HGS RESEARCH HIGHLIGHT – A Comparison of Sea-level Rise and Storm-Surge Overwash Effects on Groundwater Salinity of a Barrier Island
Frederiks, R. S., Paldor, A., Carleton, G., & Michael, H. A. (2024). A comparison of sea-level rise and storm-surge overwash effects on groundwater salinity of a barrier island. In Journal of Hydrology (Vol. 644, p. 132050). Elsevier BV. https://doi.org/10.1016/j.jhydrol.2024.132050
“Because Hydrogeosphere is an integrated model, hydraulic head is calculated across the surface domain using the Saint Venant equations. Thus, we only apply the specified head boundary to a single node in the surface domain on the bay and ocean sides of the model.”
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In this research highlight, researchers explored the impacts of storm surge overwash and sea-level rise on groundwater salinization at Assateague Island, a low-lying barrier island on the U.S. mid-Atlantic coast. The study used HydroGeoSphere (HGS) to simulate the coupled surface and subsurface flow processes that influence the island’s aquifer system. By modelling future sea-level rise and storm-surge events, the researchers aimed to better understand the long-term effects of climate change on groundwater resources, particularly the vulnerability of freshwater lenses to salinization.
Fig 4: Salinity distribution as a fraction of seawater salinity. a) 80 years of sea-level rise b) 40 future 2-year storm events.
The HGS model incorporated multiple scenarios, including steady-state conditions, long-term sea-level rise, and transient storm-surge events projected for the year 2080. The simulations revealed that Assateague Island’s aquifer is highly vulnerable to both sea-level rise and storm-surge overwash, with storm surges predicted to have a more significant impact on groundwater salinity than sea-level rise alone. Specifically, future storm surges could salinize almost the entire coastal aquifer, while sea-level rise would affect up to 80% of the freshwater lens. This suggests that storm-surge events will play a critical role in the degradation of groundwater resources on barrier islands under future climate conditions.
The study also showed that hydraulic conductivity and the thickness of the unsaturated zone are key factors in determining the aquifer’s vulnerability. In low-conductivity areas, salinization due to sea-level rise progresses more slowly, but storm-surge overwash can still significantly increase salinity in the system. The HGS model allowed researchers to track saltwater infiltration and the movement of the freshwater-saltwater interface over time, providing valuable insights into how the island’s aquifer might evolve under different climate scenarios.
This research highlights the need for coastal groundwater management strategies that account for both sea-level rise and storm-surge events. The findings underscore the importance of using detailed, process-based models like HGS to assess the complex interactions between surface and groundwater systems in coastal environments. By integrating hydrological, geological, and climate data, the study offers a comprehensive assessment of the future risks to freshwater resources on Assateague Island and similar coastal systems.
Abstract:
Coastal fresh groundwater is threatened by salinization due to both sea-level rise and storm-surge overwash. Most studies of subsurface saltwater intrusion focus on sea-level rise, but storms that are becoming more frequent and intense with climate change could have more imminent and permanent effects on aquifer salinity. Few studies directly compare saltwater intrusion due to sea-level rise with saltwater intrusion due to storm-surge overwash, and no studies address this issue on barrier islands. In this study, a hydrological model simulating coupled, variable-density, surface and subsurface flow and salt transport was developed and calibrated to water level and specific conductance data at Assateague Island, MD. The calibrated model was used to calculate the mass of salt and the total volume of aquifer salinized in 2080 due to both sea-level rise and more frequent storm surges. The results suggest that the total mass of salt that intrudes into the aquifer due to changes in the 2-year storm surge height is approximately equal to that of sea-level rise (~300 kg), and the surges salinize nearly the entire aquifer, exceeding the volume salinized by sea-level rise (~80%). The influence of aquifer properties and unsaturated zone thickness was investigated by reducing aquifer hydraulic conductivity, resulting in greater salinization from storm-surge overwash (100% of aquifer volume) than from sea-level rise (~20%). Higher storm surges are expected to overtop the dunes on Assateague by 2080 causing a 75% decline in areas above the 2-year storm event while sea-level rise will only inundate ~40% of the land area on Assateague. This analysis suggests that groundwater is likely more vulnerable to storm-surge overwash than to sea-level rise induced salinization, even in systems where sea-level rise is expected to be most severe.