田壮才
, 郭秀军, 余乐
Tian Zhuangcai, Guo Xiujun, Yu Le
中图分类号: P751
文献标识码: A
文章编号: 1001-8166(2018)02-0166-13
通讯作者:
收稿日期: 2017-10-8
修回日期: 2018-01-5
网络出版日期: 2018-02-20
版权声明: 2018 地球科学进展 编辑部
基金资助:
作者简介:
作者简介:田壮才(1991-),男,博士研究生,河南商丘人,主要从事海洋地质与环境岩土工程研究.E-mail:zhuangcaitian@163.com
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摘要
内孤立波是存在于层结海洋内部的非线性、大振幅波动,其强劲的垂向和水平运动以及破碎导致的涡旋和湍流作用,对海洋环境、海底沉积物和海洋工程产生重要的影响。围绕内孤立波作用下沉积物再悬浮的发生过程,对前人开展的大量研究工作及取得的研究成果进行了系统分析与总结,从内孤立波向岸传播的破碎机制、悬浮沉积物模式以及引起的海床动力响应进行评述,并分析内孤立波悬浮沉积物判定理论和对海底的作用,对目前研究工作中尚未解决的问题进行总结。该工作对于系统深入认识内孤立波引起的海底沉积物再悬浮发生过程,以及对海底底形的改造能力具有重要价值,为进一步研究内孤立波对海洋生态环境动力过程及深海沉积过程提供借鉴。
关键词：
Abstract
Internal Solitary Waves (ISWs) are nonlinear, large amplitude motions of the interface between two fluids with different densities in the stratified ocean. Because of their strong vertical and horizontal current velocity, and the vortex, turbulent mixing caused by breaking, they affect marine environment, seabed sediment and man-made structures in the ocean. In the paper, we systematically analyzed and summarized the ISW-induced shoaling break mechanisms, models of suspension, and seabed dynamical response. Then, we discussed the ISW-induced sediment resuspension criteria, forming bottom and intermediate nepheloid layer and the capacity to suspend sediments in the seabed, and further put forward the unsolved problems based on the conducted work and related achievements. In shallow seas with complex terrain variations, shoaling can cause ISWs to deform, break, and split. Studies on the propagation of ISWs of depression over sloping topography have shown that an adverse pressure gradient causes the rotation of the flow separation, which produces vortices, and this results in global instability of the boundary layer and ISW burst. The separation vortices increase the bottom shear stress, vertical velocity, and near-bottom Reynolds stress, which leads to sediment resuspension and transport in the flow and vortex core. Although episodic, ISW-induced resuspension is hypothesized to be important enough to shape the topography. Shoaling ISWs may erode, resuspend and transport mud-like sediments, first towards shore by boluses, and subsequently offshore through the generation of intermediate nepheloid layers. Shoaling ISWs might be an important mechanism of muddy sediment dispersal along continental shelves. Furthermore, recent hypotheses suggest that sediment mobilization and transport caused by internal waves in general, and ISWs in particular, may be at the origin of some sedimentary structures found in the sedimentary rock record and also the hummocky-cross stratification. Observed on-shelf propagating frontal ISW most likely interacts with the sand waves, sediment waves or sand dunes. ISWs contribute to their generation, as they are trailed by considerable shear-induced turbulence and high-frequency internal waves close to the buoyancy frequency. This work is of great value for further understanding the process of ISW-induced sediment resuspension, transportation, and the capacity to suspend sediments in the seabed. It helps further study of the dynamic process of marine ecological environment dynamic process by ISW and the deep sea sedimentation process.
Keywords：
内孤立波(Internal Solitary Wave)又称为内孤立子(Internal soliton)[1],是一种特殊的非线性内波,发生在海洋内部密度不均匀水层间,具有周期短、振幅大、流速强、传播速度快等特点[2]。内孤立波广泛分布在全球海洋中[3],如大西洋两岸、太平洋西岸、印度洋沿岸和中国南海(图1)。内孤立波能够诱发极强的垂向流动,造成沉积物与污染物的运移,对海洋环境产生重要的影响[4~6]。
内孤立波对海底沉积物再悬浮的研究始于20世纪70年代,Southard等[7]最早对内孤立波向岸传播过程悬浮沉积物的能力进行分析和研究。内波和内孤立波破碎过程以及未破碎情况下,对沉积物的再悬浮和输运已经在理论上得到阐明,主要通过产生剪切和局地涡旋悬浮沉积物[7~12],内孤立波破碎引起的混合和输运也在实验室得到验证[13~17]。由此引发的沉积物再悬浮的事件已经在边缘海、陆架、陆坡和水下三角洲地区得到观测和研究,如大西洋中部海湾[18],葡萄牙北部陆架[19],日本Otsuchi海湾[20],中国南海[21],美国俄勒冈州陆架[22],加利福尼亚陆架[23]和纽约湾[24]。
我国对内孤立波悬浮沉积物的研究起步较晚,早期的研究聚焦于物理海洋学领域,直到近十几年才开始广泛重视其对海底沉积物的作用。2002年的亚洲海国际声学实验(Asian Seas International Acoustics Experiment,ASIAEX)在南海北部东沙群岛陆架区观测到内孤立波导致的海底沉积物再悬浮现象[21],Reeder等[25]在东沙海域观测到内孤立波悬浮海底沉积物,形成高达200 m的底部雾状层(图2)。南海的内孤立波主要是潮成内波,在吕宋海峡生成基本沿21°N向东沙海域传播[23,26,27](图3),夏季每天出现2次,并多以波包形式出现,大潮期间波包包含十几个内孤立波[5],南海是全世界最大振幅内孤立波发生的海域[28]。南海北部陆坡地形复杂多变[29,30],潮流作用频繁,且存在常年跃层,内孤立波传播过程中与陆坡地形发生相互作用,爬坡破碎由下凹型转换成上凸型[28,31,32],并可能引起内孤立波的分裂及破碎等[33]。现场观测[21,25]、室内实验[34]和数值模拟[20,35]均证实内孤立波在陆坡陆架均可以悬浮和侵蚀海底沉积物,尤其是在陆架坡折区悬浮更为剧烈。
图1 世界范围内孤立波分布图[
Fig.1 Distribution of internal solitary waves in the world[
图2 南海北部陆坡观测到内孤立波悬浮海底沉积物形成200 m厚的雾状层[
Fig.2 ISWs resuspend the seabed sediment and form the nephloid layer to nearly 200 m in the slope of northern South China Sea[
图3 南海内孤立波传播路径[
Fig.3 The processes of propagation of ISW in the South China Sea[
内孤立波是南海深海海洋动力过程的重要组成部分,是造成海底沉积物再悬浮及运移的主要动力因素。内孤立波悬浮沉积物不仅有水流的水平剪切作用,垂向速度产生的吸力同样促进再悬浮[36],内孤立波悬浮沉积物的机制十分复杂,属于海洋学、沉积学、地质学等多学科交叉研究。研究内孤立波悬浮海底沉积物和对底质的改造能力,对追溯南海沉积历史、预测南海底质演化格局、分析地质灾害触发机制等具有重要意义。
内孤立波在向岸传播过程中,随着水深变浅与海床相互作用发生摩擦耗散,内孤立波振幅变小、速度变慢,而在地形变化剧烈的海域可能直接发生破碎耗散[37~39]。对南海内孤立波近10年的跟踪研究发现,内孤立波所携带的能量大部分在陆架区耗散[26],陆架坡折区地形变化剧烈,内孤立波极易在此发生变形破碎,是能量的主要耗散区[26,28,31,40]。同时,陆架坡折区也是南海北部内孤立波的一个源区[37,39],存在局地生成的内孤立波[38]。
由于与地形作用过程不同,内波破碎也分为局部破碎和完全破碎。局部破碎是指内波传播过程中波形整体上保持不变,但波面上个别地方发生微尺度的碎裂。完全破碎一般是指内波波面的全部碎裂[41]。内孤立波的破碎机制主要是剪切不稳定和对流不稳定,以及它们之间的联合作用[2],而且内孤立波本身的性质对不稳定的影响比流体性质要大[42]。
下凹型内孤立波沿不断变浅的地形传播时,波背面不断变陡直,水平流速超过波速,密度大的水卷入到上层发生翻转引起对流不稳定[43](图4a),进而发生破碎[43~45]。剪切不稳定发生在理查德森数Ri<0.25的区域,一般为上层水体内部和分层水界面处,剪切流的加速度决定了剪切不稳定的发生[46](图4b),当加速度小时,内孤立波会通过剪切不稳定发生破碎[46,47]。
对于内孤立波沿陆架陆坡地形传播由对流不稳定引发破碎的情况,Vlasenko等[43]通过数值模拟给出判断内孤立波发生破碎的判据为:当无量纲振幅
图4 内孤立波破碎过程示意图
(a)对流不稳定[
Fig.4 Schematic diagram of ISW breaking
(a) Convective instability[
通过数值模拟、室内实验和现场观测对内孤立波破碎过程以及未破碎情况下,悬浮沉积物的过程和方式进行了大量的研究[18,22,50~56]。当前,对内孤立波导致海底沉积物再悬浮方式的认识主要有3种:
(1)在平坦、光滑、水动力复杂的海底,内孤立波主要通过局地形成的强海流起动沉积物,通常认为当海流剪切力大于沉积物临界剪切应力时,沉积物便会悬浮[18,52,53]。
(2)在平坦、光滑、安静的海底,内孤立波主要通过产生局地涡旋悬浮沉积物[50,51,54]。
(3)当内孤立波由深水区向浅水区传播时会发生破碎,此时在坡面上内孤立波以涡旋形式爬坡并起动沉积物。Hosegood等[55]通过解释Faeroe-Shetland海峡陆架内波的观测资料描述了第三种悬浮过程,并在室内实验[56]和数值模拟[50]中得到重现。现在,越来越多的研究者关注第三种悬浮方式,认为内孤立波爬坡破碎过程中沉积物发生的再悬浮最为剧烈[20,56,57]。
数值模拟[35]和室内实验[36]发现,内孤立波向岸传播的破碎过程中,边界层的不稳定和涡旋分流共同悬浮沉积物。针对向岸传播破碎过程涡旋悬浮沉积物的模式,目前有3种不同的认识。
(1)涡应力和湍流破裂导致沉积物再悬浮,涡流从底面喷射低速流体,这个再悬浮模式类似于涡流或漩涡结构侵蚀沉积物边界[56](图5),即垂向速度引起的雷诺应力垂向拖动沉积物,沉积物再悬浮主要受到底部剪切力和垂向流速引起的应力作用[50,56]。内孤立波向岸传播的破碎过程中,波谷流速增加,并向前插,造成重力失稳,底层水流沿斜坡向下分裂,在波谷后面发展成顺时针旋转的涡旋,涡应力和湍流破裂导致沉积物再悬浮并产生一个或多个倾斜通道。
(2)沿斜坡的水流和涡旋共同再悬浮沉积物[35,58],涡旋流可以增加底部剪应力、垂向流速和近地雷诺应力,底部剪应力将垂向流速与湍流涡旋相结合,进而再悬浮沉积物。其中数值模拟发现底部剪切力是再悬浮的控制因素[35],而通过室内实验三维瞬时速度场的剖面测量发现[36],内孤立波通过时,当内孤立波诱导的底部应力达到最大时,再悬浮没有发生,在波谷后半幅部分影响下的反压力梯度区域,当垂向速度场达到最大时,沉积物发生再悬浮,且垂向速度将沉积物向上推进水体(图6)。
(3)底部水流将沉积物悬浮带入涡旋[120],涡旋前缘区域的水平流速增大,并产生垂向的水流,底流悬浮沉积物,垂向水流将其向上输运进入涡旋。通过现场试验对内孤立波破碎时形成的涡旋结构进行研究发现[20],强的底流将沉积物悬浮进入涡旋,并向上运移,可以形成一个夹带沉积物的涡流环(图7)。
图5 涡应力和湍流破裂示意图[
Fig.5 Schematic diagram of eddy-stress and turbulent bursting[
图6 沿斜坡的水流和涡旋共同悬浮沉积物示意图[
Fig.6 Schematic diagram of sediment resuspension by the bottom current and vortex[
