The impact of various methods of wave transfers from deep water to nearshore when determining extreme beach erosion

Several levels of increasing complexity of transferring wave information from offshore to nearshore have been studied to quantify their influence on extreme beach erosion estimates. Beach profiles which have been monitored since 1976 were used to estimate extreme beach erosion and compared to predictions. Examination of the wave propagation assumptions revolves around two types of offshore to nearshore transfer: excluding or including wave breaking and bottom friction. A second complication is whether still water level variations (ocean tide plus storm surge) are included.The inclusion of various combinations of wave propagation processes other than shoaling and refraction in the wave transfer function changes on the extreme erosion distribution tail through lowering estimates above one year return period. This brings the predicted tails closer to the observations, but does not capture the upper limit of storm demand implied by the extensive beach profile data set. Including wave breaking has a marked effect on probabilistic estimates of beach erosion. The inclusion of bottom friction is less significant. The inclusion of still water level variability in the wave transfer calculation had minimal impact on results for the case study site, where waves were transferred from offshore to water at 20 m depth. These changes were put into perspective by comparing them to changes resulting from limiting beach erosion by adjusting the statistical distributions of peak wave height and storm duration to have maximum limits. We conclude that the proposed improvements on wave transformation methods are as significant as limiting wave erosion potential and worth including.     

江苏及邻近海域深水波浪与增水联合概率分析

采用Gumbel Logistic模型对江苏及邻近海域深水极值波浪与增水进行了联合概率分析。首先对联合概率模型参数进行拟合,得到联合超越概率与经验联合超越概率比较图。在此基础上开展有效波高和增水年极值序列联合概率分析,比较了三种经验有效波高与增水重现期组合事件的联合概率,结果表明在选定的13个代表点位处,有效波高在二者的联合概率分布中处于主导地位,采用100年一遇有效波高与10年一遇增水组合事件作为100年一遇联合事件偏保守;而50年一遇有效波高与50年一遇增水以及10年一遇有效波高与100年一遇增水组合事件的联合概率则随着点位的移动而高于或低于100年一遇的水平。     

A relationship to describe the cumulative impact of storm clusters on beach erosion

Estimation of erosion volumes for adequate dry beach buffer zones is commonly estimated on the basis of a single extreme event, such as the 1 in 100 year storm. However, the cumulative impact of several smaller, closely spaced storms can lead to equal, if not more, dry beach loss, but this is often not quantified. Here we use a calibrated model for dune erosion, XBeach, to hindcast the cumulative erosion impact of a series of historical storms that impacted the Gold Coast, Queensland region in 1967. Over a 6-month period, four named cyclones (Dinah, Barbara, Elaine, and Glenda) and three East Coast Lows caused a cumulative erosion volume greater than the predicted 1 in 100 year event. Results presented here show that XBeach was capable of reproducing the measured dry beach erosion volume to within 21% and shoreline retreat to within 10%. The storms were then run in 17 different sequences to determine if sequencing influenced final modeled erosion volumes. It is shown that storm sequencing did not significantly affect the total eroded volumes. However, individual storm volumes were influenced by the antecedent state of the beach (i.e. prior cumulative erosion). Power-law relationships between cumulative energy density (∑ E) and eroded volume (∆V) as well as cumulative wave power ((∑ P)) and eroded volume (∆V) both explained more than 94% of the modeled dry beach erosion for the 1967 storm sequences. When the relationship was compared with observed and modeled erosion volumes for similar beaches but different storm forcing, the inclusion of pre-storm beach swash slope (β_swash) in the parameterization was found to increase the applicability of the power-law relationship over a broader range of conditions.     

