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1.
《Coastal Engineering》2005,52(6):473-495
Seadikes often fail due to wave overtopping and a failure of the landward slope. Therefore, these aspects have to be taken into account for the design of seadikes. In present design, the calculation of the crest height of seadikes is essentially based on using a design water level and the corresponding wave run-up height. An average overtopping rate is generally considered for wave overtopping which can not account for the stresses and other effects due to extreme individual overtopping events. Landward slope design is more or less based on experience. It can be concluded from failure analysis that dike failures on the landward slope are rather initiated by individual overtopping events, in particular by the related overtopping flow velocities and layer thicknesses which are relevant for the prediction of erosion, infiltration and slip failure. Therefore, overtopping flow velocities and layer thicknesses are required in addition to average overtopping rates as hydraulic boundary conditions for the geotechnical stability analysis of seadikes.The objective of the present paper is the theoretical and experimental determination of overtopping flow velocities and layer thicknesses on the seaward slope, the dike crest and the landward slope of a seadike. Overtopping parameters are derived on the basis of small scale model tests which are required for the design of the landward slope and to avoid dike failures by wave overtopping in the future. For the prediction of the layer thicknesses and the velocities of the overtopping flow on the seaward slope, the dike crest and the landward slope, a set of theoretical formulas is derived and validated by hydraulic scale model tests.  相似文献   

2.
Dike resilience against wave overtopping has gained more and more attention in recent years due to the effect of expected future climate changes. The overtopping flow velocities and flow depths on dikes have recently been studied in 2D small-scale experiments. This has led to semi-empirical formulae for the estimation of flow depths and flow velocities across a dike. The results have been coupled to the actual erosion of the landward dike slope determined by full-scale 2D tests using the so-called “Overtopping Simulator”. This paper describes the results from 96 small-scale tests carried out in a shallow water basin at Aalborg University to cover the so far unknown 3D effects from oblique long-crested and short-crested waves. Based on results from the laboratory tests, expansions are proposed to the existing 2D formulae so as to cover oblique and short-crested waves. The wave obliquity is seen to significantly reduce the overtopping flow velocities and flow depths on especially the landward slope of a sea dike. Moreover, the tests showed that the average flow directions on the dike crest from oblique long-crested and short-crested waves correspond approximately to the incident wave direction. Flow depths and the squared flow velocities on the dike are concluded to be Rayleigh-distributed in case of both long-crested and short-crested waves for all considered incident wave obliquities. Findings in the present paper are needed to obtain more realistic estimates of dike erosion caused by wave overtopping.  相似文献   

3.
Spatial distribution of wave overtopping water behind coastal structures   总被引：1，自引：0，他引：1  
Spatial distribution of random wave overtopping water behind coastal structures was investigated using a numerical model based on Reynolds-Averaged Navier-Stokes solver (RANS) and Volume of Fluid (VOF) surface capturing scheme (RANS-VOF). The computed spatial distributions of wave overtopping water behind the structure agree well with the measurements by Pullen et al (2008) for a vertical wall and Lykke Andersen and Burcharth (2006) for a 1:2 sea dike. A semi-analytical model was derived to relate spatial distribution of wave overtopping water behind coastal structures to landward ground level, velocity and layer thickness on the crest. This semi-analytical model agrees reasonably well with both numerical model results and measurements close to coastal structures. Our numerical model results suggest that the proportion of wave overtopping water passing a landward location increases with a seaward slope when it is less than 1:3 and decreases with a seaward slope when it gets steeper. The proportion of wave overtopping water passing a landward location increases with landward ground level and overtopping discharge. It also increases with the product of incident wave height and wavelength, but decreases with increasing relative structure freeboard and crest width. We also found that the extent of hazard area due to wave overtopping is significantly reduced by using a permeable structure crown. Findings in this study will enable engineers to establish the extent of hazard zones due to wave overtopping behind coastal structures.  相似文献   

4.
The Nampo dike, which is located at the west coast of Korea, was destroyed by wave overtopping during the storms on 30 August and 17 September in 1959. In this paper, is performed the probabilistic assessment of wave overtopping of Nampo dike by use of Owen model, Van der Meer ＆ Janssen model and Hedges ＆ Reis model for wave overtopping of seawall. Based on the available tidal and wave data for storm surges in 1989, the risk assessment of wave overtopping of the Nampo dike has been carried out by both Level Ⅱ and Level Ⅲ reliability methods. The calculated resuhs show the general agreement of failure probability between the two methods. By utilizing the rehabilitated cross section of Nampo dike, the failure probability of wave overtopping for the Nampo dike after rehabilitation will be rapidly reduced to that of initial design at crest level of 9.0 m with the improved slope from 1 ： 2 to 1 ： 4 at seaside. Since the sea level may only rise 1.0 m in the next few decades, the failure probability of Nampo dike will be still in the safe range.  相似文献   

