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1.
Depth of activation on a mixed sediment beach 总被引：1，自引：0，他引：1
The relationship between wave height and depth of sediment activation is evaluated on an estuarine beach to determine whether activation depth is less in pebbles than sand. Rods with washers were used to monitor three excavated beach plots filled with (1) pebbles with mean grain size of 11.5 mm; (2) sand and granules; and (3) sand, granules and pebbles. Plots were monitored for 26 events over 27 days. Significant wave heights ranged from 0.18 to 0.40 m and activation depths from 0.02 to 0.12 m. Activation depths in the pebble plot were less than the other two plots when waves reworked sediment not activated during previous tidal cycles. Proportionality coefficients for activation depth to wave height, when net change was < 0.02 m, were 0.24 in the pebble plot and 0.30–0.31 in the other plots when experimental fill sediments remained and 0.22 to 0.23 in all three plots over the entire monitoring period, which included activation of newly deposited native sediment. Results suggest that for similar wave heights, activation depths in pebbles is lower than in sand, granules and pebbles or sand and granules, but once waves have reworked the sediment there is little difference in activation depths. 相似文献
2.
Permanent boat moorings have contributed to the decline of seagrasses worldwide, prompting the development of ‘seagrass-friendly’ moorings. We contrasted seagrass cover and density (predominantly Posidonia australis) in the vicinity of three mooring types and nearby reference areas lacking moorings in Jervis Bay, Australia. We examined two types of ‘seagrass-friendly’ mooring and a conventional ‘swing’ mooring. ‘Swing’ moorings produced significant seagrass scour, denuding patches of ∼9 m radius. Seagrass-friendly ‘cyclone’ moorings produced extensive denuded patches (average radius of ∼18 m). Seagrass-friendly ‘screw’ moorings, conversely, had similar seagrass cover to nearby reference areas. Our findings reinforce previous work highlighting the negative effects of ‘swing’ and ‘cyclone’ moorings. In contrast, the previously unstudied ‘screw’ moorings were highly effective. We conclude that regular maintenance of moorings and the monitoring of surrounding seagrass are required to ensure that ‘seagrass-friendly’ moorings are operating effectively. This is important, as following damage Posidonia will take many decades to recover. 相似文献
3.
Investigations of the velocity and vorticity fields in the wake of a flexible riser with a length to diameter ratio of 181 were conducted in a towing tank at moderate Reynolds numbers in the range of 9400–47,000. Wake velocity measurements were made with the riser freely vibrating in both in-line and cross-flow directions. The motion and wake field of the riser, undergoing free vibration, were simultaneously measured by accelerometers installed inside the riser and by using a digital particle image velocimetry (DPIV) system. The vortex-induced vibration (VIV) results show that the riser freely oscillated at multiple vibration frequencies and amplitudes at each Reynolds number. Mixed vortex modes, ‘2S’, ‘2P’ and ‘P+S’, were observed in the near wake of the riser at different instants of time. The occurrence of these vortex modes depended on the Reynolds number, dominant frequency and mean amplitude. At lower Reynolds number, the single stable mode ‘2S’ dominated the wake. With the increase of Reynolds number, the percentage of the ‘2S’ modes decreased while the percentage of ‘2P’ modes increased steadily except at Reynolds numbers of 14,100 and 47,000. The ‘P+S’ modes occurred mostly at a Reynolds number of 14,100 accompanied by more ‘2P’ modes and less ‘2S’ modes. At this Reynolds number, the frequency of the VIV was very close to the natural frequency of 0.72 Hz, which was obtained from a riser decay test in steady water and the average amplitude to diameter ratio reached 0.95, the highest found in these tests. 相似文献
4.
沉积物颗粒与下覆颗粒间转动角是制约沉积物颗粒起动的一个重要因素。本文样品采集于1987年。作者通过一系列实验探明了沉积物转动角同颗粒形状、相对粒径比以及迭瓦状构造等因素之间的相互关系。在均匀粒径条件下,球状砾石转动角的测量数值同理论数值相吻合,但随粒径的减小而增大。这一结果同前人利用球状砂粒进行实验所获得的结果相一致。利用球状、椭球状及棱角状砾石进行的实验表明,沉积物转动角φ随相对粒径的增大而减小,随颗粒扁平度及颗粒棱角度的增大而增大。扁平砾石的迭瓦状构造将导致转动角的增大,其增大值约等于砾石的迭瓦角。若其它因素相同,沉积物起动的难度自球状、椭球状、棱角状及迭瓦状构造而依次增强。这些结果将有助于对颗粒粒径、形状、构造同沉积物搬运临界状态以及沉积物选择性起动之间的相互关系进行定量分析。 相似文献
5.
