Environmental controls on coastal dune formation; Skallingen Spit,Denmark

Many coastal dune systems in Western Europe were emplaced during the Little Ice Age (LIA). The formation of such dune fields has generally been ascribed to a combination of low sea level and strong winds during that time period, providing a supply of sand from the exposed shoreface and sufficient wind energy to transport this sand landward. However, little information exists on the processes that controlled sediment supply to the beach and why this onshore supply was initiated at all. In this contribution, we consider the origin and development of older dune fields on a barrier spit complex (Skallingen) located in the northern part of the Danish Wadden Sea. Maps and new data on dune litho- and chrono-stratigraphy, the latter based on OSL-dating, allow a precise estimate of the initiation and termination of dune emplacement. Dune formation at Skallingen started at a relatively late stage of the LIA and it can be temporally correlated with a phase of relative sea level rise in North Western Europe and with a high frequency of storm surges along the Danish west coast. These are the conditions during which nearshore bars currently migrate onshore across the shoreface off Skallingen. The bars eventually merge with the beach and constitute a source of sand for modern foredune accretion. It is probable that the onshore bar migration occurred under similar conditions in the past and the migration was triggered, or enhanced, by the sea level recovery from the mid-LIA low-stand and the associated frequent storm surge activity. Consequently, at Skallingen onshore sand supply was caused by marine, rather than aeolian, agents; this supply provided the basis for subsequent dune formation. Indications are, however, that the sediment supply to the beach/dunes was a factor of 2–3 larger in the past. Dune aggradation eventually ceased around 1900AD because of dyke construction. Hence, the association between dune formation and sea level/surge variation at Skallingen is somewhat contrary to other statements of coastal dune development during the LIA in North Western Europe.     

Decadal-scale variation in dune erosion and accretion rates: An investigation of the significance of changing storm tide frequency and magnitude on the Sefton coast, UK

Monitoring of frontal dune erosion and accretion on the Sefton coast in northwest England over the past 50 years has revealed significant spatial and temporal variations. Previous work has shown that the spatial variations primarily reflect longshore differences in beach and nearshore morphology, energy regime and sediment budget, but the causes of temporal variations have not previously been studied in detail. This paper presents the results of work carried out to test the hypothesis that a major cause of temporal variation is changes in the frequency and magnitude of storms, surges and resulting high tides. Dune toe erosion/accretion records dating from 1958 have been compared with tide gauge records at Liverpool and Heysham. Relatively high dune erosion rates at Formby Point 1958–1968 were associated with a relatively large number of storm tides. Slower erosion at Formby, and relatively rapid accretion in areas to the north and south, occurred during the 1970's and 1980's when there were relatively few major storm tides. After 1990 rates of dune erosion at Formby increased again, and dunes to the north and south experienced slower accretion. During this period high storm tides have been more frequent, and the annual number of hours with water levels above the critical level for dune erosion has increased significantly. An increase in the rate of mean sea-level rise at both Liverpool and Heysham is evident since 1990, but we conclude that this factor is of less importance than the occurrence of extreme high tides and wave action associated with storms. The incidence of extreme high tides shows an identifiable relationship with the lunar nodal tidal cycle, but the evidence indicates that meteorological forcing has also had a significant effect. Storms and surges in the eastern Irish Sea are associated with Atlantic depressions whose direction and rate of movement have a strong influence on wind speeds, wave energy and the height of surge tides. However, preliminary analysis has indicated only a modest relationship between dune erosion/accretion rates and the North Atlantic Oscillation index.     

A wind tunnel simulation of the dynamic processes involved in sand dune formation on the western coast of Hainan Island

