Interaction of solitary waves with emergent, rigid vegetation

In this study, solitary wave interaction with emergent, rigid vegetation was studied numerically and experimentally. Laboratory experiments were carried out in a wave flume with vegetation models of different lengths and porosities; the Boussinesq equations with the effects of the vegetation being modeled by a quadratic drag law are used to simulate the wave scattering by and the wave propagation through vegetation. Effects of incident wave height, vegetation density, and vegetation length are discussed. An empirical expression for the mean drag coefficient of emergent, rigid vegetation is presented and compared with other available data. The results are useful for studying tsunami hazard mitigation by coastal forests.     

人工柔性植被场中波浪衰减特性研究

通过SWAN-VEG模型对波浪在刚性及柔性植被中传播的模拟计算结果发现,SWAN-VEG模型能较好的模拟刚性植被场对波能衰减的特性,而对于柔性植被场的情况,则缺少合适的对阻力系数的估算方法。因而在传统的植被消浪模型的基础上,通过引入有阻尼的受迫振荡模型,来考虑柔性植被在不同入射波浪作用下的晃动效果,引入柔性植物体在波浪力下的避让因子D(D为植物运动速度与水质点流速的相对值),通过转换关系式来反映植被的柔性对阻力系数CD影响。对未考虑植被晃动的SWAN-VEG模型进行了修改,用于模拟计算波浪在柔性植被场中的衰减,并采用人工柔性材料进行物理模型试验对计算结果进行对比验证。结果表明,考虑植被晃动影响的模拟结果明显好于不计植被晃动影响的情况,验证结果的相关系数从0.68提高至0.83。由此说明在进行柔性植物消浪效果的研究中需考虑柔性植被的晃动效应,同时发现植被晃动效果的强弱与植物材质的固有频率ωn、阻尼比γ、植被高度hv以及入射波要素等因素有关。     

Analysis of coastal damage of a beach profile under the protection of emergent vegetation

The primary objective of the study was to experimentally explore the protection performance of the emergent vegetation on the morphological changes of a coastal zone. The experiments were conducted under both regular and irregular waves in two different wave flumes. A dimensionless number was derived to characterize the beach profile response under the protection of emergent vegetation. Accordingly, empirical relations were derived that explained the pattern exhibited by the experimental data. The list of wave parameters and beach erosion related functions were incorporated in relation to vegetation intensity in order to define coastal zone response. The relationships of these functions followed good trends with the derived dimensionless number. The findings showed that fall speed parameter is not necessarily involved whereas depth parameter is an important factor while defining the damage. The damage parameter is also considered to formulate the limits of dynamic and static stability of beach profiles under the protection of emergent vegetation.     

Numerical study of vegetation damping effects on solitary wave run-up using the nonlinear shallow water equations

Vegetation damping effects on propagating water waves have been investigated by many researchers. This paper investigates the effects of damping due to vegetation on solitary water wave run-up via numerical simulation. The numerical model is based on an implementation of Morison's formulation for vegetation induced inertia and drag stresses in the nonlinear shallow water equations. The numerical model is solved via a finite volume method on a Cartesian cut cell mesh. The accuracy of the numerical scheme and the effects of the vegetation terms in the present model are validated by comparison with experiment results. The model is then applied to simulate a solitary wave propagating on a plane slope with vegetation. The sensitivity of solitary wave run-up to plant height, diameter and stem density is investigated by comparison of the numerical results for different patterns of vegetation. The numerical results show that vegetation can effectively reduce solitary wave propagation velocity and that solitary wave run-up is decreased with increase of plant height in water and also diameter and stem density.     

Wave attenuation by flexible,idealized salt marsh vegetation

Wave attenuation by vegetation is a highly dynamic process and its quantification is important for understanding shore protection potential and modeling coastal hydrodynamics. Data documenting the interactions of Spartina alterniflora, represented by polyolefin tubing, and single- and double-peaked irregular waves were collected in a large-scale laboratory flume. The laboratory provided a controlled environment to evaluate wave attenuation, including the parameters of stem density, submergence, wave height, and peak period. Wave attenuation appeared to be most dependent on stem density and the ratio of stem length to water depth. Wave attention increased slightly with wave height while no clear trend with respect to wave period was seen. Treating double-peaked spectra as superimposed wave systems revealed a preferential dissipation of the higher-frequency wave system relative to the lower-frequency wave system under emergent conditions. Wave energy loss occurred at all frequencies of both spectral types, with dissipation increasing with frequency above the spectral peak. Parameterizing the spectral equilibrium range as a function of frequency showed a steepening of the spectral tail compared to the − 4 power law under emergent conditions. An empirical relationship defining the bulk drag coefficient for S. alterniflora as a function of the stem Reynolds number is found to serve as a first estimate for engineering applications.     

