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1.
The formation time of alongshore morphological variability in surf zone sand bars has long been known to differ from one beach to the other and from one post-storm period to another. Here we investigate whether the type of sea state, i.e. distant swell waves or locally generated short period wind sea, affects the formation time of the emerging alongshore topographic variability.A numerical modeling approach is used to examine the emergence of alongshore variability under different shore-normal wave forcing. A research version of Delft3D, operating on the time-scale of wave groups, is applied to a schematised bathymetry with a single bar. The model is then used to investigate several wave scenarios, examining the impact of peak period, frequency spread and directional spread on the formation time of alongshore variability.Results show that an increase in wave period has a large effect, changing the formation time up to O (250%) in case the wave period is changed from a representative value for the Dutch coast (Tp ~ 5–6 s) to an Australian South East coast value (Tp ~ 10–12 s). In contrast, modifications in the directional and frequency spread of the wave field result only in a minor change in the formation time.Examination of hydrodynamics and potential sediment transport shows that the variations in formation time are primarily related to changes in the magnitude of the time-averaged flow conditions. Variations in the magnitude of very low frequency (f < 0.004 Hz) or infragravity (0.004 < f < 0.04 Hz) surf zone flow velocities do not affect the mean sediment transport capacity. Consequently the formation speed of patterns is primarily governed by positive feedback between mean flow and morphology, and low frequency flow fluctuations are of minor importance.These findings indicate that the development of alongshore topographic variability may be faster at swell dominated open coasts, primarily due to the occurrence of longer period swell. Also, at a given site, the arrival of a long wave period swell after a storm can accelerate the emergence of variability. 相似文献
2.
Large-scale wave reanalysis databases (0.1°–1° spatial resolution) provide valuable information for wave climate research and ocean applications which require long-term time series (> 20 years) of hourly sea state parameters. However, coastal studies need a more detailed spatial resolution (50–500 m) including wave transformation processes in shallow waters. This specific problem, called downscaling, is usually solved applying a dynamical approach by means of numerical wave propagation models requiring a high computational time effort. Besides, the use of atmospheric reanalysis and wave generation and propagation numerical models introduce some uncertainties and errors that must be dealt with. In this work, we present a global framework to downscale wave reanalysis to coastal areas, taking into account the correction of open sea significant wave height (directional calibration) and drastically reducing the CPU time effort (about 1000 ×) by using a hybrid methodology which combines numerical models (dynamical downscaling) and mathematical tools (statistical downscaling). The spatial wave variability along the boundaries of the propagation domain and the simultaneous wind fields are taking into account in the numerical propagations to performance similarly to the dynamical downscaling approach. The principal component analysis is applied to the model forcings to reduce the data dimension simplifying the selection of a subset of numerical simulations and the definition of the wave transfer function which incorporates the dependency of the wave spatial variability and the non-uniform wind forcings. The methodology has been tested in a case study on the northern coast of Spain and validated using shallow water buoys, confirming a good reproduction of the hourly time series structure and the different statistical parameters. 相似文献
3.
We have employed laboratory and numerical experiments in order to investigate propagation of waves in both long and short-crested wave fields in deep water. For long-crested waves with steepness, ϵ = kcac = 0.1 (a fairly extreme case), reliable prediction can be performed with the modified nonlinear Schrödinger equation up to about 40 characteristic wavelengths. For short-crested waves the accuracy of prediction is strongly reduced with increasing directional spread. 相似文献
4.
