平直沙坝海岸叠加波浪的裂流试验

为研究叠加波浪场的裂流特征,在平直沙坝海岸地形进行了叠加波浪形成的沿岸波高周期性变化的裂流试验研究。试验中叠加波浪是由波浪在垂直岸线的丁坝反射所形成的两列交叉波浪叠加产生,交叉波浪是具有等频率但入射角相反的两波列。通过对叠加波浪节腹点垂直岸线位置浪高的测量和沙坝范围内沿岸布置的声学多普勒测速仪流速测量结果来分析沙坝海岸丁坝反射波形成的裂流特性,讨论了波浪节腹点对裂流位置和裂流空间尺度的影响。对不同周期情况在x=5 m沙坝顶处的速度剖面对比,分析了不同周期对裂流的影响。     

近岸波生流运动三维数值模拟及验证

开发建立了近岸波生流运动三维数值计算模式。模式中,引入了三维时均剩余动量、破波表面水滚、波浪水平与垂向紊动作为主要驱动力,同时考虑了波流共同作用的底部剪切力。推导了可综合反映底坡、能量传递率和密度影响的水滚能量传输方程;将Larson-Kraus的二维波浪水平紊动系数表达式拓展至三维。采用大量实测数据和文献资料测试验证了所建模式,表明所建模式可有效模拟波浪增减水、底部离岸流、沿岸流、裂流、堤后环流等不同维度的波生流现象。此外,研究也表明破波水滚效应可解释波生流峰值向岸推移的物理现象,从而在模拟中不能忽略;破波带内沿岸流速垂向较为均匀的现象与波浪附加垂向紊动有关。     

波生沿岸流数值模拟

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

甘肃厂坝地区中—晚泥盆世早期三种不同的海岸沉积类型及其找矿意义

现代大陆边缘海岸类型是多姿多态的,但若按照影响海岸类型的古地形和水动力作用过程来划分,大致可以分为三种类型:一种是以波浪作用为主,称为无障壁的海岸;第二种是以潮汐作用为主,称为障壁海岸;第三种是两者兼而有之的以波浪和潮汐共同作用的,称为过渡海岸。这三种海岸类型都有其独特的一套标志,这对于古代已经消失的不同时期众多海岸类型是一种很好识别标志。我国特大型铅锌矿床所在的厂坝地区,就是一个在中一晚泥盆世早期同一地区并存三种海岸类型的地区。尽管它们有许多共同特征,如水体活     

燕山中、新元古代裂陷槽构造旋回层序研究──兼论裂陷槽构造旋回概念及级序的划分

本文依据裂陷槽构造控制及演化规律的原则提出了裂陷槽构造旋回层序的概念，并同时给出了相应的三级划分方案。应用裂陷槽构造旋回层序的概念研讨了燕山地区中、新元古代裂陷槽构造旋回层序的发育特征。燕山裂陷槽发育４个构造阶段旋回层序，即裂张断陷、裂张均衡、均衡负载和盖层构造阶段旋回层序，其中又有序叠加了１８个构造期和８２个构造幕旋回层序。前一类层序主要受控于导致裂陷槽形成、发育的区域构造；后两类构造旋回层序受裂陷槽构造演化及所驱动的构造海平面双重控制。     

沙坝地形上沿岸流不稳定运动模式变化特性

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

波流共存场中多向随机波浪传播变形数学模型

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

南海北部长湾风暴潮贝壳堤的沉积特征及发育模式

南海北部长湾贝壳堤是一典型的风暴潮成因贝壳堤。该贝壳堤的横向、纵向露头和钻孔所显示的地层结构、层理特征、粒度特征、14C测年、物质组成、贝屑种属生境及当地风暴增水高度和地壳升降幅度表明:①贝壳堤由风暴沉积组成,是风暴潮增水期间海岸沉积物堆积的结果,由于堆积在平均高潮面之上,增水退却后不受正常天气下波浪的影响得以保存;②理想剖面由下而上分4个部分:潮滩堆积、激浪带堆积、增水面下波浪堆积和增水面上波浪爬高堆积,各个部分具有不同的沉积特征,分别代表贝壳堤向上增长的各个阶段的沉积环境和风暴作用方式;③风暴潮贝壳堤与非风暴潮贝壳堤不仅在发育过程,而且在沉积结构、粒度特征、贝屑种属、堆积部位、堆积高度及环境意义等方面都有所不同。     

中国地台区张裂盆地沉积

本文着重讨论中国地台区某些张裂盆地的沉积与演化特征。张裂盆地可分为陆内断陷和裂谷、克拉通边缘裂陷槽和被动大陆边缘。裂谷在发展过程中依次形成玄武岩-红层-蒸发盐组合或玄武岩-红层-煤系组合→深湖相组合→红层组合。中小型断陷的岩石组合类似但多不完整。豫陕裂陷槽的岩石组合为下部硅碎屑岩组合,以钾玄岩系为主体的火山岩组合,上部硅碎屑岩-碳酸盐组合,以及较深水相泥硅质岩组合。地处被动大陆边缘的扬子地台南部早寒武世海是以波浪作用为主的陆架,其东南边缘的石煤可能是远洋、半远洋沉积。本文还扼要讨论了张裂盆地在控制某些矿产资源的形成与分布方面的一些问题。     

波流共同作用下流速垂线分布及其影响因素分析

大量波流相互作用的实验表明,当波浪顺流传播时,平均水面附近的流速减小,而当波浪逆流传播时,平均水面附近的流速增加.为了从理论上解释这种现象,从垂向二维Navier-Stokes方程出发,推导出波浪、潮流共同作用下的水流控制方程;采用微幅波理论简化控制方程,并引入Grant-Madsen波流边界层模型,得到波流共同作用下的紊动切应力及流速垂线分布表达式.通过和实验结果及数值结果的对比,结果预测值和实测值较为吻合,表明公式能反映波高变化及涡粘系数共同对流速垂线分布的影响,且简单实用.利用该公式分析了波浪顺流、逆流流速变化的原因.     