5.
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   

6.
Risk analysis and assessment relating coastal structures has been one of the hot topics in the area of coastal protection recently. In this paper, from three aspects of joint return period of multiple loads, dike failure rate and dike continuous risk prevention respectively, three new risk analysis methods concerning overtopping of sea dikes are developed. It is worth noting that the factors of storm surge which leads to overtopping are also considered in the three methods. In order to verify and estimate the effectiveness and reliability of the newly developed methods, quantified mutual information is adopted. By means of case testing, it can be found that different prior variables might be selected dividedly, according to the requirement of special engineering application or the dominance of loads. Based on the selection of prior variables, the correlating risk analysis method can be successfully applied to practical engineering.  相似文献   

7.
Based on the filtered Navier-Stokes equations and Smagorinsky turbulence model,a numerical wave flume is developed to investigate the overtopping process of irregular waves over smooth sea dikes.Simulations of fully nonlinear standing wave and regular wave’s run-up on a sea dike are carried out to validate the implementation of the numerical wave flume with wave generation and absorbing modules.To model stationary ergodic stochastic processes,several cases with different random seeds are computed for each specified irregular wave spectrum.It turns out that the statistical mean overtopping discharge shows good agreement with empirical formulas,other numerical results and experimental data.  相似文献   

8.
Samples collected in the deep Nazaré Canyon and at the adjacent slope, during the HERMES RRS Discovery D297 cruise (2005), were analysed for metazoan meiofauna, nematode structure and diversity and its relation to quality and quantity of sedimentary organic material. The amount and quality of organic matter available for direct consumption was much higher in the canyon compared to the slope and positively correlated with high nematode abundances (795–1171 ind. 10 cm?2) and biomass (93.2–343.5 μg dry weight 10 cm?2), thus leading to higher standing stocks. Canyon nematode assemblages also showed particular adaptations (e.g. higher trophic complexity, variability of nematode morphology, and presence of opportunistic genera) to canyon conditions, particularly in the deeper sediment layers. The Nazaré Canyon's nematode diversity was slightly lower than that of the adjacent slope and its assemblages were characterised by a higher dominance of certain genera. Still, the canyon contributes considerably to total Western Iberian Margin diversity due to different assemblages present compared to the slope. Furthermore, the harsh conditions in terms of hydrodynamic disturbance and the high organic matter flux are likely to have a negative impact on the establishment of species rich meiobenthic communities, especially in the canyon axis.  相似文献   

9.
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10.
This paper presents a methodological approach to calculate runup from the analysis of morphodynamic conditions on a macrotidal sandy beach. The method is based on measurements of the elevation of high-tide deposits and on the analysis of morphological and hydrodynamic changes. A series of measurements has been carried out on the beach of Vougot (Brittany, France) under different wave conditions. This allowed to assess runup formula effectiveness on a macrotidal sandy beach and to determine the best slope parameters to estimate runup. The results suggest that on that macrotidal sandy beach the slope of the active section of the upper beach should be used instead of the entire slope of the foreshore, the latter resulting in an underestimation of runup elevations when used in predictive equations from the literature. Results obtained with widely used equations are relatively well correlated with observed values (r2 = 0.63). An analysis of the relationship between observed runup elevations and various variables has enabled the establishment of a runup estimation formula with a relatively good fit to the study site (r2 = 0.86).  相似文献   

