Quantification of dune dynamics during a tidal cycle in an inlet channel of the Danish Wadden Sea

High-resolution swath bathymetry measurements at centimetre-scale precision conducted during a tidal cycle in the Grådyb tidal inlet channel in the Danish Wadden Sea reveal the short-term dynamics of a large, ebb-directed compound dune with superimposed small to medium dunes, all composed of medium sand. Dune dynamics were related to simultaneous measurements of flow using an acoustic Doppler current profiler. Spatially, dune crests displayed greater mobility than did dune troughs, due to higher flow velocities at the crests than in the troughs. Temporally, superimposed lower lee-side dunes migrated more during the flood than the ebb tide, due to higher near-bed trough flow velocities during the flood phase, resulting in varying exposure to flow. Net dune migration was flood-directed over the tidal cycle, despite annual net migration being ebb-directed. Hence, extrapolation of short-term migration rates is not possible in this case. The superimposed dunes reversed direction during each half tidal cycle whereas the compound dune only developed a flood cap during flood tide, i.e. the time required for complete reversal of the compound dune was much longer than that available in a half tidal cycle. Over the tidal cycle, the bed level was stable but significant erosion and accretion occurred during the tidal phases. During the ebb tide, bed material was brought into suspension with accelerating flow and settled with decelerating flow, resulting in an average erosion and accretion of the bed of ~7 cm in each case. During the flood tide, the bed of the compound dune was overall stable, although bed material was eroded from the exposed lower lee side, being partly transported to the crest in bedload and partly brought into suspension. In general, dune height fluctuated during the tidal cycle whereas dune length remained stable. The height of the compound dune responded to changes in water depth, which acts as a limiting factor to dune growth. By contrast, the height of the stoss-side dunes responded to flow velocity, i.e. the stoss-side dunes were water depth-independent.     

海南岛西部岸外沙波的高分辨率形态特征

利用SIMRAD-EM3000多波束探测系统和DGPS定位系统,对海南岛东方岸外的沙波沙脊区进行了高精度探测,分析结果表明:从海岸到陆架底形具有明显的分带性,依次出现弱侵蚀底形段、沙波沙脊底形段和平坦底形段。沙波仅发育于沙波沙脊段,介于水深20~50 m之间,沙波形态有二维与三维两种,沙波波高多为0.7~2.5 m,波长20~70 m,沙波指数(L/H)为20~60,对称指数为1~3;沙波沙脊区沉积物的搬运方向有明显的规律性,在沙脊的西侧,沉积物主要向北搬运;在沙脊的东侧,沉积物主要向南搬运;沙波的形成和发育主要受潮流场控制,热带风暴对其有改造作用。     

南海北部湾海洋工程地质特征

根据地球物理、声学探测及海底取样等实测资料详细分析,发现北部湾油气勘探开发区具有近岸、水浅等优越自然条件以及复杂的海洋工程地质特征。海底地形地貌较为复杂,存在潮流沙脊与潮沟、海底沙坡、埋藏古河道、浅层气、埋藏古陡坡、滑塌断层及可能的砂土液化层等潜在灾害地质因素,对海上构筑物存在直接或潜在的危险性。尤其在东部,海底坎坷不平,活动性的潮流消脊、侵蚀沟槽、海底沙波及浅层气等很发育,是海底工程建设的危险区,应引起高度重视。     

多元统计及物源分析支持的北部湾东部海域沉积物输运趋势

通过对北部湾东部海域表层底质样的粒度和重矿物分析,获得了粒度和矿物组成特征。对粒度进行因子分析获得了4个主控因子,其中因子2和3分别代表细颗粒的悬移质组分和陆源粗颗粒推移质组分,指示来自琼州海峡和北部湾西岸、西北岸的物源贡献,而且因子2的高得分区与弱环流背景相吻合;因子4反映了粤西和海南岛东岸沿岸流的物源贡献。采用重矿物组合,将研究区沉积物的物质来源划为4个分区。分别对4个物源分区使用粒径趋势分析模型,把它们合并后获得了整个研究区的趋势矢量,其中海湾北部沉积物输运汇聚中心的位置与余环流的中心位置基本一致,海南岛西部海域沉积物显示为向北输运的特征,与地貌、水动力特征基本吻合,在一定程度上可消除物源混合造成的影响。在使用粒径趋势分析模型获取沉积物净输运方向时,需要综合考虑物源差异和空间尺度对沉积物粒径趋势的影响。     