The western coast of Hainan Island exhibits a savanna landscape. Many types of sand dunes, including transverse dune ridges, longitudinal dune ridges, elliptical dunes, coppice dunes, and climbing dunes, are widely distributed in the coastal zone. In winter, high-frequency and high-energy NE winds (dominant winds) are prevalent, with a resultant drift direction (RDD) of S35.6°W. In spring, low-frequency and low-energy SW secondary winds prevail, with a RDD of N25.1°E. Wind tunnel simulations revealed that the airflow over the dune surface is the main factor controlling the erosion and deposition patterns of dune surfaces and the morphological development of dunes. In the region's bidirectional wind en-vironment, with two seasonally distinct energy levels, the airflow over the surface of elliptical dunes, barchan dunes, and transverse dune ridges will exhibit a transverse pattern, whereas the airflow over longitudinal dunes ridges exhibits a lateral pattern and that over climbing dunes exhibits a climbing-circumfluent pattern. These patterns represent different dynamic processes. The coastal dunes on the western coast of Hainan Island are influenced by factors such as onshore winds, sand sources, coastal slopes, rivers, and forest shelter belts. The source of the sand that supplements these dunes particularly influences the development pattern: when there is more sand, the pattern shows positive equilibrium deposition between dune ridges and dunes; otherwise, it shows negative equilibrium deposition. The presence or absence of forest shelter belts also influences deposition and dune development patterns and transformation of dune forms. Coastal dunes and inland desert dunes experience similar dynamic processes, but the former have more diversified shapes and more complex forma-tion mechanisms.     

海南岛西海岸沙丘形成动力过程的风洞模拟试验

The western coast of Hainan Island exhibits a savanna landscape. Many types of sand dunes, including transverse dune ridges, longitudinal dune ridges, elliptical dunes, coppice dunes, and climbing dunes, are widely distributed in the coastal zone. In winter, high-frequency and high-energy NE winds (dominant winds) are prevalent, with a resultant drift direction (RDD) of S35.6°W. In spring, low-frequency and low-energy SW secondary winds prevail, with a RDD of N25.1°E. Wind tunnel simulations revealed that the airflow over the dune surface is the main factor controlling the erosion and deposition patterns of dune surfaces and the morphological development of dunes. In the region’s bidirectional wind en- vironment, with two seasonally distinct energy levels, the airflow over the surface of elliptical dunes, barchan dunes, and transverse dune ridges will exhibit a transverse pattern, whereas the airflow over longitudinal dunes ridges exhibits a lateral pattern and that over climbing dunes exhibits a climbing-circumfluent pattern. These patterns represent different dynamic processes. The coastal dunes on the western coast of Hainan Island are influenced by factors such as onshore winds, sand sources, coastal slopes, rivers, and forest shelter belts. The source of the sand that supplements these dunes particularly influences the development pattern: when there is more sand, the pattern shows positive equilibrium deposition between dune ridges and dunes; otherwise, it shows negative equilibrium deposition. The presence or absence of forest shelter belts also influences deposition and dune development patterns and transformation of dune forms. Coastal dunes and inland desert dunes experience similar dynamic processes, but the former have more diversified shapes and more complex formation mechanisms.     

Development and setting of the Alcudia Bay beach–dune system (Mallorca, Spain)

The beach–dune system of Alcudia Bay is located in the north of the island of Mallorca. The system includes both simple and compound parabolic dunes formed by N to NNE winds and is made up of two dune areas with different dynamics: 1) the northern area, where, despite the fact that the source of beach sediment is from the south, dunes are formed by northerly winds and develop a narrow and linear barrier which separates a marsh from the bay; and 2) the southern area, where complex parabolic dunes formed by north-northeasterly winds, prograde south-southwest landward extending several kilometers inland. The broad pattern of the dunefield size is limited to the southeast by a mountain range and to the west by the effects of a topographic corridor, oriented north–south, which channels the prevailing northerly wind, causing a southward dune progression and limiting the westward extension of the dunefield.     

海南岛东北部海岸沙丘的沉积构造特征及其发育模式

海岸沙丘沉积构造中既有大型交错层理,也具某些区别于内陆沙丘构造的特征,如厚层顶积层的保存及变形层理的普遍存在。 本区海岸沙丘形成于东北季风,沙源来自海滩砂。从海滩后滨向陆地的沙丘类型依次是海岸前丘、横向沙丘和新月形沙丘、抛物线沙丘、纵向沙丘、海岸沙席。其中海岸前丘和沙席为海岸沙丘区别于内陆沙丘的特有类型。本区海岸沙丘的发育开始于3000年前。     

