莱州湾悬沙输运机制研究

基于2012年实测的潮流、含沙量及表层沉积物数据及资料等,分析了潮流、余流、潮流底应力及底质类型对含沙量变化的影响,并运用物质通量分析方法,探讨了莱州湾悬浮泥沙的输运机制.研究结果表明:研究海域受半日潮控制呈往复流特征,涨、落潮期间近底含沙量与流速及潮流底应力显著相关,存在明显的再悬浮现象,含沙量呈现潮周期变化特征;底质类型与含沙量大小密切相关,细颗粒物质更容易发生悬浮;平流输运与潮泵效应是莱州湾海域的悬沙输运的主要动力因素.     

福建省东北部沿海罗源湾互花米草盐沼环境下粘性沉积物的侵蚀-沉降过程

根据在福建罗源湾互花米草盐沼内的沉积动力要素(水深、流速、悬沙浓度和沉降速率)的观测及表层沉积物的粒度分析结果,罗源湾互花米草盐沼表层沉积物组份以粉砂和粘土为主,为粘性沉积物;互花米草盐沼内的潮流流速很小,最大值不超过7cm/s,相应地,底部切应力也相对较小,悬沙在潮周期内多处于沉降状态,表层沉积物在涨潮初期和落潮后期多发生侵蚀.在台风显著影响下,潮流流速平均值变化很小,但水流紊动能量和底部切应力显著增大,导致表层沉积物多处于侵蚀状态,在台风登陆期间潮周期内净侵蚀通量可达到40.099kg/m2;台风影响过后,底部切应力逐渐减小,沉积物逐渐不发生侵蚀,悬沙多发生沉降,整个潮周期内以悬沙沉降为主.     

长江河口区上段水、悬沙通量及其对北支演化的意义

2007年9月在长江河口3条控制断面的全潮观测结果表明,长江口南支均以落潮流和落潮输沙占优,而北支大、小潮期间的水沙输运特征迥异;其特征与长江径流、河口地形地貌特征密切相关.小潮期间,长江口北支以落潮流占优;大潮期间,则以涨潮流占优,且悬沙输运率比小潮期间增大一个数量级.分析结果进一步表明,从1958年至今,长江口北支的分流比呈下降的趋势,已由11.8％降至目前的1.9％;长江口北支也由早期的悬沙输入(与径流方向相反)通道变为输出通道,目前其分沙比仍达6.4％ ～7.9％左右.总体上,分流分沙比呈显著减小趋势,这是长江口北支萎缩的重要特征之一.此外,北支分沙比显著大于分流比,将可能造成北支的进一步淤积.     

长江口泥沙输移扩散与沉降过程高分辨率声学现场观测

河口悬沙属性的现场观测对于泥沙输移和沉积过程研究具有重要的意义,现场超声探测较其他观测技术和方法具有较大的优势.本文应用ASSM-Ⅱ型声学悬浮泥沙观测系统对长江口悬沙输移扩散进行高分辨率观测,洪季涨潮期现场观测发现:①在潮流和颗粒重力共同作用下,抛泥泥沙同时存在输移扩散和沉降过程;②抛泥泥沙输移扩散存在2种基本的模式,即表层低密度羽状流和底层高密度羽状流.在正常天气涨潮流作用下,泥沙沉降可形成二级泥跃层,平流输送和沿程密度差产生的压力梯度形成底层高密度羽状流,它是泥沙输移扩散的主要方式.     

海岸与陆架沉积:动力过程、全球变化影响和地层记录

文章试图在海岸与陆架沉积的基本动力过程、全球变化影响和地层记录等方面提出新的科学问题和基础研究建议.在基本动力过程研究上,底部边界层过程的垂向尺度和悬沙浓度影响、陆架环流和水团运动的悬沙输运效应、河口与陆架密度流形成机理、大型河流的水下三角洲和斜坡沉积的形成过程、浊流运动特征以及物理海洋过程响应等是值得重视的问题;在全球变化影响方面,需深入研究流域变化和生物礁演化与碳埋藏的关系、沉积环境的系统状况转换及动力过程变异、沉积体系演化的定量模拟方法等问题;在地层记录方面,应加强陆架与海岸沉积体系中会球变化记录的高分辨率信息提取、多种沉积记录信息的整合方法、沉积体系和记录形成的数值模拟、气候变化效应的地层记录模拟、沉积体系的信息分析模型等研究.     