A numerical study of the effect of hurricane wind asymmetry on storm surge and inundation

The influence of the asymmetric structure of hurricane wind field on storm surge is studied with five types of numerical experiments using a three-dimensional storm surge model. The results from the case of Hurricane Floyd (1999) show that Floyd-induced peak surge would have been much higher had the region of maximum wind rotated 30–90° counterclockwise. The idealized cases (the hypothetical hurricanes) with a wind speed asymmetry of 20 m s^?1 show that the peak (negative) surge varied from 4.7 to 6.0 m (?5 to ?5.7 m) or equivalent to ?8.8% and 16.3% (2.8% and ?10.4%) differences as compared to the control experiment. The area of flooding varied from 3552 to 3660 km². The results from two other idealized cases of varying degree of wind speed asymmetry further show that with decreasing (increasing) asymmetry of wind fields, the variations of peak surge and peak negative surge caused by the rotation of wind fields decrease (increase) accordingly. The results suggest that in storm surge simulations forced by winds derived from balanced models, considerable uncertainty in storm surge and inundation can result from wind asymmetries. This is true even if all other storm parameters, including maximum wind speed, the radius of maximum winds (storm size), minimum central pressure, storm translation speed, drag coefficient, and model settings (domain size and resolution) are identical. Thus, when constructing ensemble and probabilistic storm surge forecasts, uncertainty in wind asymmetry should be considered in conjunction with variations in storm track, storm intensity and size.     

Probabilistic design of dunes; an example from the Netherlands

Dunes act as primary sea defences in some regions over the world and they have then the same function as common sea dikes; they prevent flooding and protect people against drowning (e.g. in the Netherlands). In other regions dunes form only the transition zone between sea and land and are sometimes part of important holiday resorts. In those cases high investment costs have been made frequently in building houses and hotels close to the edge of the sea. Under storm (surge) conditions, however, dunes can be damaged by the sea. Knowing the amount of retreat of the dune's toe is in both cases important. A design method for dunes, primarily directed at the problem of the loss of life safety, has been developed in the Netherlands in recent years. Due to the large number of factors determining dune erosion, a probabilistic design method was developed.     

Joint modelling of extreme ocean environments incorporating covariate effects

Characterising the joint distribution of extremes of ocean environmental variables such as significant wave height (H_S) and spectral peak period (T_P) is important for understanding extreme ocean environments and in the design and assessment of marine and coastal structures. Many applications of multivariate extreme value analysis adopt models that assume a particular form of extremal dependence between variables without justification. Models are also typically restricted to joint regions in which all variables are extreme, but regions where only a subset of variables is extreme can be equally important for design. The conditional extremes model of Heffernan and Tawn (2004) provides one approach to overcoming these difficulties.Here, we extend the conditional extremes model to incorporate covariate effects in all of threshold selection, marginal and dependence modelling. Quantile regression is used to select appropriate covariate-dependent extreme value thresholds. Marginal and dependence modelling of extremes is performed within a penalised likelihood framework, using a Fourier parameterisation of marginal and dependence model parameters, with cross-validation to estimate suitable model parameter roughness, and bootstrapping to estimate parameter uncertainty with respect to covariate.We illustrate the approach in application to joint modelling of storm peak H_S and T_P at a Northern North Sea location with storm direction as covariate. We evaluate the impact of incorporating directional effects on estimates for return values, including those of a structure variable, similar to the structural response of a floating structure. We believe the approach offers the ocean engineer a straightforward procedure, based on sound statistics, to incorporate covariate effects in estimation of joint extreme environmental conditions.     

现代黄河三角洲地面沉降对洲体演变的影响

对现代黄河三角洲地区地面沉降的特点和对三角洲发育演变的影响进行了分析研究。分析认为,现代黄河三角洲地区普遍发生的地面沉降具有多因素和时空不连续性的特点,给沉降区带来了一系列的危害,使多种海岸带灾害的破坏和影响加剧;地面沉降不仅可以造成地面标高损失,影响三角洲的垂向发育,还通过改变地面坡降来影响河道的演变和尾闾的摆动,同...     