The ∼8.15 ka Storegga submarine slide was a large (∼3000 km3), tsunamigenic slide off the coast of Norway. The resulting tsunami had run-up heights of around 10–20 m on the Norwegian coast, over 12 m in Shetland, 3–6 m on the Scottish mainland coast and reached as far as Greenland. Accurate numerical simulations of Storegga require high spatial resolution near the coasts, particularly near tsunami run-up observations, and also in the slide region. However, as the computational domain must span the whole of the Norwegian-Greenland sea, employing uniformly high spatial resolution is computationally prohibitive. To overcome this problem, we present a multiscale numerical model of the Storegga slide-generated tsunami where spatial resolution varies from 500 m to 50 km across the entire Norwegian-Greenland sea domain to optimally resolve the slide region, important coastlines and bathymetric changes. We compare results from our multiscale model to previous results using constant-resolution models and show that accounting for changes in bathymetry since 8.15 ka, neglected in previous numerical studies of the Storegga slide-tsunami, improves the agreement between the model and inferred run-up heights in specific locations, especially in the Shetlands, where maximum run-up height increased from 8 m (modern bathymetry) to 13 m (palaeobathymetry). By tracking the Storegga tsunami as far south as the southern North sea, we also found that wave heights were high enough to inundate Doggerland, an island in the southern North Sea prior to sea level rise over the last 8 ka. 相似文献
5.
The detailed reconstruction of the directional spectrum of wind waves from measurements of the wave field is an essential requirement for several applications, including the numerical modeling of wave evolution. Three reconstruction techniques that provide estimates of the directional distribution function D(f,θ), given the one-dimensional frequency spectrum, are compared using data from a coastal locality at the southern Brazilian coast. The techniques are the maximum entropy method (MEM), the Fourier Expansion Method using a cos2 type function (FEMcos) and the Fourier Expansion Method using a sech type function (FEMsech). The main patterns of the wave climate at the study site are qualitatively assessed. Three main sea states, including swell, transition between local sea and swell, and directionally bimodal wind sea, are identified. Time series from three events associated with the main sea states provide test cases for inter comparison of the three reconstruction techniques. Maximum entropy estimates of D(f,θ) provide results that are more consistent than those obtained from the two FEM techniques in all cases considered. 相似文献
6.
In this paper, the evolution of focused waves using different paddle displacements (piston type) under laboratory conditions is presented. It is well known that in intermediate water depths, linear paddle displacements will generate spurious, free, sub and super harmonics. Thus, a second order correction to suppress these spurious free sub and super harmonics was used to generate the focused waves. The focused waves were generated in the laboratory using a linear superimposition principle, in which the wave paddle displacement is derived based on the sum of a number of sinusoidal components at discrete frequencies, whose phases are accordingly set to focus at a particular location. For this method of generation, the second order wave maker theory proposed by Schäffer [24] can be easily adopted and was used in the present study. Two different centre frequencies (fc = 0.68 Hz and 1.08 Hz) with three different bandwidth ratios (Δf/fc = 0.5, 0.75 and 1.0) were tested in a constant water depth, to consider both narrow and broadband spectra. These test cases correspond to wave focusing packets propagating in intermediate and deep water regions. Further, for each wave packet, two different amplitudes were considered in order to analyze non-breaking and breaking cases. Thus, by systematically generating the wave packets using the linear and second order paddle displacements, the analysis was carried out for the spectral and temporal evolution of selected long waves. The temporal evolution of the selected harmonics was analyzed using the Inverse Fast Fourier Transform (IFFT), to show the propagation of the spurious, free, long waves. Further, the variations in energy for the lower, higher and primary frequency ranges are reported for different wave paddle displacements. The analysis revealed that for the broadband spectrum the differences are more pronounced when using linear paddle displacements. We have also noticed a shift in focusing/breaking location and time (i.e. premature) due to the increase in crest height using linear displacements. The experiment data used in this paper has been provided as a supplementary, which can be used to validate the numerical models. 相似文献
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8.
