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1.
With the development of high-resolution multibeam echosounder systems (MBES) for surveying shallow-water areas a new tool is available to monitor rapid changes in seabed morphology as, e.g., caused by the dumping of dredge spoil in coastal waters. In this study, four data sets of repeated bathymetric surveys with a MBES were processed and analyzed. The data were collected in a 1.94-km2 dumping site in the outer Weser Estuary (German Bight). Between June and December 1998, 2.6 million m3 of dredged sediment were deposited there. The bathymetric maps generated in the course of this study reveal features such as subaqueous dunes, scour holes, and mounds of dumped dredge spoil. The mean water depth decreased by about 1 m during the dumping period. Furthermore, difference grids showing changes in sediment volume allowed a calculation of the sediment budget for the monitored area. After a time period of only 5 months, 0.5 million m3 of the originally dumped 2.6 million m3 of dredge spoil had already been removed from the dumping site. 相似文献
2.
Between 1970 to 2000, the annual mean suspended matter (SPM) concentrations in the Vlie and Marsdiep tidal inlets of the Wadden Sea varied over five times. The present paper examines the possible relationship between SPM in the Wadden Sea and changing river Rhine discharges and dredging operations. The major short-term variations in annual mean SPM in part of the Wadden Sea appears to be a non-linear, exponential, function of river Rhine discharge and dredge spoil disposal (110 km to over 200 km from the area in front of the Dutch coast near the river Rhine outlet). Correlation coefficients (with SPM as the fixed and dredge disposal as the independent variable) ranged from R=0·8 (deep tidal inlet of Marsdiep) to R=0·2 (shallow inner area of Vlie) and weakened mainly as a function of distance to the disposal site. The best correlation with river discharge was for Marsdiep tidal inlet (r=0·45), indicating the superior effect of dredge disposal over river discharge-related processes. Taking the estimated regression equation as an explorative model, indicates that, without any disposal of dredge spoil, the expected SPM concentration levels in the tidal inlets of the Wadden Sea will be <15 g m−3 (comparable to the 1950s). The overall mean (and the highest mean) annual concentrations for the investigation period reached 42 (90) g m−3 at Marsdiep and 35 (75) g m−3 at Vlie. Assuming a 10% (220 m−3 s−1) increase in river Rhine discharge over the next 50 years, and unchanged dredging policy and other circumstances, SPM concentrations would increase 5–15% for Marsdiep and Vlie. Compared with the calculated (12·4 g m−3 SPM in Marsdiep and 14·8 g m−3 SPM in Vlie) and measured (15 g m−3 SPM in Marsdiep) background SPM concentrations, the expected overall mean increase since 1950 is at least 250% of background. The natural variation in river Rhine discharge will cause further inter-annual variation. Changes in SPM concentrations, due to expected changes in wind climate, combined with river discharge are estimated to increase SPM concentrations 20% above the present situation. The possible implications of changes in land use in combination with further increasing river discharge, changed wind fields and increased temperature are discussed. An important management conclusion is that increasing mean river discharge will significantly increase the need for dredging and spoil disposal, and result in further elevated SPM concentrations in the Dutch coastal zone and the Wadden Sea. The presented relationships offer possibilities for developing new management strategies in relation to dredging and its effects. 相似文献
3.
Distribution of particulate organic carbon (POC) and dissolved organic carbon (DOC) in seawater, and chemical composition of surficial sediments were studied in relation to pulpmill effluent and dumping of dredge spoil containing wood debris in Alberni Inlet, British Columbia, Canada. The maximum concentration of POC (377–584µg Cl–1) was observed at the surface around the dumping area (5–7 km seaward of the inlet's head), and at the location immediately adjacent to the dump site POC was elevated throughout the water column (50 m). While POC tended to decrease in the surface layer for a distance of about 25km down-inlet, measureable effects of POC in the deeper water did not extend beyond 600m from the dump site. The dump site was conspicuous by the large maximum in C:N ratio (46.3). In contrast, DOC was observed to be highest (4.25mg Cl–1) at the head of the inlet where pulpmill effluent was being discharged and a secondary maximum was found about 10km down-inlet from the dump site. The data suggest that a considerable proportion of the dredge spoil sinks rapidly near the dump site, probably within several hundred metres. Some of the spoil, perhaps low density wood debris may travel considerably further with the surface water where by leaching it may contribute to some extent to the surface DOC before sinking into deeper water. 相似文献
4.