Quantifying the storm erosion hazard for coastal planning

The quantity of coastline retreat resulting from storm erosion is one of the most important phenomena that needs to be accurately quantified to facilitate effective coastal management strategies. Historically, the volume of storm erosion (and coastline retreat) accommodated for coastal planning decisions has been directly linked to the storm (usually defined by considering wave height and duration only) with a certain pre-defined return period, known as a Synthetic Design Storm (SDS) (e.g. 1 in 100 year storm). The SDS method of estimating storm erosion volumes for coastal planning thus assumes that, for example, the 1 in 100 year storm event also results in a 1 in 100 year erosion event. This communication discusses the physical reality of this assumption and demonstrates the improved performance of a new method, based on Joint Probability Distributions (JPD) for estimating storm erosion volumes proposed by Callaghan et al. [Callaghan, D.P., Nielsen, P., Short, A.D. and Ranasinghe, R., 2008. Statistical simulation of wave climate and extreme beach erosion. Coastal Engineering, 55(5): 375–390] using one of the world's longest beach profile surveys from Sydney, Australia.     

Estimating coastal design extremes for given risks: A new approach

For many reasons, e.g., port operations, coastal construction planning, undersea structure survival, and underwater transport, man wishes to know the extreme values that are likely to occur in coastal oceanographic variables. This paper presents a hybrid statistics/ computer simulation method that uses archived oceanographic observations to estimate confidence levels on the most extreme values likely to occur over a given period in the future. The difference from previously developed methods is the ability to estimate the most extreme value over a time period for a given probability (as opposed to estimating the probability of exceeding a given value) and the ability to obtain results from empirical data without a great deal of theoretical oceanography. The method is applied to the California coast for a period of 100 years on the following variables: bottom surge particle velocity by water depth, wave height by water depth, wavelength by water depth, wave period, current velocity, regions of high density, regions of low density, and earthquake magnitude. Values are given for the 99- and 99.9-percent probability levels.     

Post-Little Ice Age Development of Coast in the Locality of Kapp Napier,Central Spitsbergen,Svalbard Archipelago

Abstract

Changes in the position of the shore in the vicinity of Kapp Napier in central Svalbard was described. The overall advance of the shore was probably related to high input of the sediment material originating from erosion of the coastal areas south of the Kapp Napier locality and high input of material from adjacent glacifluvial system of Nordenskiöld glacier with its marginal water streams. Fast evolution of glacier retreat related processes after the Little Ice Age was a secondary driver of the dynamic changes in the central Svalbard coastal areas especially in the first half of the 20th century. The highly dynamic longshore currents in the area altogether with still ongoing glacio-isostatic uplift played an important role in the process as well. The most active parts of the shore experienced advance of almost 100 m since 1908 to 2009. On the other hand, a small part of the coast retreated of about 20 m. Most of the study area experienced aggradation (65%), 30% of the coast was stable and about 5% of the coast has undergone minor retreat. The maximum aggradation rate of 0.96?m/year corresponds well with similar sites in the vicinity.     

基于Copula函数的海岸增水高度与相应风速的遭遇概率分析

简要论述了Copula理论与几种常用的二维Archimedean Copula函数的性质和适用性。以粤东汕头海域妈屿历年最高增水高度与相应风速的遭遇为研究实例,在分别采用3个三参数的概率分布模式:广义极值分布(GEV)、Weibull分布(WBL)和皮尔逊Ⅲ型(P-Ⅲ)分布对两个边缘分布(年最高增水高度与相应风速)拟合优度检验基础上构建了两变量联合概率分布模型。主要结果如下:(1) 年最大增水高度与相应风速的边缘分布分别服从Weibull分布和P-Ⅲ型分布;(2) 拟合优度检验指标表明二者的最优连接函数为Archimedean Copula类的Gumbel-Hougaard Copula;(3) 重现期介于2~200 a之间的边缘分布与同频率的联合分布的重现水平相对差值大约介于6.7%~22.2%之间;(4) 特定风速设计频率条件下,随年最大增水设计频率的减小,二者的遭遇概率也随之迅速减小;反之,特定增水设计频率随风速条件频率的减小,二者的遭遇概率随之明显增大。     