Sedimentology and evolution of Omaro Spit,Coromandel Peninsula

The sedimentary structures, composition, and texture of sediments from the barrier coast complex (Matarangi Beach—Omaro Spit—Whangapoua Harbour) at Whangapoua, Coromandel Peninsula, are described. Sediments are mainly fine sands, rarely muddy or silty, and most are plagioclase feldsarenites, reflecting derivation from a predominantly Tertiary volcanic hinterland. Sediments from each of the modern environments, namely nearshore, foreshore, back‐shore, frontal dunes, tidal flats, and tidal channels, are characterised by a particular combination of sedimentary structures and subtle textural parameters. Dune ridge and barrier flat paleoenvironments on Omaro Spit were successfully identified by comparing their lithologic properties with the modern sediments. ‘Surficial’ sediments of the well‐preserved dune ridge system developed immediately inland from Matarangi Beach closely resemble those in the modern frontal dunes, and the ‘in depth’ dune ridge sediments are more analogous to the present foreshore sands. The barrier flat deposits separating the dune ridge system from Whangapoua Harbour have similar characteristics to the modern tidal flat sediments in the harbour.

Omaro Spit probably began as an offshore bar across the mouth of Whangapoua Harbour, an embayment formed by the post‐glacial drowning of a Late Tertiary dislocated fault‐block. Tidal flat sedimentation within the harbour formed the ancient barrier flat deposits which rise to at least 2 m above the modern harbour flats, suggesting local sea level at the time was higher than at present. During a subsequent cyclic fall in sea level, supratidal aeolian deposition led to a succession of 15 to 18 parallel dune ridges developed on high‐tide berms. Linear regression analyses of dune ridge and swale heights and the height distribution of positive (aeolian) and negative (beach foreshore) skewness values and of contrasting sedimentary structures in dune ridgL paleosediments, together with the stages in dune soil development across the barrier, suggest initial sedimentation occurred from 4000–5000 years ago when local sea level was 2–3 m above present mean high water level. Barrier progradation was interrupted by an important period of coastal erosion during a temporary rise in sea level immediately before deposition in the dune ridge system of a layer of 2000‐year‐old sea‐rafted Leigh Pumice. Sea level probably reached its modern position at Whangapoua about 1000 years ago, since when some evidence suggests the barrier spit may have experienced minor uplift.     

Can beach dune ridges of the Texas Gulf Coast preserve climate signals?

A study of the evolution of North Padre Island (southern Texas Gulf Coast) dunes was carried out using LIDAR topographic data, dune vibracores through the center of the dunes, and grab samples of shoreface sand at four locations along a cross-shore profile. Grain-size analyses of the vibracores show vertical variations in shoreface sand deposition over decimeter depth intervals. A dune ridge growth model is introduced that describes the dune vertical accretion rate as a function of island progradation and freshwater lens expansion. This model allows indirect dating of the dune core samples based on a known island progradation rate (1?m/year), and height and spacing of the dunes calculated from the topographic data. A sand provenance model is also proposed that links the sand deposition in the dunes with sand sourced from various depths along the shoreface profile, depending on storm activity. We present evidence linking the changes in storm-sand deposition in the dune cores with yearly climatic fluctuations in the Gulf of Mexico associated with landfalling tropical storm activity in the period from 1942?C1965. This record of storm-induced sand variability is negatively correlated with El Ni?o-Southern Oscillation (Pacific) sea surface temperature variability, and positively correlated with North Atlantic decadal sea surface temperature variability.     

