太湖湍流特征及其对表层沉积物起悬机制的影响

太湖是一个典型的浅水大湖泊且富营养化严重,动力扰动引起的水-土界面沉积物起悬与太湖蓝藻水华的暴发有密切联系。基于声学多普勒流速仪、OBS浊度仪、pH-ⅡHandheld手持式气象站等仪器监测得到的原位高频气象、水动力、水质同步数据,分析了湖泊水-土界面悬浮物浓度与风速、切应力及湍流特征的关系。研究结果表明:观测期间,湖心区底泥沉积物再悬浮过程中由波切应力起主导作用,观测的切应力临界值为0.08 N/m~2,临界风速为4 m/s。底泥再悬浮主要是由拟序结构中短时内间歇性猝发引起的,短时大振幅事件对底泥垂向扩散起关键作用。研究结果可为进一步研究与湍流密切相关的太湖内源释放发生机制提供参考。     

太湖湖流对波浪的影响机制研究

根据基于动谱平衡方程的SWAN模型和湖流三维模型,分别对太湖波浪和湖流的生消过程进行了动态模拟,并研究了太湖湖流对波浪的影响。结果表明:湖流模型和SWAN波浪模型可以较好地模拟风作用下太湖湖流和波浪的生成和传播过程;太湖流场对波浪场具有一定影响,风速越大,影响越显著,同时也受风向影响;沿岸区波浪受湖流影响更加显著;湖流对波浪的影响基本上呈现湖流波浪同向时,有效波高增大,波速增大,波长增长;反向时有效波高减小,波速减小,波长缩短的特征。     

太湖沉积物的分布和动力扰动下最大侵蚀深度的确定

利用 2 0 0 2年对太湖沉积物空间分布的调查数据 ,将太湖划分成 69× 69个网格 ,利用最优插值法对整个太湖的沉积物总量进行了计算 ,得出太湖沉积物的蓄积量大约在 1 8 5 7亿m3左右 ,主要分布在西部沿岸和北部的梅梁湾 ,在湖心和东太湖 ,沉积物覆盖量很小 ,有泥区面积占整个水面积的 47 45 % ,在 60 %的有泥区中 ,沉积物厚度集中在 2 5m以下 ,小于 5 0cm和大于 3 5m的有泥区面积所占比例不大。此外 ,利用Shields方法计算了太湖沉积物上层 1m内不同深度上的临界切应力 ,采用SMB浅水波动模式 ,计算了夏季受东南风和冬季受西北风影响下的波切应力值 ,确定了不同扰动所能引起的最大侵蚀深度。计算结果说明 ,无论上述何种风向情况下 ,能产生悬浮的临界风速大约在 5 0m s左右 ,当风速大于临界风速时 ,悬浮深度随着风速的增加而增大 ,当风速达到 2 0 0m s时所能引起的最大侵蚀深度均在 3 0cm左右。     

内外源共同作用对太湖营养盐贡献量研究

本文根据环太湖25条主要入湖河流的监测结果对入湖水量及污染物输入量进行分析,计算了不同季节外源对太湖营养物质的贡献量。分别于春、夏、秋、冬4个季节对太湖梅梁湾水体开展5次野外试验,利用沉积物捕获器收集沉积物,采用Gansith公式法计算沉积物的再悬浮通量,并建立其与风速的关系;对代表不同风浪作用下的太湖悬浮物进行7次静沉降试验,计算悬浮物的静沉降通量,并建立其与风速关系。以3.7m/s为界对底泥悬浮沉降过程进行分解和概化,并利用近10年的风速资料估算太湖年均内源释放量。计算结果表明,全年外源负荷和内源释放对太湖营养盐的贡献总量COD为24.23万t、总氮为3.80万t、总磷为2 045.20t,其中内源所占比例分别为20.47%,20.44%和13.47%外源、内源对太湖的贡献量在不同季节有较大差异,夏季贡献量最大,春季次之。风浪作用下的COD、TN释放量只相当于外源输入量的25.7%,而TP释放量仅相当于外源输入量的15.6%。外源输入量对营养盐的贡献占据相当高的比重,太湖河道入湖污染负荷的增加是太湖水质恶化的根本原因。     