The ‘dissolved’ iron (that passed through a 0·4-μm filter) varied nonconservatively with salinity in the Connecticut River estuary. However, the total iron appeared to be conservative. Measurements of Fe(II) and Fe(III) showed that oxidation of Fe(II) was not a factor in the decrease of ‘dissolved’ iron in the low salinity region. A solubility model and analyses based on different pore-size filters indicated that a substantial amount of the ‘dissolved’ iron in the low salinity region was colloidal iron. The coagulation of fine colloidal particles led to the non-conservative behavior of ‘dissolved’ iron during estuarine mixing, but it did not necessarily lead to removal of total iron from the waters. Particulate iron was 80–90% of the total iron and it covaried with the total suspended matter during mixing and sediment resuspension. The residence time of water in the Connecticut River estuary was too short to allow removal of iron from the water column within the estuary. 相似文献
6.
In addition to high hydrostatic pressure, scarcity of food is viewed as a factor that limits the abundance and activity of heterotrophic organisms at great ocean depths, including hadal trenches. Supply of nutritious food largely relies on the flux of organic-rich particulate matter from the surface ocean. It has been speculated that the shape of hadal trenches helps to ‘funnel’ particulate matter into the deeper parts of the trench, leading to sediment ‘focussing’ and improved benthic food supply. Here we investigate for five Northwest Pacific trenches the efficiency of sediment focussing by evaluating ratios of measured (sediment-derived) and expected (water-column-derived) sedimentary inventories of the naturally occurring and radioactive particulate-matter tracer 210Pbxs. The sites comprise a broad range of surface-ocean productivity and physical-oceanographic regimes. Across the five trench-axis settings the inventory ratio varies between 0.5 and 4.1, with four trench-axis settings having ratios>1 (sediment focussing) and one trench-axis setting a ratio<1 (sediment winnowing). Although the fluid- and sediment-dynamical forcing behind sediment focussing remains unclear, this study finds evidence for another mechanism that is superimposed on, and counteracts, the focussing mechanism. This superimposed mechanism is related to higher-frequency (tidal, near-inertial) fluid dynamics. In particular, there is evidence for a strong and negative relation between the intensity of propagating internal tides and the extent of sediment focussing in the trench-axis. The relation can be approximated by a power function and the most intense drop in sediment focussing already occurs at moderate internal-tide intensities. This suggests that propagating internal tides may have a subtle but significant influence on particulate-matter dynamics and food supply in hadal trenches in particular, but possibly also in the deep seas in general. A mechanism for the influence of internal tides on sediment dynamics is proposed. 相似文献
7.
《Marine Policy》2013
This contribution is a response to the rebuttal of Agnew et al. (2012) to Froese and Proelss (2012) “Evaluation and legal assessment of certified seafood”. It corrects some factually wrong statements in the rebuttal, revisits the definitions of ‘depleted’ and ‘overfished’, and notes that the rebuttal agrees with the international definition of ‘overfishing’ (F>FMSY) that was used by Froese and Proelss (2012). The rebuttal presents an analysis of 45 MSC-certified stocks. Of these, 27% are ‘depleted’ (according to the definition used by MSC) or ‘overfished’ (according to the definition used by Froese and Proelss 2012) and 16% are subject to ‘overfishing’, basically confirming the critique of Froese and Proelss (2012). This response concludes that MSC has to change its rules for certification such that (1) overfishing is not allowed and (2) ‘depleted’ stocks are marked as such. 相似文献
8.