当前的研究主要从水动力学的角度出发,认为内孤立波悬浮沉积物与底部剪切力、垂向流速、涡旋分流等有关,而底部剪切力和垂向流速是沉积物再悬浮的关键因素,垂向流速还会将沉积物向上运移,但悬浮沉积物的直接动力究竟是底部剪切力还是垂向流速,目前还没有一致的看法,内孤立波引起再悬浮的模式并没有统一的认识。
波土作用过程会在沉积物中产生超孔隙水压力甚至引起液化,导致沉积物受力平衡破坏,促使沉积物再悬浮以及海底滑坡发生[59,60],这种机制在表面波与海底沉积物作用过程的研究中已被证实[61~63]。
在内波—海床相互作用的研究中,早期的研究者将内波模型和海床响应模型结合起来研究内波对海床的动力响应,建立线性表面波、内波的双层流体系统以及无限深度多孔弹性海床模型[64~66](图8)。Chen等[64]结合二维Biot固结理论,使用此模型求解波致海床响应的超孔隙水压力,并探讨不同固结海床和内波参数对海床响应的影响。研究发现双层流系统中内波产生的孔隙水压力是相同幅度表面波孔隙水压力的4倍,内波的波长受海床土性质的影响,并可定量计算,与表面波相比,多孔海床对内波的影响更小;结合南海东沙群岛附近的一个内波实例,计算土体饱和度、上下水层厚度和内波波高与海床响应位移值的关系,发现粗砂海床与细砂海床相比,海床响应的能量消散更大;沉积物饱和度增加或水深变浅,内波能量损失更大[64]。Williams等[65]采用同样的模型,并用摄动法求解了非线性波的解,研究发现内波引起的孔压衰减随着土壤骨架刚度和孔隙流体饱和度的增加而减小,波长、波能量和波形在较粗糙的底质中衰减得更快。Williams等[66]还考虑流体黏滞作用建立内波—海床作用模型,发现孔压衰减由下层水的相对波长和无量纲深度决定,当下层流体层厚度接近斯托克斯边界层厚度的50%时,多孔海床引起的衰减达到最大。
图8 表面波、内波的双层流体系统与海床互相作用示意图[
Fig.8 Two-layer fluid wave, ISW-seabed interaction on a porous infinite thickness seabed[
最新的研究开始涉及内孤立波引起海床动力响应[67~69],乔路正等[68]基于刚盖假设建立双层流体内孤立波—无限深度海床模型(图9),利用二维Biot固结理论,使用南海北部实测内孤立波和沉积物参数,探讨不同变量下内孤立波引发的沉积物动力响应特征;数值模拟发现渗透率变化对动力响应特征的影响较小,孔隙度影响程度中等,剪切模量和饱和度影响较大,且饱和度是首要影响因素。乔路正[69]利用有限元模型计算内孤立波作用下海床沉积物的瞬态孔隙水压力和残余孔隙水压力,发现细砂、砂质粉砂和黏土质粉砂3种沉积物在海床水平情况下均未达到剪切破坏条件,且不会发生累积液化,但在海床最浅表可能发生瞬态液化。Rivera-Rosario等[67]考虑含气的沉积物海床,利用数值模拟发现低渗透性海床厚度的2%可能发生破坏,在大陆架典型的深度(h<100 m),只有含有大量气体的淤泥海床才可能失稳;而在深水区(h≥100 m)无论饱和度如何,均能够引起淤泥海床失稳,此外,内孤立波波长也是一个重要的影响因素。垂直孔隙压力梯度起到降低海床沉积物浮重的作用,影响初始运动的标准,并且促进内孤立波悬浮海底沉积物[67~69]。
图9 双层流体内孤立波与海床互相作用示意图[
Fig.9 Two-layer fluid ISW-seabed interaction on a porous infinite thickness seabed[
乔路正[69]研究斜坡海床沉积物再悬浮,以密度跃层结构和斜坡海床为背景,对内孤立波破碎转换阶段海床沉积物动力响应特征进行物理模拟,发现内孤立波在斜坡海床上的破碎转换能够延长海床的动力响应时间,不同振幅内孤立波作用下海床均发生弹性形变。室内实验还发现,海床沉积物类型会影响超孔隙水压力场在海床中的分布特征,分布规律与其物理性质相关,但不影响海床动力响应规律[69]。
当前对内孤立波引起动力响应的研究主要参考表面波的研究成果,多以水平海床为模型进行数值模拟分析,室内试验较少,且尚未见到动力响应的现场监测数据,急需开展更加全面细致的研究,验证和发展内孤立波诱发的海床沉积物动力响应。
沉积物再悬浮隶属于泥沙动力学的研究范围,是将海床沉积物看作单个颗粒,用静力平衡理论来分析其受力状况,当波致剪切力大于自身的临界剪切力时,沉积物颗粒便会发生再悬浮。海床沉积物
再悬浮的研究最早起源于河流的流致再悬浮[70],之后研究者分别针对海洋条件、非黏性沉积物、斜坡沉积物和黏性土等多种条件研究再悬浮理论公式[71~74],但目前还未见到基于此发展的内孤立波致沉积物再悬浮理论。
在海洋学领域,内孤立波的研究非常充分,从水动力学的角度出发,Aghsaee等[36]通过室内实验基于垂向流速,定义了沉积物悬浮的垂向床面应力,提出内孤立波致沉积物再悬浮参数判断标准:
式中:ρs为泥沙的密度,ρ2为下层海水的密度,g为重力加速度,d50为沉积物的中值粒径,最大垂向流速Wmax/c0=0.09ln(ReISW)-0.44,c0为波速,雷诺数ReISW=|U2|
Tian等[34]通过室内实验模拟内孤立波悬浮斜坡沉积物,发现公式(1)与其实验结果相差很大,该公式计算结果偏保守[34,75],同时也有人提出该公式仅考虑垂向流速,并不适用于海洋环境[76]。
通过对内孤立波破碎悬浮的研究,发现破碎时形成的涡旋不仅有垂向速度引起的吸力,波致海床动力响应方面的研究发现渗流力的存在,已经证实渗流作用也会促进沉积物的再悬浮[58,59,67~69],这与内孤立波的数值模拟发现的泵送作用一致[67,69],据此可得出内孤立波破碎过程沉积物颗粒受力体系(图10)。内孤立波不仅可以产生上举力和拖拽力,还会有垂向速度引起的吸力以及渗流力,因此,在判断沉积物再悬浮时,还应该增大沉积物悬浮的水动力影响部分,表征渗流力和吸力的作用。同时,考虑到陆坡海床是倾斜的,还应该增加一个系数,表示对沉积物再悬浮的影响,减少沉积物再悬浮的阻碍作用。实质上,沉积物再悬浮与底形及沉积物的物理力学性质等密切相关,但公式(1)没有考虑这些影响因素,因此适用性不强。
图10 内孤立波破碎过程沉积物颗粒受力示意图
Fig.10 Stress distributions of sediment particle in the ISW breaking process
由于对内孤立波作用下沉积物再悬浮的研究不够全面深入,各种影响因素的作用尚未完全明白,再悬浮判别理论尚不完善,需要发展一种普遍适用的判定理论。
内孤立波在陆架坡折区悬浮沉积物可以形成底部雾状层和中部雾状层[77~79],这种机制已经得到广泛的研究,并且发现内孤立波不仅可以悬浮沉积物,当海床坡角达到临界坡度时,悬浮沉积物再沉积受到抑制,引起的沉积物悬浮量和侵蚀会显著增大[17,80]。
现场实验证实内孤立波悬浮运移沉积物有2个过程:首先是内孤立波向岸传播产生的涡旋将沉积物悬浮,并向上穿过密度层向岸边运移,粗粒悬浮物再次沉积形成沙波,随后细粒悬浮物通过生成的中间和底部雾状层向海中运移,并沿等密度层向海中扩散[20,55](图11)。数值模拟也获得类似的结论,内孤立波向岸传播过程中会侵蚀、悬浮和输运泥状沉积物,并通过生成的中间和底部雾状层向海中运移[35]。内孤立波向岸传播过程可能是泥沙沿大陆架扩散的一个重要机制,并且沿斜坡向下输运泥沙[35,55],这对深海沉积物输运过程和深海记录具有重要意义。
内孤立波搬运沉积物对海底沉积地貌形成具有重要影响[81],但是关于中间和底部雾状层这2种通道的输运贡献量还不清楚,具体运移距离还有待研究,悬浮沉积物的再沉积过程尚缺乏详细的研究。
已有的研究认为内孤立波可以改造海底底质形成沙波[82~86],在地中海墨西拿海峡[85]和巴塞罗那海港[86]已经通过现场观测对其进行了理论分析,并发现内孤立波可以影响深水沉积过程与海底地形,还发现内孤立波可使临界海床坡角的沉积物悬浮量和侵蚀显著增大,内波可能塑造整个陆坡海底形态,世界上陆坡的坡度与内孤立波能量保持平衡状态[17,80]。同时,有学者认为南海北部陆架及上陆坡的沙波地貌[25,87,88],以及陆坡、深水区的沉积物波[89,90],均由内孤立波造成,此外内孤立波可能是引发海底滑坡等地质灾害的重要因素[91]。当前对于南海北部陆坡内孤立波与海底底质作用的研究主要集中在地形地貌、底质、沉积作用等方面[92~94],并没有探索内孤立波对海底沉积物的具体作用机制。虽然缺乏更为直接的证据,但都不否认内孤立波对海底拥有巨大的改造能力。
南海底质调查显示,东沙海域存在广泛的海底剥蚀,已经形成海底冲蚀沟[88,95],研究发现冲蚀沟是由底流冲蚀作用形成的[88]。内孤立波是可以与海底直接作用的深海海洋动力,通过声学多普勒流速剖面仪(Acoustic Doppler Current Profilers,ADCP)、雷达等数据计算得到东沙海域附近内孤立波传播速度通常为50~300 m/s[96],振幅最大可达170 m[38],内孤立波在南海北部诱发的最大波致流速为1.5~2.9 m/s[97,98],垂向流速达到0.3 m/s。实验和观测表明,海底以上1 m以内的流速超过10 cm/s,再悬浮的作用就会大为增强,一般认为东沙海域沉积物再悬浮的临界流速为20~40 cm/s[99],在南海陆坡经过现场观测认为内孤立波起动沉积物的水平速度约为0.5 m/s[25],因此,内孤立波完全有能力在此悬浮沉积物、侵蚀海底。海底冲蚀沟与南海内孤立波分布图[100]相比较发现(图12),冲蚀沟位于内孤立波分布的密集区,此处海底地形变化较大,已经证实内孤立波在此处破碎耗散。我们发现冲蚀沟分布方向和内孤立波峰线基本一致(图12),且冲蚀沟范围内的波峰线有扭曲现象,这是与地形相互作用引起的[101],可以推断冲蚀沟的形成与内孤立波密切相关。而在南海观测到内孤立波悬浮沉积物形成200 m厚的底部雾状层[25],该观测点恰好处于冲蚀沟内部(图12),这是内孤立波形成海底冲蚀沟的另一个证据。
图11 内孤立波悬浮沉积物的输运趋势[
Fig.11 Transport trend of suspended sediment by ISW[
图12 南海北部内孤立波、沙丘和海底冲蚀沟分布区[
Fig.12 Distribution of ISW, sand wave and scour channels in the northern South China Sea[
内孤立波是海底沉积物再悬浮的重要动力,这种复杂的沉积物运动过程伴随着海床内部的动力响应。本文围绕内孤立波悬浮沉积物的发生过程,对前人开展的大量研究工作及取得的研究成果进行了系统分析与总结,得出以下几方面认识。
(1)内孤立波向岸传播的破碎机制主要是剪切不稳定和对流不稳定,2种破碎机制的判据已经得到详细研究。内孤立波破碎过程以及未破碎情况下悬浮沉积物的方式主要有3种,其中,内孤立波向岸传播爬坡破碎并悬浮沉积物的方式是现在研究的焦点,并被认为是内孤立波悬浮沉积物最为剧烈的方式。但是,内孤立波破碎过程悬浮沉积物的模式已有3种不同的观点,至今没达成一致的看法,有待通过细致的室内实验和现场观测,进一步研究确定垂向流速和底部剪应力对沉积物再悬浮的作用。
(2)内孤立波悬浮沉积物的判别理论缺乏更多的研究,已有的判别理论没有得到广泛的认可。由于垂向流速的作用不够清楚,而内孤立波诱发海床动力响应的研究进展缓慢,尚缺乏现场观测对成果的验证,以确定渗流力对再悬浮的影响程度,因此,更多的研究应该集中在确定垂向流速的作用和渗流力的影响,借鉴泥沙动力学的研究理论,探索内孤立波悬浮海底沉积物的具体作用机制。
(3)内孤立波悬浮海底沉积物可以形成底部和中间雾状层,并沿斜坡向下输运沉积物,通过形成海底沙波和输运沉积物来塑造陆坡形态,影响深海沉积过程。但是对于雾状层输运沉积物的输运距离、输运量、输运机制以及与海底地形改变的关系缺乏定量的认识,影响因素也没有定量的评价。需要设计完整的现场观测方案,通过长期观测,并综合海底地形的改变和悬浮沉积物输运量的测量,得到直接的证据,确定内孤立波对海底的改造能力。
(4)南海是最大振幅内孤立波的发生海域,海底沙波、沉积物波等地貌广泛发育,内孤立波具有塑造陆坡形态的潜力,需要对内孤立波引起的地质灾害进行关注和预防,这个过程不能仅靠波土相互作用理论所能描述的,需要多学科交叉的融入与渗透。
The authors have declared that no competing interests exist.
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| [17] |
Boundary layer intrusions from a sloping bottom:A mechanism for generating intermediate nepheloid layers [J].
[1] Laboratory experiments were used to investigate the growth of intrusions due to internal-wave reflection from a sloping boundary. When normalized by the incident energy density flux, the average intrusion spreading velocity was found to be a linear function of the frequency ratio 0309/0309c, where 0309 is the frequency of the incident wave and 0309c is the critical frequency, at which the wave characteristic has the same angle as the bottom slope. Evenly spaced layers, indicating thin perturbations in the background density gradient, developed within the mixing region and spread into the tank interior. The vertical spacing of these layers also bore a linear relationship to 0309/0309c. A linear model of internal-wave reflection suggests that these layers may be related to an isopycnal displacement, or overturn, scale. Intrusion growth occurred at a range around the critical frequency and was strongest at slightly supercritical conditions. A balance relating the spreading rate of intrusions to the divergence of energy density flux across the boundary layer is derived. Fitting the laboratory results to this theoretical prediction suggested a weak net buoyancy flux. This balance might be of use in predicting spreading rates of intermediate nepheloid layers generated by internal-wave mixing at oceanic margins.