台风风暴潮影响下潮滩沉积动力模拟初探--以江苏如东海岸为例

以江苏如东潮滩为研究区,采用沉积动力学垂向二维概念模型来模拟正常天气和台风期间潮滩沉积的空间分布特征,探讨台风风暴潮对潮滩正常沉积层序的改造作用.模拟结果表明,在涨落潮时间-流速对称特征明显的如东海岸,潮汐作用使潮滩沉积呈显著的分带性,且剖面形态向“双凸形”演化,两个“凸点”分别位于平均高潮位和平均低潮位附近.在台风期间风暴增水效应下,开边界悬沙浓度差异将导致潮滩冲淤和沉积分布格局的变化,潮上带和潮间带上部均堆积泥质沉积物,潮间带中下部在风暴过程中普遍遭受不同程度的砂质沉积物侵蚀或之后堆积泥质沉积物,在沉积层序中形成风暴冲刷面.因此,潮滩的风暴沉积记录存在于潮间带上部或更高部位.以此模型为基础,可进一步综合考虑极浅水边界层水动力结构、沉积物粒度分布变化、波-流联合作用、台风降水、互花米草等生物活动、潮沟摆动及人工围垦等因素,从而建立风暴事件在沉积层序中的时间序列,更好地解译沉积记录中的古环境信息.     

波流边界层水沙运动数值模拟——II.泥沙运动模拟

为解析波流边界层内泥沙运动,建立了基于水动力-泥沙-床面互馈过程的波流边界层1DV泥沙数学模型,可用于模拟不同床面形态下粉沙-沙的含沙量过程。床面形态模块提供床面形态类型和相应参数;给出了平底和沙波床面粗糙高度和泥沙扩散系数的确定方法;采用了适宜粉沙及沙的制约沉速、底部参考浓度和起动剪切应力等公式;引入含沙量层化效应和制约沉降反映水动力与泥沙之间的相互影响。水槽试验资料验证表明,建立的模型较好地模拟了不同床面不同波流组合条件下的含沙量剖面。在此基础上,讨论了不同床面含沙量剖面模拟方法的差异,指出床面形态是决定含沙量变化的重要因素之一,仅通过改变床面粗糙高度不足以反映漩涡沙波床面的含沙量剖面特征。该模型可为研究波流边界层内泥沙运动和物质输运提供工具。     

台风期间港湾海岸湿地侵蚀、淤积的环境动力学机制初探 --以福建罗源湾为例

为了探讨港湾海岸湿地在台风期间发生冲淤变化的过程和机制,利用流速仪观测了台风“凤凰”过境期间福建罗源湾海岸湿地（包括盐沼中下部和光滩）的环境动力过程。结果表明,台风登陆期间盐沼中下部和光滩的滩面淹没时间增长约2 h,近底层流速平均值变化很小,但流速波动变化幅度很大,底部切应力显著增大,滩面发生侵蚀;台风登陆后,随着风浪作用的减弱,流速波动显著减小,底部切应力逐渐减小,悬沙发生沉降,滩面表现为淤积。台风期间的滩面侵蚀、淤积主要受底部切应力的控制。     