This study investigated the spatial and temporal variations of sediment accumulation and their impacts on coral communities in four sites at two or three depths (3 m, 6 m and 9 m) at the Sanya Coral Reef Reserve by deploying sediment traps on the sea floor during 2007–2009. Rainfall and typhoon events, which appeared to control sediment accumulation in the sea floor of the coral reef, were positively correlated with total sediment and sand-sized (i.e. 63–2000 µm) sediment accumulation. Sediment accumulation rate significantly decreased with the distance far away from the coast in Sanya. The mean sediment accumulation rates in Ximaozhou, Luhuitou and Xiaodonghai during 2007 to 2009 were close to 20 mg cm−2 d−1, and they were significantly higher than that in Yalongwan, probably as a result of terrestrial soil erosion caused by strong coast human activities (e.g. coastal construction, dredging and hillside clearing). Correlation analysis revealed that silt-clay-sized sediment accumulation rate was highly negatively correlated with total live coral cover and coral cover in some taxa, such as Montipora and branching Porites. whereas, Diploastrea heliopora was positively correlated with silt-clay-sized sediment accumulation. Correlation analysis also suggested that silt-clay-sized sediment accumulation had a higher efficiency in predicting the spatial variation of total live coral cover in Sanya than did the total sediment accumulation. Based on this investigation, we conclude that high rates of sediment accumulation pose a severe threat to the Sanya Coral Reef Reserve, highlighting the importance of integrated watershed management practices in the Sanya Coral Reef Reserve. 相似文献
9.
We describe experiments with multi-directional focused waves interacted with a vertical circular cylinder in a 3D wave basin. The focus of this study is on the run-up of multi-directional focused waves, wave forces, and wave pressures on the cylinder. Part I, the study on wave run-up, has already been presented by Li et al. (2012). In this paper, the analysis of the wave force on the vertical cylinder is presented.In this experiment, a cylinder with 0.25 m in diameter was adopted and different wave parameters, such as focused wave amplitude, peak frequency, frequency bandwidth and directional spreading index, are considered. The model scale kpa (kp is the wave number corresponding to peak frequency, a is the radium of the cylinder) varies from 0.32 to 0.65. The maximum forces of multi-directional focused wave on cylinder were measured and investigated. The results showed that the wave parameters have a significant influence on the wave force, and that the spatial profile of the surface of multi-directional focused wave can also affect its force on the cylinder, which is different from two-dimensional wave. In addition, the ‘secondary loading cycle’ phenomenon was also observed and discussed. In our experiments, the ‘secondary loading cycles’ occur when kA > 0.36 for all cases. While in some referred small scale experiments, the secondary load cycles are observed even for kA = 0.2, when the waves are longer enough. To larger model scale, the pronounced secondary load cycle occurs with larger wave steepness waves. 相似文献
10.
Shandong province is located on the east coast of China and has a coastline of about 3100 km. There are only a few tsunami events recorded in the history of Shandong Province, but the tsunami hazard assessment is still necessary as the rapid economic development and increasing population of this area. The objective of this study was to evaluate the potential danger posed by tsunamis for Shandong Province. The numerical simulation method was adopted to assess the tsunami hazard for coastal areas of Shandong Province. The Cornell multi-grid coupled tsunami numerical model (COMCOT) was used and its efficacy was verified by comparison with three historical tsunami events. The simulated maximum tsunami wave height agreed well with the observational data. Based on previous studies and statistical analyses, multiple earthquake scenarios in eight seismic zones were designed, the magnitudes of which were set as the potential maximum values. Then, the tsunamis they induced were simulated using the COMCOT model to investigate their impact on the coastal areas of Shandong Province. The numerical results showed that the maximum tsunami wave height, which was caused by the earthquake scenario located in the sea area of the Mariana Islands, could reach up to 1.39 m off the eastern coast of Weihai city. The tsunamis from the seismic zones of the Bohai Sea, Okinawa Trough, and Manila Trench could also reach heights of >1 m in some areas, meaning that earthquakes in these zones should not be ignored. The inundation hazard was distributed primarily in some northern coastal areas near Yantai and southeastern coastal areas of Shandong Peninsula. When considering both the magnitude and arrival time of tsunamis, it is suggested that greater attention be paid to earthquakes that occur in the Bohai Sea. In conclusion, the tsunami hazard facing the coastal area of Shandong Province is not very serious; however, disasters could occur if such events coincided with spring tides or other extreme oceanic conditions. The results of this study will be useful for the design of coastal engineering projects and the establishment of a tsunami warning system for Shandong Province. 相似文献
11.