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| [18] |
Internal solitary waves in the coastal mixing and optics 1996 experiment:Multimodal structure and resuspension [J].
[1] Observations of internal solitary waves (ISWs) and of their role in sediment resuspension during the Coastal Mixing and Optics 1996 (CMO 96) experiment are reported. The largest resuspension events observed in the experiment can be related to retarded flow under the wave footprint. Two distinctly different periods of resuspension events could be distinguished based on the prevailing atmospheric conditions: a calm period, which we consider characteristic of nominal conditions, and an energetic period, during which two major storms occurred. ISWs arrived at the mooring site from a variety of sources, though a common intermodal dynamics seemed to occur repeatedly. Both mode 1 and mode 2 ISWs have been observed. The present analysis of the month-long portion of the CMO 96 data set constitutes the first reported observations, insofar as we are aware, of mode 2 waves on the continental shelf. Both mode 1 and mode 2 ISWs were found to stimulate resuspension. The mode 2 waves seem to be generated locally by resonant or topographic interactions with mode 1 ISWs. Internal wave fields of the type described here are expected to exist in a variety of other shallow seas as well.
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| [19] |
Evidence of sediment resuspension by nonlinear internal waves on the western Portuguese mid-shelf [J].
In-situ observations of nonlinear internal waves (NIWs) propagating over the northern shelf of Portugal, near the Nazaré submarine canyon (39°47.4′N/009°11.4′W), and evidence of their action on the bottom, are presented and discussed. Synthetic Aperture Radar (SAR) images from this margin suggest the generation of NIWs over the canyon head (39°35′N/009°25′W) and the subsequent propagation directly to the mid-shelf. An observational program was conducted in this area in the summer of 2004, between August 29th and September 1st, to study these short-period internal waves. Temperature and current high resolution measurements were made with thermistor chains and acoustic Doppler current profilers (ADCPs), moored along the canyon generated NIW track, at the 8202m isobath. A simultaneous ENVISAT SAR image was obtained during the experiment. These observations were complemented by a CTD survey and bottom sediment sampling. Collected data show the propagation of large amplitude internal solitons forcing strong bottom current pulses. Turbidity profiles and ADCP echo variability suggest the remobilization of the mid-shelf bottom sediments induced by these short-period events. Calculations of bottom shear stress, induced by the observed canyon generated NIWs, reveal an energetic internal wave activity that can complement, during summer, the role of winter swell in sediment resuspension over the mid-shelf. This work is a contribution to the European project EUROSTRATAFORM, whose objectives include the study of the specific canyon systems dynamics and related sedimentary impacts on the European continental margin.
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| [20] |
Mixing and sediment resuspension associated with internal bores in a shallow bay [J].
Observations of the run-up of internal bores in a shallow bay were made with a tow-yo instrument and mooring arrays with high spatial and temporal resolution. Shoreward propagating internal bores have been studied with laboratory experiments and numerical models, but few observational studies have shown the detailed structure of the run-up of internal bores induced by internal tides. Our observations showed that internal bores propagate along the slope, accompanied by strong turbulent mixing and strong sediment resuspension in a shallow bay. The isothermal displacement due to the bores reached 2002m vertically in a water depth of 4002m. Turbidity measurements showed suspended particles transported from the sloping bottom and offshore above the thermocline, forming an intermediate nepheloid layer (INL). At the head of the bore (dense water), a vortex accompanied by strong vertical motion induced strong vertical sediment resuspension and a steep isothermal displacement. The rate of turbulent kinetic energy dissipation reached 10 616 02W02kg 611 at the head of the wave. A nonhydrostatic numerical simulation in a two-dimensional domain reproduced fine features associated with the run-up of an internal bore and the vortex motion at its head.
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| [21] |
Laboratory observations on internal solitary wave evolution on steep and inverse uniform slopes [J].
Laboratory experiments were conducted to investigate the evolution of internal solitary waves of depression or elevation type reflecting from steep slope in a stratified two-layer fluid system. Environmental settings considered in these experiments included the upper and lower layer thickness, the difference in interface levels and inclination of uniform slope, etc. Physical phenomenon and dynamic mechanism of wave fluctuations are discussed in the course of wave breaking and evolution on the slope. Based on the experimental results available, criteria governing wave evolutions are proposed for an internal wave of depression or elevation encountering uniform slope from normal to inverse configuration. A mirror-image model is hypothesized to provide a generic description of the physical consequences leading to wave breaking and mixing on a wide range of uniform slopes. In addition, mixing efficiency resulting from wave breaking was found to approach a maximum value of 25% when the characteristic length ratio was around 0.5, with a reduction in efficiency for slopes on either side of this peak.
|
| [22] |
Internal solitary waves of elevation advancing on a shoaling shelf [J].
A sequence of three internal solitary waves of elevation were observed propagating shoreward along a near-bottom density interface over Oregon's continental shelf. These waves are highly turbulent and coincide with enhanced optical backscatter, consistent with increased suspended sediments in the bottom boundary layer. Non-linear solitary wave solutions are employed to estimate wave speeds and energy. The waves are rank ordered in amplitude, phase speed, and energy, and inversely ordered in width. Wave kinetic energy is roughly twice the potential energy. The observed turbulence is not sufficiently large to dissipate the waves' energy before the waves reach the shore. Because of high wave velocities at the sea bed, bottom stress is inferred to be an important source of wave energy loss, unlike near-surface solitary waves. The wave solution suggests that the lead wave has a trapped core, implying enhanced cross-shelf transport of fluid and biology.
|
| [23] |
Sediment resuspension and mixing by resonantly generated internal solitary waves [J].
The observation of internal solitary waves (ISWs) propagating upstream along a strongly stratified bottom layer on the California shelf is reported. An increased concentration of particulates in the water column accompanies the passage of these ISW packets. The estimated local Richardson number in the bottom vicinity is around 1/4, and a vertical coefficient of eddy diffusivity of order 10msis associated with the upstream propagating leading ISW. The leading ISW gave rise to reversed flow in an 8-m layer above the bottom. It is argued that the upstream propagating ISWs were generated by resonant flow over bottom topography. Internal waves generated in this way seem to be frequent in the record of a month-long experiment. Model results suggest that the ISWs can carry up to 73% of such generated long wave energy. The ocean conditions at the site are similar to those of other coastal sites, which suggests that the phenomenon described here may be common.
|
| [24] |
On the use of high-frequency acoustics for the study of internal waves and microstructure [J].
Experimental data and theoretical calculations on the scattering of high-frequency acoustic signals from oceanic internal waves are presented. Acoustic data on internal waves are compared with simultaneous temperature (towed thermistor) data. The comparisons have shown a high degree of correspondence between the temperature and the acoustic data. Theoretical calculations for the acoustic scattering cross section 蟽 are made by assuming that temperature fluctuations give rise to the acoustic scattering. An enhanced cross section for scattering from layered temperature fluctuations is to be expected, in agreement with the 1973 calculations of W. H. Munk and C. Garrett.
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| [25] |
Very large subaqueous sand dunes on the upper continental slope in the South China Sea generated by episodic, shoaling deep-water internal solitary waves [J].
Very large subaqueous sand dunes were discovered on the upper continental slope of the northern South China Sea. The dunes were observed along a single 40km long transect southeast of 21.93掳N, 117.53掳E on the upper continental slope in water depths of 160m to 600m. The sand dunes are composed of fine to medium sand, with amplitudes exceeding 16m and crest-to-crest wavelengths exceeding 350m. The dunes' apparent formation mechanism is the world's largest observed internal solitary waves which generate from tidal forcing on the Luzon Ridge on the east side of the South China Sea, propagate west across the deep basin with amplitudes regularly exceeding 100m, and dissipate extremely large amounts of energy via turbulent interaction with the continental slope, suspending and redistributing the bottom sediment. While subaqueous dunes are found in many locations throughout the world's oceans and coastal zones, these particular dunes appear to be unique for two principal reasons: their location on the upper continental slope (away from the influence of shallow-water tidal forcing, deep basin bottom currents and topographically-amplified canyon flows), and their distinctive formation mechanism (approximately 60 episodic, extremely energetic, large amplitude events each lunar cycle).
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| [26] |
Corrigendum: The formation and fate of internal waves in the South China Sea [J].
Internal gravity waves, the subsurface analogue of the familiar surface gravitywaves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by theirbreaking, they impact a panoply of ocean processes, such as the supply of nutrientsfor photosynthesis1, sediment and pollutant transport2 and acoustic transmission3;they also pose hazards for manmade structures in the ocean4. Generated primarilyby the wind and the tides, internal waves can travel thousands of kilometres fromtheir sources before breaking5, posing severe challenges for their observation andtheir inclusion in numerical climate models, which are sensitive to their effects6-7.Over a decade of studies8-11 have targeted the South China Sea, where the oceans鈥檓ost powerful internal waves are generated in the Luzon Strait and steependramatically as they propagate west. Confusion has persisted regarding theirgeneration mechanism, variability and energy budget, however, due to the lack ofin-situ data from the Luzon Strait, where extreme flow conditions makemeasurements challenging. Here we employ new observations and numericalmodels to (i) show that the waves begin as sinusoidal disturbances rather thanfrom sharp hydraulic phenomena, (ii) reveal the existence of >200-m-highbreaking internal waves in the generation region that give rise to turbulence levels>10,000 times that in the open ocean, (iii) determine that the Kuroshio westernboundary current significantly refracts the internal wave field emanating from theLuzon Strait, and (iv) demonstrate a factor-of-two agreement between modelledand observed energy fluxes that enables the first observationally-supported energybudget of the region. Together, these findings give a cradle-to-grave picture ofinternal waves on a basin scale, which will support further improvements of theirrepresentation in numerical climate predictions.
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| [27] |
Recent decadal progress of the study on internal solitons in the South China Sea [J].近10年来南海孤立内波的研究进展 [J].
孤立内波是存在于海洋密度跃层中的非线性大振幅波动。综述了近10年来随着卫星遥感资料、现场观测资料的日益增多和数值模拟技术的发展，关于南海北部孤立内波的特征、形成的动力机制和传播演变规律及孤立内波在海洋工程中的载荷估算等方面所取得的研究成果，并讨论了未来南海孤立内波研究中值得关注的一些问题。
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| [28] |
Internal solitons in the northeastern South China Sea, Part I: Sources and deep water propagation [J].
A moored array of current, temperature, conductivity, and pressure sensors was deployed across the Chinese continental shelf and slope in support of the Asian Seas International Acoustics Experiment. The goal of the observations was to quantify the water column variability in order to understand the along and across-shore low-frequency acoustic propagation in shallow water. The moorings were deployed from April 21-May 19, 2001 and sampled at 1-5 min intervals to capture the full range of temporal variability without aliasing the internal wave field. The dominant oceanographic signal by far was in fact the highly nonlinear internal waves (or solitons) which were generated near the Batan Islands in the Luzon Strait and propagated 485 km across deep water to the observation region. Dubbed trans-basin waves, to distinguish them from other, smaller nonlinear waves generated locally near the shelf break, these waves had amplitudes ranging from 29 to greater than 140 m and were among the largest such waves ever observed in the world's oceans. The waves arrived at the most offshore mooring in two clusters lasting 7-8 days each separated by five days when no waves were observed. Within each cluster, two types of waves arrived which have been named type-a and type-b. The type-a waves had greater amplitude than the type-b waves and arrived with remarkable regularity at the same time each day, 24 h apart. The type-b waves were weaker than the type-a waves, arrived an hour later each day, and generally consisted of a single soliton growing out of the center of the wave packet. Comparison with modeled barotropic tides from the generation region revealed that: 1) The two clusters were generated around the time of the spring tides in the Luzon strait; and 2) The type-a waves were generated on the strong side of the diurnal inequality while the type-b waves were generated on the weaker beat. The position of the Kuroshio intrusion into the Luzon Strait may modulate the strength of the waves being produced. As the waves shoaled, the huge lead solitons first split into two solitons then merged together into a broad region of thermocline depression at depths less than 120 m. Elevation waves sprang up behind them as they continued to propagate onshore. The elevation waves also grew out of regions where the locally-generated internal tide forced the main thermocline down near the bottom. The "critical point" /spl alpha/ where the upper and lower layers were equal was a good indicator of when the depression or elevation waves would form, however this was not a static point, but rather varied in both space and time according to the presence or absence of the internal tides and the incoming trans-basin waves themselves.
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| [29] |
Classification and genesis of continental slopes on the northern South China Sea [J].南海北部陆坡分类及成因分析 [J].
陆坡形态隐含了丰富的地质信息，其差异性是大陆边缘沉积、侵蚀过程长期交互作用的结果。利用横跨南海北部的二维地震测线，采取曲线拟合的研究手段，在南海北部识别出了下凹型、平直型、“S”型等三种类型的陆坡。下凹型陆坡发育在莺歌海-琼东南西部、珠江口中部两个陆坡区，但其成因不同，前者主要受控于快速的沉积物供给，而后者受到陆架边缘三角洲进积和海底峡谷侵蚀的联合作用；平直型陆坡仅见于琼东南东部地区，其主控因素为弱的沉积物供给和较快的构造沉降；“S”型陆坡发育在珠江口地区的两翼，其形成明显受到海流和内波等外作用的改造。不同类型的陆坡具有特定的地层叠置样式、陆架坡折迁移轨迹类型和沉积体系分布特征。对陆坡类型的研究有助于建立沉积过程和产物的预测模式，从而指导古代陆坡的深水油气勘探。
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| [30] |
Genesis of the Nanbeiwei Shoal on the shelf of the Northern South China Sea and its petroliferous significance [J].南海北部陆架南北卫浅滩的成因及油气地质意义 [J].
