A modelling study of residence time in a macro-tidal estuary

This paper outlines a numerical modelling study to predict the average residence time of a conservative tracer in a macro-tidal estuary, namely the Mersey Estuary, UK. An integrated hydrodynamic-dispersion model was used to predict the average residence time in the estuary for various tidal level and freshwater discharge conditions. The numerical model was verified against six sets of field measured hydrodynamic data, with the model-predicted water elevations and salinity levels generally agreeing well with the field measurements. The numerical model results show that in the Mersey Estuary both the tidal level and river discharge affect significantly the predicted average residence time. The value of the average residence time is also shown to be closely linked to the intensity of the residual tidal current. This is due to the fact that a large proportion of the Upper and Inner Estuary dries out during low tides, thus a significant amount of the tracer material is transported through the deep channels. An increase in the freshwater discharge causes a considerable increase in the intensity of the residual current along the main channels and thus a reduction in the average residence time. The predicted overall tracer residence time for the whole estuary is relatively short for a relatively large estuary, ranging from less than 1 day to 4 days for various tidal level and freshwater flow combinations. When the tidal range and freshwater discharge are both small, then the local tracer residence time in the upper part of the estuary can be significantly longer than the values predicted for the middle and lower reaches of the estuary.     

胶州湾水交换及湾口潮余流特征的数值研究

利用基于普林斯顿海洋模式建立的胶州湾及临近海域潮汐潮流数值模型,结合胶州湾口走航式声学多普勒海流剖面仪(ADCP)测流资料,研究了胶州湾口的潮(余)流特征,并在潮流模型的基础上耦合建立了水质模块,模拟了胶州湾的水交换过程。考虑M₂, S₂, K₁, O₁, M₄和MS₄六个主要分潮,胶州湾口潮流场的模拟与ADCP观测数据吻合较好。外湾口水道上的潮流非常强,大潮期间观测到201 cm/s的峰值流速。团岛岬角的两侧分别存在一个流向相反的余流涡旋,两涡旋在团岛附近辐合,形成了57 cm/s的离岸强余流。整个胶州湾平均水体存留时间为71 d,平均半交换时间为25 d。胶州湾水体交换能力在空间分布上有很大差异:湾口海域最强,向湾顶逐渐减弱。湾内存在两个弱交换区,分别位于湾的西-西南部和东北端,水体存留时间多超过80 d,湾西局部水域最长达120 d,而半交换时间也大多超过40 d。潮流场的结构、强度,以及与湾口距离的远近是造成湾内水交换能力空间差异的主要原因。     

钦州湾水交换能力数值模拟研究

基于普林斯顿海洋模式（Princeton Ocean Model,POM）,以M₂、S₂、K₁、O₁、M₄和MS₄ 6个分潮为驱动,建立了包含漫滩处理的高分辨率钦州湾水动力模式。与现场观测的数据对比表明,该模式能较好地刻画钦州湾的水动力特征。在此基础上建立了水质模型,模拟钦州湾的水交换过程。模拟结果表明:钦州湾水交换能力整体上较强,整个湾平均的水体半交换时间约为18 d,水体平均存留时间为45 d。空间分布上,钦州保税港区以南海域水交换能力最强,半交换时间小于1 d;沿着水道向北,水交换能力逐渐减弱;茅尾海中部半交换时间为26~28 d;茅尾海的东、西、北3个部分存在水交换滞缓区,半交换时间超过50 d。数值实验表明,采用漫滩技术对准确模拟钦州湾潮流速度和水交换能力非常重要,不考虑漫滩过程会低估钦州湾的潮流速度和水体交换能力。水平扩散系数对流速及交换时间都有影响,但影响有限。     

潮及台风作用下海南小海潟湖水交换的数值研究

小海潟湖位于海南岛东南部,是海南岛最大的潟湖,通过狭窄的通道与外海相连。为了解小海潟湖水交换特征,本文通过数值模拟方法,模拟了潮及典型台风影响下的潟湖水动力场,运用水体存留率、半交换时间等概念量化小海潟湖的水交换能力。研究结果表明：潮汐潮流影响下,小海潟湖水交换能力在空间上具有显著差异,距离口门越近,水交换时间越短,潟湖南部海域水交换能力最弱。台风对潟湖内水体交换具有重要的影响,典型台风影响下,小海潟湖出水量增加了1.4～8.8倍,平均半交换时间由60.4 d缩短至1.3～34.3 d。即使在台风作用下,潟湖西南部也难以与外海发生水交换。本研究可为生态环境保护部门制定精准的小海污染控制策略提供参考。     

