A general framework for modeling sediment supply to coastal dunes including wind angle,beach geometry,and fetch effects

A series of spatially explicit equations are derived that form the foundation of a modeling framework that provides insight into how the interaction of the fetch effect and angle of wind approach leads to tradeoffs that govern the magnitude of aeolian sediment transport across beaches of different geometry. The spatial distribution of sediment transport rate per unit width at any point on the beach is shown to vary predictably as a function of wind angle, critical fetch, and beach geometry; and this has evident implications for the total volume and distribution of sediment transport into the dunes behind the beach as well as the proportion of sediment lost from the beach-dune system at the downwind margin. As the wind field shifts from onshore (shore perpendicular) to oblique (shore parallel) approach angle, total sediment transport rate across a dune line segment will reflect a tradeoff between transport reduction because of the cosine effect and transport enhancement because of potentially longer fetch distances traversed by the wind prior to encountering the dune line. This tradeoff is most evident on long, narrow beaches when beach width is less than the critical fetch. For such beaches and with onshore winds, the total sediment transport rate across the dune line will be less than that predicted for a wide beach because of the constraint imposed by the fetch effect. However, as the angle of wind approach becomes oblique, the available fetch becomes progressively longer, transport limitations imposed by the fetch effect are negated, and transport enhancement across the dune line is to be expected. With very large angles of wind approach, the cosine effect dominates the interaction and transport reductions across the dune line occur until the wind is shore parallel and sediment supply to the dunes ceases altogether. This sequence of adjustments and tradeoffs were only partially understood prior to this study, and yet they form the foundation of coastal dune modeling. The framework proposed in this paper serves to place future studies of process-form interaction in beach-dune systems on a robust theoretical foundation. It also facilitates the testing of various alternative hypotheses regarding the uneven spatial and temporal distribution of dune height and growth rate in coastal environments.     

海滩养护影响下的海岸风沙堆积——以福建平潭岛龙凤头海滩养护工程为例

海滩养护工程实施后,在保护海岸抵御侵蚀的同时往往还产生其他的环境问题。尤其在强风区海岸,海滩养护造成了滩面环境的改变,使滩面风沙搬运和沉积特征都发生了显著变化。对福建平潭岛龙凤头海滩养护工程实施前后的风沙沉积对比研究表明,养护后海滩的干滩宽度、滩面沉积物粒径、表层湿度、海滩高程等环境要素变化对滩面风沙搬运与沉积过程都产生了重要的影响。主要结论为:(1)养护海滩干滩滩肩高程的提高增加了滩面风速,增大了滩面风沙输沙率;(2)养护海滩干滩宽度的增加既扩大了风区长度也为风沙搬运提供了充足的物源,且不受潮汐过程影响,提供了持续的风沙物源供给,增加了风沙作用时间;(3)养护海滩剖面形态的变化使得海滩滩面沉积物含水率减小,降低风沙起动风速,增强了滩面风沙搬运。     

农田近地表风沙流风程效应变化特征研究

风沙流的风程效应研究是定量获取风沙流沿程变化的核心和难点,风程效应是指输沙率随沙床表面或地块长度的增加而不断增大,而后趋于稳定的变化特征,饱和输沙率（f_max）和饱和路径长度（L_sat）是风程效应的重要参数。采用自动连续称重式集沙仪,以河北坝上地区康保县境内典型旱作农田为研究对象,观测了2017、2018年和2021年内4次典型风蚀事件,分析近地表5 cm高度风沙流的风程效应在5 min时间尺度下的变化特征。结果表明：（1） 近地表输沙通量随风程距离的增大而增大。（2） 4次风蚀事件中L_sat的变化范围在11~280 m之间,并存在明显差异,其变化与风速无关。（3） 近地表风沙流的f_max与风速（U）呈幂函数关系。（4） 风程效应的变化特征与地表可蚀性因子、地表微地貌变化有着紧密联系,未来应对不同的土壤类型和质地农田的风程效应进行深入研究。     

The blown sand flux over a sandy surface: a wind tunnel investigation on the fetch effect