On the displacement of marked pebbles on two coarse-clastic beaches during short fair-weather periods (Marina di Pisa and Portonovo, Italy) 总被引：1，自引：1，他引：0
Duccio Bertoni Edoardo Grottoli Paolo Ciavola Giovanni Sarti Giuliano Benelli Alessandro Pozzebon 《Geo-Marine Letters》2013,33(6):463-476
The aim of the investigation was to define the mechanisms of sediment transport in the swash zone of microtidal coarse-clastic beaches in the very short term by evaluating the displacement rates of marked pebbles under low-energy wave conditions. Tests were performed at two sites (Marina di Pisa, Ligurian Sea, and Portonovo, central Adriatic Sea) to check the consistency of the data over a range of different grain sizes. Two recovery campaigns were carried out at both sites, one 6 h and the other 24 h after the injection. During the experiments wave action was at a minimum (wave heights never exceeded 0.3 m). The results show that 20% of pebbles ranging in diameter from 30–90 mm moved significantly (more than 0.5 m) already 6 h after the injection, with some tracers being lost (3%). After 24 h, 40% of the pebbles were significantly displaced and 10% were lost. The preferential downslope movement of tracers, which suggests that coarse sediment movement under low-energy conditions is mainly controlled by gravity processes enhanced by steep beachface slopes, represents the novelty of the results reported here. It would appear that swash processes on low-energy beaches cause a significant rate of pebble displacement through the destabilization induced by wave uprush and backwash. Despite the microtidal range, the position of the mean water level plays a major role in changing the beach level at which swash processes can actually trigger pebble movement. The results of this study show that considerable, and mostly seaward-directed, coarse sediment transport takes place even during short fair-weather periods. 相似文献
9.
《Marine Policy》2015
Improving the social acceptability or ‘social licence’ of Marine Protected Areas (MPAs) is a key challenge facing countries all around the world. As the world moves slowly towards the establishment of a global network of MPAs, it is increasingly apparent that a greater understanding of social responses to MPAs is required, given they are often met with resistance from local communities. A series of in-depth, semi structured interviews were conducted across coastal users in New South Wales, Australia, including surfers, recreational fishers, professional fishers, spearfishers, walkers, divers, snorkellers, kayakers and other community members. The research identified the values, images and principles at work amongst coastal users to determine the dominant ‘cultural models’ within the community and how these models influenced attitudes towards MPAs. This research indicates that traditional consultation models may not be sufficient to address the full spectrum of community needs, and in fact suggests the need to re-conceive the make -up of ‘the community’ itself. In the context of MPA planning ‘the community’ is not an amalgamation of a range of homogenous stakeholder groups but instead a diverse and complex mix of identities and value systems which are not confined to particular interest groups. Incorporating consideration of the diverse range of values, images and principles found within and across stakeholder groups will require new and innovative approaches to participation and management. 相似文献
10.
《African Journal of Marine Science》2013,35(2):269-281
Fish were surveyed by visual census on offshore reefs in Mozambique and eastern South Africa to compare (a) fully-protected ‘sanctuary’ areas, (b) ‘partly protected’ areas where recreational diving and limited fishing are permitted, and (c) ‘open’ unprotected areas. Community composition differed between coral-dominated reefs and those covered mainly by algae and sponges, but not among treatments. Larger size classes were significantly diminished in unprotected areas compared to protected and sanctuary zones. Within coral-dominated reefs, abundances of indicator taxa showed three patterns: (1) greatest abundance in sanctuaries, intermediate in partly protected and least in open areas; (2) greatest abundance in sanctuaries but equal diminishment in partly protected and open areas; and (3) greater depletion in partly protected than either sanctuary or open areas. We conclude that partial protection does not effectively conserve reef fish, and there are indications that partly protected areas concentrate fishing effort on ‘pelagic’ gamefish. Sanctioned shore-angling and offshore ‘gamefishing’, illegal fishing and diver disturbance may collectively undermine the efficacy of partially protected areas. The depletion of reef fish species inside protected areas and reconsideration of the classification of pelagic gamefish require managerial attention. Partly protected areas in Mozambique need to be supplemented with no-take zones. 相似文献
11.
Optimization of the indophenol blue method for the automated determination of ammonia in estuarine waters 总被引：1，自引：0，他引：1
Existing automated methods for the determination of ammonia in natural waters suffer from serious ‘salt error’ in estuaries because of changes in pH, ionic strength and optical properties with salinity. A modified automated indophenol blue method is described which minimizes the ‘salt error’ to less than 8% over the entire salinity of estuaries. 相似文献
12.
The problem addressed in this paper is that of the fluid-structure interaction in offshore structures for which effects due to fluid viscosity are negligible. A method for evaluating the dynamic response of structures is described in which the generalized coordinates for the analysis are associated with the ‘dry’ modes of the structure. All hydrodynamic actions, including those induced by motions of the structure, are retained on the right-hand side of the equations of motion. This formulation requires the determination of generalized added mass and damping matrices associated with motions of the structure in its first few ‘dry’ modes. By these means, free surface and three dimensional fluid flow effects are retained in the analysis. Examples of the resulting frequency dependent matrices, computed using the boundary integral method, are presented for some common structural forms, and the validity of this approach is demonstrated through some experiments on a deforming column structure. Finally, results for the wave-excited dynamic response of a typical offshore structure obtained using (i) conventional strip theory ‘wet’ modes, and (ii) ‘dry’ modes, are compared. 相似文献
13.