海南岛东北部海岸极端波浪事件沉积记录

建立长时间尺度台风序列对于预测未来超强台风的活动规律具有重要科学和实践意义。历史上影响海南岛地区的台风十分频繁,但因缺少确切而详实的记载往往无法判断其是否为超强台风,因此迫切需要寻找千百年尺度的超强台风沉积记录。本文对海南岛东北部翁田镇沿海地区进行详细野外调查,选定白石堡海岸沙丘剖面为研究对象,在该沙丘剖面中识别出风暴越岸沉积和海滩岩巨砾沉积。根据沉积学分析和动力过程分析,这两种类型沉积极有可能是由历史上的台风事件形成,形成年代可能达到距今3 400 a;将该沉积层中的海滩岩巨砾与台风"威马逊"搬运的最大海滩岩巨砾进行对比,发现形成该沉积层海滩岩巨砾的台风事件强度应比台风"威马逊"更强,表明该区域历史上超强台风的存在,这对建立千百年尺度的台风序列有很重要的意义。同时,研究区岸外珊瑚岸礁发育良好,动力分析表明礁坪宽度对于波浪消减、海岸防护具有显著作用。然而,随着海南岛珊瑚岸礁日益衰亡、风暴强度逐渐加大和海面持续上升,未来海南岛地区的海岸极端风浪危害和海岸侵蚀形式威胁正日益加大,亟待加强海南岛珊瑚岸礁保护。     

Shoreface morphodynamics on wave-dominated coasts

An open ocean shoreface typical of long, wave-dominated sandy coasts has been examined through a combination of extensive field measurements of wave and current patterns with computations of marine bedload transport and sedimentation. Sand transport on the upper shoreface is dominantly controlled by waves with only secondary transport by currents. Sand on the middle and lower shoreface, as well as the inner continental shelf is entrained by storm waves and transported by a complex pattern of bottom boundary layer currents.

Storm events have been studied and modeled for the shoreface off Tiana Beach, Long Island. The dominant effect of coastal frontal storms is to cause significant shore-parallel bedload transport with important shore-normal secondary components. These storms tend to result in net offshore transport of sand removed from the beach and surf zone systems. The bedload transport during a storm is convergent on the shoreface leading to accretion. Most accretion occurs on the upper shoreface with lesser deposits covering the middle and lower shoreface as well as the inner continental shelf. Longer-term equilibrium can be maintained by slow return of sand up the shoreface during non-storm conditions.

Annual and geologic time-scale budgets of shoreface sand transport and sedimentation yield equilibrium, net accretion or net deposition. The annual balance results from an integration of the event-scale bedload transport patterns and morphologic responses. These processes and responses have feedback mechanisms which stabilize the system over longer, but not geologic, time scales. Geologic time scale balances are controlled by relative sea level changes and relative availability of sediment supply with the event-scale shoreface and transporting processes providing the mechanism to produce the changes in long-term morphology and sedimentation patterns. In the area of study, the long-term pattern is one of net shoreface erosion, and the permanent loss of sand to the shelf floor.     

西地中海陆架砂质底形

西地中海加的斯湾、埃布罗湾、巴塞罗那岸外和利翁湾等外陆架和陆坡上部分布大片的砂质水下沙丘、沙带、沙脊以及沙席等砂质底形。沙丘长为150～760m,最长为3km;高一般为0.1～5.0m,最高为20m。背流坡指向SW,丘长与丘高相关斜率为H=0.934L0.006 3,低于1978年世界标准的F氏斜率线;沙脊长为4～24km,宽为1～2.3km,高出海底10～30m。砂层厚约12～30m,其成因与冰消期古岸线相关。以水深350m的直布罗陀海峡为中心,大西洋低盐水团和地中海高盐水团相交换而形成的地中海环流是导致海底砂质底形发育的主要动力,陆架外侧普遍分布的垂岸沟谷及顺谷流也起一定作用。据14C年代测定,大型沙丘沙脊形成于距今13～11ka的冰消期,当时海面波动式趋稳。现代洋流只能在暴风浪期间、底流速较大时才能带动泥沙运动且进行局部修饰、破坏和蚀低原砂质底形。     

北部湾冷水团的季节变化及其机制的数值研究

A wave-tide-circulation coupled model based on the Princeton Ocean Model is established to explore the seasonal variation of the cold water mass in the Beibu Gulf and its mechanisms. The results show that the cold water mass starts forming in March, reaches the maximum strength during June and July, and fades away since October. Strong mixing in winter transports the cold water from sea surface to bottom. The cold water mass remains in the bottom layer as the thermocline strengthens during spring, except for the shallow water where the themocline is broken by strong tidal mixing, which gradually separate the cold water mass from its surrounding warm water. Further analysis on the ocean current and stream function confirms that the cold water mass in the Beibu Gulf is locally developed, with an anticlockwise circulation caused by a strong temperature gradient. Sensitivity experiments reveal that the cold water mass is controlled by the sea surface heat flux, while the terrain and tidal mixing also play important roles.     

