有槽沙坝海岸交叉波浪作用下裂流特征

研究叠加波浪场作用下的裂流特征,有助于全面深入开展海岸裂流的致灾机理及风险评估。为分析水动力和水底地形共同作用下形成的裂流特征,开展了交叉叠加波浪作用下有槽沙坝海岸的裂流流动特征试验,即同时考虑沿岸地形变化（存在沟槽）和交叉波浪共同作用所形成裂流。通过对裂流槽附近ADV（Acoustic Doppler Velocimetry）流速测量结果,分析叠加波浪场和裂流沟槽同时存在时所产生的裂流分布特征。裂流中心位置和宽度受裂流槽和波浪节点共同控制,裂流长度依赖于波高和海岸坡度。无槽情况裂流的驱动力主要是由波浪沿岸不均匀所产生的平均水面沿岸压力梯度,有槽情况裂流的驱动力由平均水面沿岸压力梯度和辐射应力沿岸梯度共同决定,二者量值相对大小依赖于波浪周期。     

波生沿岸流数值模拟

为了更好地研究近岸海域波生沿岸流,建立了基于高阶Boussinesq水波方程的波生沿岸流时域数值模型。控制方程在中等水深范围内具有较好的色散性和变浅作用性能,同时具有二阶完全非线性特征,适合描述近岸区域波浪强非线性运动。通过采用松弛造波方法实现了非线性波浪的无反射入射,采用周期性侧边界条件模拟开敞边界。通过数值试验,讨论了模型中主要参数对数值结果的影响。利用率定后的参数模拟了均匀坡度海岸上产生的沿岸流,通过和实验数据的对比验证了模型的准确性和适用性。利用模型数值模拟了不同波浪入射条件(包括周期、波高和波浪入射角度)对波生沿岸流的影响。     

河海交互作用沉积与平原地貌发育*

河流是搬运陆源泥沙的主要动力,对相邻的海岸海洋沉积动力有巨大影响。中国河流汇入海洋中的泥沙曾占全球入海泥沙的10 ％ ,现代中国边缘海大陆架在晚更新世时曾是海岸平原,河-海交互作用是形成海岸平原与浅海输积泥沙的主要因素。本文选择5个不同类型的河流展示其不同的泥沙运动与河口沉积的特性以及对相邻陆架之影响,包括: 1)强潮型动力的鸭绿江河口湾,形成从陆向海与从海向陆的双向水流交汇沉积,海岸形成潮流脊体系。 2)季风波浪为主导动力的滦河口,以泥沙的横向运动为主,形成沙坝环绕的双重海岸,沉积粒径自海向陆减小; 沿岸浪流携运泥沙向河口两侧分布,使沙坝具有沙咀状的复合特点。 3)弱潮型、多沙的黄河口,径流于两侧堆积指状沙咀,沙咀下风侧形成粉砂粘土淤泥湾,沿岸流携运泥沙向渤海湾延伸为淤泥舌。 4)径流与沿岸流组合作用的沉积模式,以长江口为代表,泥沙沿岸向南输运为主导,向海岸与向内陆架构成颗粒变细的带状沉积,外陆架出露残留砂。 5)充填河口湾的三角洲,以珠江为代表,河流分汊与会潮点泥沙堆积,悬移质扩散至湾外,被沿岸流携带沿海岸向SW运移,外陆架为残留砂沉积。20世纪80年代以来,上述河流均受到人为活动的改造影响,河流自然过程与河海交互作用效应均发生改变。本文主要以滦河三角洲为例阐述河-海交互作用与平原的地貌特征。     

大型水库沟口冲淤现象的观测

Ⅰ.概述在松散岩层修建水库后,岸线的再造不单是外形的演变同时也发生了水下地形的改变,在岬角及沟口造成凹地和沙洲,堵塞和封闭沟口,影响到港湾船泊的停驾、对护岸、码头、防波堤等场地的选择有实际意义。造成沿岸泥沙运动的研究,首先是要观测水库波浪要素以及波浪在岸沿的变形情况。岸沿附近之波浪水分子常沿椭圆轨道运动,逐渐向岸缘移动,浅水波对物质输送,于破波时最大。如波浪沿岸线斜交进行,发生破碎形成沿岸水流,其流动方向与岸线大致平行。此项沿岸流之速度在水库测得为10-15公分/秒     