世界范围内的陆架调查已经清楚，陆架浅滩是普遍发育于陆架区的一种海底地貌类型。在中国陆架上发育多个知名浅滩，这些浅滩和现今动力条件处于动态平衡状态，为现今潮流沉积体系。然而，南海北部陆架的南北卫浅滩在地貌上被称为陆架浅滩，但其成因却和其它陆架浅滩有着根本不同。根据地震剖面解释结果，本文指出，南海北部珠江口东南陆架上发育的南卫滩、北卫滩和惠州滩都属于构造地形，其下部各对应一个底辟构造。这三个底辟构造并不是孤立的，而是在深部属于同一个较大的背冲构造。这个背冲构造位于南北卫滩的下方，其上几个突出的底辟，则和本文讨论的南卫滩、北卫滩和惠州滩相对应。南卫滩、北卫滩和惠州滩在成因上具有统一的深部背景，从而在平面上构成一个统一的底辟系统，本文称为南北卫滩底辟系统。该底辟系统以北卫滩为中心，包括南卫滩、惠州滩和陆丰滩，形成一个大致呈圆形，直径约50km的区域。在构造上，该底辟系统位于珠一坳陷和东沙隆起之间。南北卫滩底辟系统的发育起因于东沙隆起后在南海东北部形成的挤压应力环境。底辟系统的发育活化了惠陆-东沙含油气系统的输导层，并重新调整了油气运移的势差和势梯度，在环绕南北卫滩底辟系统一定距离的圆周上形成一个有利于油气聚集的环带。已知的油气田基本分布于这个环带上。下一步的油气勘探方向应该考虑围绕这个环带重点展开。同时，应该考虑在南北卫滩底辟系统的中心带内开展气藏的勘探工作。
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| [31] |
Turbulent properties of internal waves in the South China Sea [J].
Luzon Strait and South China Sea waters are among the most energetic internal wave environments in the global ocean. Strong tides and stratification in Luzon Strait give rise to internal waves that propagate west into the South China Sea. The energy carried by the waves is dissipated via turbulent processes. Here, we present and contrast the relatively few direct observations of turbulent dissipation in South China Sea internal waves. Frictional processes active in the bottom boundary layer dissipate some of the energy along Chinas continental shelf. It appears that more energy is lost in Taiwanese waters of the Dongsha Plateau, where the waves reach their maximum amplitudes, and where the bottom topography abruptly shoals from 3,000 m in the deep basin to 1,000 m and shallower on the plateau. There, energy dissipation by turbulence reaches 1 W m鈥2, on par with the conversion rates of Luzon Strait.
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| [32] |
Solitons northeast of Tung-Sha Isl and during the ASIAEX pilot studies [J].
In a recent study, satellite images have shown that internal solitons are active in the northern South China Sea (SCS). During the Asian Seas International Acoustic Experiment (ASIAEX) pilot studies, current profiler and thermistor chain moorings were deployed in the spring of 1999 and 2000 to investigate internal solitons northeast of Tung-Sha Island on the continental slope of the northern SCS. Most of the observed internal solitons were first baroclinic mode depression waves. The largest horizontal current velocity, vertical displacement, and temperature variation induced by the internal solitons were around 240 cm/s, 106 m, and 11/spl deg/C, respectively, while the estimated nonlinear phase speed was primarily westward at 152 /spl plusmn/ 4 cm/s. The observed internal solitons could be categorized as four types. The first type is the incoming wave from deep water and can be described reasonably well with the KdV equation. The second and third types are in the transition zone before and close to the turning point (where the upper and lower layer depths are equal), respectively. These two types of solitons were generally near the wave-breaking stage. The fourth type of soliton is a second baroclinic mode and probably was locally generated. The time evolutions are asymmetric, especially at the middle depths. A temperature kink following the main pulse of the soliton is often seen. Higher order nonlinear and shallow topographic effects could be the primary cause for these features. The appearance/disappearance of internal solitons coincides mostly with spring/neap tide. The internal soliton is irregularly seen during the neap tide period and its amplitude is generally small. The time interval between two leading solitons is generally around 12 h. The first baroclinic mode of the semidiurnal tide has a larger amplitude than the diurnal tide and could redistribute its energy into the soliton.
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| [33] |
Breaking and energy analysis of internal solitary wave over a gentle slope [J].缓坡地形上内孤立波的破碎及能量分析 [J].
在大型重力式分层流水槽中对内孤立波沿缓坡地形的演化特征进行了实验研究, 利用分层染色标识方法和多点组合探头阵列技术对其传播特性做了定性分析和定量测量。实验表明: 下凹型内孤立波沿缓坡地形传播过程中的破碎先从波背部发生, 继而演化出上凸型内孤立波; 内孤立波破碎不仅与入射波波幅相关, 而且受到地形坡度的强烈影响; 入射波幅参数α>0.4 是内孤立波不稳定及破碎的实验判据, 内孤立波能量损失出现跃升是其发生破碎的重要特征。研究进一步获得了内孤立波沿缓坡地形的三维演化结构、破碎发生条件和能量变化特性, 对于复杂海洋环境中非线性内波传播特性认识及其动力学建模具有重要的科学意义。
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| [34] |
Experimental investigation of slope sediment resuspension characteristics and influencing factors beneath the internal solitary wave-breaking process [J].
To define the transformation steps and understand the influence of the breaking process of internal solitary waves (ISWs) on slope sediments, experiments were conducted to study the interactions betwe
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| [35] |
Sediment resuspension and nepheloid layers induced by long internal solitary waves shoaling orthogonally on uniform slopes [J].DOI URL [本文引用: 10] 摘要
61We examine sediment resuspension induced by shoaling internal solitary waves.61These waves can resuspend muddy-like sediment and cause nepheloid layers.61The length of the internal swash zone (Ls) is an important parameter.61The length of nepheloid layers depends on impacting wave energy and Ls.61Results may provide guidance for interpreting sedimentary structures.
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| [36] |
Experimental investigation of sediment resuspension beneath internal solitary waves of depression [J].
Abstract Internal solitary waves (ISWs) of depression are common features of coastal environments and believed to resuspend sediments where they shoal. In this study, the sediment resupension process associated with ISWs propagating over a flat bed was investigated in the laboratory. The first-ever profile measurements of the three-dimensional instantaneous velocity field beneath the ISWs revealed that resuspension occurs during burst like vertical velocity events, which lift sediments into the water column, in the adverse pressure gradient region beneath the trailing part of the wave. Resuspension was not observed when the wave-induced viscous bed stress was maximal directly beneath the ISW trough. Prediction of wave-induced resuspension was, therefore, unsuccessful using a traditional viscous bed stress-based Shields diagram. A parameterization for ISW-induced resuspension is proposed as a function of the maximum instantaneous vertical velocity in the bursts . Here we have replaced the viscous bed stress with , where is the instantaneous resuspending bed stress and is the near-bed fluid density. From these results, it is possible for field-oceanographers to predict the occurrence of ISW-induced resuspension from the bulk wave and stratifications characteristics in a two-layer stratification. Further research is required to extend the parameterization to larger Reynolds numbers at field-scale.
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| [37] |
A study of internal waves in the China Seas and Yellow Sea using SAR [J].
Synthetic aperture radar (SAR) images from ERS-1 and ERS-2 have been used to study the characteristics of internal waves in the East China Sea. Rank-ordered packets of nonlinear internal waves in the East China Sea are often observed in the SAR images, especially in the northeast of Taiwan. In this region, the internal wave field is very complicated, and its generation mechanisms include the influence of the tide and the upwelling, which is induced by the intrusion of the Kuroshio across the continental shelf. The internal wave distributions in the East and South China Seas have been compiled based on the SAR observations from satellites. The Kortweg–deVries (KdV) type equation has been used to study the evolution of internal wave packets generated in the upwelling area. Depending on the mixed layer depth, both elevation and depression waves can be generated based on numerical simulations as observed in the SAR images. The merging of two wave packets from nonlinear wave–wave interaction in the Yellow Sea has been observed in the SAR image and is demonstrated by numerical results.
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| [38] |
Energy of nonlinear internal waves in the South China Sea [J].
Four sets of ADCP measurements were taken in the South China Sea (SCS); these results were combined with previous satellite observations and internal-tide numerical model results. Analysis suggests that strong internal tides are generated in Luzon Strait, propagate as a narrow tidal beam into the SCS, are amplified by the shoaling continental slope near TungSha Island, become nonlinear, and evolve into high-frequency nonlinear internal waves (NIW). Internal waves in the SCS have geographically distinct characteristics. (1) West of Luzon Strait the total internal wave energy (E) is 10 脳 that predicted by Garrett-Munk spectra (E) (Levine, 2002). There is no sign of NIW. (2) Near TungSha Island E= 13 脳 E. Strong nonlinear and high-harmonic tides are present. Repetitive trains of large-amplitude NIW appear primarily at a semidiurnal periodicity with their amplitudes modulated at a fortnightly tidal cycle. The rms vertical velocity of NIW shows a clear spring-neap tidal cycle and is linearly proportional to the barotropic tidal height in Luzon Strait with a 1.85-day time lag, consistent with the travel time of internal tides from Luzon Strait to TungSha Island. (3) At the northern SCS shelfbreak E= 4 脳 E. Single depression waves are found, but no multiple-waves packets are evident. (4) On the continental shelf E= 2 脳 E. Both depression and elevation NIW exist.
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| [39] |
Evolution of nonlinear internal waves in the East and South China Seas [J].
Synthetic Aperture Radar (SAR) images from ERS-I have been used to study the characteristics of internal waves northeast and south of Taiwan in the East China Sea, and east of Hainan Island in the South China Sea. Rank-ordered packets of internal solitons propagating shoreward from the edge of the continental shelf were observed in the SAR images. On the basis of the assumption of a semidiurnal tidal origin, the wave speed can be estimated and is consistent with the internal wave theory. By using the SAR images and hydrographic data, internal waves of elevation have been identified in shallow water by a thicker mixed layer as compared with the bottom layer on the continental shelf. The generation mechanism includes the influences of the tide and the Kuroshio intrusion across the continental shelf for the formations of elevation internal waves. The effects of water depth on the evolution of solitons and wave packets are modeled by the nonlinear Kortweg-deVries (KdV) type equation and linked to satellite image observations. The numerical calculations of internal wave evolution on the continental shelf have been performed and compared with the SAR observations. For a case of depression waves in deep water, the solitons first disintegrate into dispersive wave trains and then evolve to a packet of elevation waves in the shallow water area after they pass through a 090008turning point090009 of approximately equal layer depths that has been observed in the SAR image and simulated by the numerical model. The importance of the dissipation effect in the coastal area is also discussed and demonstrated.
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| [40] |
Evolution and breaking of apropagating internal wave instratified ocean [J]. |
| [41] |
Progress of the studies on ocean internal wave breaking [J].海洋内波破碎问题的研究 [J]. |
| [42] |
The advance of internal IW aves breaking and energy dissipation in the ocean [J].海洋内波破碎及其能量耗散的研究进展 [J].
海洋内波是海洋中普遍存在的波 动形式。它的不稳定和破碎会对海洋能量的再分配产生本质的影响,直接将能量从大尺度向小尺度过程传递,产生湍流混合。内波破碎产生的湍流混合又会显著地影 响海洋环境。该文就近十五年来对内波的破碎机制以及内波破碎导致的能量耗散研究作一简要综述。
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| [43] |
Numerical experiments on the breaking of solitary internal waves over a slope shelf topography [J].
Abstract A theoretical study of the transformation of large amplitude internal solitary waves (ISW) of permanent form over a slope–shelf topography is considered using as basis the Reynolds equations. The vertical fluid stratification, amplitudes of the propagating ISWs, and the bottom parameters were taken close to those observed in the Andaman and Sulu Seas. The problem was solved numerically. It was found that, when an intense ISW of depression propagates from a deep part of a basin onto the shelf with water depth Hs, a breaking event will arise whenever the wave amplitude am is larger than 0.4(Hs 61 Hm), where Hm is the undisturbed depth of the isopycnal of maximum depression. The cumulative effect of nonlinearity in a propagating ISW leads to a steepening and overturning of a rear wave face over the inclined bottom. Immediately before breaking the horizontal orbital velocity at the site of instability exceeds the phase speed of the ISW. So, the strong breaking is caused by a kinematic instability of t...
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| [44] |
Internal wave breaking at concave and convex slopes [
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| [45] |
Numerical study of the breaking of an internal soliton and its interaction with a slope [J].
The full Navier-Stokes and diffusion equations are applied to study the breaking of an internal soliton on the continuously stratified pycnocline in a two-layer system and its interaction with a slope. First, these equations are solved numerically to study the limiting height and breaking of the soliton in the case of constant total depth. Breaking occurs when the particle velocity in a region of flow field exceeds the wave celerity. This results in a gravitational instability with a patch of dense water entraining into the upper layer in the lee of the wave. The numerically determined breaking criterion is supported by an estimate using the first-order Korteweg-de Vries (KdV) theory. Then, the model is used to examine the interaction of the soliton with a slope-shelf topography and a uniform slope. In both cases, the relative depths of the layers change at the turning point along the slope. Mechanisms of the wave breaking and wave propagation processes for both cases are described. Scaled bottom stresses and total wave run-up on the slope are also presented.