Analysis of characteristics and mechanism of current system on the west coast of Guangdong of China in summer

On the basis of data of drifting bottles＇ tracks and the current measured in anchored stations, as well as temperature and salinity observed in cruise investigations and coastal stations, ADCP current data and AVHRR surface sea temperature （SST） data on the western coast of Guangdong, synthetic results of analysis showed that the coastal currents in the west of the mouth of the Zhujiang River were mainly westward in summer, which constituted the north branch of cyclonic gyre in the east of the Qiongzhou Straits. Part of its water flowed westward into the Beibu Gulf through the Qiongzhou Straits. The coastal current pattern was not identical with the traditional current system which flowed westward in the Qiongzhou Straits in winter and eastward in summer. The summertime＇s coastal current was always westward, maybe temporarily turning northeast only when the southwest wind was strong. The important characteristics of coastal current on the western coast of Guangdong, in the Qiongzhou Straits and in the north of the Beibu Gulf were analyzed and their mechanisms also were explained.     

1960?2017年北部湾珊瑚礁区海洋热浪增强原因分析

在全球气候变化、区域气候因素的共同作用下,海洋热浪频发,北部湾珊瑚礁正处于快速退化之中。探究海洋热浪逐年增强的机制,对预测珊瑚礁未来生长环境具有重要意义。根据北部湾东部4个海洋站(北海、涠洲、海口、东方)实测海表温度历史资料和该海域的相关气候资料,选取SSTA、DHM、DHW、DHD 4种指标,采用相关分析和对比分析法探讨该海域近58年海洋热浪的变化趋势及原因,结果表明:(1)1960?2017年北部湾东部珊瑚礁区海洋热浪爆发的强度及频率逐年波动上升;(2)海洋热浪逐年增强源于全球性的厄尔尼诺事件增多、区域性的高压系统增强及风场减弱等多因素共同作用;(3)全球变暖背景下造成的海洋热浪爆发对北部湾东部海域的珊瑚礁生态系统已经造成不利影响。     

广西北海银滩侵蚀动力研究

广西北海银滩是一处典型的强潮海滩,以滩面宽阔、砂质纯白为特征,为当地的一处著名旅游景点。近年来银滩也出现了岸线后退、滩肩消失、沉积粗化等侵蚀迹象。为了研究银滩侵蚀的动力机制,本文对银滩海水浴场当前的滩面高程、坡度以及滩面后退距离特征进行了调查和分析。并基于2016?2018年间的滩面高程动态变化,分析了冬、春季风浪和台风期间水位、波浪特征与滩面侵蚀之间的关系,并进而探讨了最有可能造成银滩侵蚀的台风路径。研究表明,1985年以来的滩面最大后退距离为80 m,侵蚀造成海滩上部滩面坡度降低,使得海滩上部的剖面形态向上凸状转化。冬、春季节的风浪叠加高水位可能会造成一定程度的滩面侵蚀,并在海滩上部形成脊槽地貌,但是这种侵蚀会被夏季西南向涌浪导致的回淤所恢复。造成银滩侵蚀后退的主要原因是2003年以来风暴潮重现频率增加。发源于西太平洋,经海南岛北部或雷州半岛进入北部湾的台风可以导致显著的海岸增水,最易于引起强烈的海滩侵蚀事件。银滩东侧的渔港码头阻断了自东向西的泥沙沿岸净输运,也是加剧银滩侵蚀的重要因素。     

Morphological characteristics and sand volumes of different coastal dune types in Essaouira Province, Atlantic Morocco