Altogether three coastal dune fields, one located north and two south of the city of Essaouira, Atlantic Morocco, have been investigated to establish the distribution and overall sand volumes of various dune types. The purpose of the study was to characterize and classify the aeolian landforms of the coastal dune belt, to estimate their sand volumes and to assess the effectiveness of coastal dune stabilization measures. The northern dune field is 9 km long and lined by a wide artificial foredune complex fixed by vegetation, fences and branches forming a rectangular grid. Active and ephemeral aklé dunes border the inner backshore, while some intrusive dunes have crossed the foredune belt and are migrating farther inland. The total sand volume of the northern dune belt amounts 13,910,255 m3. The central coastal sector comprises a much smaller dune field located just south of the city. It is only 1.2 km long and, with the exception of intrusive dunes, shows all other dune types. The overall sand volume of the central dune field amounts to about 172,463 m3. The southern dune field is characterized by a narrower foredune belt and overall lower dunes that, in addition, become progressively smaller towards the south. In this sector, embryonic dunes (coppice, shadow dunes), tongue-like and tabular dunes, and sand sheets intrude from the beach, the profile of which has a stepped appearance controlled by irregular outcrops of old aeolianite and beach rock. The total volume of the southern dune field amounts 1,446,389 m3. For the whole study area, i.e. for all three dune fields combined, a sand volume of about 15,529,389 m3 has been estimated. The sand of the dune fields is derived from coastal erosion and especially the Tensift River, which enters the sea at Souira Qedima some 70 km north of Essaouira. After entering the sea, the sand is transported southwards by littoral drift driven by the mainly north-westerly swell climate and the Trade Winds blowing from the NNE. This sand feeds the beaches along the coast, from where it is blown obliquely onshore to generate the dune fields. The maximum sand input occurs in the north, from where it gradually decreases southwards, this being also reflected in the grain size and dune typologies. The study shows that dune stabilization measures have been reasonably effective along most of the coast, with the exception of a large area immediately north of Essaouira where the almost complete destruction of the plant cover has reactivated sand mobility—this may in the future threaten the city. 相似文献
5.
6.
The size (wavelength and height) and the factors influencing the characteristics of subaqueous dunes in the river Scheldt between Antwerp and the Belgian-Dutch border were studied. More than 60 dune clusters (each cluster represented by a mean height and wavelength of mostly 10 to 20 or more observations) were analysed. A very good exponential correlation between dune height and wavelength was observed (r=0.90) which can be described by H
mean=0.0321L
0.918. Length/depth ratios vary between 0.2 and 9, height/depth ratios between 0.25 and 0.01. There was no clear relationship between current velocity and dune size, as dunes of various sizes co-exist at the same current velocities. Twenty-two dunes were sampled and show grain sizes (d50) ranging between 0.1 and 0.7 mm. Seven out of eight large scale dune fields were found in sandy and/or hard bottom (gravel, coarse sand and shell fragments) environments, whereas small and medium dunes were found in muddy sand and sandy mud environments. 相似文献
7.