Beach–dune interactions on the dry–temperate Danube delta coast

Beach–dune seasonal elevation changes, aeolian sand transport measurements, bathymetric surveys and shoreline evolution assessments were used to investigate annual and seasonal patterns of dune development on Sfântu Gheorghe beach, the Danube delta coast, from 1997 to 2004. Dune volume increased consistently (1.96 m³ m^− 1 y^− 1 to 5.1 m³ m^− 1 y^− 1) over this 7-year period with higher rates in the southward (downdrift) direction. Dune aggradation is periodically limited by storms, each of which marks a new evolutionary phase of the beach–dune system. As a consequence of the variable beach morphology and vegetation density during a year, foredune growth occurs during the April–December interval while between December and April a slightly erosive tendency is present. The pattern of erosion and deposition shown by the topographical surveys is in good agreement with the sand transport measurements and demonstrates the presence of a vigorous sand flux over the foredunes which is 20–50% smaller than on the beach. This high sand flux, due to low precipitation and sparse vegetation cover, creates an aerodynamically efficient morphology on the seaward dune slope. The seaward dune face accretes during low to medium onshore winds (5.5–12 m s^− 1) and erodes during high winds (> 12 m s^− 1).     

黑河流域中游沙漠风能环境与风沙地貌

在黑河流域中游,沙漠分布在绿洲附近或绿洲之间。近年来,由于气候变化和人类活动的影响,该地区的生态与环境恶化。目前,虽然对该地区的生态与环境等问题进行了大量的研究,但关于绿洲及沙漠风能环境的研究较少。本文利用自动气象站的风资料、环境减灾卫星影像（HJ-1A/B）和Google Earth高清影像,对黑河流域中游沙漠的风能环境与风沙地貌进行探讨。结果表明：黑河流域中游沙漠的北部为高风能环境（>400）,中部为中风能环境（200～400）,南部为低风能环境（<200）;合成输沙势方向总体为东南方向,但在不同区域有所差异;方向变率在北部和中部属于中等变率,南部属于低变率。内陆河流域下游的冲积-洪积物是该地区沙漠形成的物质基础。风能环境与风况对风沙地貌沙丘的形成起着重要作用,但沙源供应对沙丘形态特征的作用不可忽略,在相同的风况下,新月形沙丘（链）、格状沙丘和金字塔沙丘可能共同存在,造成这种格局的主要原因在于沙源的供应程度,按新月形沙丘（链）—格状沙丘—金字塔沙丘顺序,沙源供应逐渐增加。研究区的沙丘类型包括新月形沙丘链、格状沙丘、灌丛沙丘、沙垄、金字塔沙丘、线形沙丘等。     

新月形沙丘与线性沙丘共存区域风况特征

与火星类似,柴达木盆地的新月形沙丘和线性沙丘共存现象引起了众多学者的关注。为探究该现象的发育环境和形成条件,以全球典型新月形沙丘和线性沙丘共存区域为研究对象,选取研究区域附近气象站点3a风速、风向数据,分析这些典型区的风况特征。结果表明:不同的新月形沙丘与线性沙丘共存区域,风速存在明显差异,柴达木盆地和塔克拉玛干沙漠共存区域年平均风速和最大风速均小于沙特阿拉伯沙漠和撒哈拉沙漠;起沙风风向控制沙丘走向,多数共存区域全年起沙风风向较单一,部分区域存在明显的主次风,且主次风风向夹角为锐角;新月形沙丘和线性沙丘可共存于在高、中、低风能环境,中、低风向变率锐双峰或宽单峰风况,沙丘发育受风能环境影响较小,可能受风向变率、下垫面和沙源供应影响大;合成输沙势方向与沙丘走向一致且季节变化小,输沙方向稳定。部分气象站点距离研究区较远,对于研究区的风况指示意义有限。     

腾格里沙漠东南缘不同发育阶段横向沙丘粒度特征

选取了腾格里沙漠东南缘风沙观测场内经人工平整后流动沙地上出现的新生横向沙丘和观测场外围未经人工干预的原始横向沙丘,采集了其表层的沉积物进行试验分析,探讨了不同发育阶段沙丘之间和沙丘表层不同部位之间沉积物粒度组成的空间差异及其所反映的近地层风沙运动过程.结果表明:腾格里沙漠东南缘横向沙丘以细沙为主(53.87%),其次为中沙(42.57%),粗沙和极细沙含量很少;新生沙丘表层沙粒的平均粒径(1.98 Φ)略大于原始沙丘(2.16 Φ),而分选性都很好.两类沙丘的偏度都表现为近对称,峰度都表现为中等类型;由于沙丘所处发育阶段和尺度不同,即使在同一种风能环境下,两类沙丘及其表层不同部位沙粒的粒径分布也存在明显差异.     