海洋环境沉积物输运研究进展

海洋环境中沉积物的输运涉及复杂的过程和机制。20世纪后半叶发展起来的悬沙输运数学模型已经成为海洋沉积动力学的一个有力的研究工具。悬沙输运数学模型的有效运行需要正确的数值解法和模型中所含参数的确定,包括悬沙沉降速度、扩散系数、底床糙度和切应力,以及底边界上的沉降-再悬浮通量。由于复杂的水动力条件、屏蔽效应以及海底生物扰动等因素的作用,海洋环境推移质输运的经验、半经验公式具有一定的局限性。因此,充分考虑以上各种因素是正确预测海洋环境中推移质输运的关键。海洋环境沉积物输运理论的进一步发展需要着重进行各种过程和机制的研究,而这项工作依赖于高精度、高分辨率现场观测仪器的发展和更先进的颗粒态物质运动理论的建立。     

波浪作用下的床面切应力研究进展

波浪作用下的床面切应力是估算近岸泥沙起动、输移的重要基本参数之一,其形成的底部摩阻效应也会对近岸水动力环境产生影响。由于现场观测较为困难,对波浪边界层与波浪作用下床面切应力的认识主要建立在室内试验观测基础上。回顾国内外相关理论模型和试验测量研究,梳理各类研究方法与测量技术的优缺点及适用条件;整理20世纪70年代至今的大量试验资料,对已有研究成果进行归纳分析,包括波浪非线性、波浪破碎等因素对床面切应力的影响;总结现有研究存在的局限性,提出今后的研究重点。超大型水槽是未来突破波浪边界层理论研究瓶颈的重要设施;高适应性水下二维切应力传感器的发展是复杂动力条件下床面切应力研究取得突破的关键。     

Sediment dynamics of turbidity maximum in Changjiang River mouth in dry season

High-resolution current velocity and suspended sediment concentration (SSC) data were collected by using an Acoustic Doppler Current Profiler (ADCP) at two anchor stations and a cross-section in the South Channel of the Changjiang River mouth during meso and neap tides on Nov. 16, 2003. In addition, tidal cycle (13-hour) observation at two stations was carried out with traditional methods during the spring tide. Results indicated that resuspension occurred not only at the flood and ebb maximum, but also in the early phase of ebb in the meso and neap tide. When tidal current transited from high to ebb phase, current speed accelerated. Subsequently, fine-grained sediment with low critical threshold was resuspended and increased concentration. The river mouth area remained in siltation in the meso and neap tidal phase during the observation season, with calculated resuspension flux in the order of magnitude of 10⁻⁴–10⁻⁷ kg·m−2/s. Suspended sediment transport in the South Channel was dominated by freshwater discharge, but the Storks drift, vertical circulation and vertical shear effect due to tidal oscillation also played an important role in resuspension and associated sediment transport. In contrast, resuspension sediment flux in the spring tide was larger than that in meso and neap tide, especially at the ebb maximum and flood maximum. The present study revealed that intensive resuspension corresponded well with the larger current velocity during winter. In addition, the ‘tidal pumping’ effect and tidal gravity circulation were also vital for forming the turbidity maximum in the Changjiang River estuary.     

A study of the dynamics of a marine sandwave

The movement of the crest of a sandwave was studied using cross-sectional profiles obtained from lines of sea-bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest. The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea-bed by storm waves, caused major reductions in crestal heights. Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.     

Investigation and numerical simulation of summer sedimentation in Jiaozhou Bay,China

In recent years, development activities have had a significant impact on the environment of the Jiaozhou Bay, China. To ensure the sustainable economic and social development of the Jiaozhou Bay area, it is necessary to strengthen corresponding control measures. The important prerequisite is to properly understand the environmental conditions laws of natural change, especially the dynamic processes of sediment and the characteristics of landform evolution. Based on the data of continuous observation at 6 stations in Jiaozhou Bay for 25 hours, the Hydrodynamic Eutrophication Model (HEM-3D) was used to simulate the sediment erosion and deposition. The results show that the maximum suspended sediment concentration in the sea area of Jiaozhou Bay is about 40 mg/L, which appears in the northwestern area of the bay top and the Cangkou watercourse area, and the low concentration is located in the area of the central Jiaozhou Bay towards the bay mouth. The suspended sediment is 6–10 mg/L. Affected by a decrease in seawater material, the direction of the prevailing current in the Jiaozhou Bay area is different from that of the sediment transport. The velocity of the flood current is higher than that of the ebb current. However, during flood tide, the flux of resuspended seafloor sediment outside and at the mouth of the bay is limited and cannot contribute significantly to the suspended sediment in the bay. During ebb tide, the resuspended sediment at the shallow-water bay head and the east and west sides spreads toward the bay mouth with the ebb current, although it extends beyond the bay through the bay mouth. The research results can provide scientific support for the Jiaozhou Bay project construction and environmental protection.     