破碎波作用下沙质海床床面形态变化试验

波浪在斜坡沙质海床上破碎会加剧泥沙输移导致海床形态变化,研究破碎波作用下沙质海床形态变化机制对于岸滩演变分析极为重要。在波浪水槽中采用中值粒径0.47 mm原型沙铺设1∶20坡度的底床模型进行试验研究,测量不同波浪条件下床面形态和沙坝顶端悬浮泥沙浓度变化。通过测量和计算破碎带输沙率、沙坝尺度和沙坝移动速度,分析破碎波作用下沙质斜坡海床上床面形态变化规律。试验结果表明,破碎带沙坝顶端的悬浮泥沙浓度与水深和底部床面密切相关,在形成沙坝和沙坝水平方向移动时,悬浮泥沙浓度较大;斜坡上沙坝前后来回运动的周期大小具有随机性,沙坝既有向岸又有离岸移动;在多组波长时间作用后沙坝尺度趋于稳定,底床净输沙量趋于0。     

破碎波作用下沙质海床床面形态变化试验

波浪在斜坡沙质海床上破碎会加剧泥沙输移导致海床形态变化,研究破碎波作用下沙质海床形态变化机制对于岸滩演变分析极为重要。在波浪水槽中采用中值粒径0.47 mm原型沙铺设1∶20坡度的底床模型进行试验研究,测量不同波浪条件下床面形态和沙坝顶端悬浮泥沙浓度变化。通过测量和计算破碎带输沙率、沙坝尺度和沙坝移动速度,分析破碎波作用下沙质斜坡海床上床面形态变化规律。试验结果表明,破碎带沙坝顶端的悬浮泥沙浓度与水深和底部床面密切相关,在形成沙坝和沙坝水平方向移动时,悬浮泥沙浓度较大;斜坡上沙坝前后来回运动的周期大小具有随机性,沙坝既有向岸又有离岸移动;在多组波长时间作用后沙坝尺度趋于稳定,底床净输沙量趋于0。     

海岸沙坝横向迁移研究综述

沙坝是波浪作用下砂质海岸地貌的主要表现形态之一,对于海滩的动力稳定状态具有重要的指示意义。从沙坝的形态参数、沙坝形成理论、沙坝迁移的动力机制以及数值模拟方法等方面概述了水下沙坝横向迁移的研究进展和存在的不足。虽然国内外学者对沙坝横向迁移进行了大量研究,然而由于海滩复杂的动力过程以及沙坝迁移与泥沙输运之间的耦合作用,目前仍存在许多问题尚待解决,未来进一步研究的方向主要包括沙坝迁移过程的空间差异、沙坝迁移的临界条件、波浪破碎对近底泥沙运动和沙坝迁移的影响、数学模型的改进以及沙坝迁移与泥沙输运的耦合机制等方面。     

利用滩坝砂体规模研究古风力的定量恢复*

古风力是一项重要的古气候指标,其定量恢复是一个难题。风作用于水体产生的波浪大小间接地反映了风力,能够为古风力的恢复提供思路。发育于破浪带和冲浪回流带的破浪沙坝、沿岸沙坝分别记录了破浪和冲浪过程,作者分别介绍利用古湖泊中发育的破浪沙坝和沿岸沙坝进行古波况和古风力恢复的原理和操作流程。(1)根据破浪沙坝的几何形态,可以将其厚度与破浪水深建立函数关系,而破浪水深又由破浪波高决定,因此破浪沙坝厚度可以恢复破浪波高,据此可以进一步根据波浪统计关系恢复有效波高、根据风浪关系恢复风力。此方法依托以下3个参数: 单期次的破浪沙坝厚度、破浪沙坝的基座坡角、古风程。(2)沿岸沙坝厚度近似记录了冲浪的极限高度,后者受控于有效波高,据此也可以恢复有效波高和风力。此方法依托以下5个参数: 单期次的沿岸沙坝厚度、古(平均)水深、古风程、古风向相对于岸线的入射角、组成沿岸沙坝的沉积物粒度。上述2种方法综合性较强,涉及古风向、古地形坡度、风程或盆地直径、古水深等参数的恢复,需要综合运用古地貌恢复、去压实校正、古岸线识别、古水深恢复等技术,并需要结合波浪理论。古湖泊滨岸带地层中保存有大量的滩坝沉积,利用其恢复古波况和古风况具有一定的应用前景,能够有助于更详细地重建沉积盆地的古地理背景。     

