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1.
R.T. McCall J.S.M. Van Thiel de Vries N.G. Plant A.R. Van Dongeren J.A. Roelvink D.M. Thompson A.J.H.M. Reniers 《Coastal Engineering》2010
A 2DH numerical, model which is capable of computing nearshore circulation and morphodynamics, including dune erosion, breaching and overwash, is used to simulate overwash caused by Hurricane Ivan (2004) on a barrier island. The model is forced using parametric wave and surge time series based on field data and large-scale numerical model results. The model predicted beach face and dune erosion reasonably well as well as the development of washover fans. Furthermore, the model demonstrated considerable quantitative skill (upwards of 66% of variance explained, maximum bias − 0.21 m) in hindcasting the post-storm shape and elevation of the subaerial barrier island when a sheet flow sediment transport limiter was applied. The prediction skill ranged between 0.66 and 0.77 in a series of sensitivity tests in which several hydraulic forcing parameters were varied. The sensitivity studies showed that the variations in the incident wave height and wave period affected the entire simulated island morphology while variations in the surge level gradient between the ocean and back barrier bay affected the amount of deposition on the back barrier and in the back barrier bay. The model sensitivity to the sheet flow sediment transport limiter, which served as a proxy for unknown factors controlling the resistance to erosion, was significantly greater than the sensitivity to the hydraulic forcing parameters. If no limiter was applied the simulated morphological response of the barrier island was an order of magnitude greater than the measured morphological response. 相似文献
2.
《Coastal Engineering》2005,52(7):633-645
New experimental laboratory data are presented on swash overtopping and sediment overwash on a truncated beach, approximating the conditions at the crest of a beach berm or inter-tidal ridge-runnel. The experiments provide a measure of the uprush sediment transport rate in the swash zone that is unaffected by the difficulties inherent in deploying instrumentation or sediment trapping techniques at laboratory scale. Overtopping flow volumes are compared with an analytical solution for swash flows as well as a simple numerical model, both of which are restricted to individual swash events. The analytical solution underestimates the overtopping volume by an order of magnitude while the model provides good overall agreement with the data and the reason for this difference is discussed. Modelled flow velocities are input to simple sediment transport formulae appropriate to the swash zone in order to predict the overwash sediment transport rates. Calculations performed with traditional expressions for the wave friction factor tend to underestimate the measured transport. Additional sediment transport calculations using standard total load equations are used to derive an optimum constant wave friction factor of fw = 0.024. This is in good agreement with a broad range of published field and laboratory data. However, the influence of long waves and irregular wave run-up on the overtopping and overwash remains to be assessed. The good agreement between modelled and measured sediment transport rates suggests that the model provides accurate predictions of the uprush sediment transport rates in the swash zone, which has application in predicting the growth and height of beach berms. 相似文献
3.
Prediction of coupled storm hydrodynamics and morphodynamics is essential for accurately designing coastal flood protection measures. A numerical simulation methodology was developed and implemented to evaluate and assess engineering design solutions for storm damage reduction along the south shore of Long Island, New York, USA. This simulation methodology was applied to compute bay water levels for two major coastal storms taking into account contributions from storm surge, waves, wind, and barrier island overwash and breaching. Simulation results for breaching and bay flooding compare well to historically documented barrier island morphological change and bay high water marks. 相似文献
4.
Dune erosion is shown to occur at the embayment of beach mega-cusps O(200 m alongshore) that are associated with rip currents. The beach is the narrowest at the embayment of the mega-cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune, to undercut the dune and to cause dune erosion. Field measurements of dune, beach, and rip current morphology are acquired along an 18 km shoreline in southern Monterey Bay, California. This section of the bay consists of a sandy shoreline backed by extensive dunes, rising to heights exceeding 40 m. There is a large increase in wave height going from small wave heights in the shadow of a headland, to the center of the bay where convergence of waves owing to refraction over the Monterey Bay submarine canyon results in larger wave heights. The large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale. The strongly refracted waves and narrow bay aperture result in near normal wave incidence, resulting in well-developed, persistent rip currents along the entire shoreline.
The alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95% confidence. The alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95% confidence. Therefore, it is concluded the mega-cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega-cusp. 相似文献
5.
《Coastal Engineering》2005,52(3):285-302
Modifications to a model describing swash motion based on solutions to the non-linear shallow water equations were made to account for interaction between up-rush and back-wash at the still water shoreline and within the swash zone. Inputs to the model are wave heights and arrival times at the still water shoreline. The model was tested against wave groups representing idealized vessel-generated wave trains run in a small wave tank experiment. Accounting for swash interaction markedly improved results with respect to the maximum run-up length for cases with rather gentle foreshore slopes (tanβ=0.07). For the case with a steep foreshore slope (tanβ=0.20) there was very little improvement compared to model results if swash interaction was not accounted for. In addition, an equation was developed to predict the onset and degree of swash interaction including the effects of bed friction. 相似文献
6.
This paper presents a mathematical approach and numerical model that simulates beach and dune change in response to cross-shore processes of dune growth by wind and dune erosion by storms, and by gradients in longshore sand transport that will alter shoreline position. Sub-aerial transport processes are represented, whereas sub-aqueous transport is neglected. The system is tightly coupled morphologically, with the berm playing a central role. For example, the potential for sand to be transported to the dune by wind depends on berm width, and sand lost in erosion of the dune during storms can widen the berm. Morphologic equilibrium considerations are introduced to improve reliability of predictions and stability of the non-linear model. An analytical solution is given under simplification to illustrate properties of the model. Sensitivity tests with the numerical solution of the coupled equations demonstrate model performance, with one test exploring beach and dune response to potential increase in storm-wave height with global warming. Finally, the numerical model is applied to examine the consequences of groin shortening at Westhampton Beach, Long Island, New York, as an alternative for providing a sand supply to the down-drift beach. Results indicate that the sand will be released over several decades as the shoreline and dune move landward in adjustment to the new equilibrium condition with the shortened groins. 相似文献
7.
A new model for the boundary layer development and associated skin friction coefficients and shear stress within the swash zone is presented. The model is developed within a Lagrangian reference frame, following fluid trajectories, and can be applied to both laminar flow and smooth turbulent flow. The model is based on the momentum integral approach for steady, flat-plate boundary layers, with appropriate modifications to account for the unsteady flow regime and flow history. The model results are consistent with previous measurements of bed shear stress and skin friction coefficients within the swash zone. These indicate strong temporal and spatial variation throughout the swash cycle, and a clear distinction between the uprush and backwash phase. This variation has been previously attributed the unsteady flow regime and flow history effects, both of which are accounted for in the new model. Fluid particle trajectories and velocity are computed using the non-linear shallow water wave equations and the boundary layer growth across the entire swash zone is estimated. Predictions of the bed shear stress and skin friction coefficients agree reasonably well with direct bed shear stress measurements reported by Barnes et al. (Barnes, M.P., O’Donaghue, T., Alsina, J.M., Baldock, T.E., 2009. Direct bed shear stress measurements in bore-driven swash. Coastal Engineering 56 (8), 853–867) and, for a given flow velocity, give stresses which are consistent with the bias toward uprush sediment transport which has consistently been observed in measurements. The data and modelling suggest that the backwash boundary layer is initially laminar, which results in the late development of significant bed shear during the backwash, with a transition to a turbulent boundary layer later in the backwash. A new conceptual model for the boundary layer structure at the leading edge of the swash is proposed, which accounts for both the no-slip condition at the bed and the moving wet–dry interface. However, further development of the Lagrangian Boundary Layer Model is required in order to include bore-generated turbulence and to account for variable roughness and mobile beds. 相似文献
8.