11.
We examined bacterioplankton biomass and heterotrophic production (BHP) during summer stratification in the northwestern Mediterranean in four successive stratification seasons (June–July of 1993–1996). Values of phytoplankton biomass and primary production were determined simultaneously so that the data sets for autotrophic and heterotrophic microbial plankton could be compared. Three standard stations were set along a transect from Barcelona to the channel between Mallorca and Menorca, representing coastally influenced shelf waters, frontal waters over the slope front, and open sea waters. Conversion factors from 3H-leucine incorporation to BHP were empirically determined and varied between 0.29 and 3.25 kg C mol-1. Bacterial biomass values were among the lowest found in any marine environment. BHP values (between 0.02 and 2.5 μg C L-1 d-1) were larger than those of low nutrient low chlorophyll areas such as the Sargasso Sea and lower than those from high nutrient low chlorophyll areas such as the equatorial Pacific. Growth rates of bacterioplankton were highest at the slope front (0.20 d-1) and lowest at the open sea station (0.04 d-1). Phytoplankton growth rates were similar at the three stations (∼0.50 d-1). Integrated values of bacterioplankton biomass, BHP and bacterial growth rates did not show significant differences among years, but differences between the three stations were clearly significant. Phytoplankton biomass, primary production, and phytoplankton growth rates did not show significant differences either with year or with station. As a consequence the bacterioplankton to phytoplankton biomass (BB/BPHY) and production (BHP/PP) ratios varied from the coastal to the open sea stations. The BB/BPHY ratio was 0.98 at the coast and ∼0.70 at the other two stations. These ratios are similar to those found in other oligotrophic marine environments. The BHP/PP ratio was 0.83 at the coast, 0.36 at the slope and 0.09 at the open sea station. The last value is also similar to values found in other oligotrophic marine environments. Vertical distribution of these ratios was also examined.The comparison of microbial parameters at the three stations indicates a different kind of relationship between bacterioplankton and phytoplankton in oligotrophic open sea waters and in coastal, nutrient-richer waters. According to such parameters and to the values of the BB/BPHY and BHP/PP ratios, open waters in the northwestern Mediterranean (despite their relatively short distance from the shore) were intermediate between the extremely oligotrophic waters of the eastern Mediterranean or the Sargasso Sea and the more productive waters of the equatorial Pacific.  相似文献   

12.
Under the numerical modelling work package of the EU funded CLASH project, the time accurate, free surface capturing, incompressible Navier–Stokes solver AMAZON-SC has been applied to study impulsive wave overtopping at Samphire Hoe, near Dover in the United Kingdom. The simulations show that the overtopping process on this vertical, sheet pile, seawall is dominated by impulsive, aerated, near vertical overtopping jets. In order to perform the simulations AMAZON-SC has been extended to incorporate an isotropic porosity model and for validation purposes the solver has been applied to study overtopping of a low crested sea dike and a 10:1 battered wall. The results obtained for the battered wall and Samphire Hoe tests are in good agreement both with predicted overtopping discharges calculated using the UK overtopping manual and with available experimental results.  相似文献   

13.
Windblown sand control in coastal areas of China began in the 1950s, but was restricted to coastal afforestation 500 to 1000 m landward from the storm-tide line (the maximum height reached by waves during a powerful storm); there were no studies of the supratidal and intertidal areas, where mobile sands create windblown sand damage problems. We therefore studied the formation conditions, characteristics, and damage mechanisms for aeolian sand activity in these areas. Based on the results, we propose suitable sand control measures that combine blocking of the moving sand with stabilization, initially using engineering measures then concluding with biological measures. Control measures include establishing dikes in the intertidal area, erecting nylon nets, and planting Cynodon dactylon behind the dikes on gentle sandy slopes above the high-tide line and below the storm-tide line; spraying sand stabilizer on steep slopes beside moving sand and erecting sand fences on both sides; and spreading mulch nets and mixing clay into the surface, thereby creating a comprehensive protective system that functions as a breakwater, and blocks and stabilizes sand. This protected installations along the beach and improved the ecological environment, leading to the establishment of mangrove seedlings that also stabilized beach areas. These results fill a large gap in the research on controlling windblown sand damage in China's coastal dune areas, and will have significant implications for controlling such areas in other climatic areas.  相似文献   

14.
Long-term (⩽1-year) records obtained by seabed observatories (BOBO) and repeated (24-h) CTD casts show the presence of a highly energetic environment in and around two cold-water carbonate-mound provinces, on the Southwest and Southeast Rockall Trough (SW and SE RT) margin. Carbonate mounds, covered with a thriving coral cover, are embedded mainly in the Eastern North Atlantic Water (ENAW) and are observed in a confined bathymetric zone between 600 and 1000 m water depth. Cold-water corals seem to be restricted in their growth by temperature and food availability. The presence of living corals on top of the carbonate mounds appears linked to the presence of internal waves and tidal currents in the water column, and consequently carbonate mound structures are shaped by the local hydrodynamic regime. Mound clusters have an elongated shape perpendicular to the regional contours and corresponding to the direction of the highest current speeds. On the SW RT margin temperature, salinity and current speed reflect a diurnal tidal pattern, causing maximum temperature variations at 900 m depth of more than 3 °C. Current speeds up to 45 cm s−1 occur, and a residual current of 10 cm s−1 is directed along the slope to the southwest. At the SE RT margin the temperature of the bottom water fluctuates more than 1 °C with a semi-diurnal tidal cyclicity. Amplitudes of average and peak current speeds here are comparable with those measured on the southwest margin, but the residual current in this area is directed to the northeast. Tidal currents and internal waves at both margins force the formation of intermediate and bottom nepheloid layers and bring fresh food particles with increased velocity to the mounds. The distribution of corals in both mound areas is considered directly related to the presence of enhanced turbidity. An increase in temperature can be directly related to an increase in the amount of particles in the water column. Current velocity increases when a transition occurs from cold to warm waters. High current velocities prevent local sedimentation but provide sufficient food particles to the corals, so that the corals thrive at the mound summits.  相似文献   