《Coastal Engineering》2006,53(2-3):277-287
In this paper different strategies of morphological updating, as an important component of integrated modelling, are discussed, starting with the classical ‘tide-averaging’ approach in combination with the ‘continuity correction.’ An extension of the ‘continuity correction’ method, the so-called ‘RAM’ method, is then discussed. Next, the ‘online’ or ‘morphological factor’ approach is evaluated and compared with the previous approaches. Finally, a new, ‘parallel online’ method is presented which overcomes the problem of large intra-tidal variations in bathymetry when applying high morphological factors. The pros and cons of the different methods are compared and illustrated for a hypothetical case of a tidal inlet developing from an initially very schematised geometry. 相似文献
14.
This paper investigates mercury contamination in recent marine sediment of the Tyrrhenian continental shelf near the mouths of three rivers draining an area with cinnabar deposits (M. Amiata, Italy). By means of chemical leaching, two fractions of the total mercury were distinguished: a ‘non-leachable’ fraction, consisting of mercury held in relatively stable forms and a ‘leachable’ fraction, composed of forms that are more weakly bound to the sediments. The two Hg fractions are correlated to the organic matter content, the distribution of which is strictly dependent on the sediment surface area.In order to eliminate the effects of grain size variations among the samples, the Hg concentrations were normalized to the unit of surface area (ng/m2). Following this procedure, the ‘non-leachable’ Hg was found to be concentrated mostly in a belt along the coast, while the ‘leachable’ Hg fraction was shown to accumulate largely in the zones with high depositional dynamics near the mouths of the rivers.Hypotheses to account for the discrepancy between the low concentrations of ‘leachable’ Hg in the sediment of the outer continental shelf and the high concentrations in the fauna of the same area, reported in previous papers, are presented. 相似文献
15.
《Deep Sea Research Part I: Oceanographic Research Papers》2007,54(8):1221-1230
Otoliths of pre-recruit orange roughy (Hoplostethus atlanticus) were sampled from different deep-water habitats (‘flat’ and ‘hill’) and areas (north and south) on the Porcupine Bank. Age-based profiles for stable isotopes of carbon (δ13C) and oxygen (δ18O) in these otoliths were developed, which provide a fish life-history record of water depth and metabolic activity. These profiles were consistent among all individuals implying endogenous (ontogenic) influences on the pattern. The data indicate that post-larval orange roughy are mesopelagic active foragers, early juvenile fish move into a low energy deep-demersal phase, and older pre-recruit orange roughy assume the habitat depth and metabolic rate typical of adults. Comparison of otolith stable isotope profiles among areas and habitats on the Porcupine Bank suggest that juvenile orange roughy from South ‘hill’ and ‘flat’ habitats experience differing temperature and metabolic status at certain life stages. This may reflect oceanographic and ecological divergence between the two environments and suggests fine-scale population structure that may reduce resilience to exploitation. 相似文献
16.
Highly dependent on boundary conditions, the behaviors of underwater explosion (UNDEX) bubbles would be quite unusual near boundaries that are discontinuous with abrupt changes in shape, e.g. ship structures that have already been deformed by previous attacks. The oscillation features of the UNDEX bubble near the bilge with a circular opening representing previous deformation are studied experimentally with electric-spark-generated bubbles and high-speed photographing. The bubble behaviors are found highly dependent on two non-dimensional variables, D and Φ, representing the opening-bubble distance and the opening diameter, respectively. Seven distinctive oscillation scenarios are summarized from 180 experiments, namely the ‘rim-constrained oscillation’, the ‘inward jet’, the ‘outward jet’, the ‘bump and dimple’, the ‘quasi-spherical oscillation’, the ‘spherical oscillation with jet’ and the ‘spherical oscillation without jet’. The occurrence domains of the scenarios are identified as functions of D and Φ. Significantly affected by the opening, the bubble behaviors are quite different from that near a non-opening bilge; the bubble jet might not be formed, or even develop from inside the bilge, which indicates that the bubble load on the bilge should be re-evaluated. Finally the speeds, initiation time and displacements of the jets in different scenarios are measured and noticeable variation trends are found. 相似文献
17.