Altogether three coastal dune fields, one located north and two south of the city of Essaouira, Atlantic Morocco, have been investigated to establish the distribution and overall sand volumes of various dune types. The purpose of the study was to characterize and classify the aeolian landforms of the coastal dune belt, to estimate their sand volumes and to assess the effectiveness of coastal dune stabilization measures. The northern dune field is 9 km long and lined by a wide artificial foredune complex fixed by vegetation, fences and branches forming a rectangular grid. Active and ephemeral aklé dunes border the inner backshore, while some intrusive dunes have crossed the foredune belt and are migrating farther inland. The total sand volume of the northern dune belt amounts 13,910,255 m³. The central coastal sector comprises a much smaller dune field located just south of the city. It is only 1.2 km long and, with the exception of intrusive dunes, shows all other dune types. The overall sand volume of the central dune field amounts to about 172,463 m³. The southern dune field is characterized by a narrower foredune belt and overall lower dunes that, in addition, become progressively smaller towards the south. In this sector, embryonic dunes (coppice, shadow dunes), tongue-like and tabular dunes, and sand sheets intrude from the beach, the profile of which has a stepped appearance controlled by irregular outcrops of old aeolianite and beach rock. The total volume of the southern dune field amounts 1,446,389 m³. For the whole study area, i.e. for all three dune fields combined, a sand volume of about 15,529,389 m³ has been estimated. The sand of the dune fields is derived from coastal erosion and especially the Tensift River, which enters the sea at Souira Qedima some 70 km north of Essaouira. After entering the sea, the sand is transported southwards by littoral drift driven by the mainly north-westerly swell climate and the Trade Winds blowing from the NNE. This sand feeds the beaches along the coast, from where it is blown obliquely onshore to generate the dune fields. The maximum sand input occurs in the north, from where it gradually decreases southwards, this being also reflected in the grain size and dune typologies. The study shows that dune stabilization measures have been reasonably effective along most of the coast, with the exception of a large area immediately north of Essaouira where the almost complete destruction of the plant cover has reactivated sand mobility—this may in the future threaten the city.     

北部湾海域叶绿素a质量浓度时空分布研究

叶绿素a质量浓度是水体富营养化的重要指标,及时和有效地对水体叶绿素a质量浓度进行监测和评价,对环境保护十分必要。本文基于2011年至2016年的卫星遥感数据,分析了北部湾海表叶绿素a质量浓度的时空分布特征。研究结果表明:北部湾叶绿素a质量浓度有明显的月变化和季节性变化,北部湾叶绿素a月平均质量浓度在8月和1月分别达到峰值,北部湾冬季与夏季的叶绿素a季平均质量浓度高于春季和秋季,夏季北部湾海域表层的叶绿素a质量浓度开始迅速升高,秋季北部湾中南部叶绿素a质量浓度呈下降的趋势。在空间分布上,北部湾近岸海域叶绿素a质量浓度平均水平较高,叶绿素a质量浓度高值区集中在湾北部雷州半岛西部沿岸的浅水区海域,而低值区在湾中部以及湾南部的深水区海域。     

Storms and shoreline retreat in the southern Gulf of St. Lawrence

Storms play a major role in shoreline recession on transgressive coasts. In the southern Gulf of St. Lawrence (GSL), southeastern Canada, long-term relative sea-level rise off the North Shore of Prince Edward Island has averaged 0.3 m/century over the past 6000 years (>0.2 m/century over 2000 years). This has driven long-term coastal retreat at mean rates >0.5 m/a but the variance and details of coastal profile response remain poorly understood. Despite extensive sandy shores, sediment supply is limited and sand is transferred landward into multidecadal to century-scale storage in coastal dunes, barrier washover deposits, and flood-tidal delta sinks. Charlottetown tide-gauge records show mean relative sea-level rise of 3.2 mm/a (0.32 m/century) since 1911. A further rise of 0.7±0.4 m is projected over the next 100 years. When differenced from tidal predictions, the water-level data provide a 90-year record of storm-surge occurrence. Combined with wind, wave hindcast, and sea-ice data, this provides a catalogue of potentially significant coastal storms. We also document coastal impacts from three recent storms of great severity in January and October 2000 and November 2001. Digital photogrammetry (1935–1990) and shore-zone surveys (1989–2001) show large spatial and temporal variance in coastal recession rates, weakly correlated with the storm record, in part because of wave suppression or coastal protection by sea ice. Large storms cause rapid erosion from which recovery depends in part on local sand supply, but barrier volume may be conserved by washover deposition. Barrier shores with dunes show high longshore and interdecadal variance, with extensive multidecadal healing of former inlet and overwash gaps. This reflects recovery from an episode of widespread overwash prior to 1935, possibly initiated by intense storms or groups of storms in the latter half of the 19th century. With evidence from the storms of 2000–2001, this points to the importance of storm clustering on scales of weeks to years in determining erosion vulnerability, as well as the need for a long-term, large-scale perspective in assessing coastal stability. The expected acceleration in relative sea-level rise, together with projections of increasing storm intensity and greatly diminished winter ice cover in the southern GSL, implies a significant increase in coastal erosion hazards in future.     