非淹没刚性植物对海啸作用下海堤水动力特性影响数值模拟研究

基于非静压单相流模型NHWAVE建立了高精度数值波浪水槽。通过设计不同的计算工况,系统研究了非淹没刚性植物对海啸作用下海堤周围水动力特性的影响。着重分析了不同入射波高、不同植物分布密度以及不同植物分布宽度条件下海啸波沿程波形特征以及海堤堤顶越浪流空间分布特征。结果表明:不同入射波高、植物分布密度以及植物宽度条件下,堤顶流厚度和水体流速具有明显单调的变化趋势,并且随着植物分布密度和宽度增大,波能衰减增大;随着入射波非线性增强,植物分布密度和分布宽度对堤顶前段水流厚度的影响也随之增强,而对堤顶后段水流厚度的影响则减弱,且堤顶后缘水流厚度约为堤顶前缘厚度的二分之一;在波浪非线性较大情况下,植物的存在对堤顶流速度的空间分布趋势几乎没有影响,但相对堤顶流速度增加程度均大于无植物情况,且堤顶后缘水流速度约为堤顶前缘的1.6倍。     

Wave dissipation by vegetation with layer schematization in SWAN

The energy of waves propagating through vegetation is dissipated due to the work done by the waves on the vegetation. Dalrymple et al. (1984) estimated wave dissipation by integrating the force on a cylinder over its vertical extent. This was extended by Mendez and Losada (2004) to include varying depths and the effects of wave damping due to vegetation and wave breaking for narrow-banded random waves. This paper describes the wave dissipation over a vegetation field by the implementation of the Mendez and Losada formulation in a full spectrum model SWAN, with an extension to include a vertical layer schematization for the vegetation. The present model is validated with the original equation and results from Mendez and Losada (2004). The sensitivity of the model to the shape of the frequency spectrum, directional spreading and layer schematization are investigated. The model is then applied to field measurements by using a vegetation factor. This model has the ability to calculate two-dimensional wave dissipation over a vegetation field including some important aspects such as breaking and diffraction as used in SWAN model.     

台风过程影响下的滨海湿地物理变量观测及湿地系统响应

如何研究台风等极端天气事件影响下的湿地系统响应过程,进而提出有效的生态完整性维护和管理方案,对关键区域的湿地管理及生态安全维护具有重要意义。本文于2021年9月“灿都”台风期间在南汇东滩南岸设置水动力观测点,采集表层沉积物、测量滩面高程并用无人机获得植被影像,运用ArcGIS空间分析,探讨了台风过程影响下的南汇东滩水动力、滩面沉积变化与植被分布面积响应。结果表明:台风中,观测点近底层平均流速为0.23 m/s,植被边缘平均有效波高和波能是台风前后的1.54倍和2.14倍,近底层1 m的滩面出现“高悬沙浓度层”（>10 g/L）且存在时长为8.13 h。台风后高程低于4 m的稀疏海三棱藨草和互花米草滩面侵蚀0～4.8 cm,高程高于4 m的茂盛互花米草和芦苇滩面淤积0～14.7 cm;研究区植被分布面积共减少1 827.67 m2,减少量占台风前植被总量的1.63%,其中侵蚀滩面植被分布面积减少31.9%,淤积滩面减少68.1%。对台风过程影响后的湿地管理,可以总结为:（1）湿地在台风过程后滩面基本表现为明显的侵蚀、淤积区域共存的特征;（2）对高程低于4 m的侵蚀滩面,建议确定植被适宜生长的高程,结合台风过程冲淤变化通过“微生物膜”和植被斑块移植的方法消浪、固滩和促淤,加速湿地在台风过程影响后的修复。     

Static wave setup with emphasis on damping effects by vegetation and bottom friction