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| [46] |
Numerical experiments on internal gravity waves in an accelerating shear flow [J].
The influence of an accelerating shear flow on the propagation of an internal gravity wave in a continuously stratified fluid is studied by means of two-dimensional numerical simulations. These are motivated by earlier laboratory experiments [Thorpe, S.A. 1978b. On internal gravity waves in an accelerating shear flow, Vol. 88. J. Fluid Mech. pp. 623鈥639]. In these experiments the mean flow is an accelerated Couette flow and the mean density profile is linear. The laboratory experiments revealed the striking effect of the unsteady shear flow in the evolution of an internal gravity wave leading to the wave focusing in a region where the flow is extremum. This phenomenon is associated with the growth of small scale density fluctuations. As a result density overturns are sometimes observed. This behaviour is well reproduced by the numerical simulations. We provide insights on the flow dynamics in particular on the possible occurrence of wavebreaking. We show that the dynamics is characterized by two competitive mechanisms that is a damping of the wave and a local enhancement of its steepness leading sometimes to density overturns. The budget for the energy of the wave reveals that the initial damping of the wave results from wave-mean flow interactions. These interactions lead to the development of a fine scale vertical density structure which is associated with high vertical shear. We find that in some cases wavebreaking occurs as a result of shear instability. The value of the acceleration of the mean flow is very likely to influence the onset of the instability. The scaling laws of the wave evolution, in particular the rate of decrease of its energy, are determined. From these laws the lifetime of the wave is found as a function of the acceleration of the shear. It may be expected that, in the ocean, this development will result in the largest fluctuations derived from wave-flow interactions occurring where the mean flow in the wave direction is greatest. Waves travelling normal to a two-dimensional shear flow will be unchanged. Waves travelling parallel will be damped. This may have particular application at the continental shelf where flow, mainly parallel to the isobaths, will damp waves travelling along-slope, but allows waves travelling normal to the isobaths (e.g., directly across the shelf-break) to be transmitted without attenuation. Similar effects are expected for the evolution of a high frequency wave interacting with a lower frequency (e.g., near inertial) motion.
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| [47] |
Simulations of shear instabilities in interfacial gravity waves [J].
An adaptive numerical method is employed to simulate shear instabilities in open-ocean internal solitary-like gravity waves. The method is second-order accurate, employs block-structured adaptive mesh refinement, solves the incompressible Navier090009Stokes equations and allows for the simulation of all of the length scales of interest by dynamically tracking important regions with recursively-nested finer grids. Two-dimensional simulations are performed over a range of parameters, which allows us to assess the conditions under which the shear instabilities in the waves occur, including a method to evaluate the critical Richardson number for instability based on the bulk wave parameters. The results show that although the minimum Richardson number is well below the canonical value of 1/4 in all simulations, this value is not a sufficient condition for instability, but instead a much lower Richardson number of 0.1 is required. When the Richardson number falls below 0.1, shear instabilities develop and grow into two-dimensional billows of the Kelvin090009Helmholtz type. A linear stability analysis with the Taylor090009Goldstein equation indicates that an alternate criterion for instability is given by 0303i Tw > 5, where 0303i is the growth rate of the instability averaged over Tw, the period in which parcels of fluid are subjected to a Richardson number of less than 1/4. A third criterion for instability requires that Lw/L > 0.86, where Lw is half the length of the region in which the Richardson number falls below 1/4 and L is the solitary wave half-width. An eigendecomposition of the rate-of-strain tensor is performed to show that the pycnocline thickness increases within the wave because of pycnocline-normal strain and not because of diffusion, which has important ramifications for stability. A three-dimensional simulation indicates that the primary instability is two-dimensional and that secondary, three-dimensional instabilities occur thereafter and lead to strong dissipation and mixing.
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| [48] |
Internal tide and nonlinear internal wave behavior at the continental slope in the Northern South China Sea [J].
A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.
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| [49] |
Shear-induced breaking of large internal solitary waves [J].
The stability properties of 24 experimentally generated internal solitary waves (ISWs) of extremely large amplitude, all with minimum Richardson number less than 1/4, are investigated. The study is supplemented by fully nonlinear calculations in a three-layer fluid. The waves move along a linearly stratified pycnocline (depthh2) sandwiched between a thin upper layer (depthh1) and a deep lower layer (depthh3), both homogeneous. In particular, the wave-induced velocity profile through the pycnocline is measured by particle image velocimetry (PIV) and obtained in computation. Breaking ISWs were found to have amplitudes (a1) in the range$a_1\,{>}\,2.24\sqrt{h_1h_2}(1+h_2/h_1)$, while stable waves were on or below this limit. Breaking ISWs were investigated for 0.27 <h2/h1< 1 and 4.14 <h3/(h1+h2) < 7.14 and stable waves for 0.36 <h2/h1< 3.67 and 3.22 <h3/(h1+h2) < 7.25. Kelvin–Helmholtz-like billows were observed in the breaking cases. They had a length of 7.9h2and a propagation speed 0.09 times the wave speed. These measured values compared well with predicted values from a stability analysis, assuming steady shear flow withU(z) and ρ(z) taken at the wave maximum (U(z) horizontal velocity profile, ρ(z) density along the verticalz). Only unstable modes in waves of sufficient strength have the chance to grow sufficiently fast to develop breaking: the waves that broke had an estimated growth (of unstable modes) more than 3.3–3.7 times than in the strongest stable case. Evaluation of the minimum Richardson number (Rimin, in the pycnocline), the horizontal length of a pocket of possible instability, with wave-inducedRi< 14, (Lx) and the wavelength (λ), showed that all measurements fall within the rangeRimin= 610.23Lx/λ + 0.298 ± 0.016 in the (Lx/λ,Rimin)-plane. Breaking ISWs were found forLx/λ > 0.86 and stable waves forLx/λ < 0.86. The results show a sort of threshold-like behaviour in terms ofLx/λ. The results demonstrate that the breaking threshold ofLx/λ = 0.86 was sharper than one based on a minimum Richardson number and reveal that the Richardson number was found to become almost antisymmetric across relatively thick pycnoclines, with the minimum occurring towards the top part of the pycnocline.
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| [50] |
Boundary layer separation driven vortex shedding beneath internal solitary waves of depression [J].
We investigate global instability and vortex shedding in the separated laminar boundary layer beneath internal solitary waves (ISWs) of depression in a two-layer stratified fluid by performing high-resolution two-dimensional direct numerical simulations. The simulations were conducted with waves propagating over a flat bottom and shoaling over relatively mild $(S= 0. 05)$ and steep $(S= 0. 1)$ slopes. Over a flat bottom, the potential for vortex shedding is shown to be directly dependent on wave amplitude, for a particular stratification, owing to increase of the adverse pressure gradient ($\mathrm{d} P/ \mathrm{d} x\gt 0$ for leftward propagating waves) beneath the trailing edge of larger amplitude waves. The generated eddies can ascend from the bottom boundary to as high as 33 % of the total depth in two-dimensional simulations. Over sloping boundaries, global instability occurs beneath all waves as they steepen. For the slopes considered, vortex shedding begins before wave breaking and the vortices, shed from the bottom boundary, can reach the pycnocline, modifying the wave breaking mechanism. Combining the results over flat and sloping boundaries, a unified criterion for vortex shedding in arbitrary two-layer continuous stratifications is proposed, which depends on the momentum-thickness Reynolds number and the non-dimensionalized ISW-induced pressure gradient at the point of separation. The criterion is generalized to a form that may be readily computed from field data and compared to published laboratory experiments and field observations. During vortex shedding events, the bed shear stress, vertical velocity and near-bed Reynolds stress were elevated, in agreement with laboratory observations during re-suspension events, indicating that boundary layer instability is an important mechanism leading to sediment re-suspension.
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| [51] |
Experimental evidence of internal solitary wave-induced global instability in shallow water benthic boundary layers [J].
Experimental evidence is presented in support of the theoretical prediction ofDiamessis and Redekopp [J. Phys. Oceanogr.36,784(2006)] for wave-induced vortex shedding at the lower solid boundary of a stratified fluid system as a result of global instability. The time-dependent boundary layer induced by a strongly nonlinear internal wave of depression in shallow water is examined experimentally. Measurements of the velocity field close to the bottom boundary illustrate coherent periodic shedding of vortex structures at the lower boundary in the adverse pressure gradient region aft of the wave. The vortical structures ascend high into the water column and cause significant benthic turbulence. It is shown that global instability has a critical threshold dependent on the Reynolds number of the flow and the amplitude of the wave. The critical amplitudes observed are approximately half that predicted byDiamessis and Redekopp [J. Phys. Oceanogr.36,784(2006)], indicating that internal wave-induced benthic mixing may be even more prominent than previously thought.
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| [52] |
Large fully nonlinear internal solitary waves: The effect of background current [J].
In this paper we consider what effect the presence of a nonconstant background current has on the properties of large, fully nonlinear solitary internal waves in a shallow, stratified ocean. In particular, we discuss how the amplitude of the largest nonbreaking wave that it is possible to calculate depends on the background current as well as the nature of the upper bound. We find that the maximum wave amplitude is given by one of three possibilities: The onset of wave breaking, the conjugate flow amplitude or a failure of the wave calculating algorithm to converge (associated with shear instability). We also discuss how wave properties such as propagation speed, half-width, etc. vary with background current amplitude.
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| [53] |
Sediment resuspension mechanisms associated with internal waves in coastal waters [J].
[1] Large-amplitude, vertically trapped internal waves can induce sizable velocities and trigger hydrodynamic instabilities in the bottom boundary layer, thereby contributing to the resuspension of sediments and the maintenance of sediment concentration in the water column. We discuss numerical simulations of several different situations in which the boundary layer in the wave footprint undergoes hydrodynamic instability, with a resultant increase in the incidence of spatiotemporal structures that could facilitate sediment resuspension. For the case of internal solitary waves we provide bounds in parameter space separating regions in which internal waves can be expected to efficiently resuspend sediment from those in which the boundary layer in the wave footprint is both laminar and stable. A notable finding is that the onset of instability is a strong function of the background current. The Lagrangian transport of passive particles due to the instability is explored, and some quantitative measures of the efficiency of the particle transport process are provided. We subsequently discuss the evolution of the power spectra of the bottom shear stress with time and find that while the general characteristics of the instability are robust, lowering either the Reynolds number or the strength of the background current leads to an increase in the typical length scales associated with the mature instability. Finally, we discuss instabilities during the internal wave generation process and alternative instability mechanisms when the bottom is not flat.
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| [54] |
Numerical investigation of solitary internal wave-induced global instability in Shallow Water Benthic boundary layers [J].
ABSTRACT The time-dependent boundary layer induced by a weakly nonlinear solitary internal wave in shallow water is examined through direct numerical simulation. Waves of depression and elevation are both considered. The mean density field corresponds to that typical of the coastal ocean and lakes where the lower fraction of the water column is subject to the stabilizing effect of a diffuse stratification. Sufficient resolution of the "inviscid" dynamics of the boundary layer is ensured through use of a Legendre spectral multidomain discretization scheme in the vertical direction. At higher Reynolds numbers, where the simulations become underresolved, because of restrictions in available computational resources, spectral accuracy and numerical stability at the scales of physical interest are preserved through use of a penalty scheme in the vertical and explicit spectral filtering. Thus, a highly accurate description of the qualitative dynamics of the wave-induced global instability is possible and finescale physical mechanisms critical to the appearance of this instability are not smeared out by the high artificial dissipation inherent in lower-order finite-difference schemes. Results indicate that, for a wave amplitude exceeding a critical value, the global instability occurs in regions near the bottom boundary where the wave induces an adverse pressure gradient. The structure of the associated separation bubble is modified through the establishment of coherent and synchronous dynamics, characterized by elevated levels of bottom shear stress and a periodic shedding of coherent vortex structures. Although details of the vortex shedding depend on the particular wave forcing involved, these vortical structures always ascend high into the water column. All findings suggest that this global instability is a potent mechanism for benthic turbulence, mixing, and possible sediment resuspension in shallow waters, presumably even more intense than the nominal turbulent boundary layer.
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| [55] |
Near-bed solibores over the continental slope in the Faeroe-Shetland Channel [J].
On two occasions within a 12-day measurement period in the Faeroe-Shetland Channel, strongly nonlinear wave trains were observed at the sea-bed, propagating up the continental slope in water depths >450 m. The events were separated by a period of 4 days and, whilst resembling in appearance a density current running up a slope, are termed ‘solibores’, displaying the properties of both turbulent internal bores and nonlinear internal solitary waves (ISW). Each solibore displays a steep leading edge followed by a train of nonlinear waves with amplitudes of O(10 m) and periods of 655–20 min. Wave-induced particle velocities are consistent with ISW, whilst a zone of strong horizontal convergence at the leading edge of the solibores causes the formation of a rotor with flow in the opposite sense to a ‘forward overturning’ surface wave. In both cases upward vertical velocities >10 cm s 611 are immediately followed by a return downward flow of equal magnitude. A CTD and microstructure transect conducted during the passage of the first solibore illustrates its behaviour as an up-slope intrusion of cold, dense water with concurrent rates of turbulent dissipation, ε, of O(10 617 W kg 611) and resulting in short-term maximum vertical diffusivities, K z, of O(10 611 m 2 s 611). In the long-term however, K z<10 614 m 2 s 611, implying that the solibores are not important for sustaining deep-sea mixing. In contrast, sediment fluxes at 2 and 30 m above the bed during the first solibore are O(10 2) larger than the background value, implying solibores are the dominant sediment transport mechanism despite their intermittent occurrence. The transport of sediment, up the slope in the direction of propagation of the solibore, is contrary to the usually considered pathway for sediment from coastal seas to the abyss via the continental slope. That sediment transport is dominated by short-term events appears to be corroborated by long-term data (>140 days) which again indicate intermittency in periods of enhanced sediment flux. The solibores are consistent with the results of numerical and laboratory experiments on hydraulic jumps resulting from shoaling ISW. The influence of the slope is proposed to be the reason for their slightly different forms and effects on sediment transport; thus the first solibore, propagating directly up the slope, is proposed to result from an overturning hydraulic jump caused by kinematic instabilities and which subsequently forms a horizontal density intrusion. The second solibore represents a dispersive wave train due its oblique direction of propagation which reduces the effective bottom-slope, allowing dispersion to balance nonlinear effects and prohibiting overturning.