Tidal sediment transport versus freshwater flood events in the Konkouré Estuary, Republic of Guinea

In comparison to their temperate counterparts, sediment processes in tropical estuaries are poorly known and especially in African ones. The hydrodynamics of such environments is controlled by a combination of multiple processes including morphology, salinity, mangrove vegetation, tidal processes, river discharge, settling and erosion of mud and by physico-chemical processes as well as sediment dynamics.The aim of this study is to understand the sediment processes in this transitional stage of the estuary when the balance between river discharges and marine processes is reversing. Studying the hydrodynamics and sediment dynamics of the Konkouré Estuary has recently been made possible thanks to new data on bathymetry, sedimentary cover, salinity, water elevations, and current velocities. The Lower Konkouré is a shallow, funnel shaped, mesotidal mangrove-fringed, tide-dominated estuary, well mixed during low river discharge and stratified during high river discharge. The Konkouré Estuary is turbid despite the small amount of terrestrial input and its residual velocity at the mouth during low river discharges, landwards for two of the three branches, suggests a landward migration by tidal pumping of the suspended particulate matter. A Turbidity Maximum Zone (TMZ) is identified for typical states of the estuary with regard to fluvial and tidal components. Suspended sediment transport during a transitional stage between the rainy and dry seasons is known thanks to current velocity and Suspended Sediment Concentration (SSC) measurements taken in November 2003. The Richardson layered number calculation assesses that turbulence is the major mixing process in the water column, at least during the flood and ebb stages, whereas stratification occurs during the slack water periods. Tidal currents generate bottom erosion, and turbulence mixes the suspended sediment throughout the water column. As a result, a net sediment input is calculated from the western Konkouré outlet for two consecutive tidal cycles. Despite the net water export, almost 300 tons per tide reach the estuary through this outlet, for a moderate river flow.     

Effect of a broad shallow sill on tidal circulation and salt transport in the entrance to a coastal plain estuary (Mira—Vila Nova de Milfontes, Portugal)

We describe the tidal circulation and salinity regime of a coastal plain estuary that connects to the ocean through a flood tide delta. The delta acts as a sill, and we examine the mechanisms through which the sill affects exchange of estuarine water with the ocean. Given enough buoyancy, the dynamics of tidal intrusion fronts across the sill and selective withdrawal (aspiration) in the deeper channel landward appear to control the exchange of seawater with estuarine water. Comparison of currents on the sill and stratification in the channel reveals aspiration depths smaller than channel depth during neap tide. During neap tide and strong vertical stratification, seawater plunges beneath the less dense estuarine water somewhere on the sill. Turbulence in the intruding bottom layer on the sill promotes entrainment of fluid from the surface layer, and the seawater along the sill bottom is diluted with estuarine water. During ebb flow, salt is effectively trapped landward of the sill in a stagnant zone between the aspiration depth and the bottom where it can be advected farther upstream by flood currents. During spring tide, the plunge point moves landward and off the sill, stratification is weakened in the deep channel, and aspiration during ebb extends to the bottom. This prevents the formation of stagnant water near the bottom, and the estuary is flooded with high salinity water far inland. The neapspring cycle of tidal intrusion fronts on flood coupled with aspiration during ebb interacts with the sill to play an important role in the transport and retention of salt within the estuary.     