Modeling erosion of sedimentary coasts in the western Russian Arctic

Morphodynamic modeling is employed in the present work to predict the long-term evolution (over the next 100 years) of typical sedimentary coasts in the western Russian Arctic. The studied objects are the coasts of Varandey (the Barents Sea), Baydaratskaya Bay and Harasavey (the Kara Sea). The model developed takes into account both the short-term processes (storm events) and long-term factors (for example, changes in sea level, inter-annual variations in gross sediment flux, lack or excess of sediment supply). Predicted and observed morphological changes in coastal profiles are shown to agree well for time scales ranging from weeks to decades. It is revealed that under given environmental conditions, the morphological evolution is strongly influenced by storm surges and associated wind-driven circulation. The water level gradient created by a surge generates a seaward flow at the bed. This outflow is shown to be an important destructive mechanism contributing to the erosion and recession of Arctic coasts. The rate of change is found to depend on both the exposure of the coast (relative to the direction of dominant winds) and its height above the sea. The open coast of Varandey is expected to retreat as much as 300–500 m over 100 years, while recession of the less exposed coasts of Baydaratskaya Bay would not exceed about 100 m/century. If long-term sediment losses are insignificant, the rate of erosion decays with time and the morphodynamic system may tend toward equilibrium. It is concluded that the expected relative sea-level rise (up to 1 m over the nearest 100 years) is non-crucial to the future coastal evolution if an erosion activity is already high enough.     

Long-term regional hurricane hazard analysis for wind and storm surge

This paper introduces a new method to estimate the long-term regional hurricane wind and storm surge hazard. The output is a relatively small set of hurricane scenarios that together represent the regional hazard. For each scenario, the method produces a hazard-consistent annual occurrence probability, and wind speeds and surge levels throughout the study area. These scenarios can be used for subsequent evacuation or loss estimation modeling. This optimization-based probabilistic scenario (OPS) method involves first simulating tens of thousands of candidate hurricane scenarios with wind speeds and approximate surge depths. A mixed-integer linear optimization is then used to select a subset of scenarios and assign hazard-consistent annual occurrence probabilities to each. Finally, a surge model is used to estimate accurate surge depths for the reduced set of events. The method considers the correlation between winds and surge depths and the spatial correlations of each; it is computationally efficient; and it makes explicit the tradeoff between the number of scenarios selected and the errors introduced by using a reduced set of events. A case study for Eastern North Carolina is presented in which a final set of 97 hurricanes provides unbiased results with errors small enough for many practical uses.     

A heuristic examination of cohesive sediment bed exchange in turbulent flows

Prediction of the concentration of suspended cohesive sediment in the marine environment is constrained by difficulties in interpreting experimental evidence on bed exchange, i.e. erosion and deposition of particles, which remains sparse in mechanistic details. In this paper, conditions under which bed exchange in turbulent flows collectively determines the concentration of suspended matter have been examined in the heuristic sense based on selective experimental data. It is argued that interpretation of such data can be significantly facilitated when multi-class representation of particle size, collisional interaction between suspended particles and probabilistic representations of the bed shear stress along with variables describing particle behavior (critical shear stress for deposition, bed floc shear strength) are taken into account. Aggregation—floc growth and breakup kinetics—brings about shifts in the suspended particle size distribution; bed exchange is accordingly modulated and this in turn determines concentration dynamics. Probabilistic representation of the governing variables broadens the suspended sediment size spectrum by increasing the possibilities of inter-particle interactions relative to the mean-value representation. Simple models of bed exchange, which essentially rely on single-size assumption and mean-value representation of variables, overlook the mechanistic basis underpinning particle dynamics.     

Distributions for storm surge extremes

The present paper is an attempt to assess a variety of potential probabilistic models for the meteorologically driven storm surge component of a long term water level observation record at Sandy Hook, New Jersey. Simple assumptions are made with regard to the record, and the fitting of the extreme values of the data are assessed graphically in observation space for determining quality of fit in the upper tail of the data. Results are presented which show a comparison of typical distributions utilized along with some distributions not before utilized for coastal storm surge.     