Detailed wind tunnel tests were conducted to examine the fetch effect of a sandy surface on a sand cloud blowing over it. The results suggest that the fetch length of a sandy surface has a significant effect on both the vertical flux profile and total horizontal flux. The sand flux over a sandy surface increases with height in the very near surface layer, but then decays exponentially. In agreement with the widely accepted conclusion, the decay function can be expressed by q=aexp(−h/b), where q is the sand flux at height h. Coefficient a that tends to increase with wind speed implies the influence of wind, while coefficient b that defines the relative decay rate shows the influence of both the fetch and wind. The relative decay rate increases with fetch when the fetch length is short, then becomes constant when the fetch reaches a certain length. The threshold fetch length over which the relative decay rate keeps constant increases with wind speed. The average saltation height generally increases with fetch. Both the relative decay rate and average saltation height show that the fetch effect on the flux profile becomes more significant when the wind speed increases. The total sand transport equation for the total fetch can be expressed by Q=C(1−U_t/U)²U³(ρ/g), where Q is the total sand transport rate, U and U_t are the wind velocity and threshold wind velocity at the centerline height of the wind tunnel, respectively, g is gravitational acceleration, ρ is the density of air, and C is a proportionality coefficient that increases with the fetch length, implying that the total sand flux increases with the fetch length.     

潮汐作用下的风沙运动过程研究进展

潮汐作用下的滩面干湿交替是影响风沙运动的重要过程之一。从潮汐作用干湿交替影响下的海滩表层湿度时空变化及其对滩面风沙运动的影响、潮汐作用干湿交替影响下的海滩风区长度变化及其对滩面风沙运动的影响等方面,较为系统地概括和总结了关于潮汐作用下海滩风沙运动过程的研究进展和主要成果。主要结论为：（1）表层湿度与潮汐水位变化之间的关系是非线性的,存在地下水和毛细作用双重的滞后效应;（2）海滩临界起沙风速随表层湿度的变化关系在低于临界表层湿度时正相关,超过临界表层湿度则滩面风沙活动近乎停滞;（3）海滩风沙输送量随风区长度的变化关系按达到平衡状态前、中、后,可分为正相关、不相关和负相关3个不同的阶段。最后,针对当前国内外潮汐-海滩风沙作用模式的研究现状,提出中国未来急需加强的研究方向。     

Evidence of aeolian sediment transport across an Ammophila arenaria‐dominated foredune,Mason Bay,Stewart Island

Aeolian sedimentation across an Ammophila arenaria‐dominated foredune was assessed for eight months using sand traps. This research aimed to ascertain if such foredunes act as a barrier to transport between beaches and hinterland dune systems. At Mason Bay, sand was transported across the foredune at rates of up to 10 kg.hour^?1per 100 m length of coast at wind velocities of 31 m.s^?1. Results were compared with computational fluid dynamics modelled airflows and foredune volume changes. Saltation was important for up to 17 m past the stoss face. Rates of transport were well short of rates of volume gain. Approximately 2% or less of stoss face suspended sediment transport reached the hinterland. The foredune is primarily a sand sink.     

Short-term beach–dune sand budgets on the north sea coast of France: Sand supply from shoreface to dunes, and the role of wind and fetch

Three experimental plots, covering the transition from the upper beach to the dune, on the North Sea coast of France were monitored at various intervals over a period of 18–24 months via high resolution terrain surveys in order to determine inter-site sand budget variability, as well as patterns and processes involved in sand exchanges between the upper beach and dune. The wind regime consists of a fairly balanced mix of moderate (80% of winds are below 8 m/s) onshore, offshore and shore-parallel winds. Sustained dune accretion over several years depends on the periodic local onshore welding of shoreface tidal banks that have developed in the storm- and tide-dominated setting of the southern North Sea. The only site where this has occurred in the recent past is Calais, where bank welding has created a wide accreting upper beach sand flat. At this site, significant sand supply from the subtidal sand bank reservoir to the upper beach flat occurred only once over the 18-month survey following a major storm. The bulk of the sand deposited over this large flat is not directly integrated into the adjacent embryo dunes by onshore winds but is progressively reworked in situ into developing dunes or transported alongshore by the balanced wind regime, thus resulting in alongshore stretching of the embryo dune system. The Leffrinckoucke site near Belgium shows moderate beach–dune mobility and accretion, while the Wissant site exhibits significant upper beach bedform mobility controlled by strong longshore currents that result in large beach budget fluctuations with little net budget change, to the detriment of the adjacent dunes. Accretion at these two sites, which are representative of the rest of the North Sea coast of France, is presently constrained by the absence of a shore-attached sand bank supply reservoir, while upper beach–dune sand exchanges are further limited by the narrow wave-affected upper beach, the intertidal morphology of bars and troughs which segments the aeolian fetch, and the moderate wind energy conditions. The balanced wind regime limits net sand mobilisation in favour of either the beach or the dune, and may explain the relatively narrow longshore morphology of the dune ridges bounding this coast.     