Longitudinal dispersion processes in the upper tamar estuary

Measurements of velocity, salinity, and suspended solids concentration have been used to investigate the intra-tidal variation of vertical and transverse shear-induced dispersion. For the study research the interaction of the longitudinal density gradient and vertical shear during the early part of the ebb tide accounted for much of the net longitudinal dispersion of solute landward. The same mechanism also is shown to lead to a net particulate transport landward. The landward flux, however, takes place during the flood tide. The field data are also used to elucidate the tidally averaged tidal pumping mechanism.     

A modeling study of the Satilla River estuary,Georgia. II: suspended sediment

A three-dimensional (3-D) suspended sediment model was coupled with a 3-D hydrodynamic numerical model and used to examine the spatial and temporal distribution of suspended sediments in the Satilla River estuary of Georgia. The hydrodynamic model was a modified ECOM-si model with inclusion of the flooding-drying cycle over intertidal salt marshes. The suspended sediment model consisted of a simple passive tracer equation with inclusion of sinking, resuspension, and sedimentation processes. The coupled model was driven by tidal forcing at the open boundary over the inner shelf of the South Atlantic Bight and real-time river discharge at the upstream end of the estuary, with a uniform initial distribution of total suspended sediment (TSS). The initial conditions for salinity were specified using observations taken along the estuary. The coupled model provided a reasonable simulation of both the spatial and temporal distributions of observed TSS concentration. Model-predicted TSS concentrations varied over a tidal cycle; they were highest at maximum flood and ebb tidal phases and lowest at slack tides. Model-guided process studies suggest that the spatial distribution of TSS concentration in the Satilla River estuary is controlled by a complex nonlinear physical process associated with the convergence and divergence of residual flow, a non-uniform along-estuary distribution of bottom stress, and the inertial effects of a curved shoreline.     

Flow,stress and sediment resuspension in a shallow tidal channel

In October of 2004, a 3-d observational program to measure flow and sediment resuspension within a coastal intertidal salt marsh was conducted in the North Inlet/Winyah Bay National Estuarine Research Reserve located near Georgetown, South Carolina. Current and acoustic backscatter profiles were obtained from a moored acoustic Doppler current profiler (ADCP) deployed in a shallow tidal channel during the spring phase of the tidal cycle under high discharge conditions. The channel serves as a conduit between Winyah Bay, a large brackish estuary, and North Inlet, a saline intertidal coastal salt marsh with little freshwater input. Salinity measurements indicate that the water column is vertically well mixed during flood, but becomes vertically stratified during early ebb. The stratification results from brackish (15 psu) Winyah Bay water entering North Inlet via the tidal channel, suggesting an exchange mechanism that permits North Inlet to receive a fraction of the poor water quality and high discharge flow from upland rivers. Although maximum flood currents exceed maximum ebb currents by 0.2 m s⁻¹, suspended sediment concentrations are highest during the latter ebb phase and persist for a longer fraction of the ebb cycle. Even though the channel is flood-dominated, the higher concentrations occurring over a longer fraction of the ebb phase indicate net particulate transport from Winyah Bay to North Inlet during spring tide accompanied by high discharge. Our evidence suggests that the higher concentrations during ebb result from increased bed friction caused by flow asymmetries and variations in water depth in which the highest stresses occur near the end of ebb near low water despite stronger maximum currents during flood.     

Dynamics of suspended matter concentration in Chupa Inlet (Karelian Coast,the White Sea) in summer and autumn

Studies on the water and suspended matter concentration dynamics were performed for the summer and autumn periods in Kruglaya Bay, Chupa Inlet (Karelian Coast, the White Sea). The tidal cycle appeared as the major factor defining the water salinity, temperature, and suspended matter concentration. The water temperature and suspended matter concentration increased during low tide, and the opposite pattern was observed for water salinity; all the processes were true for both surface and bottom water layers. Therefore, we conclude that major bulk of the allochtonous suspended matter is carried into the sea by the ebb stream.