Transience of Coastal Water Table Rise in Response to Sea-Level Rise

Coastal shallow groundwater is susceptible to adverse sea-level rise (SLR) impacts. Existing research primarily focuses on SLR-induced salinization of coastal aquifers. There is limited understanding of the magnitudes and rates of water table rise in response to SLR, which could lead to groundwater flooding and associated infrastructure challenges. This study used a variable-density groundwater flow model to quantify the transient movement of the water table in response to various SLR scenarios and rates, considering a range of aquifer parameters for both fixed-head and fixed-flux inland boundary conditions. The SLR scenario based on realistic and progressive SLR projections resulted in a smaller water table rise than the instantaneous or gradual SLR scenarios at 100 years, despite a final identical SLR. Rates of water table rise were always less than SLR, decreased with distance from the coastline, and were proportional to SLR. The magnitude and rate of water table rise in response to SLR were largest for fixed-flux conditions. It also took longer for the rate of water table rise to equilibrate after the commencement of SLR for fixed-flux conditions than for fixed-head conditions. As such, fixed-flux conditions represent a greater hazard for water table rise, and the maximum impact may not be experienced for decades. This delayed response poses challenges to planners and managers of coastal groundwater systems. Introducing a drain reduced water table rise more on the inland side of the drain than on the coastal side. Subsurface infrastructure may limit SLR impacts, but further effects need to be carefully considered.