偏西风作用下敦煌月牙泉金字塔沙山顶部风沙流初步观测研究

金字塔沙山丘体高大,形态复杂,坡面风沙动力过程实地观测困难,至今尚无坡面风沙流野外观测数据。通过新研制自动集沙仪,对偏西风作用下敦煌月牙泉北侧金字塔沙山顶部风沙流进行了实时观测,共观测8组近地表51.4 cm高度内输沙数据,各观测时段中2 m高度平均风速7.46～14.15 m·s^-1,各组观测时长18~95 min。结果表明：偏西风纵向气流作用下金字塔沙山风沙流结构与平坦地表一致,即输沙量随高度增加呈单一的指数规律递减,且风沙流主要在近地表30 cm高度内输移。风沙流结构特征值λ以大于1为主,且随风速增大而增大,表明金字塔沙山坡面过程主要表现为风蚀过程。偏西风作用下金字塔沙山风沙流中沙粒以细沙为主,平均粒径随高度增加而减小;沙山迎风坡沙粒从坡脚到沙脊线逐渐粗化,风蚀作用增强,细沙从而随风沙流搬运至沙山顶,对金字塔沙山多以细沙为主的格局形成起重要作用。     

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.     

基于GPR图像的河北昌黎海岸横向沙脊北段沉积构造及其成因

运用探地雷达对河北昌黎海岸带横向沙脊的沉积构造进行探测,通过对探地雷达探测影像进行处理与解译,获得海岸沙丘约20 m深度范围内的海岸横向沙脊沉积构造图像,包括沉积层理的倾向、倾角与平面展布信息等。研究发现,昌黎海岸横向沙脊北段的沉积构造主要由高角度交错层理和波状交错层理组成,两者之间界面应为古海滩沿岸沙坝的顶面。界面以上发育稳定的向SWW倾斜的高角度交错层理,倾角约为30°~32°,为向岸风作用形成的前积层理;横向沙脊的丘顶可见槽状交错层理,可能是沙丘脊顶部沉积作用对风向季节性变化的响应。界面以下波状交错层理发育,为波浪作用形成的海滩沉积构造。结合前人对探测区地层调查以及全新世晚期以来海岸线变迁研究成果综合分析,认为海岸横向沙脊是大约2000 aBP以来,以澙湖海滩沿岸沙坝为基础经长期风力吹扬作用持续增长而发育的风成沙丘。     

腾格里沙漠东南缘沙丘迎风坡风速变化的初步研究

对腾格里沙漠东南缘格状沙丘表面气流的观测结果表明,迎风坡风速的增加程度因坡成形态和原始气流的方向,强度而不同。迎风坡风速放大率介于１．０６～１．９１之间,且在格状沙丘主梁,主要与坡形系数和高度有关;在副梁随原始风入射角,坡形系数而变化,风速放大率对沙丘形态的影响主一要增加丘顶区域的输沙率和活动性,且因区域气流的方向和强度而不同,在横向气流条件下,其迎风坡随区域气流强度和增加而变缓,变长,在双向（斜     

新月形沙丘表面风速廓线与风沙流结构变异研究

在风沙地貌学中,跃移沙粒与风场的耦合作用下,其最直观的表现形式为风速廓线和风沙流结构的变异。通过对塔克拉玛干沙漠腹地新月形沙丘迎风坡坡脚、沙丘顶部、沙丘的两个兽角前端、沙丘背风坡坡脚5个典型部位的风速廓线和风沙流结构进行了实地观测,并与参照点（丘间粗沙地）的风速廓线和风沙流结构进行了对比。发现受地形扰动作用,沙丘背风坡坡脚处和沙丘的两个兽角前端的风速廓线形式均呈现非对数形式分布。除迎风坡坡脚处风沙流结构与参照点处相似之外,其余各个部位风沙流均表现出不同于参照点的结构形式。直接采用曲线拟合方法对风沙流结构函数进行拟合,并针对有明显分段现象的风沙流结构形式,采取分段拟合,并对造成风速廓线和风沙流结构变异的原因进行了讨论。     

