海湾水库底边界层对水体突然泛咸的双重影响机制与效应

The salinity in a estuary reservoir usually tends to meet the standard of industrial water for several years, but it would abrupt salinization under special environmental conditions. This issue of abrupt salinization restricts the key role of estuary reservoirs in alleviating water crisis for coastal areas. The project explores the mechanism of water salinization by charactering the three layer structure, including salinization sediment, thick benthic boundary layer and fresh surface layer. The velocity and conductance for the benthic boundary layer is monitored with high resolution on vertical transects in the moderate tank and large annular flume experiments, and then the stability of density stratification and the interfacial flux of salt are quantified. The proposed research is focus on (1) effects of the resistance and salt-enrichment of benthic boundary layer, and taking water from reservoir on the quality of surface water; (2) mixing conditions of boundary layer with obvious gradients of density and viscosity; (3) effects duration of salts released from sediments on water quality. The goal of the project is to explore the relationship between the length scale of the vertical overturning and the degree of abrupt salinization, and to elucidate the dual influence of the benthic boundary layer on water salinization. At last, we provide alternative measures for different stage of the reservoir aiding to diminish the salts in the benthic boundary layer. Thus, there is a potential economic benefit from the proposed research, in that proper management of water salinization may ensure the safety of water supply for agricultural, industrial and domestic water in coastal areas.

海湾水库在运行管理中存在"多年库水盐分达标，而个别时段突然泛咸"的特殊现象，水体的突然泛咸限制了海湾水库在缓解滨海地区水资源短缺中的重要作用。课题从海湾水库具有的"咸化底质-厚浓度边界层-淡水表层"三层结构入手，在中型水槽、大型环形水槽等模拟实验中通过边界层流速与电导率垂向剖面的同步高分辨率测试，密度分层稳定性评价、界面盐分扩散通量准确计算、数值模拟等方法，重点研究：⑴ 边界层的阻滞、盐分富集效应及水库取水与库水盐分达标的关系；⑵ 具有显著密度与粘度梯度的厚边界层混合发生的条件；⑶ 沉积物释放盐分对水质影响的时间尺度。明确边界层的混合尺度与水体突然泛咸的关系，揭示海湾水库底边界层对水体泛咸的双重影响机制。在此基础上，提出在水库的不同时期采取相应的调控边界层盐分蓄积的对策，为滨海地区水资源的"开源"决策、为海湾水库水体咸化的防治提供新思路和理论依据。

海湾水库建设已成为世界各国解决滨海地区水资源短缺的重要途径。但海湾水库运营管理中存在“多年库水盐分达标，个别时段突然泛咸”的特殊现象。本研究从海湾水库具有的“咸化底质-厚浓度边界层-淡水表层”三层结构入手，以青岛拟建的沐官岛水库为例，通过野外调查、室内实验、数值模拟等手段探讨海湾水库的咸化机制。研究结果表明，海湾水库由沉积物盐分释放（通量在10^0 ~10^1 g/(m2•d)量级）引起的底部浮力通量（10^-10 W/kg）大于分层湖库底层水体的常规湍流浮力通量(10^-13~10^-11 W/kg)，因此，在海湾水库底部常形成较厚的浓度边界层。较厚浓度边界层形成过程具有的三个特征（盐分的分子扩散传输机制、盐分累积效应以及边界层形成过程施密特稳定性逐渐增大）促使海湾水库库水盐分多年达标。边界层稳定后水体的施密特稳定性逐渐降低，边界层易于混合；内假潮扰动计算表明，15.6 m/s的风速将使2.5 m厚的底边界层发生混合。水体混合后原边界层累积的盐分与表层沉积物增强释放的盐分（使水体盐分增加了9.3%）促使水体突然泛咸，并且表层沉积物的扰动深度远大于Brinkman层的尺度。在斜坡沉积物条件下，蓄水初期由于进水冲刷效应存在单次往复密度流现象，这将加速坡底边界层盐分的累积速度。模拟结果表明，咸化沉积物将在海湾水库的整个生命周期内对库水咸化产生重要影响。受水位变化影响，需要综合考虑水量-盐分因素进行水库的日取水量调算。为防治库水咸化，在海湾水库管理的不同阶段应采取相应的调控边界层盐分蓄积的工程措施，蓄水前库底开挖排咸沟（建议间距100 m），蓄水过程排除初期蓄水，运营管理过程抽排边界层咸水（通过监测垂向盐分动态，掌握抽排时机）。研究结果对于海湾水库水体咸化的预测和防治具有较重要的理论意义和实用价值。