Wind tunnel Study on Aeolian Saltation Dynamics and Mass flow

Aeolian saltation dynamics and mass flows were studied in a wind tunnel. Evaluation of the measured flux rates suggested a new sand transport equation. The theoretical rate data of this equation fitted well with the measured flux rates, as well as with those predicted by other equations reported in literature. Also, an expression was established that predicted accurately the observed roughness heights as well as the roughness data reported in Bagnold (1941). Similarly, the grain-borne drag expression was developed and used to establish the height above the surface at which the grain-borne drag is at equilibrium with the aerodynamic traction.     

The dynamic effects of moisture on the entrainment and transport of sand by wind

There is an increasing awareness of the influence of surface moisture on aeolian entrainment and transport of sediment. Existing wind tunnel studies have shown the impact of a limited range of moisture contents on entrainment thresholds but similar investigations are lacking in the field. The research reported here investigated the influence of changes in surface moisture content on sand entrainment and transport on a meso-tidal beach in Anglesey, North Wales.High frequency (1 Hz) wind velocities measured with hot-wire anemometers were combined with grain impact data from a Sensit monitor and mass flux measurements from a standard sand trap. Surface and near-surface moisture contents were assessed gravimetrically from surface sand scrapes and also directly by using a ThetaProbe. Critical threshold values for entrainment were specified using a modified form of the time fraction equivalence method (Stout, J.E., Zobeck, T.M., 1996a. Establishing the threshold condition for soil movement in wind-eroding fields. Proceedings of the International Conference on Air Pollution from Agricultural Operations. MWPS C-3, Kansas City, 7–9 February 1996, pp. 65–71).Results indicate a time-dependent change in dominant control of the sand transport system from moisture to wind speed, dependent upon the moisture content of the surface sediment. This interchange between controlling parameters on both entrainment and transport was very sensitive to prevailing moisture conditions and took place over a period of minutes to hours. Under conditions experienced in the experiments presented here, the critical moisture threshold for sediment entrainment was determined to be between 4% and 6%, higher than the 1–4% specified in previous wind tunnel experiments. Furthermore, a moisture content of nearly 2% (where moisture was adhered to transported sediment) appeared to have little or no impact on the rate of sand flux.     

不同下垫面对风沙流特性影响的风洞模拟研究

通过对戈壁、草方格和流沙地表风沙流特性的风洞模拟实验,探讨不同性质下垫面对风沙流结构和风沙活动层风速廓线的影响,从而为野外工程防沙的优化设计及其应用提供理论依据。实验结果表明：戈壁地表风沙活动层主要集中在距地表20cm高度范围内;由于沙颗粒与戈壁地表的砾石发生碰撞．输沙率随高度的分布不再简单的遵循对数关系,其极值出现的高度随风速的增加而上移。由于草方格构成的下垫面复杂多变,其对风沙流结构与风沙活动层风速廓线的影响很难确定。但对于特定的下垫面,在不同风速下．同一高度层含沙量具有很大的相关性。     

TRANSFORMATION PATTERNS OF SUBAERIAL MICRO-TIDAL BEACHES

Seven subaerial, low energy beaches in the SE Mediterranean were surveyed biweekly for 13 months. Beach level data were computer-processed and plotted as time-series profile diagrams that differentiated the subaerial beach into three basic subenvironments: backshore, berm, and swash-zone. Heterogeneous seasonal trends in beach sand budget, erosion/accretion patterns, occurrence of ridges and berm-crests, and in pollution by seaborne tar were observed. The profile stations also showed very different degrees of seasonality, although located along similar beaches. Local beachrock protection, when evident, effectively masked seasonality by significantly lowering beach dynamics. Biweekly, seasonal, and annual fluctuations of the beach sand-budget were computed. Average annual net sand flux at the seven beaches was 66 m³/m of beach front, though wide variations occurred. Annual changes in the sand budget along the study area exceeded volumetric changes within the profile, indicating longshore sand transfer. Synchronism of beach behavior prevailed only at the seasonal level. However, intra-seasonal fluctuations for the different profile stations were out of phase, indicating poor synchronism of beach response due to longshore movement of rhythmic topography. Profile changes were thus often unrelated to concomitant wave-climate changes. The overall heterogeneous beach response was in sharp contrast to the identical wave climate, similar bathymetry, and sedimentology of the studied beaches.     