沙坝地形上沿岸流不稳定运动模式变化特性

为研究沙坝地形上沿岸流不稳定运动特征模式变化特性,建立了考虑底摩擦影响的不稳定模型,并用该模型计算了两种沙坝地形(坡度分别为1:40和1:100)上的不稳定特征模式,这些模式内容包括不稳定运动波长、传播速度、不稳定增长率和扰动速度场。将这些结果与忽略底摩擦计算结果进行了比较,分析了底摩擦对不稳定特征模式的影响。上述研究表明沙坝地形上不稳定运动传播速度约为时均沿岸流最大值的0.71~0.79;1:40沙坝地形上产生不稳定运动所用的时间比1:100地形上更短;底部摩擦可以很大程度上抑制沿岸流不稳定运动的产生;一般观测到的(准)韵律形、新月形沙坝都在后剪切区(紧邻时均沿岸流最大值向海方向区域)产生;不稳定运动在沙坝处运动最强,并且坡度越缓,不稳定运动沿岸方向波长越长。     

湖泊碎屑岩沙坝成因机制及构型表征研究进展*

湖泊碎屑岩沙坝因波浪和沿岸流的反复冲刷,成熟度较高,显示出巨大的油气勘探潜力,已成为隐蔽油气藏勘探的重点。作者在大量文献调研的基础上,总结了近年来前人在滩坝定义、沉积特征、主控因素、成因机制及内部构型研究中的重要进展,在此基础上提出了沙坝演化阶段及沉积构型的新认识。结果表明,沙坝完整的演化历程主要包括4个阶段,分别是弱波浪作用阶段、动荡水流—冲洗回流阶段、风成阶段及沼泽化阶段。影响沙坝发育的控制因素多样,其中物源和风场是沙坝发育的根本因素,两者分别影响了沙坝发育的物质来源和能量供给;而风场所决定的水动力是影响沙坝储集体发育的最直接因素;构造运动和层序演化则控制沙坝的分布位置及保存程度;各因素综合作用决定了坝的形态、发育规模及保存状况。依据前人构型理论,将沙坝构型按照正序方案依次划分为6级,对构型表征方法和沙坝3~5级构型定量表征进行了初步探索,以期为滩坝砂体勘探开发提供理论和实践支持。     

Perception of the rip current hazard on Galveston Island and North Padre Island,Texas, USA

The hazard posed by a rip current depends in part on the ability of beach users to identify a rip current and to associate surf conditions with the potential for rip currents. Understanding which visual features beach users associated with rip currents is an important step in the development of appropriate programs and educational materials aimed at improving the ability of beach users to identify a rip current. A face-to-face survey (n = 392) was conducted to assess the ability of beach users to identify a rip current using five near eye-level photographs that simulate the view of the beach and surf zone as the respondent approached the beach. The survey was conducted on three heavily used public beaches in Texas (Galveston, Port Aransas, and Corpus Christi) at the height of the summer beach season in 2012. Only 13 % of respondents correctly selected the photograph showing the most hazardous conditions and correctly identified the precise location of the rip current on the photograph. The majority of beach users (87 %) incorrectly indicated that the photograph with the heaviest surf represented the most hazardous surf conditions and greatest potential for the development of rip currents, or failed to identify rip currents in photographs. Respondents who were able to correctly identify the rip current tended to recognize the breaking wave pattern, areas of darker water, or the proximity to structures as key visual characteristics of rip currents and also had higher self-reported abilities to swim in rough water and escape a rip current.     