水下沙丘形态演化的数值模拟实验 总被引：1，自引：0，他引：1
水下沙丘在海洋、湖泊、河流等浅水砂质沉积区广泛分布。基于过程的数值实验方法探讨了沙丘形态演化问题,试图解释在沙丘形成过程中各因素的作用及它们之间的关系。模拟中考虑了以下的变量:水深、沉积物粒度、沉积层厚度以及台风作用。模拟结果显示,沙丘的空间分布控制了空间流场的参数k2,沙丘波高受水深、沉积物粒度以及沉积层厚度等因素的影响,沉积层厚度决定了沙丘的形态是否饱满。在台风作用中,沙丘波峰的沉积物被侵蚀,高程降低,波高渐小;台风作用后,沉积物被重新输运至波峰,沙丘高度逐渐恢复。因此,沙丘的高度取决于台风作用的时间以及2次台风作用之间的间隔。沙丘形态和尺度在台风作用前后变化较小,但沙丘演化的速度却有所提高。根据台湾浅滩和北海南部地貌系统数据的验证,模拟具有较好的效果。 相似文献
8.
水下沙丘形态演化的数值模拟实验 总被引：1，自引：0，他引：1
水下沙丘在海洋、湖泊、河流等浅水砂质沉积区广泛分布。基于过程的数值实验方法探讨了沙丘形态演化问题,试图解释在沙丘形成过程中各因素的作用及它们之间的关系。模拟中考虑了以下的变量:水深、沉积物粒度、沉积层厚度以及台风作用。模拟结果显示,沙丘的空间分布控制了空间流场的参数k2,沙丘波高受水深、沉积物粒度以及沉积层厚度等因素的影响,沉积层厚度决定了沙丘的形态是否饱满。在台风作用中,沙丘波峰的沉积物被侵蚀,高程降低,波高渐小;台风作用后,沉积物被重新输运至波峰,沙丘高度逐渐恢复。因此,沙丘的高度取决于台风作用的时间以及2次台风作用之间的间隔。沙丘形态和尺度在台风作用前后变化较小,但沙丘演化的速度却有所提高。根据台湾浅滩和北海南部地貌系统数据的验证,模拟具有较好的效果。 相似文献
9.
Large and very large subaqueous dunes on the continental shelf off southern Vietnam, South China Sea 总被引：1，自引：1，他引：0
A. Kubicki 《Geo-Marine Letters》2008,28(4):229-238
In the course of two regional side-scan sonar surveys on the continental shelf off southern Vietnam after the winter monsoon
seasons of 2003 and 2004, and covering a total distance of over 1,000 km, the widespread occurrence of large and very large
subaqueous dunes was discovered. On the basis of their size, shape, depth of occurrence and orientation, the dunes were grouped
into five spatially distinct regions. In each case, a different height/wavelength relationship is observed. With the exception
of region no. 3 where dune dimensions follow the mean global trend, the dimensions in the other regions lie below the mean
global trend. The most plausible explanation for this is sediment starvation and/or insufficient time for the larger dunes
to fully adjust to changing flow conditions. A good correlation is observed between average dune height in each region and
water depth, although this is not the case for dune length. The orientation of the dunes corresponds to the direction of the
current pattern induced by the regional winter monsoon winds (NE to SW and S). The generally well-developed asymmetrical shapes
and the large size of the dunes suggest that the wind-induced currents are strong enough to reactivate most of the dunes during
the winter monsoon season, a conclusion supported also by theoretical calculations of critical current velocities. The largest
dunes, which seem to have reached their maximum sizes for the local water depths, may not be reactivated regularly but rather
only by exceptionally strong episodic flows. 相似文献
10.
M.R.A. van Gent J.S.M. van Thiel de Vries E.M. Coeveld J.H. de Vroeg J. van de Graaff 《Coastal Engineering》2008,55(12):1041-1051
Large-scale physical model tests were performed to quantify the effects of the wave period on dune erosion. Attention was focussed on 2D cross-shore effects in a situation with sandy dunes and extreme water levels and wave conditions. Besides profile measurements, detailed measurements in time and space of water pressure, flow velocities and sediment concentrations were performed in the near near-shore area. It was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face. Tests with double-peaked wave spectra showed that the influence of the spectral shape on dune erosion was best represented by the Tm − 1,0 spectral mean wave period, better than the peak wave period, Tp. The effect of the wave period on dune erosion was implemented in a dune erosion prediction method that estimates erosion volumes during normative storm conditions for the Dutch coast. More details of the measurements and additional analyses of physical processes are described in an accompanying paper by Van Thiel de Vries et al. [Van Thiel de Vries, J.S.M., van Gent, M.R.A., Reniers, A.J.H.M. and Walstra, D.J.R., submitted for publication. Analysis of dune erosion processes in large scale flume experiments, In this volume of Coastal Engineering.]. 相似文献
11.