15.
Investigation of the bottom slope effects on the nonlinear transformation of irregular waves, which are generated based on JONSWAP spectra, is carried out in a physical wave flume with three slopes (β = 1/15, 1/30, 1/45). The slope effects on the estimation of representative wave height are examined first. To obtain a better estimation of wave height, the slope effect should be considered when slope is larger than 1/30. The nonlinear parameters (bicoherence, skewness and asymmetry) are estimated by using the wavelet-based bispectrum, and the empirical formulae regarding these nonlinear parameters as a function of the local Ursell number are derived based on the present data measured on each slope. The results indicate that the slopes have a negligible effect on the variations of the skewness. The fitted coefficients of the formulae for the other parameters on slope β = 1/15 are clearly different from the results on the slopes β = 1/30 and 1/45, indicating that slope influence on the parameterization cannot be ignored when β > 1/30. Hence, new formulae considering the slope effect are presented. Furthermore, the empirical formulae for the data in surf zone are recommended.  相似文献   

16.
A novel autonomous free-fall lander vehicle, with a capability down to 6000 m, was deployed off Cape Verde for studies on bioluminescence in the deep sea. The system was equipped with a high-sensitivity Intensified Silicon Intensified Target (ISIT) video camera, a programmable control-recording unit and an acoustic current meter with depth and temperature sensors. The ISIT lander was used in three modes: (1) free falling at 34 m min−1, with the camera looking downwards at a mesh screen, recording impacts of luminescent organisms to obtain a vertical profile down to the abyssal sea floor, sampling at >100 l s−1; (2) rotating, with the lander on the sea floor and the camera orienting to the bottom current using a servo-controlled turntable, impacts of luminescent organisms carried by the bottom current onto a mesh screen mounted 0.5 m in front of the camera were recorded to estimate abundance in the benthic boundary layer; (3) baited, with the camera focused on a bait placed on the sea floor.Profiles recorded abundance of luminescent organisms as 26.7 m−3 at 500–999 m depth, decreasing to 1.6 m−3 at 2000–2499 m and 0.5 m−3 between 2500 m and the sea floor at 4046 m, with no further detectable significant change with depth. Rotator measurements at a 0.5 m height above the sea floor gave a mean abundance of 0.47 m−3 in the benthic boundary layer at 4046 m and of 2.04 m−3 at 3200 m. Thirty five minutes after the bait was placed on the sea floor at 3200 m, bioluminescent fauna apparently arrived at the bait and produced luminescent displays at a rate of 2 min−1. Moving, flashing light sources were observed and luminescent material was released into the bottom current.  相似文献   

17.
An autonomous upwardly-moving microstructure profiler was used to collect measurements of the rate of dissipation of turbulent kinetic energy (ε) in the tropical Indian Ocean during a single diurnal cycle, from about 50 m depth to the sea surface. This dataset is one of only a few to resolve upper ocean ε over a diurnal cycle from below the active mixing layer up to the air–sea interface. Wind speed was weak with an average value of ~5 m s−1 and the wave field was swell-dominated. Within the wind and wave affected surface layer (WWSL), ε values were on the order of 10−7–10−6 W kg−1 at a depth of 0.75 m and when averaged, were almost a factor of two above classical law of the wall theory, possibly indicative of an additional source of energy from the wave field. Below this depth, ε values were closer to wall layer scaling, suggesting that the work of the Reynolds stress on the wind-induced vertical shear was the major source of turbulence within this layer. No evidence of persistent elevated near-surface ε characteristic of wave-breaking conditions was found. Profiles collected during night-time displayed relatively constant ε values at depths between the WWSL and the base of the mixing layer, characteristic of mixing by convective overturning. Within the remnant layer, depth-averaged values of ε started decaying exponentially with an e-folding time of 47 min, about 30 min after the reversal of the total surface net heat flux from oceanic loss to gain.  相似文献   