基于激光垂直照射沙床面的风成沙波纹二维形态特征分析

风成沙波纹因其规则的波浪形几何图案吸引着研究者的兴趣,但是对其发育过程的认识还比较薄弱。原因之一就是缺少高精度和高分辨率的沙波纹形态动态测量方法。传统的风成沙波纹形态动态测量方法采用的倾斜光源照射方式忽略了沙波纹脊线高度和走向的变化,会导致沙波纹高度测量错误和形态变形。本研究建立的激光片光源垂直照射沙波纹表面的方式纠正了这一问题,并分析了沙波纹形态及其参数随时间的变化规律。结果表明：（1）沙波纹发育形成过程的初始形成阶段、增长阶段和稳定阶段的时长都随风程长度的增加呈减小趋势,且风程长度越短沙波纹的发育进程越滞后,形成 “倒游现象”。（2）沙波纹的波高和波长都随时间先快速增大然后趋于稳定,而且波高增长速度随风程长度增加而增大。稳定阶段沙波纹平均高度和波长呈现随风程长度的增加而增大的趋势。（3）沙波纹指数、迎风坡指数和背风坡指数都随时间先快速减小然后基本保持不变,服从指数函数分布。稳定阶段平均沙波纹指数随风程长度增加而减小。（4）沙波纹移动速度总体上随高度增大而减小,平均移动速度随时间按幂函数减小,而且在风沙流未饱和时平均移动速度总体上呈随风程长度增加而减小的趋势。可见,风程长度是沙波纹定量中不可忽略的一个因素。     

海岸沙丘沙运动特征若干问题的研究——以闽江口南岸为例

在海岸带,沙粒粒径和水分含量是决定沙粒起动风速的最主要因素。闽江口南岸海滩沙的平均粒径为2.0Φ,水分含量多在0.3%~1.0%之间。运用Johnson的经验公式,可计算出海滩沙的起动风速为6~7m/s。这一结果与实际观测值十分吻合。由于海岸环境因素的影响,使得同等条件下海岸带沙粒的起动风速明显大于内陆沙漠沙,而风沙流的输沙率明显小于内陆沙漠沙。海岸带是风速剧变带,在引用陆地气象台站的风速记录分析海岸风况和研究风沙运动规律时,必须对风速进行订正。     

砂质海滩地形动力分类研究进展

海滩状态是一个地形与水动力相互作用的三维地形动力问题.国外对海滩状态的地形动力分类已开展了大量卓有成效的研究工作,既包括波控、小潮、开阔海岸海滩状态的研究,也包括波控、不同潮差海岸海滩类型的研究和岬间海岸海滩类型的研究.文中回顾并概括了自20世纪70年代至今国外在砂质海滩地形动力分类方面代表性的研究工作,这些对于我国的...     

海岸湿沙表面风沙传输特征的风洞实验研究

通过湿沙表面风沙传输的风洞实验,研究了湿润海岸风沙流的垂直结构、输沙率随风速与表层湿度的变化规律。输沙测量使用60 cm高直立式积沙仪,湿度（M）为沙面表层1 mm厚的重量湿度值。结果表明,湿沙表面的输沙量和高度呈指数关系。一般,湿度增大,整体输沙率降低,高湿度床面的沙粒有相对更大的比例被传输到更高的位置。比起低湿度（M＜0.587%）沙粒,高湿度（0.587%＜M＜1.448%）沙粒的垂直运动对湿度变化的影响更加敏感,尤其是在跃移层的底部,当M＞1.448%时,输沙率已经很低,小于0.99 g·cm-1·s-1。伴随湿度0.587%和1.448%的过渡,风沙流垂向分布被分为3个不同坡度的区域,曲线坡度反映了沙粒间不同水分存在形式的影响差异。对于跃移沙粒,高湿度表面（M＞1.448%）仅起到了一个传输平台的作用;当表面变干到某种程度（M=0.587%）之前,表面湿度是跃移运动的主要控制因子,然后风速才重新开始影响输沙。     