Progradational gravel beach sequences in a moderate- to high-energy, microtidal marine environment

The anatomy of progradational gravel beaches was analysed in two different tectonic settings: the strongly subsiding Messinian foreland basin of the eastern Southern Alps and the rising Pleistocene marine terraces of the outer margin of the Apenninic chain. Progradation took place near the mouths of fan-delta or braid-delta distributaries debouching in microtidal landlocked basins (palaeo-Adriatic and palaeo-Ionian seas). A wind-wave climate, comparable to that of the present-day, was characterized by periodic intense storm activity. Most of the beachface progradation is thought to have taken place during the post-storm recovery and fair-weather stages, whereas the impact of storms is mostly recorded by the cutting of erosional surfaces on the beachface and by emplacement of poorly-sorted and coarse-grained gravels on the shoreface by storm-intensified seaward-trending flows and behind the highest berms by washover processes. Analysis of the sequences led to the identification of a number of typical divisions which are differently developed in the two settings compared here. The characteristics of the Apenninic sequences suggest a strongly variable wind-wave climate, with periodic changes from a relatively dissipative barred morphology during storms, accompanied by strong longshore currents and rip development, to a more reflective state, typical of recovery and fair-weather stages. The Southern Alpine sequences, on the other hand, are characterized by poor evidence of barred morphology and of longshore currents and widespread effects of shear- and gravity-sorting in the lower-beachface gravels. These features indicate a comparatively more reflective average state, due to location of beaches along the deeply embayed head of the palaeo-Adriatic, and the very narrow directional width of the incoming wave spectrum, which was almost shore-normal most of the time. In addition to well-developed shape zonation of gravels, the common presence of wave-generated gravel megaripples in the Southern Alpine sequences indicates the greater influence of long-period waves, due to the greater available fetch distances. In both settings the cyclicity is thought to be genetically related to minor changes of sea-level.     

波流共存场中多向随机波浪传播变形数学模型

基于波作用量守恒方程建立了波流共存场中多向随机波浪传播变形数学模型,模型中考虑了波浪绕射的影响和水流引起的波浪弥散多普勒效应,应用包含水流和地形影响的激破波模式计算波浪破碎的能量耗散,采用一阶上迎风有限差分格式离散控制方程。分别计算了有无近岸流情况下单向和多向随机波浪的波高分布,考虑水流影响的数值计算结果与物理模型实验数据吻合良好,比较分析表明,所建立的数学模型能够复演由于离岸流引起的波高增大,可用于波流共存场多向随机波浪传播变形的模拟和预报。     

An analytical model for the prediction of rip spacing in intermediate beaches

Considering the diversity of rip currents based on the classification of Castelle et al. (2016), it is crucial to study the characteristics of each individual rip current type, which are classified based on different dominant controlling forces and physical driving mechanisms. In this study, an analytical model was presented to predict the spacing of channel rip currents \((S_{\mathrm{rip}})\) in intermediate beaches using the equations of continuity and momentum. Then, the analytical model predictions were compared with the results of numerical simulations calibrated with field studies of other researchers, that showed good agreements. The main results of this study showed that rip spacing was simultaneously related to the characteristics of wave and bed in the surf zone. In addition, it was shown that due to inevitable changes of the hydrodynamic and morphological conditions of rip channel in the beach, the parameter \(H_{b0}/h_{c}\) (where \(H_{b0}\) is the wave height before approaching the sand bar and \(h_{c}\) is channel depth) is an important factor in predicting the rip current situation and the changes of the beach state with time.     

Evaluation of swimmer-based rip current escape strategies

Rip currents are the primary hazard on surf beaches, and early studies described them as fast, shore-normal flows that extended seaward of the surf zone. Based on this traditional view, commonly promoted safety advice was to escape a rip current by swimming parallel to the beach. However, recent studies have shown dominant rip current re-circulation within the surf zone and have endorsed floating as an appropriate escape strategy. Here, a first quantitative assessment of the efficacy of various rip current escape strategies, with a focus on the underlying physical processes, is presented. A field study was conducted at Shelly Beach, NSW, Australia, measuring three rip currents (two open beaches, one topographic) over 3 days in varying wave conditions. Floating was found to be a longer duration, more variable escape strategy ( $ \overline{t} $  = 3.8 min, σ = 2.4 min), than swimming parallel ( $ \overline{t} $  = 2.2 min, σ = 1.0 min). Neither of the scenarios is 100 % foolproof, and both fail in some scenarios, making simplified safety recommendations difficult. Swim parallel failures are related to swimming against the alongshore current of the rip circulation. Float failures related to surf zone exits, with the highest exit rate occurring in the topographic rip. Float failures also occurred due to multiple re-circulations without the person attaining safe footing on the bar. The variable spatial and temporal behaviour of rip currents suggests that a single escape strategy safety message is inappropriate. Instead, a combined approach and scenario-specific safety advice should be considered by beach safety practitioners to promote to the public.     

