水动力扰动下太湖藻源性湖泛起悬机理研究

"Black water cluster" is water blackening and smelly phenomenon caused by cyanobacteria and fermented floating sludge mixture decomposed under anaerobic environment. Organic-based heavily polluted sediment and its suspension process is one of the necessary conditions for the occurrence of "Black water cluster". And currently research about "Black water cluster" mainly focuses on meteorological trigger condition, key environmental factor threshold, characteristics of black smelly matter and emergency treatment. But the hydrodynamic driving mechanism for pollutants suspension remains unclear. The project aims to reveal turbulence characteristics of water-sediment interface and the driving mechanism for pollutants suspension through field observation, indoor simulation and numerical simulation. The objectives are: (1) Obtain the water-sediment interface micro scale high spatial and temporal resolution of the instantaneous velocity, concentration, turbulent mixing dynamic parameters, temperature and wind velocity time series when “Black water cluster” occurs. The wave and flow turbulence characteristics of the near bottom layer and its internal relationship between suspended pollutants will be analyzed; (2) Collect in situ sediment sample and high-density algal samples to simulate field "Black water cluster" environment, conduct the lab flume experiment to study the impact of hydrodynamic process induced by the individual wave or flow. Quantitatively distinguish wave and current turbulence effect on suspension of "Black water cluster" pollution; (3) Correct parameters of the existing 3D coupled model of Lake Taihu, predict the effects of hydrodynamic disturbance on pollution suspension, transportation and faded out. The project contributes to provide technological support for “Black water cluster” prediction and control.

“湖泛”是蓝藻与发酵上浮的淤泥混合物在厌氧状态下分解造成水质发黑发臭的现象。以有机污染物为主的严重污染底泥及其起悬过程是湖泛发生的必要条件之一。而目前对湖泛的研究主要集中在气象触发条件、关键环境因子阈值、致黑致臭物质特性等方面。关于水动力扰动对湖泛污染物起悬的机理尚不明确。本项目旨在通过野外观测、室内实验和数值模拟的方法，揭示水-土界面波流紊动特性对太湖湖泛起悬的驱动机制，具体为：（1）获取湖泛发生时水-土界面微尺度、高频瞬时三维流速、紊流混合动力参数、藻密度、温度和风场等时间序列数据，研究野外“真实”环境下的近底层波流紊动特性及其与沉积物起悬的内在联系；（2）采集原位底泥及高密度实验藻样进行培养模拟野外湖泛环境，在受控的实验室环境下，定量区分波、流紊动对湖泛污染物起悬影响；（3）对已有太湖三维耦合模型进行参数校正，模拟并预测湖泛污染物起悬和输移过程。该项目能为湖泛预测及治理提供科学依据。

本研究围绕水动力扰动下太湖藻源性湖泛起悬机理开展研究。获取野外水-土界面微尺度紊流、浊度等时间序列参量，分析波、流切应力特征及对沉积物起悬的影响，阐明湖泛易发区紊流对沉积物起悬猝发微机制。以切应力为中间变量，模拟不同湖流、波浪组合扰动下污染物起悬过程，并计算释放率，定量区分波、流对湖泛污染物起悬的影响与贡献。开发多元耦合富营养化模型，对动力参数进行校正，在气象场的驱动下模拟蓝藻水华暴发，并据此预测藻源性湖泛可能发生位置及几率。主要结论如下：（1）湖泛发生前，风向短时间发生突变，风速逐渐减小至1-3m/s，气温出现短时骤降，气压不断降低短时间内出现较大幅度的波动；DO异常升高随即在24小时之内迅速降低至厌氧状态（1mg/L以下），浊度较正常水体高出3-4倍主要为非溶解性悬浮颗粒物，NH4+、PO4--P，TN、TP指标均远高于非湖泛区，显示出极高的富营养化水平；存在气象条件突变而产生污染物突然大量起悬的现象。（2）由波浪产生的切应力在多数情况下大于湖流产生的切应力，悬浮物浓度对波切应力的响应关系较好，说明波浪对底泥起悬过程中起至关重要的作用；拟序结构中的间歇性猝发过程是底泥起悬的主要动力来源，沉积物起悬主要由持续时间很短的大振幅事件“猝发”完成。（3）波浪和湖流叠加后，水体混合加强，扰动更为剧烈；水流产生的切应力对综合切应力的贡献随着流速的增大而增加，风速小于3m/s时，以波产生的切应力为主导，风速4-7m/s之间综合切应力主要由波高和水流比重决定，但是总体而言两者的贡献率在同一个量级大体相当；当风速超过8m/s后，水流占主导地位；沉积物及氮磷营养盐起悬率与风速呈正线性相关。（4）构建的太湖多元耦合模型在0-36小时区间内对蓝藻水华预报率能达到60%以上，能较好的模拟出蓝藻水华的发生位置、大致面积及变化趋势。从蓝藻水华暴发的程度与大致位置可以初步预判藻源性湖泛可能发生的位置及几率。研究成果以期为湖泛污染控制提供支持。