象山港水交换特性研究

在验证良好的三维斜压潮流数学模型的基础上,以溶解态的保守性物质为示踪剂,建立对流-扩散型的海湾水交换数值模型,计算了象山港水体半交换时间和平均滞留时间,并研究了斜压动力对湾内外水交换的贡献.研究结果表明,象山港水交换速度的区域性变化较大,水体半交换时间和平均滞留时间由象山港口门向湾顶逐渐增加,口门附近半交换时间在5d以内,平均滞留时间为5～10 d;湾顶水交换速度缓慢,水体半交换时间为30～35 d,平均滞留时间为35～40 d.斜压动力对狭湾外段水交换影响较弱,对狭湾内段有较大的影响.     

河口冲刷时间的研究进展与问题

冲刷时间是表征河口水体交换的一个非常重要的物理量．本文在回顾潮汐河口冲刷时间研究现状的基础上，分析了影响冲刷时间的主要影响因子．结论表明冲刷时间不仅是潮棱体和淡水径流量的函数，而且与河口形态、天气要素密切相关，各要素之间的非线性关系更增加了冲刷时间研究的复杂性．在今后的研究中，应以河口界面的观点，加强河口冲刷时间界面机制的研究，并注重人类活动对冲刷时间的影响．     

半封闭海湾潮间带部分围垦后纳潮量计算的商榷——以胶州湾为例

传统的海湾(泻湖)纳潮量计算方法不能精确地反映潮间带被部分围垦后纳潮量的实际情况,本考虑围堤的影响,将纳潮量分解为堤基以下的“梯形”区和堤基以上的“矩形”区两部分之和,在理论上更加严密。根据改进后方法算出的胶州湾近期纳潮量比传统方法算出的纳潮量大15％左右。可见,半个多世纪以来胶州湾的纳潮量只减少了约15％．而不是传统方法得出的约25％。     

湄洲湾潮流特性的数值研究

本文运用一个含动边界的二维河口海岸动力模型模拟了湄洲湾潮汐潮流的基本特征,模拟结果与实测数据吻合较好．在此基础上估算了湄洲湾大、小潮过程的纳潮量,并根据湾内保守示踪物的质量（浓度）随涨、落潮流周期性的变化,进一步估算了湄洲湾的水交换周期,其半交换和80％的交换周期分别为5d和15d．同时可以看出,主航道深水区的水交换特性明显强于湾顶浅水区．     

湾口建闸海湾水体交换及景观水位数值模拟

大幅度的潮位变动对近岸景观和亲水性有较大影响,为此往往在湾口建闸控制湾内潮位变动,通过设置景观水位改善以上问题,但这将带来湾内的水动力及水体交换条件下降。本研究以马銮湾为例通过潮流数学模型模拟,研究湾口开闸孔数、开闸位置及不同景观水位与湾内水动力和水体交换的关系。利用湾口闸门调度,在湾内形成大尺度环流提高水体交换效率,对马銮湾景观水位和闸门调度方式提出了建议,有效地解决了湾内潮差减小引起水体交换效率减低的问题。     

Transport timescales for identifying seasonal variation in Bass Strait, south-eastern Australia

A three-dimensional σ-coordinate ocean model with realistic forcing is used to derive and compare flushing time, residence time and water age distributions in Bass Strait, a broad shallow shelf sea in south-eastern Australia which exhibits seasonal variation in water mass properties and circulation. Results illustrate flushing out of most old water occurs each winter–spring period and the circulation in summer increases age of remnant older water in the Strait. A correlation between water age and salinity is found in winter which is the result of advective “erosion” of the interior water mass. The presence of an area in the south-eastern part of the interior with low flushing is suggested to result from a number of physical factors including the presence of quasi-stationary eddies which entrain age, leading to relatively older water residing there.     