Prasad G. Thoppil Patrick J. Hogan 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(8):946-955
The results from a～1 km resolution HYbrid Coordinate Ocean Model (HYCOM), forced by 1/2° Navy Operational Global Atmospheric Prediction System (NOGAPS) atmospheric data, were used in order to study the dynamic response of the Persian Gulf to wintertime shamal forcing. Shamal winds are strong northwesterly winds that occur in the Persian Gulf area behind southeast moving cold fronts. The period from 20 November to 5 December 2004 included a well defined shamal event that lasted 4–5 days. In addition to strong winds (16 m s?1) the winter shamal also brought cold dry air (Ta=20 °C, qa=10 g kg?1) which led to a net heat loss in excess of 1000 W m?2 by increasing the latent heat flux. This resulted in SST cooling of up to 10 °C most notably in the northern and shallower shelf regions. A sensitivity experiment with a constant specific humidity of qa=15 g kg?1 confirmed that about 38% of net heat loss was due to the air–sea humidity differences. The time integral of SST cooling closely followed the air–sea heat loss, indicating an approximate one-dimensional vertical heat balance. It was found that the shamal induced convective vertical mixing provided a direct mechanism for the erosion of stratification and deepening of the mixed layer by 30 m. The strong wind not only strengthened the circulation in the entire Persian Gulf but also established a northwestward flowing Iranian Coastal Current (ICC, 25–30 cm s?1) from the Strait of Hormuz to about 52°E, where it veered offshore. The strongest negative sea level of 25–40 cm was generated in the northernmost portion of the Gulf while the wind setup against the coast of the United Arab Emirates established a positive sea level of 15–30 cm. The transport through the Strait of Hormuz at 56.2°E indicated an enhanced outflow of 0.25 Sv (Sv≡106 m3 s?1) during 24 November followed by an equivalent inflow on the next day. 相似文献
12.
《Deep Sea Research Part I: Oceanographic Research Papers》2006,53(4):713-725
We tested the idea that bacterial cells with high nucleic acid content (HNA cells) are the active component of marine bacterioplankton assemblages, while bacteria with low nucleic acid content (LNA cells) are inactive, with a large data set (>1700 discrete samples) based on flow cytometric analysis of bacterioplankton in the Northeast Pacific Ocean off the coast of Oregon and northern California, USA. Samples were collected in the upper 150 m of the water column from the coast to 250 km offshore during 14 cruises from March 2001 to September 2003. During this period, a wide range of trophic states was encountered, from dense diatom blooms (chlorophyll-a concentrations up to 43 μg l−1) at shelf stations during upwelling season (March–September) to lower chlorophyll-a concentrations (0.1–5 μg l−1) during winter (November–February) and at basin stations (>1700 m depth). We found only weakly positive relations of log total bacterial abundance to log chlorophyll-a concentration (as a proxy for availability of organic substrate), and of HNA bacteria as a fraction of total bacteria to log chlorophyll-a. Abundance of HNA and LNA bacteria co-varied positively in all regions, although HNA bacteria were more responsive to high phytoplankton biomass in shelf waters than in slope and basin waters. Since LNA cell abundance in general showed responses similar to those of HNA cell abundance to changes in phytoplankton biomass, our data do not support the hypothesis that HNA cells are the sole active component of marine bacterioplankton. 相似文献
13.
