Preliminary studies of offshore placer deposits, eastern Australia

Offshore exploration during the 1960's for gold off southern New South Wales and for tin in Tasmanian waters did not result in the discovery of economic deposits. Although very rich gold-bearing beach placers were worked in the past, individual deposits were small and rested on bed rock; the chances of locating and exploiting similar deposits offshore appear to be remote. In the case of tin, sub-economic resources were outlined in submerged river channels at a number of places off northeastern Tasmania. Such channels can be outlined by seismic methods, but to locate workable tin deposits in the buried alluvium by drilling alone is likely to be impracticable and successful exploration may depend on the development of other geophysical prospecting techniques.

Large resources of rutile- and zircon-bearing heavy-mineral sands have been indicated off the east Australian coast by mining company work, but no economic deposits have been found to date. Studies of the morphology of the eastern shelf by the Bureau of Mineral Resources have revealed linear features believed to be related to shore lines developed during Quaternary low sea-level still stands. The most persistent of these off northern N.S.W. are about 105 m, 85 m, and between 35 and 45 m below present sea level. A widespread change of slope at a depth of 20–30 m marks the base of the main body of the present-day paralic-zone wedge of sediment, but seismic profiles indicate that a veneer of recent sediment commonly extends seawards into water depths of about 100 m. Much of the outer shelf is floored by relict sediments and extensive areas of bed rock crop out on the middle shelf.

Virtually all sub-surface data from company drilling for heavy-mineral sands relates to the present-day paralic-zone wedge of sediments; this wedge includes undisturbed sedimentary sequences deposited during pre-Holocene high sea-level periods. No large economic-grade deposits have been outlined by this work offshore, and there is reason to believe that the bulk of the heavy-mineral deposits formed during Holocene and previous high sea-level stands are located above present sea level. In addition, the best-developed submerged strand lines are in deep water probably inaccessible to mining. Nevertheless, the possibility that substantial deposits occur offshore in moderate water depths exists.

Outcrops of bed rock are extensive in the mid-shelf zone in the southern part of the area, but north of 29° S they are much less common. Significant areas with sediment thicknesses greater than 20 m in water depths of less than 60 m occur to the east of Newcastle, to the southeast of Smoky Cape, and to the north of Yamba. Two sediment sequences, an upper and a lower, are recognizable. Highest heavy-mineral values in surface sediments occur offshore from the Permo-Triassic basins. Subsurface enrichment may occur at the junction of the upper and lower sequences, or where the upper sequence overlies basement. The abundance of heavy minerals is a function of the total sediment throughput, and the intensity and direction of shore-line sorting, so that the highest potential for accumulation occurs in the northern part of the area.

The most likely prospective areas occur mainly near Cape Byron and near Sugarloaf Point. These areas have been defined on the basis of the thickness of sediments, the depth to the base of the upper sequence, the distribution of ancient strand lines, and the abundance of heavy minerals in the surface sediments.     

Coastal Bathymetry from Hyperspectral Remote Sensing Data: Comparisons with High Resolution Multibeam Bathymetry

We present a large-scale quantitative test of a hyperspectral remote-sensing reflectance algorithm. We show that coastal bathymetry can be adequately derived through model inversions using data from the Airborne Visible-Infrared Imaging Spectrometer instrument. Data are analyzed from a shore-perpendicular transect 5 km offshore Sarasota, Florida at water depths ranging from 10 m to 15.5 m. Derived bottom depths are compared to a high-resolution multibeam bathymetry survey. Model-derived depths are biased 4.9% shallower than the mean of the multibeam depths with an RMS error of 7.83%. These results suggest that the model performs well for retrieving bottom depths from hyperspectral data in subtropical coastal areas in water depths ranging from 10 m to 15.5 m.     

Sensitivity of the Atlantic Ocean circulation to a hydraulic overflow parameterisation in a coarse resolution model: Response of the subpolar gyre

We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland–Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland–Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing.Special attention is paid to changes in the subpolar gyre circulation. We find it sensitive to the overflow transport and the density of these water masses through baroclinic adjustments. The analysis of the governing equations confirms the presence of positive feedbacks inherent to the subpolar gyre and allows us to isolate the influence of the overflows on its dynamics.     