《Oceanologica Acta》1998,21(2):371-381
Regular flights over the Bay of Biscay by the French Customs Air Service, have enabled us to record more precisely, from 1989 to 1995, the geographical locations of French and other European Community (EU) fishing boats. Data showed three main ‘métiers’ within the Bay of Biscay fishing fleet: French and Spanish bottom trawlers (61 %), French gill-netters and Spanish longliners. More than 80 % of gill-netters worked inside the territorial sea (12 nautical miles) and 60 % of them worked south of Noirmoutier (47°N). Longliners moved according to the seasons, from the edge of the continental shelf to the 50–100 m sounding lines. Since they work at night, only few pelagic trawlers have been recorded (records being made in daylight). The potters, liners and other small ‘métier’ boats of the bay were only located along the coast. The position of the French boats' positions according to their registration district showed that most boats worked close to their landing ports and that the overlap is small between the areas of two neighbouring fleets. Further analysis of the spatial distribution of boats in the French EEZ showed that Spanish boats were mainly recorded in the southern part of the Bay of Biscay, and that most were Basque. Belgian and Dutch beam trawlers fished their quota of sole off the ‘Pertuis Charentais’, the Gironde estuary and the ‘bassin d'Arcachon’. The 100 m sounding line appeared to constitute a demarcation line between the major fraction of the French fleet and other EU fishing boats. Although some fishing areas were only visited by boats practising a particular ‘métier’, others were coveted by everybody and, consequently, were areas with the most serious risk of conflicts. 相似文献
18.
E. Azoulay M. Colin J. Dubreuil H. Dou G. Mille G. Giusti 《Marine environmental research》1983,9(1):19-36
In marine sediments from the Mediterranean coast polluted by oil spills (Etang de Berre) high bacterial activity is responsible for self-purification, samples collected from these areas showing a bacterial density several orders of magnitude higher than samples from non-polluted areas (Isle des Embiez). Between 60 and 80 % of the heterotrophic bacteria in the polluted areas are hydrocarbon-degrading, compared with 0.01 % in the non-polluted area.Several strains of hydrocarbon utilisers were isolated, a mixture of strains from each biotope was taken to represent a ‘biotope population’ and their activity towards different types of hydrocarbons (n-alkanes, iso-alkanes, cycloalkanes, aromatics) was determined.With ammonia as the nitrogen source the ‘biotope population’ from the polluted area degraded hydrocarbons much more readily than the ‘biotope population’ from the non-polluted area. With nitrate as the nitrogen source the degradation was much reduced, and, for some hydrocarbons, ceased in both ‘populations’.Individual hydrocarbons encouraged the growth of individual strains in the ‘biotope population’. In natural mixtures of hydrocarbons all strains grew well and degraded aliphatics. There was a good correlation between the respiratory activity of the ‘biotope population’ and the ability of the ‘population’ to utilise hydrocarbons for growth. 相似文献
19.
V.N. de Jonge 《Estuarine, Coastal and Shelf Science》1985,21(5):607-622
The role of ‘sand’ (particles >55 μm) as a substratum for benthic diatoms was investigated, using sediments collected from tidal flats and main channels in the Ems estuary. On average, 13% of the sand grains were occupied by one or more diatom cells. It appeared that 80% of the diatom cells in this fraction were present in or on mud coating the sand grains. Of the sand grains in the water, 7% were occupied by diatom cells. The preference of benthic diatoms for the mud coatings rather than the bare parts of the sand grains stresses the importance of ‘mud’ (particles <55 μm) as a substratum for diatoms on the tidal flats. This was confirmed by separate cell counts for the mud and sand fractions.Of the sand grains with attached diatoms, the mean number of diatom cells per grain in the water was similar to that on the tidal flats. This indicates a large exchange of sand between tidal flats and channels. A similar exchange between tidal flats and possibly a shift in the species composition. These processes suggested by the striking similarities in cell number distribution and species composition in samples from these two habitats. However, on exposed tidal flats certain species were present in lower numbers than in the channels, indicating that loosely attached diatoms can be scoured off their substratum. Consequently, two factors are assumed to contribute to the dynamics of the species composition of benthic diatoms. The first is the adhesion capacity of diatoms and the second is sediment sorting by currents and waves. These factors cause a differential but significant flux of benthic diatoms to and from the channels, followed by changes over time in the diatom populations on the tidal flats and possibility a shift in the species composition. These processes show that the terms ‘epipelic’ and ‘epipsammic’ are not helpful to classify the main components of the diatom flora in this estuary. 相似文献