Previous studies on the impact of sea-level rise (SLR) on seawater intrusion (SWI) are mostly based on the assumption of a homogeneous coastal aquifer. In this study, we extend those studies by investigating SLR-induced SWI in a layered coastal aquifer using the analytical method developed by Strack and Ausk (2015). We provide analytical solutions for steady-state SWI in confined and unconfined coastal aquifers, where both constant-head and constant-flux inland boundary conditions are considered. The analysis based on a three-layer aquifer indicates that in general aquifer stratification affects either or both the initial location and response distance of the interface toe. Specifically, for flux-controlled unconfined coastal systems, the toe response distance driven by SLR is a linear function of the hydraulic conductivity of the top layer and independent of hydraulic conductivities of lower layers. Using an equivalent homogeneous hydraulic conductivity (derived based on the initial interface toe location before SLR) would result in overestimation or underestimation of the toe response distance, depending on the hydraulic conductivities and thicknesses of the layers. For flux-controlled confined layered coastal systems, by contrast, SLR can not cause variation of the steady-state interface toe location, which is consistent with previous findings for homogeneous coastal aquifers. The interface toe location in head-controlled layered coastal systems is only a function of relative hydraulic conductivities between the layers. Moreover, the effect of the layer thickness on the interface toe location and response distance in the head controlled system exhibits a more complicated pattern than in the flux-controlled coastal system, as changing the layer thickness changes both the overall aquifer transmissivity and inland freshwater flux. The results obtained enhance the understanding of the impact of SLR on SWI, which could provide a first-order assessment tool for relevant practitioners.

中文翻译：

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评估海平面上升对层状沿海含水层中稳态海水入侵的影响

先前关于海平面上升 (SLR) 对海水入侵 (SWI) 影响的研究大多基于均质沿海含水层的假设。在这项研究中，我们通过使用 Strack 和 Ausk（2015 年）开发的分析方法调查层状沿海含水层中 SLR 引起的 SWI 来扩展这些研究。我们为承压和无承压沿海含水层中的稳态 SWI 提供分析解决方案，其中考虑了恒定水头和恒定通量内陆边界条件。基于三层含水层的分析表明，一般而言，含水层分层影响界面趾部的初始位置和响应距离中的一个或两个。具体而言，对于受通量控制的无约束沿海系统，SLR 驱动的趾部响应距离是顶层水力传导率的线性函数，与下层水力传导率无关。使用等效的均匀水力传导率（基于 SLR 之前的初始界面脚趾位置得出）将导致脚趾响应距离的高估或低估，具体取决于水力传导率和层的厚度。相比之下，对于受通量控制的封闭层状沿海系统，SLR 不会引起稳态界面趾部位置的变化，这与先前对均质沿海含水层的发现一致。水头控制的分层海岸系统中的界面趾部位置仅是层间相对水力传导率的函数。而且，在水头控制系统中，层厚对界面趾部位置和响应距离的影响表现出比通量控制沿海系统更复杂的模式，因为改变层厚会改变整体含水层透射率和内陆淡水通量。所获得的结果增强了对 SLR 对 SWI 影响的理解，可为相关从业者提供一阶评估工具。