Dynamics of the Mackenzie River plume on the inner Beaufort shelf during an open water period in summer

The oceanographic conditions of the Mackenzie River plume in the Arctic Ocean were examined during a 12-day period in August 2007. Field observations in the river channel and the delta region (2–6 m depth), ship-based observations on the shelf and satellite observations of sea surface temperatures indicate that movements of plume density fronts cause changes in water temperatures of over 10  C over a few days. We used a 1D model to compare the strength of stratification versus surface wind stress, and a 3D numerical model to simulate the plume motions under forcing from the river flows, local wind and water level variations from tides and wind-driven surge. The results indicate that the coastal region is stratified with a ∼2 m thick surface plume even in water depths of 3–4 m, resulting in strong vertical variation of horizontal currents. Moderate easterly winds of 5–10 m/s are sufficient to induce offshore transport of the surface plume and onshore transport of the deeper shelf water, leading to large fluctuations in temperature and salinity in the coastal region. This study examined a period of offshore transport and mean water level set-down, and indicates the rapid response of the plume to wind over the shallow delta.     

Sediment facies and pathways of sand transport about a large deep water headland,Cape Rodney,New Zealand

Cape Rodney is a large headland that protrudes 3–4 km into deep water in the Hauraki Gulf and separates the Mangawhai‐Pakiri and Omaha littoral cells. Detailed swath mapping of seabed sediments around Cape Rodney was carried out using by side‐scan sonar and ground‐truthed by SCUBA, grab sampling, and video. Despite the barrier imposed by the headland two pathways of sand transport around the headland, separated by the topographic high of Leigh Reef, have been identified. One lies close to the headland, where sand from the beach and nearshore of the Mangawhai‐Pakiri embayment is driven by waves and currents along a 500‐m‐wide pathway in c. 20–25 m depth around the headland to the vicinity of Leigh Harbour. The other lies in 50 m water‐depth seawards of Leigh Reef. Here fine sand, sourced from the nearshore of the Mangawhai‐Pakiri embayment and driven offshore from the tip of the headland, is transported back and forth by tidal currents in 50 m water depth on the floor of the Jellicoe Channel. The sand bodies along both these pathways are thin and so sand leakage from the Mangawhai‐Pakiri embayment is thought to be small. Transport at these depths is dependent on both tide and wave generated currents and episodic occurring during storm events. The sediment facies associated with little sand transport about a headland in deep water is one of thin and discontinuous and patchy sand cover between rocky areas and over coarser megarippled substrate. Ocean swell, tidally driven phase eddies that spin up on both sides of the headland, and bathymetry all play a role in shaping those facies.     

圆形重力式网箱阻力性能研究

采用网箱模型试验 ,对周长 5 0 m,深 8m的 HDPE双管圆形重力式网箱 ,在不同配重、网片规格和流速条件下的阻力性能进行研究。结果表明 ,在 2 .5 kn的相对流速范围内 ,网箱的整体阻力在 75 0 0 kg以内 ;在网箱整体阻力中 ,框架及重锤阻力所占比重较小 ,而网箱箱体阻力占近90 %。不同重锤配重的网箱 ,其整体阻力与流速均呈幂函数关系 F=a Vb,网箱重锤配重增加 ,网箱阻力也随之增大 ,而重锤配重变化对网箱阻力的影响程度又随流速的增加而减小     

Simulation of Pollutant Transport in Marmaris Bay

The circulation pattern and the pollutant transport in the Marmaris Bay are simulated by the developed three-dimensional baroclinic model. The Marmaris Bay is located at the Mediterranean Sea coast of Turkey. Since the sp ring tidal range is typically 20- 30 cm, the dominant forcing for the circulation and water exchange is due to the wind action. In the Marmaris Bay, there is sea outfall discharging directly into the bay. and that threats the bay water quality significantly. The current patterns in the vicinity of the outfall have been observed by tracking drogues which are moved by currents at different water depths. In the simulations of pollutant transport, the coliforms-counts is used as the tracer. The model provides realistic predictions for the circulation and pollutant transport in the Marmaris Bay. The transport model component predictions well agree with the results of a laboratory model study.     

Contribution of hydrodynamic conditions during shallow water stages to the sediment balance on a tidal flat: Mont-Saint-Michel Bay, Normandy, France