Distribution and thickness of sedimentary facies in the coastal dune, beach and nearshore sedimentary system at Maspalomas, Canary Islands 总被引：1，自引：1，他引：0
Angela Fontán Bouzas Javier Alcántara-Carrió Isabel Montoya Montes Andrés Barranco Ojeda Silvia Albarracín Jorge Rey Díaz de Rada Jorge Rey Salgado 《Geo-Marine Letters》2013,33(2-3):117-127
Numerous studies have shown that most beaches and coastal dune systems of the world are currently eroding but very few have investigated the combined sediment budgets of subaerial and nearshore submarine systems. In the case of the dune field of the Maspalomas Natural Special Reserve (in the south of Gran Canaria), the adjacent Maspalomas and El Inglés beaches and the adjacent submarine platform, the sediment budgets have been severely affected by erosion over the past few decades. The objectives of this study were to investigate the availability of sand within the modern sedimentary system, including the coastal dunes, the beaches and the submerged shelf, but also to assess local sediment sinks. An isopach map generated on the basis of topo-bathymetric data and seismic-reflection profiles revealed that sediment thickness varies from 0–22 m in the study area. Expanses of relatively low sediment thickness were identified in the south-western sector of the coastal dune field along Maspalomas beach, and in the nearshore region to the south of this beach. These localized sediment-deficit areas earmark Maspalomas beach as the most vulnerable shore strip threatened by erosion. The shallow seismic data also revealed that the submarine platform south of Maspalomas represents a marine terrace cut into an ancient alluvial fan, thus documenting an influence of the geomorphological heritage on the present-day morphodynamics. A side-scan sonar mosaic of this nearshore platform enabled the delimitation of areas covered by rock, boulders and gravel, vegetated sand patches and a mobile sand facies, the latter including ripple and megaripple fields. The megaripple field in a valley close to the talus of the marine terrace has been identified as a major sediment sink of the Maspalomas sedimentary system. It is fed by south-westerly storm-wave events. The sediment deficit in the coastal dune field and along Maspalomas beach can therefore only be explained by a currently faster loss of sediment to an offshore sink than can be compensated by the supply of sand from outside the system. 相似文献
12.
The response of subaqueous dunes to the variation of riverine sediment supply in an estuary is rarely reported. Five sets of field measurements of bed sediments and topography were made in the Changjiang estuary, China during dry and flood seasons for the past 9 years. Results show that dunes were mainly three-dimensional (3D) during flood seasons and two-dimensional (2D) dunes during dry seasons. Dunes were also much larger in flood seasons. Dunes mainly occurred within the main channels of the estuary although they extended 11.7 km seaward of their normal extent in the South Channel during dry seasons from 2002 to 2006. This extension of the dune field is likely due to a sharp decline of sediment discharge caused by large reservoirs such as Three Gorges. 相似文献
13.
长江口深水航道以悬沙落淤为主,回淤泥沙较为松散、上覆有流动性较强的高密度浮泥层,航槽需进行常年维护。为提高疏浚效率、降低疏浚成本,基于耙头动力改进的思路,提出适用于长江口沉积物特性的掺气扰动耙具,并通过室内试验,探讨耙具推荐型式,推荐空压机流量及压力指标,估算掺气耙具扰动效果。结果表明,2 mm气嘴耙具掺气扰动后水体含沙量增幅高于其他孔径;相同孔径、相同气流量下,气嘴间距越大、扰动效果越明显,因此推荐耙具气嘴孔径2 mm、20 cm间距交错布置型式。对于推荐耙型,当气体压力低于4.8×105Pa、气流量小于1.1 m3/min时,扰动后水体含沙量基本呈线性增长;当气体压力超过4.8×105Pa,扰动后水体含沙量无明显变化。以室内试验推荐压力4.5×105Pa为基础,估算施工作业水深15 m,结合实际作业水深及气力损耗,现场空压机选型参考值为气流量61.7 m3/min、工作压力为0.7 MPa。据此推算设计航速1.286 m/s时,中上层(0～4 m)水体掺气扰动疏浚耙具悬扬原状土方量约800 m3/h。 相似文献
14.
