The role of the groundwater to the Chesapeake Bay Storm Surge Modeling

Increased storm activity has been observed in the Atlantic basin in recent years due to the growing effects of climate change. These storms disrupt the hydrological cycle and pose a serious threat to the communities along the coastal regions. Most recently, the Baltimore-Washington D.C. area was hit by a major storm on October 29, 2021, which caused widespread coastal inundation along the low-lying waterfront communities. A combination of heavy rainfall, up to four inches in some places, and intense winds, reaching up to 60 miles per hour, resulted in a storm surge that produced record high tides and one of the most extreme flooding events in the region since Hurricane Isabel in 2003 which brought approximately 2.1 m flooding. The water level reached 1.27 m at high tide during the 2021 storm, compared to a usual height range of 0.3-0.45 m. Along with the inundation of roads, residential and commercial infrastructure, the residual effects from the storm triggered significant environmental imbalance impacting surface and subsurface water quality. These immediate storm-driven impacts combine with more systematic pressures that include declining groundwater level resulting in seawater intrusion, land subsidence, and contamination. Besides, climate change will affect coastal groundwater resources due to the mean sea level rise and an increase in storm intensity and frequency. Increasing saltwater intrusion from the subsurface as well as intrusion into aquifers from land-surface storm surges is expected. On the contrary, nitrogen concentration from the groundwater discharge to the tidal creeks possesses significant impacts on the coastal waters.

On account of a direct correlation between the ocean and the coastal groundwater table, higher sea level is going to raise the water table as well and eventually aggravate coastal flooding. Thus, shallow coastal aquifers have a potential to undergo groundwater shoaling and emergence as sea level rises, leading to increasing coastal inundation and releasing harmful contaminants. Additionally, the impact of contaminated groundwater on the surface water quality is worth an investigation. These factors will ultimately trigger potential threats to existing infrastructure, ecosystem, and designed climate adaptations in more widespread areas compared to most flood maps. A comprehensive perspective on integrated surface water and groundwater modeling including the intrusion mechanism is essential to understand the hydrological and biogeochemical processes of these two interconnected systems.

The interaction between surface water and groundwater is an important process during water circulation in watersheds including rivers, lakes, reservoirs, wetlands and estuaries. More often, the surface water modelling system does not consider groundwater influence. Frequent storm surges increase groundwater level and specific conductivity. Connected aquifer has the greater salinized extent and shorter recovery time. Storm surge events like Katrina and Rita witnessed a decrease in the Ca/Mg ratio and elevated chloride concentration right after the storm and which returned toward pre-hurricane values in nearly about six months.

According to recent studies, sea level rise contributes to increasing occurrence of saltwater intrusion by raising the interface between intruding saltwater and overlying freshwater. Thus, shallow coastal aquifers have a potential to undergo groundwater shoaling and emergence as sea level rises, leading to increasing coastal inundation and releasing harmful contaminants. Hence, a comprehensive understanding on coupled surface water and groundwater modelling is attempted under this study to understand the seasonal hydrological interference during fair and extreme weather events.

For the study, the Finite Volume Coastal Ocean Model (FVCOM) model is implied over the Chesapeake Bay area by coupling of groundwater/hydrological modeling system and surface water modeling system. To develop a coupling modeling system for the Chesapeake Bay region, meteorological forcing is adopted from NCEP North American Regional Reanalysis (NARR). The model is stimulated with tidal elevation at the open boundary from TPXO global tidal solutions. To incorporate the effect of hydrology, and groundwater to the Chesapeake Bay dynamics, precipitation rate (NARR) is included in the model domain, river discharge (USGS) is forced at the river mouths, and groundwater flux (PARCI) is forced at the coastal boundary (Figure 1b). A schematic diagram for model input and outputs are listed in Figure 1a.

The results decipher vulnerability of Chesapeake Bay and the adjacent lands to the storm surge and coastal flooding which are noticeably influenced by precipitation, river discharge and groundwater flux. High water level (0.4 m higher above normal tidal level) was noticed at the Baltimore Harbor area even after reduced wind speed (~10 m/s) of hurricane Ida (Figure 1c).

The annual variation of groundwater in the Chesapeake Bay conditioned to climatological mean groundwater flux at the coastal boundaries. Initial results show monthly variations (Figure 1d) after the model spin up for about six months. The physical mechanism and governing factors associated with the monthly variations are still under investigation. In future, the project looks forward to understand the impacts of the storm surges and precipitation on groundwater flux and vice versa in the watersheds. Scientific questions such as the impact of anthropogenic activities and changing climate scenarios on coupled groundwater and storm surge system are yet to be explored.