Rip current-related fatalities in India: a new predictive risk scale for forecasting rip currents

This study analyzes fatalities caused by rip currents in important locations along east and west coasts of India for the period 2000–2010. Results include the frequency of fatalities from rip currents, their cause, and their spatial distributions. Rip current fatalities are most common in the southeastern India, with a non-uniform spatial distribution. August and October are identified as most favorable for rip current generation. In India, rip current-related drowning is estimated as 39 per year during the last decade. East coast of India averaged 30–40 drownings, and west coast of India averaged 5–10 drownings per year. In coastal Andhra Pradesh, more than 350 people had been drowned due to rip currents and only 10 people were rescued. Visakhapatnam recorded highest drowning (293); R. K. beach is the most vulnerable potential rip current zones. From 2007 onward, rip current deaths increased at an alarming rate. The frequency of rip current drownings increased markedly during shore-normal wave incidence and mid-low tidal stages. A new empirical forecasting technique has been developed for prediction of rip current risk in India for the first time. FORTRAN-based software was developed to generate automatic rip current forecast report for any given location. The scale’s performance was tested with field data and by using different statistical methods. The new predictive scale is predicting rip current occurrences reasonably well.     

Rip currents as a geological tool

This paper considers the nature and sedimentary significance of rip currents. Rip currents are the main factor responsible for the transport of coarse sediments from the littoral zone to greater depths. Such sediments, deposited outside the zone of wave deformation, may be identified as storm rip current increments within sediments deposited during fair weather. Composite beds deposited during a given wave cycle by storm rip currents are closely similar to ‘turbidites’ and many so called ‘fluxo-turbidites’ described from flysch deposits. Using data gathered in studies conducted on the modern Baltic coast, supplemented by experimental work and theoretical considerations, a sedimentary model is proposed. The model may be used to interpret possible rip current deposits among shallow water ‘turbidites’, and both modern storm sediments and ancient ‘tempestites’.     

台湾东部海域沉积物波特征及其成因探讨

利用地震剖面对沉积物波的分布、形态和内部结构进行了分析,结合区域地质背景对沉积物来源和成因进行了探讨。识别出的沉积物波域主要位于台东峡谷与陆坡其他峡谷的交汇区,单个波形的波长为0.8～7.2 km,波高为18～75 m左右,呈NE—SW向展布。台东峡谷弯曲段内侧向上坡迁移的沉积物波,其底界发育块体流沉积,内部可细分为下部过渡单元和上部波形单元。弯曲段外侧的沉积物波呈垂向加积的特征,底部无块体流沉积。基于沉积物波的几何形态,估算整个波域的流体厚度在196～356 m之间,流体速度在15～21 cm/s之间。沉积物波的形态特征、内部结构、分布规律以及数值计算表明这些沉积物波为浊流成因。台湾东部海域沉积物波域的发育与台湾西南部的沉积物波域一样,是台湾造山运动的沉积响应。距今3.5 Ma以来花东海脊的形成以及广燠火山岛—绿岛—兰屿火山岛间闸口的抬升和封闭使得沉积物经由卑南溪及海下水道向南输送到绿岛西侧的台东海槽残留弧前盆地时受阻,转而沿台东峡谷及陆坡冲沟体系向东输送入花东海盆。浊流沉积物沿着峡谷/沟谷体系向下坡方向输送的过程中,在峡谷/冲沟的嘴部等地形限制性降低的位置卸载,或在台东峡谷的高弯曲段漫溢出来,从而形成沉积物波域。