The impact of water jet dredging for razor clams,Ensis spp., in a shallow sandy subtidal environment
《Journal of Sea Research》2000,43(1):65-81
The effects of water jet dredging for Ensis spp. on the seabed and benthos were examined through experimental fishing. Immediate physical effects were apparent, with the dredge leaving visible trenches in the seabed. While these trenches had started to fill after five days, and were no longer visible after 11 weeks, the sediment in fished tracks remained fluidised beyond this period. The majority of the studied infaunal community is adapted morphologically and behaviourally to a dynamic environment, and other than initial removal through dispersal, is not greatly affected by the dredge at the site studied. Species that are likely to be affected (e.g. the heart urchin Echinocarium cordatum, Arctica islandica and other large bivalves) were very rare in infaunal samples, but present in dredge catches, where damage was noted, and ranged on average from 10 to 28% of individuals. Epifauna were scarce in the study area, and unaffected by the fishing, except that epifaunal scavenging species were attracted to the fished tracks. On the evidence of the present and previous studies, it would appear that there was little difference between the biological impact of hydraulic and suction dredging, although the latter may have a greater physical effect (larger trenches). 相似文献
15.
16.
Les Watling Robert H. Findlay Lawrence M. Mayer Daniel F. Schick 《Journal of Sea Research》2001,46(3-4)
Scallops are usually obtained by means of a heavy metal dredge that is pulled over the sea bottom. Most studies of the impact of this gear type have shown that larger invertebrates, in particular, are severely disturbed. These studies, however, have been conducted on coarse sediments, ranging from sands to cobble, and have dealt only with faunal changes. In this paper the impact of a New England type scallop dredge on the fauna and sedimentary nutritional characteristics of a silty sand community is detailed. The site, in the Damariscotta River, Maine, USA, was sampled during the fall and winter prior to, then again immediately following, the dragging event, and twice more over the ensuing six months. Loss of surficial sediment, lowered food quality of the sediment (as measured by microbial populations, enzyme hydrolysable amino acids, and chlorophyll a), and changes in the faunal composition of the dragged site were observed. While some taxa returned to the drag track relatively quickly, others such as the cumaceans, phoxocephalid and photid amphipods, and nephtyid polychaetes, were not seen in abundances comparable to those of the adjacent undragged site until the food quality also recovered. 相似文献
17.
Thomas P O&#x;Connor Henry A Walker John F Paul Victor J Bierman 《Marine environmental research》1985,16(2):127-150
It has been proposed that future ocean disposal of sewage sludge from the US east coast be done at a site beyond the edge of the continental shelf. In anticipation of that, a monitoring strategy has been developed to determine the average spatial distribution of contamination. The strategy is an iterative series of measurements developed from models of sludge dispersion and settling which are based on characteristics of the disposal site and sewage sludge. Once disposal is initiated at the site, the strategy requires sampling the upper mixed layer at 36 stations, mostly within 100 km of the site, and deployment of near-bottom sediment traps along a line extending 300 km away from the site. Based on initial results, subsequent sampling locations will be selected to refine estimates of the detectable extent of sludge-derived contamination. The sludge constituents which can be used to detect sludge in water at a dilution of 106 and sludge in sediment traps when diluted by 100 with natural material include zinc, PCBs, coprostanol and spores of the bacterium Clostridium perfringens. Other synthetic organic compounds, besides PCB, may prove to be useful tracers. 相似文献
18.