Numerical Simulation of Water Exchange Characteristics of the Jiaozhou Bay Based on A Three-Dimensional Lagrangian Model

Based on theory of three-dimensional hydrodynamics,an Euler-Lagrangian particle model is established to study the transport and water exchange capability in the Jiaozhou Bay.The three-dimensional hydrodynamic model,driven by tide and wind,is used to study the effects of wetting and drying of eatuarine intertidal flats by the dry-wet grid technology based on the Estuarine,Coastal and Ocean Model (ECOM).The particle model includes the advection and the diffusion processes,of which the advection process is simulated with a certain method,and the diffusion process is simulated with the random walk method.The effect of the intertidal zone,the turbulent diffusion and the timescales of the water exchange are also discussed.The results show that a moving boundary model can simulate the transport process of the particle in the intertidal zone,where the particles are transported for a longer distance than that of the stationary result.Simulations with and without the turbulent random walk show that the effect of turbulent diffusion is very effective at spreading particles throughout the estuary and speeding up the particle movement.The spatial distribution of residence time is given to quantify the water exchange capability that has very important ramifications to water quality.The effect of wind on the water exchange is also examined and the southeasterly wind in summer tends to block the water exchange near the northeast coast,while the northerly wind in winter speeds up the transport process.These results indicate that the Lagrangian particle model is applicable and has a large potential to help understanding the water exchange capability in estuaries,which can also he useful to simulate the transport process of contaminant.     

Can start-up motives influence social-ecological resilience in community-based entrepreneurship setting? Case of coastal shrimp farmers in Sri Lanka

The aim of this paper is to understand the extent to which the start-up motives can influence social-ecological resilience in community-based entrepreneurship setting. The case study approach was used to study five coastal communities in northwestern Sri Lanka. Shrimp farmers’ start-up motives include both the desire to capitalize on opportunities to utilize their resources and skills (opportunity-driven) and the need to establish satisfactory sources of income (necessity-driven). They have come up with diverse entrepreneurial responses to face various stressors and shocks by using their existing capital types and their willingness for collaboration. Farmers’ coping capacity is demonstrated through their individual responses whereas adaptive and transformative capacities are demonstrated mostly through their community cooperative work. Findings also reveal that the start-up motives of small-scale shrimp farmers are less influential on their social-ecological resilience in the community setting. Instead, the key to social-ecological resilience lies with the understanding of the nature of stressors and their commitment for collective action.     

围填海工程对近岸海域海洋环境影响的研究进展

通过收集丁字湾附近海域水深地形、海流、潮位等实测资料,采用数值模拟的研究方法,运用Mike21数值模拟软件模拟计算了丁字湾东部不同跨海方案的水动力、纳潮量、水交换和地形地貌冲淤等沉积动力特征,研究了跨海通道工程建设对鲁岛周边海域沉积动力环境的影响。结果表明,跨海通道工程建设使鲁岛周边海域潮流流速大幅减弱约50%;工程建设增加了鲁岛周边淤积强度,淤积速率最大增加量约为6 cm/a;工程建设明显影响鲁岛周边水交换,造成水交换率下降,水交换率＞80%的区域由73.1%减小为66.9%;涨潮和落潮的纳潮量平均减小超过10%。整体而言,北侧、中间和南侧桥梁长度分别为100、100 和200 m的推荐方案充分利用了现有水道,在水动力、水交换和纳潮量等方面均优于其他方案,对海洋环境影响最小。该方案最大限度地保证了工程建设和海洋环境的协调发展。     

罗源湾海水交换三维数值模拟

利用ECOMSED模式中的示踪粒子三维追踪模块分析罗源湾的水交换情况。结果表明,水交换能力的强弱与纳潮量的大小成正比。这里分析湾内水逗留期的空间分布,然后根据湾水存留期的不同将湾内划分为4个地区。进一步的研究表明不同海区湾水逗留期的差异主要是由欧拉余流场的结构差异造成的。由于风场可以影响当地的海流,因此不同的风场也会影响水交换能力。本文结果表明西南方向的风有利于水交换;而作为罗源湾全年主导风向的东北风,在平均风速的情况下,却阻碍了湾内水体的交换。本文还发现在不同投放时刻水交换能力的不同。初始投放时刻的不同对质点总数的减少速度有较明显的影响,高潮时和落潮中间时投放的质点数要比涨潮中间时和低潮时投放的质点数减少得快。罗源湾水交换能力的垂直分布不是均匀的:表层交换率较快,底层较慢,整个海域的交换能力与中层的相当。     