风暴潮、大潮对广西涠洲岛西南沙滩侵蚀的影响分析

本文统计分析了广西涠洲岛沿海气候、潮汐和风暴潮等历史资料,利用耿贝尔方法推算了涠洲岛多年一遇年极值高潮位,并估算了其漫滩范围分布,指出近几年高潮位出现的频次和极值均越来越高是涠洲岛西南部沙滩侵蚀愈加严重的重要原因,最后结合风暴潮-海浪耦合数值模拟了研究区域内"0312"号台风风暴潮漫滩的情况,分析了风暴潮和大潮对涠洲岛西南部沙滩侵蚀的影响,对当地岸滩修复和防护具有一定的指导意义。     

Statistical modelling of the barrier height fronting a coastal lagoon and the impact of sea-level rise

A temporal stochastic modelling method for predicting exceedance probabilities of the beach barrier elevations fronting intermittently closed and open coastal lagoons is developed. The method incorporates synthetic tides generated from measured tidal harmonics, and randomly sampled values relating to rainfall, beach face slope, lake opening period, and wave height, direction and period. Samples are derived from distributions of each of these parameters formed from standard long term data records. The method is applied to Tabourie Lake, on the south coast of New South Wales. This entrance is sheltered from the dominant wave climate by an island close to shore, the impact of which is separately assessed by phase averaged wave modelling. The barrier elevation is determined from the 2% run-up level arising from constructive waves. The sensitivity of results to a variety of assumptions is tested. The methodology is applied to determine the probabilistic distribution of barrier heights for both stationary and non-stationary (i.e. sea level rise (SLR)) scenarios. Such probabilities can be adopted in a risk based assessment of catchment flooding behind an enclosing barrier for present conditions, or provide management guidelines for future climate scenario, i.e. changes in rainfall, wave climate, sea level. The model can also be used to investigate different management strategies and how these alter the barrier elevation for given probabilities of exceedance.     

辽东湾东部区块热带风暴设计增水的计算

以辽东湾东部区块出现的极值增水序列为例,考虑热带气旋过程出现频次的影响,采用泊松-最大熵复合极值分布进行了增水统计分析,计算得到辽东湾东部区块的增水重现值.最大100年一遇值为183cm,最大50年一遇值为158cm,皆出现于第1区块.由于受地形的影响,区块1、2与3的增水大小相似,区块4则略小.该结果对辽东湾东部区块...     

Appraisal of storm beach buffer width for cyclonic waves

The loss of beach sand from berm and dune due to high waves and surge is a universal phenomenon associated with sporadic storm activities. To protect the development in a coastal hazard zone, hard structures or coastal setback have been established in many countries around the world. In this paper, the requirement of a storm beach buffer, being a lesser extent landward comparing with the coastal setback to ensure the safety of infrastructures, is numerically assessed using the SBEACH model for three categories of wave conditions in terms of storm return period, median sand grain size, berm width, and design water level. Two of the key outputs from the numerical calculations, berm retreat and bar formation offshore, are then analysed, as well as beach profile change. After having performed a series of numerical studies on selected large wave tank (LWT) test results with monochromatic waves using SBEACH, we may conclude that: (1) Berm erosion increases and submerged bar develops further offshore as the storm return period increases for beach with a specific sand grain size, or as the sand grain reduces on a beach under the action of identical wave condition; (2) Higher storm waves yield a large bar to form quicker and subsequently cause wave breaking on the bar crest, which can reduce the wave energy and limit the extent of the eroding berm; (3) A larger buffer width is required for a beach comprising small sand grain, in order to effectively absorb storm wave energy; and (4) Empirical relationships can be tentatively proposed to estimate the storm beach buffer width, from the input of wave conditions and sediment grain size. These results would benefit a beach nourishment project for shore protection or design of a recreational beach.