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| [56] |
Flow separation and resuspension beneath shoaling nonlinear internal waves [J].
[1] Laboratory observations are presented showing the structure and dynamics of the turbulent bottom boundary layer beneath nonlinear internal waves (NLIWs) of depression shoaling upon sloping topography. The adverse pressure gradient beneath the shoaling waves causes the rear face to steepen, flow separation to occur, and wave-induced near-bottom vortices to suspend bed material. The resuspension is directly attributed to the near-bed viscous stress and to near-bed patches of elevated positive Reynolds stress generated by the vortical structures. These results are consistent with published field observations of resuspension events beneath shoaling NLIWs. Elevated near-bed viscous stresses are found throughout the domain at locations that are not correlated to the resuspension events. Near-bed viscous stress is thus required for incipient sediment motion but is not necessarily a precursor for resuspension. Resuspension is dependent on the vertical velocity field associated with positive Reynolds stress and is also found to occur where the mean (wave-averaged) vertical velocity is directed away from the bed. The results are interpreted by analogy to the eddy-stress and turbulent bursting resuspension models developed for turbulent channel flows.
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| [57] |
Numerical simulations of the interaction of internal waves with a shelf break [J].
The energetics of the interaction of internal gravity waves with a shelf break is investigated by means of high-resolution two-dimensional numerical simulations, with an emphasis on understanding the partitioning of the internal wave energy over the course of the interaction process and the subsequent dynamics of the onshore propagating internal waves. Some of the energy is dissipated as a result of the instabilities associated with breaking, while the remaining energy is either reflected back away from or transmitted onto the shelf. We employ an analysis of the distribution of the energy flux across the shelf break taking into account the contributions from nonhydrostatic as well as nonlinear effects to quantify the percentage of energy flux that is transmitted onto the shelf, as well as the percentages of reflected and dissipated energy fluxes, from an incoming wave field. For a given frequency of an incoming wave, we vary the amplitude of the wave to vary the incident energy flux, and we simulate conditions ranging from subcritical to supercritical slopes by varying the topographic slope angle. The results show that the cumulative transmitted energy flux is a strong function of the ratio of the topographic slope纬, to wave characteristic slopes, while the reflected energy flux is a strong function of both纬鈭晄as well as the nonlinearity. The energy flux calculations indicate that the internal boluses that form as a result of the interaction of the incident wave with the slope are very energetic, especially for critical to supercritical slopes. These nonlinear internal waves are plausible candidates for effectively transporting mass onshore, not withstanding their contribution to diapycnal mixing as well.
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| [58] |
Near-Bottom Turbulence and Sediment Resuspension Induced by Nonlinear Internal Waves[R]. |
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Wave flume experiments on the contribution of seabed fluidization to sediment re-suspension [J].
Sediment resuspension is commonly assumed to be eroded from the seabed surface by an excess bottom shear stress and evolves in layers from the top down. Although considerable investigations have argued the importance of wave-induced seabed fluidization in affecting the sediment resuspension, few studies have been able to reliably evaluate its quantitative contribution till now. Attempt is made to preliminarily quantify the contribution of fluidization to resuspension using a series of large-scale wave flume experiments. The experimental results indicated that fluidization of the sandy silts of the Huanghe Delta account for 52.5% and 66.8% of the total resuspension under model scales of 4/20 and 6/20 (i.e., relative water depth: the ratio of wave height to water depth), respectively. Some previously reported results obtained using the same flume and sediments are also summarized for a contrastive analysis, through which not only the positive correlation is confirmed, but also a parametric equation for depicting the relationship between the contribution of fluidization and the model scale is established. Finally, the contribution of fluidization is attributed to two physical mechanisms: (1) an attenuation of the erosion resistance of fluidized sediments in surface layers due to the disappearing of original cohesion and the uplifting effect resulting from upward seepage flows, and (2) seepage pumping of fines from the interior to the surface of fluidized seabed.
|
| [60] |
Verticalmigration of fine-grained sediments from interior to surface of seabed driven by seepage flows-‘sub-bottom sediment pump action’ [J]. |
| [61] |
Numerieal Analysis for Dynamic Response and Liquefaction Potential of Seabed under Wave Loading[D]. 波浪作用下海床动力响应与液化的数值分析[D] . |
| [62] |
Mechanism of seabed scour and its critical condition estimation by considering seepage forces [J].考虑渗流力的海床临界冲刷机理及计算方法 [J].
从波浪引起的海床内部渗流与床面泥沙运动耦合的角度出发，研究波浪作用下海床临界冲刷机理及计算方法。研究表明，波浪作用下床面泥沙起动及冲刷是一个不断向下发展的过程，最终达到一个临界冲刷深度。波浪引起的渗流力能够显著降低泥沙的临界起动切应力，促进泥沙起动，是影响海床冲刷的一个重要因素。将渗流力引入到传统泥沙起动公式中，推导并给出了波浪作用下海床临界冲刷深度的计算方法。结合室内和现场两个算例，很好地解释了波浪水槽试验中海床"流化"现象和黄河水下三角洲粉砂流冲沟等灾害地貌特征及成因，初步验证了该方法用于评价和计算海床冲刷的有效性。
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| [63] |
Breaking wave-induced response of composite breakwater and liquefaction in seabed foundation [J].
61Interaction between breaking wave-breakwater-porous seabed is investigated.61Biot's dynamic “u-p” formulation is used for dynamic soil behaviour.61Breaking wave-induced impact on breakwater is determined by VARANS equation.61Breaking wave-induced dynamics of breakwater and its foundation is determined quantitatively.61Breaking wave-induced transient liquefaction in seabed foundation is captured effectively.
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| [64] |
Hsu J R C. Interaction between internal waves and a permeable seabed [J].
The mechanism of sediment resuspension is also discussed based on analytical solutions for the wave-induced soil displacements, instead of empirical equation as used in previous works. The numerical results calculated from these solutions provide a preliminary insight to quantify internal wave-induced sediment resuspension observed on a continental shelf slope in the ocean. Soil displacement on the seabed is estimated to reach 40 and 15聽m in the horizontal and vertical directions, respectively, in the same order as observed in field condition in the northeastern South China Sea.
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| [65] |
The effects of a porous-elastic seabed on interfacial wave propagation [J].
A theoretical model for the decay of progressive interfacial gravity waves propagating above a porous bed is developed assuming potential flow in a two-layer system with a free surface and a sharp interface. A new wave dispersion relation for two-layer flow above a quasi-static porous seabed is derived and investigated. The solutions for the nonlinear wave profile are derived using a perturbation method and the effects of geometric and flow parameters including bed characteristics, depth ratios and the densities of the two fluids are studied and discussed. Comparisons with existing analytical solutions for viscous interfacial wave attenuation over a rigid bed demonstrate the relative importance of the porous bed as a mechanism for wave decay. It is shown that the influence of a porous seabed on wave propagation is significant when the depth of the lower layer, normalised by the wavenumber, is less than 蟺 蟺 mathContainer Loading Mathjax .
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| [66] |
Viscous attenuation of interfacial waves over a Porous Seabed [J].
Abstract WILLIAMS, S.J., and JENG, D.-S., 2007. Viscous attenuation of interfacial waves over a porous seabed. In this paper, we study the viscous damping of waves on a stably stratified fluid over a porous elastic seabed. Using the linearised Navier-Stokes equations as the governing equations of motion, the newly derived solution describes the attenuation of a progressive surface wave on a two-layer stratification due to damping caused by interfacial shear and from the wave interaction with a porous elastic seabed. The analytical results are discussed in the light of experimental observations and the viscous effects are considered by comparison with the previous solution for a rigid impermeable seabed. It is shown that the overall decay scale for viscous attenuation is determined by the relative wavelength and the non-dimensional depth of the lower fluid layer. Wave attenuation due to the porous bed is at a maximum when the lower overlying fluid layer is approximately 50% thicker than its Stokes boundary layer.
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| [67] |
Bed failure induced by internal solitary waves [J].
The pressure field inside a porous bed induced by the passage of an Internal Solitary Wave (ISW) of depression is examined using high‐accuracy numerical simulations. Assuming a two‐layer, continuously...
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| [68] |
Analysis oninternal solitary wave-induced dynamic response characteristics of surface sediments in the Northern South China Sea [J].内孤立波作用下南海北部陆坡沉积物动力响应特征分析 [J].
为探究内孤立波作用下南海北部陆坡浅表沉积物的动力响应特征,采用实测内孤立波和沉积物参数,建立两层流内孤立波-无限深度海床模型,基于二维Boit固结理论,计算不同沉积物物理力学参数和内孤立波参数条件下,无限深度内位移场和超孔隙水压力的分布情况,探讨不同变量下内孤立波引发的沉积物动力响应特征.研究表明南海北部陆坡沉积物的渗透率变化对动力响应特征的影响较小,孔隙度影响程度中等,剪切模量和饱和度影响较大,且饱和度是首要影响因素;细砂、砂质粉砂和黏土质粉砂三种南海北部陆坡浅表沉积物响应的超孔隙水压力在相同深度时逐渐减小;水平位移逐渐增大,垂向位移逐渐减小.
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| [69] |
Analysis of Internal Solitary Wave-induced Dynamic Response Characteristics of Surface Sediments in the Northern South China Sea[D]. 内孤立波作用下南海北部陆坡浅表沉积物动力响应特征分析[D] . |
| [70] |
Application of Similarity Principles and Turbulence Research to Bed-load Movement [M]. |
| [71] |
Coastal and Estuarine Sediment Dynamics [M]. |
| [72] |
Threshold of Sediment Motion in Coastal Environments [
|
| [73] |
Critical shear stress for cohesive sediment transport [J]. |
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Principles of Sediment Transport in Rivers, Estuaries and Coastal Seas [M]. |
| [75] |
Experimental Analysis on Characteristics of Sediment Resuspension, Transport and Sedimentation in the Process of Internal Solitary Wave Shoaling Breaking[D]. 内孤立波浅水破碎过程坡面沉积物悬浮、输运及沉积特征实验分析[D] . |
| [76] |
Topographically generated internal waves and boundary layer instabilities [J].
Flow over topography has been shown to generate finite amplitude internal waves upstream, over the topography and downstream. Such waves can interact with the viscous bottom boundary layer to produce vigorous instabilities. However, the strength and size of such instabilities depends on whether viscosity significantly modifies the wave generation process, which is usually treated using inviscid theory in the literature. In this work, we contrast cases in which boundary layer separation profoundly alters the wave generation process and cases for which the generated internal waves largely match inviscid theory. All results are generated using a numerical model that simulates stratified flow over topography. Several issues with using a wave-based Reynolds number to describe boundary layer properties are discussed by comparing simulations with modifications to the domain depth, background velocity, and viscosity. For hill-like topography, three-dimensional aspects of the instabilities are also discussed. Decr...
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| [77] |
An intermittent, intermediate nepheloid layer in Emerald Basin, Scotian Shelf [J].
Intermittent, intermediate nepheloid layers were observed on the Scotian Shelf in late April, 1987. These 10–30 m thick nepheloid layers were found between 140 and 200 m in the main part of Emerald Basin and 120–140 m in the northern basin, well below the surface mixed layer and about 90 m above the sea bed in both cases. Both biological and physical considerations are used to assess mechanisms for the observed intermediate nepheloid layers. Depths of intermediate nepheloid layers coincide with the critical depth for possible generation and amplification of internal waves with semi-diurnal ( M 2 ) internal tidal frequency. Intermittent particle resuspension at the “critical” depth on the Basin slope appears to be the likely cause of the observed intermittency of the intermediate nepheloid layers. Observations of intermediate nepheloid layers at the three stations in the main part of Emerald Basin and the northern basin indicate that this process is not a rare event, but rather a basin-wide phenomenon.
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| [78] |
Local and global aspects of the bottom nepheloid layers in the world ocean [J].
The bottom 1 km of the ocean is appreciably more turbid than the two or more km above it. This bottom nepheloid layer has been mapped principally by nephelometers that are most sensitive to particles 猹2 渭m in diameter. However, the layers contain larger aggregated particles which play an important role in sedimentation. The thickness of the nepheloid layer is much greater than the height above the bed to which turbulent mixing occurs, a fact explained by lateral transport of turbid mixed layers that have detached from the bottom. The overall upwards decline in turbidity is accounted for by the layers higher above the bed having come farther and being on average older than those lower down. The most intense nepheloid layers are found in areas of strong western boundary currents and in regions where the bottom current regime is thought to be highly variable due to penetration of eddy energy from the surface. Areas under the Gulf Stream and in the Argentine basin are examples. Review of the distribution and characteristics of nepheloid layers examines features contributing critically to these hypotheses. An important contributing factor to the depth distribution of turbidity is that the ratio of water volumes to bed areas in contact with them is eight times higher for water between 4 and 5 km than that between 2 to 3 km.