The purpose of this study was to evaluate related processes of sediment consolidation and resuspension in a coastal basin and how these processes influence retention of fine sediment delivered by a river diversion. Sediment samples were collected from Lake Lery, a coastal receiving basin of the Caernarvon Diversion from the Mississippi River, Louisiana. Consolidation was tested for six initial sediment concentrations (14.0–105 kg m–3) in a settling column over 15-day periods. Mud erodibility was tested at seven shear stress regimes (0.01–0.60 Pa) using a dual-core Gust erosion microcosm system, on cores containing suspensions that consolidated for 1, 2, and 4 weeks. Consolidation rates were found to be inversely and exponentially related to initial suspension concentration, over concentrations ranging from fluid mud (10–200 kg m–3) to hydraulic dredge effluent. Consolidation is best predicted by a function consisting of two exponential terms and one asymptotic constant, describing rates of rapid initial and slower subsequent settling. Coupled resuspension and consolidation tests (concentrations of 20–21 kg m–3) show that shear stresses generating the highest turbidity peaks increase from ≤0.30 Pa after 2 weeks of consolidation to ≥0.45 Pa after 4 weeks, and this strengthening cannot be attributed solely to increasing sediment concentration over time. Comparison of measured erosion shear stresses with bed shear stresses typical of coastal lakes and bays suggests that this degree of strengthening, if given time to occur, could increase the overall retention of fine sediments deposited on lake and bay floors. 相似文献
19.
This paper gives some theoretical concepts of dune management as well as practical examples of how actual measures should be carried out.Dune management is defined as all measures aimed at the preservation and restoration of the natural values of a coastal sand dune area. It is essential that beach and foreshore are seen as parts of a whole dune system. Management must take into account all processes within this system. It should aim at nature conservation and wise use of coastal resources and all forms of land use should be ecologically sustainable. Dune management should work as much as possible ‘with’ the natural processes and not ‘against’ them.Four types of Mediterranean dune systems (including Portugal and SW Spain) are distinguished:: mainland dunes, lagoon-bay barrier dunes, delta dunes and remnant dunes.Like in most parts of the world, Mediterranean dunes fulfill several functions at the same time. The following functions will be dealt with: nature management, coastal defence and erosion, dune stabilisation and afforestation, agriculture, tourism and urbanisation, recreation, golf, information and education. The concepts of management which are considered include: sustainable use, large and coherent units, compensation, management planning, environmental impact assessment and function analysis.Dune stabilisation is not always considered necessary. In case of stabilisation by planting, only indigenous species should be used, preferably pioneers and not trees. Planting of exotic species should be avoided. 相似文献
20.
Lina Fernandes G.N. Nayak D. Ilangovan D.V. Borole 《Estuarine, Coastal and Shelf Science》2011,91(3):388-399
Two core sediment samples; one from inner part (ManI) and the other closer to the mouth (ManII); were collected from the intertidal regions of Manori, a tidally influenced creek near Mumbai, India. Both the cores were subjected to various geochemical analyses to determine parameters such as pH, sediment components, total organic carbon, total nitrogen, total phosphorus and selected metals viz., Fe, Mn, Cu, Pb, Co, Ni, Zn, Cr, Al, Ca and V. Analysis of 210Pb activity was employed to assess the sediment deposition trend of the area. The data was further processed using factor and cluster analyses. The results indicate that the sediments from site ManI, had finer sediment composition, higher porosity, organic matter and metal contents but exhibited an erratic decline in 210Pb activity downcore. Also ManI showed higher C:N ratio and enrichment factor values as compared to site ManII. The inner area (ManI) probably received a greater input of organic matter from the erosion of terrestrial matter as well as domestic and industrial discharge. Sediments from site ManII had typical marine organic matter composition (lower C:N ratio). The concentration of metals at this site was also low indicating the contents were getting diluted by freshwater and seawater mixing. 相似文献