18.
This paper describes the development of a numerical model for wave overtopping on seadikes. The model is based on the flux-conservative form of the nonlinear shallow water equations (NLSW) solved with a high order total variation diminishing (TVD), Roe-type scheme. The goal is to reliably predict the hydrodynamics of wave overtopping on the dike crest and along the inner slope, necessary for the breach modelling of seadikes. Besides the mean overtopping rate, the capability of simulating individual overtopping events is also required. It is shown theoretically that the effect of wave breaking through the drastic motion of surface rollers in the surfzone is not sufficiently described by the conventional nonlinear shallow water equations, neglecting wave setup from the mean water level and thus markedly reducing the model predictive capacity for wave overtopping. This is significantly improved by including an additional source term associated with the roller energy dissipation in the depth-averaged momentum equation. The developed model has been validated against four existing laboratory datasets of wave overtopping on dikes. The first two sets are to validate the roller term performance in improving the model prediction of wave overtopping of breaking waves. The last two sets are to test the model performance under more complex but realistic hydraulic and slope geometric conditions. The results confirm the merit of the supplemented roller term and also demonstrate that the model is robust and reliable for the prediction of wave overtopping on seadikes.  相似文献   

19.
To examine the relationship between near-bottom larval surfclam concentrations and surfclam settlement at an inner continental shelf site off New Jersey (USA), four consecutive sets of settlement experiments were carried out at three stations at the Long-term Ecosystem Observatory (LEO-15) from 14 to 31 July 1997 during upwelling and downwelling. Two inshore stations were on the landward and seaward sides of Beach Haven Ridge at ∼12 m depth, and a third station was 8 km further offshore at ∼20 m depth. In each experiment, four replicate trays of azoic sand from Beach Haven Ridge were placed flush with the seafloor and exposed for 3–7 days. Larval surfclam concentrations were measured every 4 h at 1 m above the bottom (mab) using Moored, Automated, Serial Zooplankton Pumps at the three stations. At all three stations, larval surfclam concentrations (1 mab) were low during upwelling, and higher during and after downwelling. Pulses of highest larval surfclam concentrations coincided with the initial arrival of downwelled warm water. In addition, larval surfclam concentrations were higher at the two inshore stations than at the offshore station. Larval surfclam settlement in the trays was higher during and following downwelling than during upwelling at one inshore station and at the offshore station. At the other inshore station (landward of Beach Haven Ridge), surfclam settlement did not increase during and following downwelling. Overall, surfclam settlement was higher inshore than offshore. The results indicate that spatial and temporal variation in larval surfclam supply was controlled by upwelling and downwelling circulation and that surfclam settlement was influenced by larval supply. Bottom flows across Beach Haven Ridge during a storm may have reduced larval surfclam settlement on the upcurrent side of the ridge, affecting initial densities on a small (∼1 km) scale.  相似文献   

20.
Data from seven moorings deployed across the East Greenland shelfbreak and slope 280 km downstream of Denmark Strait are used to investigate the characteristics and dynamics of Denmark Strait Overflow Water (DSOW) cyclones. On average, a cyclone passes the mooring array every other day near the 900 m isobath, dominating the variability of the boundary current system. There is considerable variation in both the frequency and location of the cyclones on the slope, but no apparent seasonality. Using the year-long data set from September 2007 to October 2008, we construct a composite DSOW cyclone that reveals the average scales of the features. The composite cyclone consists of a lens of dense overflow water on the bottom, up to 300 m thick, with cyclonic flow above the lens. The azimuthal flow is intensified in the middle and upper part of the water column and has the shape of a Gaussian eddy with a peak depth-mean speed of 0.22 m/s at a radius of 7.8 km. The lens is advected by the mean flow of 0.27 m/s and self propagates at 0.45 m/s, consistent with the topographic Rossby wave speed and the Nof speed. The total translation velocity along the East Greenland slope is 0.72 m/s. The self-propagation speed exceeds the cyclonic swirl speed, indicating that the azimuthal flow cannot kinematically trap fluid in the water column above the lens. This implies that the dense water anomaly and the cyclonic swirl velocity are dynamically linked, in line with previous theory. Satellite sea surface temperature (SST) data are investigated to study the surface expression of the cyclones. Disturbances to the SST field are found to propagate less quickly than the in situ DSOW cyclones, raising the possibility that the propagation of the SST signatures is not directly associated with the cyclones.  相似文献   

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