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| [79] |
Large-amplitude internal solitary waves observed in the Northern South China Sea: Properties and energetics [J].
ABSTRACT Five large-amplitude internal solitary waves (ISWs) propagating westward on the upper continental slope in the northern South China Sea were observed in May-June 2011 with nearly full-depth measurements of velocity, temperature, salinity, and density. As they shoaled, at least three waves reached the convective breaking limit: along-wave current velocity exceeded the wave propagation speed C. Vertical overturns of ~100 m were observed within the wave cores; estimated turbulent kinetic energy was up to 1.5 x 10(-4) W kg(-1). In the cores and at the pycnocline, the gradient Richardson number was mostly KE nor the subsurface maximum of the along-wave velocity for shoaling ISWs, but does simulate the total energy and the wave shape. Including the background shear in the model results in the formation of a surface trapped core.
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| [80] |
The shaping of continental slopes by internal tides [J].
The angles of energy propagation of semidiurnal internal tides may determine the average gradient of continental slopes in ocean basins (approximately 2 to 4 degrees). Intensification of near-bottom water velocities and bottom shear stresses caused by reflection of semi-diurnal internal tides affects sedimentation patterns and bottom gradients, as indicated by recent studies of continental slopes off northern California and New Jersey. Estimates of bottom shear velocities caused by semi-diurnal internal tides are high enough to inhibit deposition of fine-grained sediment onto the slopes.
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| [81] |
Role of internal waves in the generation of nepheloid layers on the northwestern Alboran slope: Implications for continental margin shaping [J].
[1] The role of internal waves in the sediment dynamics of the northwestern Alboran continental slope was investigated in a selected area around the Guadiaro submarine canyon. Nepheloid layer distribution was identified using closely spaced CTD/transmissometer profiles collected during two hydrographic surveys. A well-defined pattern of suspended sediment distribution consisting of surface, intermediate, and near-bottom nepheloid layers was found. Intermediate and bottom nepheloid layers were always observed within the canyon and on the adjacent continental slope, spanning mainly from 200 to 500 m depth. In addition, a current meter with a turbidimeter was deployed in the lower section of the Guadiaro Canyon at 600 m depth, 25 meters above the seafloor. Time series analysis indicated that the currents, temperature, and turbidity within the canyon fluctuate mainly at semidiurnal tidal frequencies, suggesting the presence of semidiurnal internal tides affecting the near-bottom suspended sediment concentration along the canyon axis. High-resolution bathymetry from the study area was used to evaluate the internal wave reflection conditions at semidiurnal tidal frequency for the entire continental slope region. Critical slope conditions were reached on the upper continental slope and along the canyon axis, coinciding with the region in which nepheloid layers were observed. This region also coincides with a zone of erosion on the upper continental slope of the study area previously identified by Hern脙隆ndez-Molina [1993]. These results indicate that the generation of intermediate and bottom nepheloid layers, as well as the erosion and shaping of the northwestern Albor脙隆n continental slope, may result from the interaction of internal waves and the seafloor morphology.
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| [82] |
Sediment transport processes from the topset to the foreset of a crenulated clinoform (Adriatic Sea) [J].
Crenulated clinoforms of complex and uncertain origin characterize large portions of the Late-Holocene prograding mud wedge in the western Adriatic continental shelf. Sediment failure was originally postulated as the most plausible mechanism for the formation of the crenulations. Subsequent work has shown that, although the origin of the crenulations may have been related to deformation processes, their maintenance through time seems to be better explained by different sediment accumulation rates in the flat and steep flanks. In order to establish relationships between active sediment dynamics, across-shelf transport and sediment accumulation in these crenulated clinoforms, two tripods and a mooring were deployed off the Pescara River during autumn and winter 2002鈥2003 as part of the EuroSTRATAFORM program, and in combination with the Po and Apennine Sediment Transport and Accumulation (PASTA) study. The tripods were placed on the shallow topset region and close to the clinoform roll-over point (i.e., offlap break), at 12 and 20-m water depth, respectively, and the mooring was located at 50-m depth, in the crenulated foreset region. Several sediment-resuspension events were recorded, mainly related to Bora and Sirocco storms, during which wave鈥搊rbital and current velocities increased considerably. Sediment transport in the topset region was predominantly towards the SE, following the direction of the coastal current and the bathymetry, but showing a significant offshore component at the roll-over point that was intensified during storm events. Currents at the foreset region were also directed to the SE. In mid-waters they were clearly aligned with the local bathymetry, whereas near the bottom they had an important and persistent offshore component. This current behavior seems to be associated with an intense bottom Ekman transport that causes the near-bottom current to be deflected to the left (i.e., offshore) with respect to the direction of the surface current. This mechanism enhances the suspended-sediment transport from the topset down the foreset region along the Adriatic prograding mud wedge, contributing to the basinward clinoform progradation and controlling the depth of the clinoform roll-over point. In addition, activity of near-inertial internal waves was also recorded by the near-bottom instrument deployed in the foreset region. During periods characterized by a strong near-inertial signal, increases of the water turbidity clearly coincided with an intensified offshore velocity component, which suggest that this mechanism also contributes to the transport of suspended sediment across the clinoform. Both the bottom Ekman transport and the internal waves are mechanisms that could be responsible for the formation/maintenance of the Adriatic seafloor crenulations until present-day, although several arguments suggest that the latter likely plays the major role.
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| [83] |
Morphobathymetric analysis of the large fine-grained sediment waves over the Gulf of Valencia continental slope (NW Mediterranean) [J].
Detailed analysis of recently acquired swath bathymetry, together with high-resolution seismic profiles and bottom sediment samples, revealed the presence of large-scale fine-grained sediment waves over the Gulf of Valencia continental slope. As many other deep-water sediment waves, these features were previously attributed to gravitational slope failure, related to creep-like deformation, and are here reinterpreted as sediment wave fields extending from 250聽m depth to the continental rise, at ~聽850聽m depth. Geometric parameters were computed from the high-resolution multibeam dataset. Sediment wave lengths range between 500 and 1000聽m, and maximum wave heights of up to 50聽m are found on the upper slope, decreasing downslope to minimum values of 2聽m high. Sediment waves on the lower part of the slope are quasi-stationary vertically accreting, whereas they show an upslope migrating pattern from the mid-slope to the upper part of the continental slope. High-resolution seismic profiles show continuous internal reflectors, with sediment waves merging down-section and sediment wave packages decreasing in thickness downslope. These sediment packages are thicker on the crest of each individual sediment wave and thinner on the downslope flank. 210 Pb analyses conducted on sediment cores collected over the sediment wave fields also indicate slightly higher sediment accumulation rates on the wave crests. Sediment wave formation processes have been inferred from contemporary hydrodynamic observations, which reveal the presence of near-inertial internal waves interacting with the Gulf of Valencia continental slope. Internal wave activity is suggested to be the preferential mechanism for the transport and deposition of sediment, and the maintenance of the observed sediment wave fields.
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| [84] |
Spatio-temporal evolution of sediment waves developed on the Gulf of Valencia margin (NW Mediterranean) during the Plio-Quaternary [J].
Several fields of large-scale sediment waves have been observed along the Gulf of Valencia continental margin (NW Mediterranean). The largest sediment waves develop on the continental slope, extending from 250 to 85002m water depth, with wavelengths ranging between 50002m and 100002m and wave heights from ~02202m to ~025002m. On the lower part of the slope, sediment waves are quasi-stationary “vertically accreting”, becoming up-slope migrating towards the mid- and upper part of the slope. A second group of sediment waves have developed over the outer continental shelf, with wavelengths of 400 to 80002m and heights of 2 to 402m, also displaying an up-slope migrating pattern. Multi-channel seismic lines crossing the continental margin show that the sediment waves over the continental slope region have been continuously developed on the foreset region of the prograding margin clinoform. Several units of sediment waves have been identified in the sedimentary record, evolving in accordance with the margin progradation. Detailed analysis of single-channel (sparker) seismic profiles revealed the presence of several sediment depositional subunits over the outer continental shelf, some of them with successive development of sediment waves being truncated by erosive surfaces, likely related to Quaternary eustatic sea-level oscillations. These erosional surfaces can be followed downslope into paraconformable strata of the sediment waves on the continental slope, where constant bedform growth is observed, without being affected by sea level changes. Based on geophysical data, the thickness of the sediment waves mapped units show that the largest sediment waves (in wave ratio, length and height) develop where sediment deposition rates are the highest, coinciding with the upper part of the continental slope (foreset clinoforms), confined by the presence of structural highs. The development of these sediment waves has been previously explained by the interaction of internal waves over the continental slope. Because sediment waves are preserved in the sedimentary record since the Lower/Pliocene, internal waves activity could have been present in this part of the margin shortly after the Zanclean reflooding of the Mediterranean Basin, following the Messinian desiccation event ~025.6 My ago. Deep water hydrodynamic conditions were re-established at that time, modulating sediment transport and deposition over the continental slope and outer continental shelf.
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| [85] |
The role of Internal Solitary Waves on deep-water sedimentary processes: The case of up-slope migrating sediment waves off the Messina Strait [ |
| [86] |
Internal wave turbulence in the Llobregat prodelta (NW Mediterranean) under stratified conditions: A mechanism for sediment waves generation? [J].
An array of 76 high-resolution temperature sensors at 0.502m intervals between 5 and 42.502m off the bottom was moored near the Barcelona harbor buoy in 8102m water depth, between October 2013 and April 2014. The mooring was located just seaward of an extensive sediment wave area developed in the Llobregat River prodelta, with 102m high crests parallel to the coast and 50–10002m wavelengths. In the NW-Mediterranean, the thermal stratification reaches its maximum penetration through the water column in autumn until it is broken by winter convection. Such a deep stratification affects large-scale sub-inertial slope currents, which are mostly confined to the upper half of the water column, by the hampered vertical exchange of frictional turbulence, and supports near-bottom internal waves between the inertial and buoyancy frequencies. Observed onshelf propagating frontal bores most likely interact with the sediment waves and contribute to their generation, as they are trailed by considerable shear-induced turbulence and high-frequency internal waves close to the buoyancy frequency that have wavelengths matching those of the sediment waves. The bores are either driven by near-inertial or 3–702day periodic sub-inertial motions just following a brief period of large convective instability at the end of the offshelf flow phase.
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| [87] |
Internal-wave characteristics of strong bottom currents at the sand-wave zone of the northern South China Sea and its role in sand-wave motion [J].南海北部沙波区海底强流的内波特征及其对沙波运动的影响 [J].
2008年3月6日至2008年4月9日, 在南海北部外陆架与陆坡上的沙波区进行了海底流速的连续观测，观测结果表明潮流与海流较弱，但时有流速达30—77cm<em>.</em>s-1的海底强流发生。强流方向与南海北部内波传播方向相对应，多分布在偏NW向与偏SE向。偏SE向流强于偏NW向流，与内波在传播方向上的下坡流大于上坡流的特征一致。对流速序列进行了旋转功率谱分析，结果表明，高于M2分潮的频率中，众多的振荡分量具有内波流性质，说明阵发性强流为内波所致。采用观测流速计算了沙波的移动速度，计算结果得出强流能起动海底泥沙，由于NW向传播（上坡方向）的内波导致了SE向（下坡方向）的净流动，沙波偏SE向移动，但沙波移动速度不大，小型沙波移动速度小于1.6m<em>.</em>a-1。采用潮流、风暴潮耦合模型计算了强台风驱动的海底流速过程，表明潮流、风暴潮耦合也能移动海底沙波，但沙波移动方向与台风路径相关，不一定为SE向，且移动距离更小，潮流、风暴潮耦合不是沙波移动的主要动力机制。
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| [88] |
The distribution and characteristics of very large subaqueous and dunes in the Dongsha region of the northern South China Sea [J].南海北部东沙海域巨型水下沙丘的分布及特征 [J].
本文基于多波束测深和高分辨率多道反射地震数据研究了东沙海域深水巨型水下沙丘的特征．巨型水下沙丘发育在230～830m水深的上陆坡范围内，呈斑块状分布．NW—SE向的近海底流体运动不仅冲蚀地层，形成了三条与水下沙丘间隔分布的冲蚀带，为水下沙丘提供了沉积物来源，同时也为水下沙丘的形成提供了动力源．研究区水下沙丘波长（L）范围55～510rn，波高（h）范围1．5～20m，二者呈指数关系分布．沙丘的波长随水深增大而增大，波高则在500～700m水深范围内最大．水下沙丘NE—sw向展布的脊线和几何参数关系是与现今水动力条件相平衡的结果．
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| [89] |
Internal-wave and internal-tide deposits of the Middle-Upper Ordovician in the Center Tarim Basin [J].塔中地区中晚奥陶世内波, 内潮汐沉积 [J].