柴达木盆地东北部哈勒腾河流域风况特征及其对风沙地貌发育的影响

风况是影响风沙地貌发育的重要因子,在一定程度上决定了沙丘类型.为探究区域风沙地貌发育与风况之间的关系,对哈勒腾河流域不同区域一个完整年度的风况进行分析.结果表明:流域年平均风速为2.78 m·s-1,起沙风集中在春夏季,平均风速为7.78 m· s-1,起沙风事件平均持续时间为4.1 h;流域输沙势122-254 VU...     

巴丹吉林沙漠沙山发育与环境演变研究

通过对巴丹吉林沙漠风沙地貌、沙山地层结构、古沙山层理构造、沙山TL测年和区域输沙率等的综合研究表明,巴丹吉林地区自西北而东南大致可划分为河湖为主的弱水冲洪积扇、以河湖与风沙交替作用过度区和以风沙作用为主沙丘区。弱水冲洪积扇的发育为沙漠、沙山的发育提供了丰富的沙物质来源。高大沙山是更新世期间起伏的沙质下垫面与西风环流和季风环流相互作用的结果。末次冰期以前为西风环流为主时期,地表主导风向为西风,次为西北风,河湖环境发育;末次冰期以来为东亚季风环流为主时期,地表盛行风向为西北风,次为西风,风沙活动盛行。期间,风积床面经历了新月形、新月形沙丘链、复合新月形、复合沙丘链到复合型高大沙山的一系列复合、演变与发展过程。     

乌兰布和沙漠西南部风况对穹状沙丘形成的影响

风是塑造沙丘形态的主要动力,目前对穹状沙丘的形成动力学过程尚不清楚.为探讨风况对穹状沙丘形成和演化的影响,以乌兰布和沙漠西南部穹状沙丘为例,利用吉兰泰气象站风速风向数据和Google Earth影像测量沙丘形态参数,分析风况对穹状沙丘形成的影响.结果表明:(1)乌兰布和沙漠西南部主风向为西北风,次风向为西南风和东北风,...     

库姆塔格沙漠羽毛状沙垄形成机理研究

库姆塔格沙漠像一把扇子覆盖在了阿尔金山山前的倾斜平原上,沙丘类型复杂,而尤以羽毛状沙垄最为独特成为我国独有的沙丘形态。有关羽毛状沙垄形成发育机理研究,近几十年来进展甚微。通过野外观测、高精度航片解译和风洞模拟实验研究,结果表明：羽毛状沙垄形成条件是：①具有广大平坦倾斜的地面。②地表沙物质不甚丰富。③两组相近风向是其形成动力条件。④舌状沙丘是在新月形沙垄形成之后的次一级沙丘形态。二者组合成羽毛状沙垄。羽毛状沙垄形成过程可以归纳为下列图式：新月形沙丘→新月形沙垄→舌状沙丘→羽毛状沙垄。     

塔克拉玛干沙漠简单线形沙丘形态动力学过程研究

采样分析、风沙观测以及计算结果表明,在不同风向作用以及沙丘形态参数改变等因素的控制下,简单线形沙丘表面起动剪切速度有明显的差异。表面的气流强度随风向的改变而有所不同。气流与沙丘正交时迎风一侧的输沙率远大于背风一侧同地貌部位的输沙率,但当气流与沙丘走向斜交时,沙丘两侧同地貌部位的输沙率差异不是太大,这是线形沙丘在动力学过程中能保持其形态的最主要原因之一。风向的多变以及沙丘形态的变化使风沙流内边界层内随高度的风速与所观测的输沙率虽也表现为幂函数关系,但相关性较低。沙丘表面被输移的沙物质的粒度分布与下垫面物质组成有一定的差异,在中低强度的起沙风范围内,被输移的沙物质的平均粒径小于下垫面沙物质的平均粒径,且随风速或输沙率的增大,粒度组成也趋于统一。总体上,地表沙物质的输移仍是一随机过程。