Hendrik M. van Aken 《Journal of Sea Research》2010,63(2):143-151
Time series of observations of the sea surface temperature (SST) at 12 stations in the Dutch coastal zone are analyzed to establish whether an earlier published nearly 150 year long SST time series from the Marsdiep tidal inlet is representative for the whole Dutch coastal zone. The annual cycles (SST range and phase) as well as the long-term SST trends at decadal scales from other estuaries agree with the Marsdiep time series. An increasing SST trend since 1982 is a phenomenon of the whole Dutch coastal zone. In order to increase the understanding of the causes of the observed SST variability, a multiple linear regression model is constructed, which links locally determined seasonal meteorological and oceanographic forcing factors to the seasonal mean SST. The oceanographic forcing factor is the SST value from the preceding season, representing persistence due to thermal inertia of the sea. Season to season changes of the atmospheric circulation, connected with SST variability, are represented by seasonal mean wind components as forcing factors, e.g. the western winds in winter which bring relatively warm air masses to Western Europe. For the seasons where shortwave solar radiation is the dominant term in the local heat budget (spring and summer), the number of bright sun hours is used as forcing factor, roughly representing the effects of changing cloudiness. The annual mean SST, derived from the regression models for the four seasons, applied to 4 locations along the Dutch coast, correlates quite well, not only for the year to year variability (R = 0.88) but also for the longer-term SST trends (R = 0.95). An explicit local greenhouse effect is not required as separate forcing factor to explain the recent warming trend of Dutch coastal waters starting in the early 1980s; coincident variations in wind statistics and cloudiness are a sufficient explanation. 相似文献
14.
《Deep Sea Research Part II: Topical Studies in Oceanography》1999,46(6-7):1319-1333
The ambient noise produced by large areas of ice floes, such as the Odden and the marginal ice zone, was recorded and analysed. A model based on the collision, compression and shearing of floes driven by ocean swell was constructed. This model allows the level and variance of the ambient noise to be predicted provided the conditions of the ice and the sea state are known. The frequency spectrum of the ambient noise in the range 100 Hz to 100 kHz is explained in terms of acoustic propagation and absorption by sea water. The probability distribution of ambient noise intensity, obtained from a statistical analysis of collision frequencies, is a form of K distribution and agrees with observations. 相似文献
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16.
As concluded from physical theory and laboratory experiment,it is widely accepted that nonlinearities of sea state play an important role in the formation of rogue waves;however,the sea states and corresponding nonlinearities of real-world rogue wave events remain poorly understood.Three rogue waves were recorded by a directional buoy located in the East China Sea during Typhoon Trami in August 2013.This study used the WAVEWATCHⅢmodel to simulate the sea state conditions pertaining to when and where those rogue waves were observed,based on which a comprehensive and full-scale analysis was performed.From the perspectives of wind and wave fields,wave system tracking,High-Order Spectral method simulation,and some characteristic sea state parameters,we concluded that the rogue waves occurred in sea states dominated by second-order nonlinearities.Moreover,third-order modulational instabilities were suppressed in these events because of the developed or fully developed sea state determined by the typhoon wave system.The method adopted in this study can provide comprehensive and full-scale analysis of rogue waves in the real world.The case studied in this paper is not considered unique,and rules could be found and confirmed in relation to other typhoon sea states through the application of our proposed method. 相似文献
17.
The relationship between the spring bloom along the Primorye coast and the sea ice of the Tatarskiy Strait in the northern region of the East/Japan Sea, a semi-enclosed marginal sea in the North Pacific, was investigated using the ten-year SeaWiFS chlorophyll-a concentration data and DMSP/SSMI sea ice concentration data from 1998 to 2007. Year-to-year variations in the chlorophyll-a concentrations in the spring were positively correlated with those of the sea ice concentrations in the Tatarskiy Strait in the previous winter with a correlation coefficient of 0.77. Abrupt increases in nutrients, essential for the spring bloom in the upper ocean during spring, were supplied from sea ice-melted waters. Time series of vertical distributions of the nutrients indicated that phosphate concentrations were extremely elevated in the upper ocean (less than 100 m) without any connection to high concentrations in the deep waters below. The water mass from sea ice provided preferable conditions for the spring bloom through changes in the vertical stratification structure of the water columns. Along-coast ratios of stability parameters between two neighboring months clearly showed the rapid progression of the generation of a shallow pycnocline due to fresh water originating from sea ice. This study addressed the importance of the physical environment for biogeochemical processes in semi-enclosed marginal seas affected by local sea ice. 相似文献
18.