基于三维浅水模式的太湖水动力数值试验-盛行风作用下的太湖流场特征

基于三维浅水动力模式，对太湖不同定常风作用下的整层平均流场和分层流场特征做了模拟，结果发现在无旋风场的情况下，可产生有旋流场。经分析发现，有旋流场为湖底地形变化所致，若在均匀平坦的湖底地形情况下，流场中除边界以外任一点的涡度为零。在方向相反的不同定常风作用下，其环流形势相似，环流个数相等，但涡度值呈现反位相特征。数值试验表明，在定常风作用下，太湖表层流完全为风生流，流向与风向平行，中间为过渡层，底层流向相反，为补偿流，在增水区域，下沉运动强烈，而减水区域则出现较强的上升补偿流，形成封闭环流。     

壳聚糖改性黏土作用下湖泛致黑悬浮物的沉降特性研究

为了探讨湖泛致黑物质在壳聚糖改性黏土作用下的絮凝沉降以及再悬浮的规律,利用柱状水槽再悬浮发生模拟装置,选取壳聚糖改性黏土(1 g/L硅藻土+0.2 g/L壳聚糖)作为絮凝剂,进行了风浪扰动作用下太湖湖泛的絮凝及再悬浮过程的室内模拟试验。结果表明:当模拟流速产生的切应力超过0.042 N/m~2时,低浊度组中的黏土絮体产生了明显的再悬浮现象;当流速产生的切应力超过0.109 N/m~2时,高浊度组发生再悬浮。对比太湖不同风速所产生的切应力,发现4 m/s以下的太湖风情产生的湖流切应力不会使黏土絮体发生再悬浮。研究旨在为太湖湖泛的应急处置提供参考。     

太湖湖流、波浪、沉积物的三维数值模拟

运用ECOMSED模拟风速为6.5 m/s定常东南风作用下太湖流场及波浪场,对模型的适用性及可靠性进行验证;在此基础上进行波流共同作用下沉积物再悬浮数值模拟,得出沉积物-水界面切应力及水体中悬浮物浓度分布。结果表明:太湖有效波高范围在9~29 cm之间,有效波高的空间分布与风速、风向和水深有很大的关系,同时波浪对切应力会产生很大影响,而切应力的分布决定了悬浮物浓度的分布,风浪及地形要素是导致太湖悬浮物浓度变化的重要因子。模拟结果与现有实验成果较吻合,说明运用该模型模拟太湖沉积物再悬浮的结果可靠。     

波浪辐射应力张量的垂向变化

将垂向考察区域分为波谷以下部分，波谷至平均水面部分以及平均水面以上部分，应用线性波理论，分段推求波浪辐射应力张量沿水深分布函数，通过欧拉坐标变换，建立了任意波向角时波浪辐射应力张量的计算表达式，给出深水及浅水条件下的渐近表达式，讨论了波浪辐射应力张量的垂向变化情况。     

间歇性波浪扰动下河口底泥中磷释放特性研究

作为反映河流、海洋水体受污历史的载体,河口底泥在污染达到一定程度时易在海洋动力作用下释放沉积其中的污染物,影响水体水质。为了明确秦皇岛近岸海域内污源染,量化波浪扰动下底泥释放对水质的影响,该研究以波流水槽为动力试验设备,选取秦皇岛污染最严重的河流之一大蒲河的河口底泥为研究对象,试验研究河口底泥在间歇性波浪扰动下磷的释放特性。总磷在波浪扰动下的释放可分为浓度迅速增加的初期和浓度随时间线性增加的稳定期两个阶段,浓度随波高增大而升高,且在波高大于8 cm时与浊度呈显著正相关。溶解性总磷受浓度梯度和波浪引起的孔隙水压力影响向上覆水体迁移扩散,随时间呈小幅增加。总磷在波浪扰动初期的释放强度为2.00×10~(-3)–8.75×10~(-3) mg/m2/s,是稳定期释放强度0.50×10-4–2.25×10~(-4) mg/m~2/s的18–40倍,且两个阶段的释放强度均随波高呈指数增长。当波浪强至起动底泥,原先的泥水界面释放转为底泥起动悬浮释放,水体中颗粒态磷取代溶解态磷成为磷的主要赋存形态。间歇性波浪扰动静置期,颗粒态磷的吸附沉降使得水体中总磷浓度和颗粒态磷的比例大幅降低。     