Wave-induced nearshore circulation along the Calangute-Candolim beach,Goa, west coast of India

The wave-induced nearshore circulation model suggested by Noda has been modified and applied for three small segments along the coast of Goa. The present model incorporates the prevailing bottom topography and considers its variation along with the radiation stress as the driving force for the circulation. We find that the flow pattern is strongly dependent on bottom topography. While normal incidence of waves results in a cellular pattern of flow, meandering flows prevail for oblique incidence along the coast. The shoreward flows are always located over shoals while the rip currents prevail over channels. The onshore/offshore flows show magnitudes as high as 3·1 m/s, while those alongshore reach a maximum of 1·1 m/s. When compared with field observations these values are slightly higher.     

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

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

Alongshore variation in the rip current hazard at Pensacola Beach, Florida

Many drowning and near drownings at Pensacola Beach, Florida are attributed to rip currents, the strong seaward-flowing currents that extend from the shoreline to the line of breakers and sometimes beyond. While surf forecasts assume that the rip hazard is uniform alongshore and that the (erosion) rips are ephemeral features, evidence is presented to suggest that the rip hazard at Pensacola Beach is not uniform alongshore. Rather the rip current “hotspots” develop as a consequence of an alongshore variation in the surf similarity parameter and nearshore state on the order of ~1,450 m. The variation is forced by transverse ridges on the inner shelf that force wave refraction and focusing at the ridge crests. This creates a more dissipative, rhythmic bar and beach morphology at the ridges and rougher surf. Between ridges, where wave heights and periods are smaller and the outermost bar is forced closer to the shoreline, the nearshore is in a (more reflective) bar and rip state during red flag conditions. Drownings between 2000 and 2009 are shown to be clustered between transverse ridges and in the years following a hurricane or tropical storm (2000–2003 and 2005–2008) when the bar and rip morphology first develops as the shore face recovers. This continues until the innermost bar attaches to the beach face unless the bar system is reset by another tropical storm or hurricane. It is argued that the rip hazard is dependent on the alongshore covariation of the environmental forcing with the individual and group behavior in both time and space, even on what appears to be a relatively uniform beach environment.     

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

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

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

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

Sediment density flow distribution on wave-influenced deltas

Deltas are at the transition between fluvial and marine sedimentary environments where sediment density flows are often triggered during high river discharge events, forming submarine channels and sediment waves. On wave-influenced deltas, longshore currents are particularly efficient at transporting sediment alongshore, reducing the likelihood of sediment density flows from occurring at river mouths. This study describes four deltaic sedimentary systems at different stages of their evolution on a formerly glaciated continental inner shelf of eastern Canada in order to better understand the distribution of sediment density flows on wave-influenced deltas. Three types of settings are recognized as being prone to sediment density flows: (i) in the early stages of wave-influence and on large deltas, converging longshore currents can lead to offshelf sediment transport; (ii) on wave-influenced to wave-dominated deltas, a sandy spit can re-route the river mouth and sediment density flows form where the spit intersects the delta lip; (iii) in advanced stages of wave-dominated deltas and during their demise, rocky headlands are exposed and can intersect the slope, where off-shelf sediment transport occurs. These types of sediment density flows were all characterized by debris flows or surge-type turbidity currents which have limited offshore run-out. More rarely, hyperpycnal flows form at the river mouths, especially where the river incises glaciomarine clays prone to landsliding in the river, which increases fine-grained fluvial suspended sediment concentration. Overall, these results highlight the predominance of fluvial-dominated deltas during a phase of relative sea-level fall combined with high sediment supply. However, as soon as sediment supply diminishes, wave action remobilizes sediment alongshore modifying the distribution and types of sediment density flows occurring on wave-influenced deltas.     

Evaluation of swimmer-based rip current escape strategies

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

Computer model of wind, waves, and longshore currents during a coastal storm

A mathematical model has been developed to forecast or hindcast wind, waves, and longshore currents during the passage of a coastal storm. Storm intensity is a function of the barometric pressure gradient which is modeled by rotating an inverted normal curve around the center of an ellipse. The length and orientation of the major and minor axes of the ellipse control the size and shape of the storm. The path of the storm is determined by a sequence of storm positions for the hindcast mode, and by interpolated positions assuming constant speed and direction for the forecast mode. The site location, shoreline orientation, and nearshore bottom slope provide input data for the shore position. The geostrophic wind speed and direction at the shore site are computed from the latitude and barometric pressure gradient. The geostrophic wind is converted into surface wind speed and direction by applying corrections for frictional effects over land and sea. The surface wind speed and direction, effective fetch, and wind duration are used to compute wave period, breaker height, and breaker angle at the shore site. The longshore current velocity is computed as a function of wave period, breaker height and angle, and nearshore slope. The model was tested by comparing observed data for several coastal locations with predicted values for wind speed, wave period and height, and longshore current velocity. Forecasts were made for actual storms and for hypothetical circular and elliptical storms.