Aeolian sand transport along beaches

Storm‐generated ephemeral transverse sand ridges were observed developing along the beach fronting Sir Richard Peninsula, South Australia during 25 March 1984. The ridges displayed a mean height of 10 cm and a wavelength of 12 m; their breadth was approximately 2 m, and length varied with beach width but ranged up to 40 m over 10 km of coastline. The steeper sides of the ridges faced upwind due to erosion after initial ridge development. Damp sand in the swales inhibited wind scour and restricted sand supply, but provided a firm substrate over which the sand could saltate. Approximately 5000 m³ of sand were incorporated into the ridges by westerly winds blowing at velocities between 45 and 69 km/hr. This observation emphasises the role of alongshore winds in transporting beach sediments and developing essentially ephemeral forms, which might, nevertheless, be preserved in the geological record. The significance of these forms varies with coastal orientation and local wind regimes.     

沙源供给条件对风沙流结构的影响

 针对风沙运动的气固两相流特点,利用FLUNET 软件,对不同沙源供给条件下各种床面的风沙积蚀平衡进行了数值模拟,给出了风速分布和沙粒体积分数随沙源供给条件和下垫面变化的线图,比较了流动边界条件对流场的影响。计算结果揭示了风沙运动中沙粒的体积浓度分布在风速变化和床面形态变化下的变化规律,为沙漠化的防治和理论研究提供了基础。     

灌丛沙堆表面压力分布特征的实验研究

风成沙丘表面压力分布特征直接反映沙丘表面风蚀风积特点,因而是研究沙丘表面地貌过程的重要依据。依据风沙运动实验相似性理论,以新疆和田河流域实测柽柳沙堆为基础,按一定比例缩小制作成四组模型,选用起沙风速区间在6~14 m/s的5组不同风速分别对四组模型在风洞中作纯气流表面压力的模拟实验。结果表明,无植物覆盖的半球形沙堆和圆锥形沙堆的表面压力分布特征存在着较大差异,其中半球形沙堆迎风坡下部有较明显反射涡流,丘顶存在高压剪切气流区域,圆锥形沙堆则不存在这种风压变化,此外半球形和圆锥形沙堆两翼的高低压风蚀区域分布特点也不一致。人工“植物”引发的沙堆丘顶和背风坡区域表面风压的显著变化有利于截留沙尘、保护沙堆免受风蚀并促进沙堆增长。     

A rationale for beach selection by the public on the coast of Wales, UK

Questionnaires were completed by 2306 beach users at 19 Welsh beaches with respect to beach selection parameters. The modal group of respondents was female aged 30–39. Beach choice was primarily determined by clean litter-free sand and seawater, followed by safety. Refreshment facilities and beach awards were deemed minor considerations by the public when choosing a beach to visit. Approximately 58 per cent of respondents were aware of beach award and rating schemes. Of coastal visitors interviewed for this paper, 67 per cent rated a beach as 'important' or 'very important' to their holiday, with just 2 per cent replying that they were unimportant.     

Cyclic and Seasonal Beach Response: A Comparison of Oceanside and Bayside Beaches

This study compares the cyclic and seasonal geomorphic responses of oceanside and bayside beaches on Sandy Hook Spit, New Jersey. It was hypothesized that the different nature of onshore wind regimes at bayside and oceanside beaches would cause different types of beach change. On the east-facing ocean shoreline, rates of beach change are related to weather patterns associated with the passage of mid-latitude cyclonic storms. Bayside beach change is related to the influence of the prevailing westerlies. Storm erosion and post-storm deposition is more rapid on oceanside beaches. Swell waves occurring between storms rapidly reinstate pre-storm equilibrium conditions. Lower bay-side wave energies occurring between storms have little effect on profile development, and foreshore slopes inherited from previous storms undergo little change. Bayside beaches therefore appear to be more in equilibrium with storm conditions than oceanside beaches.

Changes in foreshore slope, beach volume, and beach position associated with individual storm events are compared to seasonal trends to test the applicability of the Hayes cyclic model of beach development to both oceanside and bayside beaches. There is evidence of a cyclic trend of develop-ment on the more exposed ocean sites and seasonal development on the more sheltered bayside sites, which suggests that cyclic development may be causally related to the difference among the energy levels of the storm and nonstorm wave regimes.