Wave setup can contribute significantly to elevated water levels during severe storms. In Florida we have found that wave setup can be 30% to 60% of the total 100-year storm surge. In areas with relatively narrow continental shelves, such as many locations along the Pacific Coast of the United States, wave setup can be an even larger proportionate contributor of anomalous water levels during major storms. Wave setup can be considered as comprising two components, with the first being the well-known static wave setup resulting from the transfer of breaking wave momentum to the water column. The second, oscillating component, is a result of nonlinear transfer of energy and momentum from the primary (linear) spectrum to waves with length and time scales on the order of the wave groups.Static wave setup is the focus of this paper with emphasis on effects due to internal or surface forces that act on the wave system and cause both dissipation of wave energy and transfer of momentum. In particular, the effects of wave damping by vegetation and bottom friction are considered. Linear wave theory is applied to illustrate these effects and, for shallow water waves, the setup is reduced by two-thirds the amount that would occur if the same amount of energy dissipation occurred in the absence of forces. Effects of nonlinear waves are then considered and it is found, for a shallow water wave of approximately one-half breaking height, that a wave setdown rather than setup occurs due to damping by vegetation and bottom friction.The problem of wave setup as waves propagate through vegetation was stimulated by studies to establish hazard zones associated with 100-year storm events along the shorelines of the United States. These storms can generate elevated water levels exceeding 4 to 6 m and can result in overland wave propagation. As these waves propagate through vegetation and damp, the question arose as to the contribution of this process to elevated mean water levels through additional wave setup.     

海岸圈围对雁鸭类水鸟种群及生境的影响研究

滨海湿地是雁鸭类水鸟的重要栖息地,然而高强度的海岸圈围工程对滨海湿地生态系统和鸟类栖息地造成了显著影响,因此分析和研究海岸圈围对雁鸭类水鸟种群及栖息地的影响,对水鸟保护和滨海湿地资源可持续利用具有重要的理论和实践意义。位于长江口的横沙东滩,于2003年启动了海岸圈围促淤工程,工程的实施导致湿地生境和水鸟种群都发生着快速变化。本研究分别对2013、2015、2016和2017年,横沙东滩雁鸭类水鸟种群和生境因子的时空变化进行了调查与分析,通过拟合回归分析得到两者的相关关系,进一步探讨海岸圈围后生境的快速演替对雁鸭类水鸟种群产生的影响,从而提出海岸带圈围后水鸟栖息地的保护与管理对策。结果表明：1）海岸圈围工程的实施显著改变了滨海湿地的水鸟生境及种群特征。随着工程的推进,自然潮滩的盐沼湿地呈动态变化趋势;促淤区则随着泥沙的补给,新生了大面积的沼泽湿地;成陆区逐渐向淡水生态系统演替。伴随着生境的变化,雁鸭类水鸟种群逐渐从成陆区和自然潮滩向促淤区转移。2）不同生境类型影响雁鸭类种群数量特征的生境因子不同。未实施圈围的自然潮滩影响雁鸭类水鸟数量的主要因素是海三棱藨草/藨草群落面积;正在实施圈围的促淤区的主要影响因素是挺水植物面积、植被归一化指数（NDVI）和无植被覆盖水域面积;已圈围的成陆区主要影响因素为沉水植物面积。综上,雁鸭类水鸟偏向选择具有开阔水域、充足食物供给和相对较低植被盖度的生境类型。对于海岸圈围区域,若能合理利用圈围土地,保留部分区域为湿地,并根据水鸟的生境需求,适当营造水鸟适宜的栖息地,可在一定程度上减缓圈围工程对水鸟的影响,实现滨海湿地水鸟栖息地保护和湿地资源可持续利用的动态平衡。     

Effect of the kelp Laminaria hyperborea upon sand dune erosion and water particle velocities

This paper presents a set of results from a laboratory study on water wave propagation above submerged vegetation growing in the surf zone and the effect of submerged vegetation on dune erosion. The study has focused on the kelp Laminaria hyperborea. The reason is that this kelp is commercially harvested along the Norwegian coast and there is a need to obtain better knowledge on the possible consequences of this harvesting. Experiments were run with irregular waves over a sloping bottom, and a kelp field was simulated by 5000 artificial kelp plants in a 1:10 scale. The experiments primarily focused on the effect of kelp upon erosion of a sand dune, wave damping and water velocities. It was found that the water level is a very important factor to the degree of dune erosion, while the kelp has only a minor effect. The kelp does, however, cause significant wave damping and the degree of wave breaking is reduced. It was also found that the kelp modifies the water velocity profile. In a region above the kelp canopy layer, the time-averaged water velocity was shoreward, while the seaward undertow was confined to a region higher up in the water column.     