Relation Matrix of Water Exchange for Sea Bays and Its Application

1 .IntroductionWater exchange of sea bays is a basic issue for marine environment study. Water exchange be-tween a coastal area andthe outside area dueto advection and diffusionis animportant waytoimprovethe water qualityinthe coastal area . Withthe rapid…     

Toward a generic method for studying water renewal, with application to the epilimnion of Lake Tanganyika

We present a method, based on the concept of age and residence time, to study the water renewal in a semi-enclosed domain. We split the water of this domain into different water types. The initial water is the water initially present in the semi-enclosed domain. The renewing water is defined as the water entering the domain of interest. Several renewing water types may be considered depending on their origin. We present the equations for computing the age and the residence time of a certain water type. These timescales are of use to understand the rate at which the water renewal takes place. Computing these timescales can be achieved at an acceptable extra computer cost.The above-mentioned method is applied to study the renewal of epilimnion (i.e. the surface layer) water in Lake Tanganyika. We have built a finite element reduced-gravity model modified to take into account the water exchange between the epilimnion and the hypolimnion (i.e. the bottom layer), the water supply from precipitation and incoming rivers, and the water loss from evaporation and the only outgoing river. With our water renewal diagnoses, we show that the only significant process in the renewal of epilimnion water in Lake Tanganyika is the water exchange between the epilimnion and the hypolimnion, other phenomena being negligible.     

近70年胶州湾水动力变化的数值模拟研究

采用无结构三角形网格海洋模式FVCOM,基于胶州湾不同年代的岸线和水深地形条件,建立胶州湾及其邻近海域各年代的三维潮汐潮流数值模型,从数值模拟角度分析和比较胶州湾不同年代纳潮量、潮汐潮流、水交换率等水动力参数的变化。结果表明:随着胶州湾水域总面积不断缩小,纳潮量在逐渐减小,2008年全湾的纳潮量相对于1935年减少了31.5%,约合3.9×108 m3;海湾流场结构变化很小,流速呈减小趋势;胶州湾欧拉余流\"团团转\"的多涡结构基本保持不变,最大值都发生在团岛附近;海湾的水交换能力趋弱,对整个胶州湾水体的半交换时间进行海湾平均,不同年代5套岸线下海湾的水体半交换时间分别是37.0 d,36.7 d,39.2 d,39.7 d和40.8 d。     

Can co-management emerge spontaneously? Collaborative management in Sri Lankan shrimp aquaculture

Shrimp aquaculture in northwestern Sri Lanka shows co-management like features. To understand the reasons behind co-management and to identify the mechanisms by which co-management is carried out, the paper examines shrimp aquaculture operations in three coastal communities using a case study approach. Water from an interconnected lagoon system is the key input for shrimp ponds, but it is also the potential source of shrimp disease outbreaks that threaten all shrimp farms. Farmers try to prevent the spread of disease by co-operating to adjust the timing of water intake and wastewater release. This is done through a zonal crop calendar system which is developed and implemented by a vertically integrated institutional structure with three levels: sub-zonal/community, zonal, and national. Partnerships, overall sharing of power and authority, and learning-by-doing are key features of this collaborative management system. The case shows that adaptive co-management can develop through collaborative problem-solving over time, even in the absence of legal arrangements.     

Circulation and Material Transport in Suo-Nada during Spring and Summer

An attempt was made to reproduce the circulation pattern in Suo-Nada, Japan during spring and summer season in order to elucidate the water exchange mechanism in the basin. Two hydrographic surveys at the end of each season were conducted covering the entire Suo-Nada area. A three-dimensional hydrodynamic Princeton Ocean Model (POM) was used to compute the current resulting from the observed density and wind field. During spring, a very pronounced counter clockwise gyre is situated near the opening of the basin. This is replaced by a clockwise circulation which seemed to occupy the whole domain during summer. Within each season, however, the vertical distribution of current does not show any remarkable differences, indicating the dominance of horizontal current and a very weak estuarine flow. These observational and numerical results were used to estimate the remnant function and the corresponding average residence time of permanently dissolved matter (PDM) and transformable matter (TM). The results revealed a small difference in the average residence times of materials within each season but a large seasonal variability between spring and summer. Furthermore, calculations based on climatological density fields have indicated a similar trend of variation between the seasonal values of average residence times. This revised version was published online in July 2006 with corrections to the Cover Date.