Numerical cross-shore profile evolution models have been good at predicting beach erosion during storm conditions, but have difficulty in predicting the accretion of the beach during calm periods. This paper describes the progress made in modifying and applying the public domain XBeach code to the prediction and explanation of the observed behaviour of coarse-grained beaches in the laboratory and the field under accretive conditions. The paper outlines in details the changes made to the original code (version 12), including the introduction of a new morphological module based upon Soulsby's sediment transport equation for waves and currents, and the incorporation of Packwood's infiltration approach in the unsaturated area of the swash region. The competence of this modified model during calm conditions for describing the steepening of the profile, and the growth of the beach berm is demonstrated. Preliminary results on the behaviour of the beach subject to both waves and tides are presented. Good agreement is found between the model simulations and large-scale laboratory measurements, as well as field observations from a composite beach in the UK. The reasons for the model's capabilities are discussed. 相似文献
9.
Hurricane- or storm-generated swell waves may cause erosion and deposition along coasts which are situated thousands of kilometers outside the generating wind field. Marked beach erosion, caused by such swell waves, was observed along the micro-tidal west coast of Aruba. During the process of erosion a swash bar was formed, which moved up-beach during the waxing part of the swell event. The swash bar welded to the beach during the waning part of the event. Rapid sedimentation occurred on the upper beach. Finally, recovery of the beach was observed. The formation of a swash bar was attributed to an erosive, dissipative interval of a normally accretionary reflective beach. The sedimentary structures, although generally in line with observations on other beaches, show several peculiar characteristics: (1) the great thickness of the laminae in these calcareous sands; (2) the succession of low-angle sigmoidal and tangential sets in the swash bar; (3) the relatively steep erosional lower set boundaries and the wedge-shaped lamination in the successive stages of beach recovery; and (4) the several types of deformation structures. 相似文献
10.
The influence of the seaward boundary condition on the internal swash hydrodynamics is investigated. New numerical solutions of the characteristics form of the nonlinear shallow-water equations are presented and applied to describe the swash hydrodynamics forced by breaking wave run-up on a plane beach. The solutions depend on the specification of characteristic variables on the seaward boundary of the swash zone, equivalent to prescribing the flow depth or the flow velocity. It is shown that the analytical solution of Shen and Meyer [Shen, M.C., Meyer, R.E., 1963. Climb of a bore on a beach. Part 3. Runup. J. Fluid Mech. 16, 113–125] is a special case of the many possible solutions that can describe the swash flow, but one that does not appear appropriate for practical application for real waves. The physical significance of the boundary conditions is shown by writing the volume and momentum fluxes in terms of the characteristic variables. Results are presented that illustrate the dependence of internal flow depth and velocity on the boundary condition. This implies that the internal swash hydrodynamics depend on the shape and wavelength of the incident bore, which differs from the hydrodynamic similarity inherent in the analytical solution. A solution appropriate for long bores is compared to laboratory data to illustrate the difference from the analytical solution. The results are important in terms of determining overwash flows, flow forces and sediment dynamics in the run-up zone. 相似文献
11.
The present work analyzes the hydro-morphodynamics characterizing the swash region during the uprush stage. A comparison is illustrated between the sediment transport measured in a series of dam-break experiments and that predicted by the numerical hydro-morphodynamic model of Postacchini et al. (2012). The primary aim is to investigate the differences arising between the weakly coupled or uncoupled model and the measurements, in terms of hydrodynamics, tip celerity and sediment transport. The hydrodynamics are well described by the model and results have been used to calibrate both friction factor and subgrid turbulent viscosity. Comparison of numerically-computed tip celerity with experimental data reveals a fairly good agreement, i.e. a mean error of about 10%, while modeled sediment transport differs by about 40% from the available data. No evident differences are found between results obtained from the coupled and uncoupled model runs (2% for the celerity and 11% for the sediment transport rate at the tip), suggesting that for the specific flow under investigation, at the leading edge of the swash front, hydro-morphological coupling is not an issue of fundamental importance. However, for the special case here of a swash forced by a dam-break, scour occurs at the dam location, and in this case the erosion of the bed is significantly larger in the uncoupled model. 相似文献
12.