Probability distribution of wave heights attenuated by salt marsh vegetation during tropical cyclone

Zero-crossing wave heights, obtained from the field measurement of random waves propagating through salt marsh vegetation (Spartina alterniflora) during a tropical storm, were analyzed to examine their probability distribution. Wave data (significant wave heights up to 0.4 m in 0.8 m depth) were collected over a two-day period along a 28 m transect using three pressure transducers sampling at 10 Hz. Wave height distribution was observed to deviate from the Rayleigh distribution. The observed probability densities of the larger wave heights were reduced significantly by vegetation, producing wave heights lower than those predicted by the Rayleigh distribution. Assuming Rayleigh distributed wave heights for the incident waves to the vegetation patch, existing vegetation-induced wave attenuation formulations are used to derive a special form of two-parameter Weibull distribution for wave heights in the inundated wetland. The scale parameter of the distribution is theoretically shown to be a function of the shape parameter, which agrees with the measurements, effectively reducing the proposed distribution to a one-parameter type. The derived distribution depends on the local parameters only and fits well to the observed distribution of wave heights attenuated by vegetation. Empirical relationships are developed to estimate the shape parameter from the local wave parameters.     

An empirical model to estimate the propagation of random breaking and nonbreaking waves over vegetation fields

In this work, a model for wave transformation on vegetation fields is presented. The formulation includes wave damping and wave breaking over vegetation fields at variable depths. Based on a nonlinear formulation of the drag force, either the transformation of monochromatic waves or irregular waves can be modelled considering geometric and physical characteristics of the vegetation field. The model depends on a single parameter similar to the drag coefficient, which is parameterized as a function of the local Keulegan–Carpenter number for a specific type of plant. Given this parameterization, determined with laboratory experiments for each plant type, the model is able to reproduce the root-mean-square wave height transformation observed in experimental data with reasonable accuracy.     

Experiments on Surface Waves Interacting with Flexible Aquatic Vegetation

Surface wave interaction with aquatic vegetation appears to play a key role in coastal hydro-morpho-dynamics. As an example, the presence of a dense meadow at intermediate water depth is usually associated with a stable and resilient shore. Wave-meadow interactions are investigated here by means of physical modelling, with a focus on wave height distribution and hydrodynamics. The central part of a wave flume is covered by flexible artificial seagrass, composed of polyethylene leaves. This vegetation is tested in both near emergent and submerged conditions. The wave height reduction is evaluated by means of a drag coefficient defined from linear wave theory, which contains all the unknowns of the adopted methodology. The behaviour of such a coefficient is investigated as a function of a wave related Reynolds number. The influence of the flexibility of the leaves is also considered, together with a wave frequency parameter. The results show a complex behaviour with three different trends for near rigid, intermediate or highly flexible leaves. Amplitudes of the orbital velocities are investigated and show a fairly good match with the linear wave theory. On the contrary, the mean velocity along the water column appears to be modified by the seagrass for submerged leaves.     

Numerical study of turbulence and wave damping induced by vegetation canopies

Vegetation canopies control mean and turbulent flow structure as well as surface wave processes in coastal regions. A non-hydrostatic RANS model based on NHWAVE (Ma et al., 2012) is developed to study turbulent mixing, surface wave attenuation and nearshore circulation induced by vegetation. A nonlinear k − ϵ model accounting for vegetation-induced turbulence production is implemented to study turbulent flow within the vegetation field. The model is calibrated and validated using experimental data from vegetated open channel flow, as well as nonbreaking and breaking random wave propagation in vegetation fields. It is found that the drag-related coefficients in the k − ϵ model C_fk and C_fϵ can greatly affect turbulent flow structure, but seldom change the wave attenuation rate. The bulk drag coefficient C_D is the major parameter controlling surface wave damping by vegetation canopies. Using the empirical formula of Mendez and Losada (2004), the present model provides accurate predictions of vegetation-induced wave energy dissipation. Wave propagation through a finite patch of vegetation in the surf zone is investigated as well. It is found that the presence of a finite patch of vegetation may generate strong pressure-driven nearshore currents, with an onshore mean flow in the unvegetated zone and an offshore return flow in the vegetated zone.     