现代海底普遍发育由内波、内潮汐引起的深水牵引流,内波、内潮汐不仅可形成各种厘米级的深水牵引流沉积构造,还可建造千米级的大型沉积物波。这种大型沉积物波既可沿斜坡向下迁移,也可沿斜坡向上迁移。在古代地层记录中已发现各种厘米级的内波、内潮汐沉积单元,但尚未发现内波成因的大型沉积物波。塔中地区中上奥陶统碎屑岩段内发育内波及内潮汐形成的各种牵引流沉积构造单元。通过详细的岩心观察,识别出 4种内波、内潮汐沉积微相类型及 5种基本垂向沉积层序。此外,通过地震剖面分析,在研究区中上奥陶统陆坡相中识别出顺坡向上迁移的大型沉积物波,该沉积物波的特征与现代海底发育的沉积物波的特征类似,运用内波理论可对其成因进行合理的解释。
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| [90] |
Deep-water sediment waves formed by internal waves [J].内波单独作用形成的深水沉积物波 [J].
深水沉积物波是一种海底普遍发育、规模较大的波状沉积体,大多数学者将它们解释为等深流沉积或浊流沉积。本文结合内波理论的研究进展，考虑内波沉积作用的水动力学特征，探讨了深水沉积物波的内波成因机制。得出以下几点认识：①海底流动单独作用无法满足沉积物波形成所需的流动层厚度及流动速度，较难解释沉积物波的迁移方向及规则的内部及外部形态。②内波可以引起海底流动，内波比表面波更容易形成更大规模范围内的沉积床形。③内波可以形成大型沉积物波，用内波可以较合理地解释内波的对称波形单元、非对称波形单元及上攀波形单元的成因。波动面离海底距离较大的行进内波及内驻波可以形成对称波形的沉积物波；波动面离海底距离较近的行进内波及内孤立波可以形成非对称波形的沉积物波；内波引起的海底流动进一步增强时，可形成上攀波形沉积物波。④行进内波可以形成向内波传播相反方向迁移的沉积物波，向海盆内部传播的内波可以形成向上坡方向迁移的沉积物波。
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| [91] |
Sea bottom landslide in the SHENHU area on the north margin of south China sea and triggering mechanisms [J].南海北部神狐海域海底滑坡在地震剖面上的识别及形成机制 [J].
海底滑坡广泛发育于海底陆坡,是沉积物往深海盆地搬运的重要方式。大规模的突发性海底滑坡事件不但可以改变海底地形地貌,还可能触发海啸,给沿海地区居民的生命和财产带来巨大损失。对海底滑坡地震响应特征进行研究,可以推断其物质来源、形成机制以及演化模式,为人类的防灾减灾提供科学依据。神狐海域地处被动大陆边缘,海底构造复杂,大陆坡较陡,具有发育海底滑坡的条件。结合最新采集的三维地震数据,在南海北部神狐海域发现了多期海底滑坡,研究表明,这些海底滑坡在地形上具有以下特征:在坡度剧烈变化处,有弧形主陡坎,沿着主要陡坎的两侧向下延伸则会出现近平行的侧壁,在海底峡谷侧壁和在较顺直的海底斜坡上存在圈椅状陡坎或陡壁。在对研究区海底滑坡的地震响应特征分析的基础上,研究了海底滑坡的地貌特征、内部结构及几何形态,探讨了南海北部陆坡神狐海域海底滑坡的形成机制,对于开展区域海洋工程,防止灾害发生具有重要意义。
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| [92] |
Characteristics of hazardous geological factors on Shenhu continental slope in the northern South China Sea [J].南海北部神狐陆坡区灾害地质因素特征 [J].
南海北部神狐陆坡区富含海洋油气资源和天然气水合物,其海底地质环境对于各项资源开采活动和工程建设尤为重要,但目前专项研究较少.在大量二维地震资料解译的基础上,结合浅地层剖面和多波束测深资料,对该区海底地质环境进行整体研究,识别出20种灾害地质因素.按照动力来源,将该海域地质灾害归纳为构造应力、重力、水动力、气动力和土动力5大类灾害地质类型,每种类型包含多种灾害地质因素.依据各灾害因素总体平面分布特征,划分出埋藏三角洲密集区、海底滑坡密集区、火山密集区、软弱层密集区、浅断裂密集区和浅埋基岩面密集区6个灾害大区.还对主要灾害因素的地震反射特征和灾害性进行了研究,为该区未来工程建设的顺利开展提供科学的参考.
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| [93] |
The Mechanism and Prediction of Deepwater Geohazard in the northern of South China Sea[D].Qingdao: Institute of Oceanology,
南海北部陆坡深水区地质灾害机理与钻前预测[D] .
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| [94] |
Distribution of Cenozoic igneous rocks and its relation to submarine geological hazards in the deepwater area of the northern South China Sea [J].南海北部深水区新生代岩浆岩分布规律及其与海底地质灾害的相关性 [J].
前人对南海北部新生代陆缘盆地结构、构造样式和期次等做了大量的研究工作，但对于具体体现内动力过程的岩浆活动关注较少，尤其是北部深水区的研究鲜有涉及。本文基于二维地震剖面的解译分析，将南海北部新生代岩浆岩划分为3个集中分布区：琼东南-西沙区、神狐区和东沙区。各区域岩浆岩的产出状态有所差异，其中琼东南-西沙区岩浆岩的平面展布规律性极强，主要受右行右阶的走滑断裂控制；神狐区岩浆岩位于 NW 向与 NE 向断裂的交汇处，侵位空间受先存断裂制约；东沙区岩浆岩的产生与东沙运动期间区域伸展过程有直接关系。作为内动力的表征，岩浆作用对于海底地形地貌具有重要的改造作用，与活动断裂、海底滑坡、浅层气等海底地质灾害也具有成因上的关联。
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| [95] |
Dongsha erosive channel on northern South China Sea Shelf and its induced Kuroshio South China Sea Branch [J].南海北部东沙海底冲蚀河谷及其成因探讨 [J].
对覆盖南海北部东沙区的50多条多道地震剖面进行了解释.通过剖面解释,发现东沙隆起后,南海北部海底地形发生了较大变化.这其中包括东沙岛在陆架坡折处的隆起,以及沉积地层以较大的幅度向上翘起.向上翘起的地层在很大的范围内遭受海底剥蚀.在东沙隆起区的北侧,海底发育下切沟.本文解释为现代海底在更强烈的剥蚀条件下形成的海底冲蚀地形,称为海底冲蚀沟.海底冲蚀沟分布于东沙岛的北侧,大致与陆架坡折线,以及600m等深线相平行,其宽度一般在20km左右,NE-SW向长度可达200km,本文称为东沙海底冲蚀河谷.无论是冬季还是夏季,东沙岛以北的吕宋海峡都有黑潮分支从太平洋贯入南海.这一黑潮分支在冬季演化为太平洋-印度洋贯通流,在夏季演化为吕宋海峡次表层流.东沙海底冲蚀谷的分布与太平洋-印度洋贯通流,以及吕宋海峡次表层流的位置完全一致.本文认为,黑潮分支在遇到南海北部陆坡的阻挡后,黑潮水体由陆坡向陆架堆积,并因水深变浅,水体通过空间变小而使流速加大,从而有能力对海底产生剥蚀.东沙区海底地层的剥蚀,以及东沙海底冲蚀谷的发育都是黑潮分支在东沙陆架区形成的强底流和海底长期耦合作用的结果.
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| [96] |
Shoaling of large-amplitude nonlinear internal waves at Dongsha Atoll in the northern South China Sea [J].
Shoaling of large-amplitude (6510002m) nonlinear internal waves over a steep slope (653°) in water depths between 10002m and 28502m near Dongsha Atoll in the northern South China Sea is examined with an intensive array of thermistor moorings and a bottom mounted Acoustic Doppler Current Profiler. During the 4402h study period in May 5–7, 2008, there were four groups of large internal waves with semidiurnal modulation. In each wave group a rapid transition occurred during the shoaling, such that the front face of the leading depression wave elongated and plunged to the bottom and the rear face steepened and transformed into a bottom-trapped elevation wave. The transitions occur in water depths of 20002m and deeper, and represent the largest documented internal wave shoaling events. The observations repeatedly capture the detailed temperature and velocity structures of the incident plunging waves. Strong horizontal convergence and intense upward motion are found at the leading edge of transformed elevation waves, suggesting flow separation near the bottom. The observations are compared with the previous observations and model studies. The implication of the shoaling internal waves on coral reef ecology also is discussed.
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| [97] |
A numerical study of the generation and propagation of internal solitary waves in the Luzon Strait [J].
Un nouveau modèle composite, associant un modèle de génération de marées internes et un modèle de propagation d’ondes longues régularisée est proposé pour l’étude de la génération et de l’évolution d’ondes solitaires internes dans un détroit à seuil. Les ondes de marée internes dans le détroit à seuil sont d’abord simulées par le modèle de génération; le champ de marée interne à l’extérieur du seuil sert à initialiser le modèle de propagation. Des expériences numériques sont conduites pour étudier les effets de la marée imposée, du profil de profondeur, de la largeur du canal et des haut-fonds, etc. sur la génération et l’évolution des ondes solitaires internes. C’est seulement quand l’amplitude de la marée interne à la limite de for04age du modèle de propagation atteint une certaine valeur qu’un train d’ondes solitaires internes est induit. L’amplitude de la marée initiale dans le modèle de génération, l’effet de haut-fond, l’asymétrie du profil de profondeur et la largeur du canal agissent quelque peu sur l’amplitude de l’onde solitaire interne induite. Le flux de marée surimposé à un courant moyen constant a un effet majeur d’amortissement sur les ondes internes induites, spécialement sur celles qui se propagent contre la direction du courant moyen. La génération et la propagation d’ondes solitaires internes dans trois des seuils du détroit de Luzon sont simulées et les raisons de leur asymétrie de propagation sont également expliquées.
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| [98] |
The direct breaking of Internal Waves at Steep Topography [J].
Internal waves are often observed to break close to the seafloor topography that generates them, or from which they scatter. This breaking is often spectacular, with turbulent structures observed hundreds of meters above the seafloor, and driving turbulence dissipations and mixing up to 10,000 times open-ocean levels. This article provides an overview of efforts to observe and understand this turbulence, and to parameterize it near steep supercritical topography (i.e., topography that is steeper than internal wave energy characteristics). Using numerical models, we demonstrate that arrested lee waves are an important turbulence-producing phenomenon. Analogous to hydraulic jumps in water flowing over an obstacle in a stream, these waves are formed and then break during each tidal cycle. Similar lee waves are also observed in the atmosphere and in shallow fjords, but in those cases, their wavelengths are of similar scale to the topography, whereas in the ocean, they are small compared to the water depth and obstacle size. The simulations indicate that these nonlinear lee waves propagate against the generating flow (usually the tide) and are arrested because they have the same phase speed as the oncoming flow. This characteristic allows estimation of their size a priori and, using a linear model of internal tide generation, computation of how much energy they trap and turn into turbulence. This approach yields an accurate parameterization of mixing in numerical models, and these models are being used to guide a new generation of observations.
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| [99] |
Analysis of spatial distribution characteristics of seabed sediments critical starting velocity in the northern South China Sea [J].南海北部海底沉积物临界起动流速空间分布特征分析 [J].
为研究南海北部海底沉积物临界起动流速空间分布特征,通过自测和资料收集得到45个站位沉积物物性资料,根据沉积物类型选用适合的方法计算临界起动流速,并评判计算结果的适用性,结合沉积物空间分布规律和统计结果插值,共得到112个站位数据,绘制成沉积物临界起动流速空间分布图。研究结果显示南海北部沉积物临界起动流速存在空间差异性,其分布特征与沉积物粒径分布规律基本对应并受其控制。与南海上层流场进行对比分析,得到冬夏两季陆架区沉积物起动难易程度分布图,为研究海洋动力对南海北部海底沉积物改造机制及海底沉积演变规律提供依据。
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| [100] |
Remote sensing evidence for baroclinic tide origin of internal solitary waves in the northeastern South China Sea [J].
Evidence for baroclinic tide origin of internal solitary waves (ISWs) in the northeastern South China Sea is presented, based on 116 internal wave packets observed in satellite images from 1995 to 2001. These wave packets can be divided into two types, a single-wave ISW packet containing only one ISW with/without an oscillating tail, and a multiple-wave ISW packet composed of a group of rank-ordered ISWs. All of the 22 single-wave ISW packets occur in the deep water zone. It is suggested that the ISWs, instead of being generated by the lee-wave mechanism, are developed by nonlinear steepening of the baroclinic tides, which are produced by the strong tidal currents flowing over a ridge in Luzon Strait. This suggestion is verified by an ERS-2 SAR image, which records such an evolution process from a baroclinic tide to a single ISW in its spatial domain.
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| [101] |
Distortion and broadening of internal solitary wave front in the northeastern South China Sea deep basin [J].
Internal solitary waves (ISWs) with peculiar fronts are frequently observed in the world ocean by satellite images, though with quite few explanations. In this study a distorted and broadening ISW front across the northeastern South China Sea deep basin is presented by using synthetic aperture radar (SAR) image. To illustrate this peculiar front, a nonlinear refraction model is developed to simulate and evaluate the effects of realistic bottom topography, current, and stratification on its transformation. Simulated results in realistic oceanic environments show good agreements with this SAR-observed front. Based on separate and comparative results in different background environments, we demonstrate that the distortion is actually caused by the strong mesoscale currents at periphery of an anticyclonic eddy. Moreover, the broadening is due to the difference in change of wave half width at different rays, which is associated with the different transformation of ISWs across variable bottom topography in the deep basin.
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