Modeling sea dike breaching induced by breaking wave impact-laboratory experiments and computational model 总被引：1，自引：0，他引：1
Sea dikes are of crucial importance for the defense systems of low-lying coastal areas in countries such as Germany, The Netherlands, Denmark, etc. As sea dike breaching induced by storm surges is regarded as one of the main causes of coastal flood disasters, reliable predictions of both the breach initiation, formation and breach development is urgently needed. Although a simple preliminary model for the simulation of sea dike breaching initiated from the seaside was developed (Stanczak et al., 2006), its limitations showed the necessity for further research and for the development of a more detailed model. This paper therefore provides a summary of the results of laboratory experiments on the erosion of grass and clay revetment of a sea dike induced by breaking wave impact and describes the new detailed computational model, including an uncertainty analysis. 相似文献
13.
The morphological changes of multiple intertidal bars (ridges) on a macrotidal beach were examined under low-energy wave conditions during a spring-to-spring tidal cycle. The morphological response was coupled to the tidal water level variations and related residence times for swash processes and surf (breaking waves and bores) over the cross-shore profile. Spring tides induced a large spatial variation in water lines and small residence times for distinct processes. Neap tides narrowed the intertidal area and increased the time for certain processes to work on the sediment at one location. The observed morphological changes could be coupled to the stagnation of processes at a certain bar crest position. The action of surf (breaking waves and bores) played the major role in the onshore migration of the intertidal bars and the simultaneous erosion of the seaward flank. Swash action, responsible for the generation and migration of intertidal bars in microtidal settings, was not the dominant process in causing the observed morphological changes. Intertidal ridges on macrotidal beaches cannot be considered swash bars as suggested by most previous investigations into these morphological features. 相似文献
14.
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
15.
《Coastal Engineering》2001,43(1):25-40
Video-based swash motions from three studies (on two separate beaches) were analyzed with respect to theoretical swash trajectories assuming plane beach ballistic motions under quadratic friction. Friction coefficient values for both the uprush and backwash were estimated by comparing measured swash space–time trajectories to these theoretical expectations given an initial velocity and beach slope. Observations were made spanning high tides, and in one case, during a light rain. Analysis of over 4500 individual swash events showed that the uprush friction coefficient was nearly constant during all three studies with a mean value of roughly 0.007 and showed no trends over a tidal cycle. In contrast, backwash friction coefficient values varied over the tidal cycles ranging between 0.01 and 0.07 with minimum values corresponding to the highest tides. Although these values are close to the theoretical estimates based on a Law of the Wall formulation and values commonly referenced in the literature, these observations show a consistent tendency for backwash friction estimates to greatly exceed uprush friction estimates. The disparity between uprush and backwash friction coefficients can be partially attributed to the exclusion of a pressure gradient term in the ballistic model. However, results indicate that backwash friction coefficients adjusted to account for this effect may be three times larger than the uprush friction values during lower tides. This tidal dependence for backwash friction coefficients is attributed to a complex interaction between swash infiltration and entrained sediment loads. These findings imply that friction estimates (necessary for sediment transport calculations and hydrodynamic predictions) based solely on grain roughness may not be correct for backwash flows. 相似文献
16.
利用涌浪影响下短时段内的冲流带滩面高频高程数据和碎波带波流资料,在奇异谱分析(SSA)的基础上,以比研究了不同形态滩面的冲淤变化趋势、趋势分布形状、冲淤变化周期和冲淤变化强度,以及同一条剖面不同桩点间各因素间的变化关系;用交叉谱方法探索了每分钟滩面高频冲淤变化与碎波带长重力波间的作用关系.分析结果表明,滩角韵律地形引起的冲流分流作用促进了滩脊向滩谷的泥沙转运,冲流带滩面存在明显的长重力波频段的周期性冲淤振动,滩面冲淤振动强度由滩面下部向上部递减,碎波带长重力波对滩面高频冲淤变化起重要作用. 相似文献
17.
This paper presents results of experimental and mathematical modelling of beach and dune erosion under storm events. Re-analysis of the experimental results on dune erosion in small-scale and large-scale flumes shows that the dune erosion for extreme conditions is somewhat smaller than that based on earlier analysis results. 相似文献
18.