Field measurements were conducted in Mont-Saint-Michel Bay, a megatidal embayment (spring tidal range of 15 m), in order to monitor, over the course of a tidal cycle, sediment transport variability due to waves and tides on the upper part of a tidal flat characterised by shallow water depths. Sensors used to measure currents, water depth and turbidity were installed just above the bed (0.04 m). Two experiments were conducted under contrasting hydrodynamic conditions. The results highlight wave activity over the tidal flat even though observed wind waves were largely dissipated due to the very shallow water depths. Very high suspended sediment concentrations (up to 6 kg/m³) were recorded in the presence of wave activity at the beginning of the local flood, when significant sediment transport occurred, up to 7 times as much as under conditions of no wave activity. This influence may be attributed to the direct action of waves on bed sediments, to wave-induced liquefaction, and to the erosive action of waves on tidal channel banks. The sediment composition, comprising a clay fraction of 2-5%, may also enhance sediment transport by reducing critical shear stress through the sand lubrication effect. The results also show that antecedent meteorological conditions play an important role in suspended sediment transport on the tidal flat. Total sediment flux directions show a net transport towards the inner part of the bay that contributes to deposition over the adjacent salt marshes, and this tendency also prevails during strong wave conditions. Such sediment transport is characterised by significant variability over the course of the tidal cycle. During fair and moderate weather conditions, 83% and 71% of the total flux was observed, respectively, over only 11% and 28% of the duration of the local tidal cycle and with water depths between 0.04 and 0.3 m. These results suggest that in order to improve our understanding of sediment budgets in this type of coastal environment, it is essential to record data just at the beginning and at the end of tidal submergence close to the bed.     

Stratigraphic Model Predictions of Geoacoustic Properties

Geoacoustic properties of the seabed have a controlling role in the propagation and reverberation of sound in shallow-water environments. Several techniques are available to quantify the important properties but are usually unable to adequately sample the region of interest. In this paper, we explore the potential for obtaining geotechnical properties from a process-based stratigraphic model. Grain-size predictions from the stratigraphic model are combined with two acoustic models to estimate sound speed with distance across the New Jersey continental shelf and with depth below the seabed. Model predictions are compared to two independent sets of data: 1) Surficial sound speeds obtained through direct measurement using in situ compressional wave probes, and 2) sound speed as a function of depth obtained through inversion of seabed reflection measurements. In water depths less than 100 m, the model predictions produce a trend of decreasing grain-size and sound speed with increasing water depth as similarly observed in the measured surficial data. In water depths between 100 and 130 m, the model predictions exhibit an increase in sound speed that was not observed in the measured surficial data. A closer comparison indicates that the grain-sizes predicted for the surficial sediments are generally too small producing sound speeds that are too slow. The predicted sound speeds also tend to be too slow for sediments 0.5-20 m below the seabed in water depths greater than 100 m. However, in water depths less than 100 m, the sound speeds between 0.5-20-m subbottom depth are generally too fast. There are several reasons for the discrepancies including the stratigraphic model was limited to two dimensions, the model was unable to simulate biologic processes responsible for the high sound-speed shell material common in the model area, and incomplete geological records necessary to accurately predict grain-size     

Physical oceanographic conditions in the deepwater Gulf of Mexico in summer 2000–2002

The circulation and distribution of water properties in the water column of the Gulf of Mexico influence the flux of carbon to the benthic environment. The eddy field of the upper 1000 m creates environmental conditions that are favorable for biological productivity in an otherwise oligotrophic subtropical ocean. This eddy field results in the transport of nutrients and organic matter into the photic zone through cross-margin flow of shelf waters, upwelling in cyclones, and uplift from the interaction of anticyclones with bathymetry. These conditions then allow the productivity that becomes a possible source of carbon to the benthos.Data from four cruises during summers of 2000–2002 are used to describe the currents and water property distributions in the deepwater Gulf of Mexico, which consists of water depths greater than 400 m. Comparisons are made to historical data sets to provide an understanding of the persistence of the characteristics of the Gulf and the processes that occur there.The currents in the Gulf are surface intensified, have minimum in 800–1000 m depths, and also exhibit bottom intensification, especially near sloping topography. Historical time series records show current speeds near-bottom reach 50–100 cm s⁻¹. At basin scales, these currents tend to flow cyclonically (counter-clockwise) along the bathymetry. These near-bottom, episodic, high-speed currents provide a mechanism for the transport of organic material, in both large and small particle sizes, from one benthic area to another.The distributions of temperature, salinity, nutrients, and dissolved oxygen during the study appear to be unchanged from historical findings. The source waters for the deep Gulf are the water masses brought into the Gulf by the Loop Current system. The properties in the upper 100–200 m are the most variable of the water column, consistent with their proximity to wind mixing, river discharge mixing, and atmospheric influences. Below 1500 m, there are no major horizontal variations in these water properties.     

