complex geomorphology of the middle Adriatic basin. In the absence of wind, the Adriatic Promontories force
the current to separate from the coast and induce instabilities. Persistent 7-m s − 1 downwelling favorable northwesterly winds thicken and narrow the current. Instabilities whose size is ~10 km develop but
ultimately vanish, since there is not enough across-shore space to grow. On the contrary, 7-m s − 1 upwelling favorable southeasterly winds thin and widen the current, and instabilities can grow to form mesoscale (~35 km)
features. When realistic winds are considered, the same trends are observed, but the state of the sea set up by previous wind
events also plays a crucial role. The turbulent regimes set up by different winds affect mixing and the WAC meridional transport.
With downwelling winds, the transport is generally southward and mixing happens mostly between the fresher (S ≤ 38) salinity classes. With upwelling winds, the transport decreases and changes sign, and mixing mainly involves saltier
(S > 38) waters. In all cases, mixing is enhanced when a finer 0.5-km horizontal resolution is employed. 相似文献