Wave propagation through dense vertical cylinder arrays: Interference process and specific surface effects on damping

The purpose of this research work is to study the effect of specific surface s, the fluid–solid contact surface per volume unit, on the wave energy dissipation by porous structures consisting in dense arrays of emergent vertical cylinders. Experiments have been carried out in a 10 m long wave flume. Three cylinder diameters D are considered in order to study the effects of the specific surface while keeping the porosity constant. In a first series, the length of the porous zone is kept constant for the three cylinder diameters tested. The measurements, which include various wave steepness conditions, demonstrate the role of specific surface s on both wave attenuation and interference processes. The larger the specific surface is, the stronger the wave damping is. Damping is found to be almost proportional to 1/D when laminar, turbulent and inertial effects are of same order. Results are compared to numerical calculations based on either a constant rate of wave damping within the porous medium per unit wavelength or a quadratic damping developed using a force expression based on the work of [26]. This latter model, calibrated with drag and inertia coefficients, shows a good agreement with measurements. In a second series, both porous length and water depth are kept proportional to the cylinder diameter for the three diameters. Scale effects are then discussed and underline the importance of the flow regime within the porous medium.     

计算波浪飘移阻尼的一种切片理论方法

本文利用Ｇｅｒｒｉｔｓｍａ波浪阻力增加公式，导出一种基于切片理论的波浪漂移阻尼的计算方法。与现有理论和试验结果比较证实具有工程一致性。可以指出，这是一种简单使用，便于推广的应用的理论方法。     

Non-hydrostatic finite element model for coastal wave processes

This paper presents the application of the depth-integrated non-hydrostatic finite element model, CCHE2D-NHWAVE (Wei and Jia, 2014), for simulating several types of coastal wave processes. Specifically, the model is applied to (1) predict the swash zone hydrodynamics involving wave bore propagation, (2) resolve wave propagation, breaking, and overtopping in fringing reef environments, (3) study the vegetation effect on wave height reduction through both submerged and emergent vegetation zones using the drag force term technique, and (4) simulate tsunami wave breaking in the nearshore zone and inundation in the coastal area. Satisfactory agreement between numerical results and benchmark data shows that the non-hydrostatic model is capable of modeling a wide range of coastal wave processes. Furthermore, thanks to its simple numerical formulation, the non-hydrostatic model also demonstrates a better computation efficiency when comparing with other numerical models.     

Hydrodynamic performance of a pile-supported OWC breakwater: An analytical study

A pile-supported OWC breakwater is a novel marine structure in which an oscillating water column (OWC) is integrated into a pile-supported breakwater, with a dual function: generating carbon-free energy and providing shelter for port activities by limiting wave transmission. In this work we investigate the hydrodynamics of this novel structure by means of an analytical model based on linear wave theory and matched eigenfunction expansion method. A local increase in the back-wall draft is adopted as an effective strategy to enhance wave power extraction and reduce wave transmission. The effects of chamber breadth, wall draft and air chamber volume on the hydrodynamic performance are examined in detail. We find that optimizing power take-off (PTO) damping for maximum power leads to both satisfactory power extraction and wave transmission, whereas optimizing for minimum wave transmission penalizes power extraction excessively; the former is, therefore, preferable. An appropriate large enough air chamber volume can enhance the bandwidth of high extraction efficiency through the air compressibility effect, with minimum repercussions for wave transmission. Meanwhile, the air chamber volume is found to be not large enough for the air compressibility effect to be relevant at engineering scales. Finally, a two-level practical optimization strategy on PTO damping is adopted. We prove that this strategy yields similar wave power extraction and wave transmission as the ideal optimization approach.     

Empirical calculation of roll damping for ships and barges

For a large floating structure in waves, the damping is computed by the linear diffraction/radiation theory. For most degrees of freedom, this radiation damping is adequate for an accurate prediction of the rigid body motions of the structure at the wave frequencies. This is not particularly true for the roll motion of a long floating structure. For ships, barges and similar long offshore structures, the roll damping is highly nonlinear. In these cases the radiation damping is generally quite small compared to the total damping in the system. Moreover, the dynamic amplification in roll may be large for such structures since the roll natural period generally falls within the frequency range of a typical wave energy spectrum experienced by them. Therefore, it is of utmost importance that a good estimate of the roll damping is made for such structures. The actual prediction of roll damping is a difficult analytical task. The nonlinear components of roll damping are determined from model and full scale experiments. This paper examines the roll damping components and their empirical contributions. These empirical expressions should help the designer of such floating structures. The numerical values of roll damping components of typical ships and barges in waves and current (or forward speed) are presented.