The flow structure of a swash event over a uniform slope is studied using a RANS-VOF numerical model coupled with a v2–f turbulence closure. The model is compared with experimental data of recent laboratory experiments. The ability of the turbulence modelling for simulating swash flow and the evolution of the computed bed shear stress during run-up and run-down are investigated. The agreement between numerical results and measured data, such as water depth, depth-averaged velocity and bed shear stress is very good during run-up. Main discrepancies are found during run-down. The paper also examines the aeration of the water layer in the swash flow, taking advantage of the PLIC method for computing the air–water interfaces. Air is continuously entrapped in the swash front and released at its rear during run-up. A detailed analysis indicates that the flow reversal is initiated near the bottom at the outer boundary of the swash zone and progresses landward. The study highlights the asymmetry between run-up and run-down. During run-up, the swash front propagation determines the turbulence properties and the bed shear stress profile on the beach, whereas the flow properties are more homogeneously distributed in the swash area during run-down. 相似文献
19.
Confirmation of beach accretion by grain-size trend analysis: Camposoto beach, Cádiz, SW Spain 总被引：4，自引：4，他引：0
An application of the grain size trend analysis (GSTA) is used in an exploratory approach to characterize sediment transport on Camposoto beach (Cádiz, SW Spain). In May 2009 the mesotidal beach showed a well-developed swash bar on the upper foreshore, which was associated with fair-weather conditions prevailing just before and during the field survey. The results were tested by means of an autocorrelation statistical test (index I of Moran). Two sedimentological trends were recognized, i.e. development towards finer, better sorted and more negatively skewed sediment (FB–), and towards finer, better sorted and less negatively or more positively skewed sediment (FB+). Both vector fields were compared with results obtained from more classical approaches (sand tracers, microtopography and current measurements). This revealed that both trends can be considered as realistic, the FB+ trend being identified for the first time in a beach environment. The data demonstrate that, on the well-developed swash bar, sediment transported onshore becomes both finer and better sorted towards the coast. On the lower foreshore, which exhibits a steeper slope produced by breaking waves, the higher-energy processes winnow out finer particles and thereby produce negatively skewed grain-size distributions. The upper foreshore, which has a flatter and smoother slope, is controlled by lower-energy swash-backwash and overwash processes. As a result, the skewness of the grain-size distributions evolves towards less negative or more positive values. The skewness parameter appears to be distributed as a function of the beach slope and, thus, reflects variations in hydrodynamic energy. This has novel implications for coastal management. Figure
GSTA model for Camposoto Beach, Cádiz, with FB– (finer, better sorted, more negatively skewed) and FB+ (finer, better sorted, less negatively/more positively skewed) textural trends dominating the lower and upper foreshore respectively 相似文献
20.
A computational procedure is developed for predicting the time-dependent, two-dimensional beach and dune erosion during severe storms due to elevated water levels and waves. The model employs the equation of sediment continuity and a dynamic equation governing the cross-shore sediment transport due to a disequilibrium of wave energy dissipation levels. These equations are solved numerically by an implicit, double-sweep procedure to determine the change in position of elevation contours in the profile. Given sufficient time, the profile will evolve to a form where the depth, h, in the surf zone is related to the distance seaward of the waterline by the relationship: , which is consistent with many natural profiles and in which A depends on sediment characteristics.The model is verified qualitatively and quantitatively through application to several idealized cases and through a preliminary simulation of erosion during Hurricane Eloise. In general, the time scales for shoreline response were found to be quite long relative to natural storm systems and erosion in the early response stages was found to be sensitive to storm surge height, but much less sensitive to wave height. The model response characteristics for simulation of erosion due to time-varying storm conditions show a lag between the maximum storm surge elevation and maximum erosion with the maximum erosion rate occurring at the time of the peak surge. For the simulated erosion due to Hurricane Eloise, reasonable agreement was found between the post-hurricane dune profiles and those calculated. However, the eroded volumes were in better agreement than the profile forms as the steepening of the natural dune profiles was not reproduced in the model. 相似文献