Waves and Currents in Tide-Dominated Location Off Dahej,Gulf of Khambhat,India

Based on measurements of waves, currents, and tides off Dahej in the Gulf of Khambhat, hydrodynamics are studied. Estimated tidal constituents show that primary lunar semi-diurnal constituent M2 was the strongest constituent, and the amplitude was found to be around 4.5 times stronger than that of the major diurnal constituent K1. Currents were predominantly tide induced with speeds up to 3.3 m/s and were north-northwest during flood tide and south-southeast during ebb tide. Residual cross-shore and along-shore current was found to be varying with the corresponding change in the cross-shore and along-shore wind speed. Influence of tidal current was observed in most of the wave statistical parameters.     

Identifying storm impacts on an embayed, high-energy coastline: examples from western Ireland

Large sections of the western Irish coast are characterised by a highly compartmentalised series of headland-embayment cells in which sand and gravel beaches are backed by large vegetated dune systems. Exposure to modally high-energy swell renders most of these beaches dissipative in character. A mesotidal range (c. 3.5–4.5 m) exists along much of the coast. Analysis of instrumental wind records from three locations permitted the identification of a variety of storm types and the construction of storm catalogues. Few individual storms were recorded at all three stations indicating a lack of regional consistency in storm record. Of the total storms recorded, only a small percentage are potentially damaging (onshore directed) and even fewer span a high tide and thus potentially induce a measurable morphological response at the coast.

Through a combination of historical records, meteorological records, field observations and wave modelling we attempt to assess the impact of storms. Quantifiable records of coastal morphology (maps, air photos and beach profiles) are few in number and do not generally record responses that may be definitely attributed to specific storms. Numerical wave simulations and observations at a variety of sites on the west Irish coast, however, provide insights into instantaneous and medium term (decadal) storm responses in such systems.

We argue that beaches and dunes that are attuned to modally high-energy regimes require extreme storms to cause significant morphological impact. The varying orientation of beaches, a spatially nonuniform storm catalogue and the need for a storm to occur at high water to produce measurable change, impart site-specific storm susceptibility to these embayments. Furthermore, we argue that long-period wave energy attenuation across dissipative shorefaces and beaches reduces coastal response to distant storms whereas short-period, locally generated wind waves are more likely to cause major dune and beach erosion as they arrive at the shoreline unrefracted.

This apparently variable response of beach and dune systems to storm forcing at a decadal scale over a coastline length of 200 km urges caution in generalising regarding regional-scale coastal responses to climatic change.     

Three-dimensional numerical simulation for tide and tidal current in the Beibu Gulf

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海平面上升对北部湾风暴潮增水影响研究——以 2012 年台风“山神”为例

全球变暖引发的海平面上升将加剧风暴潮增水,进而危及沿海经济发展与社会安全保障。本文基于模型耦合与模型嵌套技术构建北部湾台风风暴潮数值模拟系统,以2012年台风"山神"为天气背景,通过设计7组情景模拟研究未来不同海平面上升背景下北部湾风暴潮增水变化。结果表明:风暴潮期间水位从南向北沿北部湾逐渐涌高,最高水位发生在广西沿岸,达2.4 m以上。天文潮和台风风场拖曳力是形成高水位的主要驱动力,其中天文大潮和最大风场拖曳力对最高水位的贡献率分别约占70%和30%。海平面上升对风暴潮增水的影响具有时空非线性和非均一性特征。其中,潮位波动和波-流耦合效应会改变实际最大增水发生时间,导致钦州湾附近高潮位大致提前1天半,海平面上升1.1 m使得最大风暴潮增水大致提前30 min;未来海平面上升0.66～1.1 m将导致北部湾大部分海域风暴潮增水幅度放大6%～10%,广西沿岸钦州湾和大风江河口出现负增加效应,可能与溺谷海湾地形特征有关。研究结果可为未来北部湾沿岸防御风暴潮灾害提供理论依据。