基于有限体积法的三维波生近岸流数值模型

波浪破碎产生近岸流是近岸浅水区域一个重要的水动力现象。该文利用辐射应力的概念,并考虑辐射应力沿水深的变化、波浪破碎引起的水滚以及波流共同作用下的水底剪应力,建立了基于有限体积法的三维波生近岸流数值模型。运用建立的数值模型模拟了两个实验室实验,并同实验的实测结果进行了对比。结果表明,建立的数值模型能够较好地模拟波浪破碎产生的近岸流。     

太湖长兜港固体悬浮物再悬浮的室内模拟

定量研究浅水湖泊动力扰动与沉积物再悬浮的关系,目前仍是湖泊研究中的难点之一。实验利用再悬浮发生装置,对太湖长兜港沉积物风浪扰动进行室内模拟,建立扰动强度与有效波高之间的关系,定量分析风浪对太湖水体中悬浮物的影响及固体悬浮物在水体中的垂向分布,初步得出风浪对太湖沉积物扰动深度一般在毫米级的结论。     

风浪对浅水湖泊水质的影响机制

以太湖重要的浅水湖湾——东太湖为例,应用经典风浪经验公式,将风速转化为浪高,以直接体现风浪强度,在分析其与污染底泥卷起、水质指标相关关系的基础上,对风浪影响湖区水质的机制进行进一步探究。结果表明:随着风速、浪高的增加,水体浑浊度增加,偏北风作用下湖区浑浊度更高;DO的质量浓度呈降低趋势,全年浓度呈现冬季偏高而夏季偏低的规律;COD的质量浓度呈增加趋势,全年浓度呈现冬季偏高而夏季偏低的规律;TN的质量浓度与风速相关关系不明显,全年浓度呈现夏季偏高而冬季偏低的规律;浑浊度与COD存在较好的相关性。确定风浪对湖区水质的影响机制,旨在为针对性地制定湖泊水质改善措施提供技术支撑。     

Impacts of wind waves on sediment transport in a large,shallow lake

Lake Okeechobee is a large, shallow subtropical lake, located in south Florida. Over the last several decades, Lake Okeechobee has experienced accelerated eutrophication due to excessive phosphorus loads from agricultural run‐off. Recycling of phosphorus from bottom sediments through resuspension is critical to addressing eutrophication of the lake and for water quality management. The present study investigates the impacts of wind waves on sediment transport in Lake Okeechobee, using measured data and the Lake Okeechobee Environmental Model (LOEM). The LOEM was fully calibrated and verified with more than 10 years of measured data in previous studies. Analysis of the measured data indicates significant wave height (SWH) and suspended sediment concentration are closely correlated to the wind speed in the lake. The nonlinear interaction of high‐frequency wind waves with relatively low‐frequency currents in the boundary layer plays a key role in sediment deposition/resuspension. Without considering the effects of wind waves, the bottom shear stress can be greatly underestimated. The spatial variations of key variables for sediment modelling, including SWH, water depth, orbital velocity, current velocity, bottom shear stress and sediment concentration, are discussed. In general, the near‐bottom wave velocity (and the associated bottom shear stress) is greater than or the same order of magnitude as the near‐bed current velocity (and the associated bottom shear stress) in this shallow water system. Although the sediment zones of Lake Okeechobee were described in previous studies, few published papers discussed its formation mechanisms. The findings of the present study include that the multiyear averaged bottom shear stress with wind‐wave effect plays a key role in forming the spatial patterns of the sediment zones. The study results are currently being used in lake management and in developing strategies for reducing phosphorus in the lake.     

太湖风生流及污染物输移扩散数值模拟

建立高精度的太湖三维风生流数值模型和风生流-污染物耦合数值模型,分析了盛行风作用下的太湖风生流特征和风生流驱动下的太湖污染物输移特征。结果表明:稳定后的太湖风生流场,表层流速大于底层流速,表层流向基本上同风向一致,底层流向则与表层大致相反,具有补偿流的特征;风向能显著影响太湖风生流的形态和结构;在垂向扩散作用下,太湖污染物质量浓度沿水深方向上近似均匀分布;在定常风作用下,太湖北部的湖湾区出现多个小尺度闭合环流,不利于污染物的输移扩散,对太湖水环境会产生不利影响。     