The depth-varying response of coastal circulation and water levels to 2D radiation stress when applied in a coupled wave–tide–surge modelling system during an extreme storm

During storm events wave setup in shallow regions can contribute significantly to the total water elevation, and radiation stress can also generate alongshore drift influencing sediment transport. In low lying coastal regions this generates the potential for flood inundation and morphological change. A coupled tide–surge–wave modelling system is therefore required for accurate forecasting. Liverpool Bay, UK, is taken as a case study because it has a resource of observations and incorporates three estuaries, thus providing conditions to assess the model performance both at the open coast and within estuarine environments. The model covers a region encompassing depths from about 50 m below the mean tidal level to shallow wetting and drying regions, and has previously given good wave and surge hindcasts both for individual storm events and multi-year studies.The present study builds on an already accepted model, to include and assess the spatial influence of 2D radiation stress when implemented in a 3D circulation model. The results show that the method is computationally efficient, so relevant for operational use, and also provides a plausible solution. The varied influence of radiation stress across a coastal domain is demonstrated, with larger impact at an estuary mouth and along the open coast, while having lesser impact within an estuary and further offshore.     

Comparative Experimental and Numerical Study of Wave Loads on A Monopile Structure Using Different Turbulence Models

This study numerically and experimentally investigates the effects of wave loads on a monopile-type offshore wind turbine placed on a 1: 25 slope at different water depths as well as the effect of choosing different turbulence models on the efficiency of the numerical model. The numerical model adopts a two-phase flow by solving Unsteady Reynolds-Averaged Navier-Stokes(URANS) equations using the Volume Of Fluid(VOF) method and three different turbulence models. Typical environmental conditions from the East China Sea are studied. The wave run-up and the wave loads applied on the monopile are investigated and compared with relevant experimental data as well as with mathematical predictions based on relevant theories. The numerical model is well validated against the experimental data at model scale. The use of different turbulence models results in different predictions on the wave height but less differences on the wave period. The baseline turbulence model and Shear-Stress Transport(SST) turbulence model exhibit better performance on the prediction of hydrodynamic load, at a model-scale water depth of 0.42 m, while the laminar model provides better results for large water depths. The SST turbulence model performs better in predicting wave run-up for water depth 0.42 m, while the laminar model and standard model perform better at water depth 0.52 m and 0.62 m, respectively.     

Gassy sediment occurrence and properties: Northern Gulf of Mexico

Free gas is ubiquitous at shallow sediment depths of the northern margin of the Gulf of Mexico. Gassy sediment patches are between 250 and 500 m in horizontal size. Often the gassy layers are within 100 m from the sea floor and are only a few meters thick. Both biogenic and thermogenic gas hydrates have been recovered. Stability values of temperature and pressure indicate that hydrates can exist in water depths less than 500 m. Gassy sediment geoacoustic parameter values are not well constrained because of a lack of concurrent measurements of acoustic properties and sediment gas content. For Gulf of Mexico gassy sediment, some reportedin situ values of sound speed are reduced by an order of magnitude below values for water saturated sediments. More commonly, sound speed is reduced from water saturated sediment values by only 15 to 50 percent.     

Medium-term frequency distributions of cross-shore suspended sediment transport rates in water depths of 3 to 9 m

A data set of several thousands of hours of near-bed flow, obtained at three cross-shore positions in 3- to 9-m water depth in the multiple bar system of Terschelling (Netherlands), was used to estimate the medium-term (≈years) frequency distribution of the cross-shore suspended sediment transport rates induced by short waves, infragravity waves and cross-shore mean flows. Predictions of an energetics-based transport model were categorised into groups of the local height-over-depth ratio with a width of 0.02 and were subsequently coupled to the discrete medium-scale probability distribution of this ratio. At all depths, the estimated medium-scale sediment transport rate by the short waves, q_ms,h, and mean flows, q_ms,mf, were of approximately equal magnitude and were about three times as large as that of the infragravity waves. In general, the medium-term sediment transport rates were dominated neither by the most extreme conditions nor by day-to-day situations. This was related to the infrequent recurrence of the most energetic events and by the predicted negligible transport rates under daily conditions. In 9-m depth, breaking conditions contributed to about 90% of both q_ms,h and q_ms,mf. In shallower water (3–5 m), non-breaking conditions became increasingly important for q_ms,h, whereas q_ms,mf remained fully dominated by surf zone conditions. This observation as well as literature findings for water depths less than about 3 m suggest that the range of small-scale conditions that contribute most to q_ms,h and q_ms,mf changes in the onshore direction from mainly breaking conditions at depths in excess of 5–7 m towards prolonged non-breaking periods for q_ms,h and short breaking events for q_ms,mf on the beach.     

Evidence for erosion and deposition by the 2011 Tohoku-oki tsunami on the nearshore shelf of Sendai Bay,Japan

Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.     