Laminar flow past a circular cylinder with 3 small control rods is investigated by numerical simulation. This study is concerned with the suppression efficacy of vortex induced vibration by small control rods located around a main cylinder. The effects of the attack angle and rod-to-cylinder gap ratio on the hydrodynamics and vibration responses of the main cylinder are investigated. The attack angle of α = 45° is performed as the critical angle for VIV suppression of 3 control rods. The 3 control rods have no effect on VIV suppression when the attack angle is less than the critical angle. The 3 control rods have an excellent VIV suppression efficacy when the attack angle is larger than the critical angle. The transverse vibration frequency of the cylinder with 3 control rods is less than that for an isolated cylinder for all the configurations. The numerical results for the configurations of α = 45° & 60°, G/D = 0.6–1.2 show excellent suppression efficient among the cases investigated in this study. The best suppression efficient is found at α = 45°, G/D = 0.9 for 3 control rods. 2 rods in behind of the main cylinder perform more efficient than that of 1 rod in front for VIV suppression as the gap ratio of G/D less than 1.0. 相似文献
19.
This study investigates a construction-induced sea level rise and tide characteristics change, using a regression analysis to separate the local construction effect such as sea-dike/seawalls and global warming from total sea level change. The study also makes it clear why and how the extreme high water level has risen just after constructions at Mokpo harbor in Korea. As a result of the regression analysis, it is found that the high water level rise for the period of 1960–2006 is ～60 cm, which is summation of four components: ～23 cm for Youngsan River sea-dike (1981), ～15 cm for Youngam seawall (1991), ～8 cm for Geumho seawall (1994), and ～14 cm for gradual rise (due mainly to global warming). Then, a numerical simulation at Mokpo coastal zone is performed to identify each component, and the results support the premise that the tidal amplification caused by constructions is due mainly to the extinguishment of the tidal choking effect at outer Mokpogu. The tidal flat effect makes the amplification greater at spring tide or extraordinary high tide, which would result in the increase of inundation risk at the Mokpo coastal zone. Frequency distribution of observed high water level data shows increasing trend for both maximum value of astronomical tide component (simulated high water level) and meteorological tide component (surge height) after the coastal constructions. A frequency analysis presents that the high water level for 50 year return period, which is often used for design in practice, is 474 cm before the construction, and while that is 553 cm after the construction. Furthermore, design height might steadily be elevated considering future global sea level rise. 相似文献
20.
Erik van Sebille Peter Jan van Leeuwen Arne Biastoch Wilhelmus P.M. de Ruijter 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(3):319-327
Estimating the magnitude of Agulhas leakage, the volume flux of water from the Indian to the Atlantic Ocean, is difficult because of the presence of other circulation systems in the Agulhas region. Indian Ocean water in the Atlantic Ocean is vigorously mixed and diluted in the Cape Basin. Eulerian integration methods, where the velocity field perpendicular to a section is integrated to yield a flux, have to be calibrated so that only the flux by Agulhas leakage is sampled. Two Eulerian methods for estimating the magnitude of Agulhas leakage are tested within a high-resolution two-way nested model with the goal to devise a mooring-based measurement strategy. At the GoodHope line, a section halfway through the Cape Basin, the integrated velocity perpendicular to that line is compared to the magnitude of Agulhas leakage as determined from the transport carried by numerical Lagrangian floats. In the first method, integration is limited to the flux of water warmer and more saline than specific threshold values. These threshold values are determined by maximizing the correlation with the float-determined time series. By using the threshold values, approximately half of the leakage can directly be measured. The total amount of Agulhas leakage can be estimated using a linear regression, within a 90% confidence band of 12 Sv. In the second method, a subregion of the GoodHope line is sought so that integration over that subregion yields an Eulerian flux as close to the float-determined leakage as possible. It appears that when integration is limited within the model to the upper 300 m of the water column within 900 km of the African coast the time series have the smallest root-mean-square difference. This method yields a root-mean-square error of only 5.2 Sv but the 90% confidence band of the estimate is 20 Sv. It is concluded that the optimum thermohaline threshold method leads to more accurate estimates even though the directly measured transport is a factor of two lower than the actual magnitude of Agulhas leakage in this model. 相似文献