EXPERIMENTAL STUDY ON SEDIMENT RESUSPENSION IN TAIHU LAKE UNDER DIFFERENT HYDRODYNAMIC DISTURBANCES

Contaminants resuspension in sediments induced by wind-wave could influence the water quality in shallow lakes. Resuspension of surface sediments from the Zhushan Bay, Taihu Lake was simulated under different wind forcing by using a pneumatic annular flume in this study. Acoustic Doppler Velocimeter (ADV) was used to measure flow velocity at each wind speed, and the characteristics of sediment resuspension were studied with the layered sampling technology. The experimental results show that the flow velocity increases with wind speed obviously and 6m/s is a critical wind speed which affected hydrodynamic conditions significantly. The distribution of flow velocity and water depth is different from that in ordinary open channel. With the enhanced hydrodynamic factors, the Suspended Solids Concentration (SSC) in water increases accordingly, and the incipient velocity of sediment resuspension is about 0.21 m/s. Based on the analysis of wind speed and average SSC in water column, the quantitative relationship is obtained. The SSC of the bottom layer is higher than the content of surface layer under different hydrodynamic conditions, and there are similar distributions between SSC and flow velocity in different water layers.     

Experiment study of the effects of hydrodynamic disturbance on the interaction between the cyanobacterial growth and the nutrients

The eutrophication of shallow lakes is sensitive to dynamic currents(i.e., disturbances) because of their shallow depths and high contents of nutrients in bed sediments. The relation between the sediment resuspension and the algae bloom is not well understood in the field scale because the interwoven influencing factors cannot be examined individually. By combining the laboratory experiment and the field observation, this paper proposes a sediment-water-algae concept to simulate the effects of hydrodynamic disturbances on the algae growth in the Taihu Lake located in east China. The sediments are sampled from the Taihu Lake while the Microcystis aeruginosa(M. aeruginosa) algae is cultured in the laboratory and then transplanted into the experiment cylinders. The temperature and the light intensity in the experiment are adjusted to be similar with the prevalent in situ conditions. The results indicate that the M. aeruginosa populations under the disturbance condition of the rotational speed ?300 rad/min in the experiment(corresponding to the bottom velocity flow ?0.059 m/s, the shear stress 2?0.069 N/m, or the wind speed ?4 m/s in the field) are higher than those under the disturbance condition of the rotational speed is 400 rad/min(corresponding to the bottom flow velocity 0.079 m/s, the shear stress 0.124 N/m2). It is suggested that a low to moderate disturbance prompts the release of the nitrogen as well as the phosphate nutrients from the bed sediments, amplifying the eutrophication of the Taihu Lake.     

Modeling of sediment and heavy metal transport in Taihu Lake, China

With the current rapid economic growth, heavy metal pollution has become one of the key issues in the Taihu Lake. Although heavy metal pollution levels and distributions of the Taihu Lake have previously been described, an effective model to describe the transport process of heavy metals between the water column and sediment bed for this lake is not available. It is known that heavy metals in the water column can be related to the resuspension of sediment in the lake bed. In this study, we set up a coupled model of relating hydrodynamics, sediment and heavy metals based on Environmental Fluid Dynamics Code (EFDC), and applied it to Taihu Lake, China. For calibration and validation of the model, we employed two series of field sampling data taken all over Taihu Lake during April and July of 2009. The results show that the hydrodynamics simulations of the coupled model agree with the observations reasonably well and the sediment and heavy metal model shows similar variation trends during the simulation. Our results indicate that the model can be used for simulating the sediment and heavy metal transport process in the Taihu Lake and here we provide an effective tool for water quality management at small time scales.     

太湖底泥主要营养物质污染特征分析

为进一步推进太湖流域水环境综合治理工作,给太湖水生态清淤提供决策参考,结合太湖流域管理局2018年组织开展的太湖污染底泥勘察项目调查结果,对太湖底泥中的有机质、氮磷等营养物质的含量及其平面分布、垂向变化情况进行了分析,并与2003年底泥调查结果进行了对比。结果表明:总磷主要集中分布在竺山湖底泥0~30 cm深度内,高含量的总氮和有机质主要分布在竺山湖和东太湖;在外源污染有效控制后,生态清淤工程有利于降低湖区表层底泥的营养物质含量。