潮汐水位和盐度变化耦合条件下河口海滩地下水排泄及盐分运移规律研究

Numerical simulations of groundwater flow and solute transport in beaches have rapidly evolved over the last three decades accounting for tidal hydraulics and for seaward boundary condition. Most of these studies focused on tide-induced variation of groundwater hydraulic head but with a constant surface water salinity on the seaward boundary. In reality, salinity in tidal creeks varies in response to the combined influences of saline tides from downstream estuary and freshwater flow from upstream river. Existing observations and reports indicated that the variations of salinity in tidal creek are typically corresponding with its water level fluctuations, referring to "tidal salinity fluctuations". The primary goal of this project is to explore the influences of tidal level and salinity fluctuations in estuary on the submarine groundwater discharge (SGD) and solute transport in shallow beaches with sloped intertidal zone. Field study in typical intertidal zone of Changjiang River estuary, laboratory experiments and numerical modelling techniques will be employed for this propose. Sensitivity analyses will be conducted to investigate the effects of seepage face, inland freshwater recharge, and beach permeability on the distribution of salt plumes (freshwater-saltwater interface/mixing zone) in the beach and the SGD across the beach-sea interface subjected to the tidal level and salinity fluctuations. Such an improved understanding of the beach groundwater hydrodynamics is desired for addressing the coastal exigent problems (e.g., seawater intrusion, shoreline contamination), and for better strategies to manage similar coastal tidal beaches and the adjacent marine environment.

河口海滩是陆地-海洋交界带的中心区域，是海水（咸水）和地下水（淡水）相互作用和转化的重要场所。近30年来，许多学者对潮汐水位变化作用下的海滩地下水-地表水交互过程进行了广泛研究，但极少关注潮汐引起的河口水域盐度变化（指感潮河地表水盐度随潮汐周期性变化的现象）对海滩地下水动力过程的影响。而盐度在维持海滩生物多样性等方面发挥着重要作用。本项目将以长江河口海滩为研究对象，结合现场原型实验及其监测、数学模型和数值模拟等综合技术手段，系统定量地研究潮汐水位和盐度变化耦合条件下海滩中地下水流和盐分运移的规律。通过敏感性分析，综合考虑该条件下海滩地表渗出面、非均质性等因素对海滩地下水-地表水交换率、海底地下水排泄量（SGD）和海滩中盐度分布（咸淡水混合带/交界面）的影响。该项目将有助于阐明长江河口海滩的海水入侵、咸淡水混合等复杂地下水动力过程，为该类河口海滩生态环境的保护和管理提供理论依据和技术支撑。

本项目以长江河口海滩为研究对象，结合现场原型实验及其监测、数学模型和数值模拟等综合技术手段，系统定量地研究潮汐水位和盐度变化耦合条件下海滩中地下水流和盐分运移的规律。项目获取了10口观测井连续一个月的水位、盐度和温度的监测数据。同时获取了6个长期监测井连续一年的水位、盐度和温度的监测数据。通过分析监测数据和潮间带水文地质参数，提出了上部淤泥层-下部含砂层的双层含水层概念模型，构建了相应的数值模型；现场观测结果与数值模拟结果表明，地表水在高潮（涨潮）期间由高潮线附近渗入含水层，低潮期间地下水从低潮带渗出地表。地表水-地下水的这一运动特征与潮间带的植被分带一致，并且潮汐盐度周期性变化有利于海底地下淡水排泄。通过敏感性分析，综合考虑该条件下海滩地表渗出面、非均质性等因素对海滩地下水-地表水交换率、海底地下水排泄量（SGD）和海滩中盐度分布（咸淡水混合带/交界面）的影响。该项目研究成果有助于阐明长江河口海滩的海水入侵、咸淡水混合等复杂地下水动力过程，为该类河口海滩生态环境的保护和管理提供理论依据和技术支撑。