基于WorldView-2遥感影像的龙湾港浅海水深反演

根据遥感水深反演原理,利用海南岛龙湾港的WorldView-2多光谱卫星数据和海图水深资料,通过对水深进行0~2,2~5,5~10,10~15和15~20 m的分区处理、潮汐改正和海图水深数据与相应图像波段反射率值的相关性分析及回归分析,建立了浅海水深线性回归反演模型,开展了浅海水深的实际计算与精度分析。结果表明:对不同水深范围分别建立线性回归模型反演的水深精度要高于未分区建立的模型;分区模型中,多波段模型在0~5 m的反演精度最高,而双波段比值模型在5~20 m的反演精度最高,但是反演水深在最浅处的精度还有待提高。本文方法提取的水深与海图水深数据变化趋势基本相似,可以满足海洋科学研究对大范围浅水水下地形探测的要求。     

吕宋海峡深层水体体积输运的诊断分析

使用WOD09, WOA05, WOA01和SODA四种水文数据,利用水力学理论对吕宋海峡的深层水体体积输运进行了估计。结果表明：由于忽略了摩擦和地形等因素的影响,水力学理论得到的输运结果是深层体积输运的上限。不同数据以及吕宋海峡东西两侧选取的不同区域,都对水力学理论的体积输运估计存在显著影响。SODA数据在吕宋海峡附近的深层海区存在偏差,得到体积输运估计明显偏大,因而不适用于水力学理论进行吕宋海峡深层水体体积输运的估计。为对于南海的环流、混合及生态环境存在重要影响的吕宋海峡深层水体体积输运进行合理估计,对利用水力学理论估计深层水体体积输运的研究方法做了进一步探讨。     

Fluctuation in volume transport distribution accompanied by the Kuroshio front migration in the Tokara Strait

A relation between migration of the Kuroshio front and fluctuation of distribution of volume transport in the Tokara Strait was described, using sea level records at five tide gauge stations around the strait and data which were composed of sea surface temperature, XBT casts, sea surface salinity and velocities at 20 m, 75 m and 150 m depths taken en route a ferryboat. The Kuroshio front extends to about 150 m depth. The sea surface salinity and the horizontal velocities abruptly change at the front. There is a good correlation in a period range from half a month to two months between the migration of the front, which is not only at the surface but also in the subsurface, and the sea level fluctuation at Nakano-shima. A northward migration of the front with a period range from 17 to 50 days decreases the transport in the southern strait between Naze and Nakano-shima but increases in the northern strait between Nakano-shima and Sata-misaki. The northward migration intensifies inflow into Kagoshima Bay and the Ohsumi Branch Current. Correlation between the transport in the northern strait and the Ohsumi Branch Current is significant in the period range from 30 to 50 days. In this significant period range, the former leads the latter by about 3 days.     

A three-dimensional numerical model of hydrodynamics and water quality in Hakata Bay

The hydrodynamics and water quality in Hakata Bay, Japan, are strongly affected by the seasonal variations in both the gravitational circulation and the stratification in the bay. The three-dimensional hydrodynamics and water quality model has been developed to simulate the long-term transport and fate of pollutants in the system. The model is unique in that it completely integrates the refined modelling of the hydrodynamics, biochemical reactions and the ecosystem in the coastal areas. It is a 3-dimensional segmented model which is capable of resolving mean daily variations in all the parameters relevant to pollution control. It predicts daily fluctuations in the oxygen content at different depths in water throughout the year. It takes into account transport and settling of pollutant particles. It predicts light penetration from computed turbidity variations. It includes interactions between the ecosystem and water quality, through nutrient cycling and photosynthesis. The model has been calibrated well against the data set of historical water quality observations in Hakata Bay.     

Shallow water depth retrieval from space-borne SAR imagery

Based on shallow water bathymetry synthetic aperture radar (SAR) imaging mechanism and the microwave scattering imaging model for oceanic surface features, we developed a new method for shallow water depth retrieval from space-borne SAR images. The first guess of surface currents and winds are estimated from the normalized radar crossing section (NRCS) profile of shallow water bathymetry SAR imagery, according to the linear theory and geophysical model function. The NRCS profile is then simulated by the microwave scattering imaging model. Both the surface currents and winds are adjusted by using the dichotomy method step by step to make the M4S-simulated NRCS profiles approach those observed by SAR. Then, the surface currents and the wind speeds are retrieved when a best fit between simulated signals and the SAR image appears. Finally, water depths are derived using the Navier–Stokes equation and finite difference method with the best estimated currents and the surface winds. The method is tested on two SAR images of the Taiwan Shoal. Results show that the simulated shallow water NRCS profile is in good agreement with those measured by SAR with the correlation coefficient as high as 85%. In addition, when water depths retrieved from the SAR image are compared with in situ measurements, both the root mean square and relative error are less than 3.0 m and 6.5%, respectively, indicating that SAR images are useful for shallow water depth retrieval and suggesting that the proposed method in this paper is convergent and applicable.