Large weather events, such as tropical cyclones and nor’easters, exacerbate coastal flooding.

Coastal inundation

Article by Adam Thomas Photos by Lisa Tossey June 02, 2022

UD professor examines how the Mid-Atlantic coast is impacted by tropical and non-tropical weather events

Residents on the Mid-Atlantic coast face a dual threat when it comes to coastal flooding, which is one of the most costly, devastating and pervasive natural hazards in the region.

Not only has the Mid-Atlantic experienced increased rates of sea level rise, the area also gets hit with large tropical weather systems, such as hurricanes, as well as battered with non-tropical weather events — midlatitude weather systems such as nor’easters like the one that hit the Delaware coast in mid-May.

These large weather events exacerbate coastal flooding, and when combined with the higher rates of sea level rise, they pose a threat to human life, damage natural and human-built critical infrastructure, erode beaches, and disrupt important ecosystems found along the coast.

John Callahan, climatologist and visiting assistant professor in the University of Delaware’s College of Earth, Ocean and Environment, was the lead author on three papers published in the past year that focused on these large-scale weather events to see just how much coastal areas — particularly the Chesapeake and Delaware Bays — are inundated by tropical and non-tropical weather events. Dan Leathers, professor and Delaware State Climatologist, was a co-author on all of the papers, and Christina Callahan, scientist for the Center for Environmental Monitoring and Analysis (CEMA), was a co-author on two of the papers.

When combined with the higher rates of sea level rise, large weather events pose a threat to human life, damage natural and human-built critical infrastructure, erode beaches, and disrupt important ecosystems found along the coast.

The most recent paper was published in the Journal of Applied Meteorology and Climatology and compared extreme coastal flooding events from tropical cyclones and mid-latitude weather systems in the Delaware and Chesapeake Bays from 1980-2019.

Callahan looked at the past 40 years of measurements from several National Oceanic and Atmospheric Administration (NOAA) tide gauges in the Delaware and Chesapeake Bays. This helped him to quantify the storm surge — the rising sea as the result of atmospheric pressure and winds associated with a storm — from these large weather events.

While coastal flooding from tropical weather events tend to get a lot of media attention — and actually have a higher average surge level — Callahan said that midlatitude weather events can produce flood levels just as severe and occur much more frequently in the Mid-Atlantic.

“About 85 to 90% of our coastal flooding events here in the Mid-Atlantic come from the midlatitude events; they don’t come from the tropical cyclones and the hurricanes,” said Callahan. “You can get strong nor’easters that have just as high coastal inundation levels and cause just as much — if not more — damage than tropical cyclones.”

One of the reasons that the midlatitude events can cause so much damage is that, unlike the tropical systems that commonly impact coastal areas in the southeastern United States before hitting the Mid-Atlantic, the intensity and size of midlatitude events are most difficult to forecast and can strengthen quickly without much warning. Also, while tropical systems usually peak and are well-formed storms before reaching the Mid-Atlantic, a nor’easter can strengthen quickly right on or just off-shore of the region. Additionally, mid-latitude systems are often bigger in size, move slower, and remain over our region for longer periods of time.

Because they happen frequently in the cold season — from November to March — not much attention is paid to how nor’easters cause coastal flooding. Instead, more attention is paid to the amount of ice and snow and wind that the nor’easters bring and not as much focus is on the coast.

“Our attention is diverted between these other impacts or factors of these storms in the winter and spring, but this is where most of our coastal flooding comes into play,” said Callahan.

Three papers published in the past year by UD faculty focused on these large-scale weather events to see just how much coastal areas — particularly the Chesapeake and Delaware Bays — are inundated by tropical and non-tropical weather events.

That is not to say that tropical cyclones don’t also bring with them lots of coastal flooding. Callahan said that while these tropical systems tend to impact areas in the southeastern and Gulf of Mexico regions of the United States most frequently and on occasion areas just north of the Mid-Atlantic like New York and New Jersey, the data shows they also bring coastal flooding to the Mid-Atlantic, they just don’t occur very often.

Of the top 10 largest coastal flooding events in the Mid-Atlantic, tropical weather systems account for only 30-45% in the Delaware and upper Chesapeake Bays and 40-45% in the lower Chesapeake Bay. If you expand out further, tropical systems make up approximately 10-15% of all coastal flooding events.

Hurricane Sandy, for instance, when averaging the water levels at the tide gauges in Delaware Bay, caused the highest amount of storm surge in the Delaware Bay out of any tropical system in the past 40 years. Hurricane Gloria, in 1985, was second.

In the Chesapeake, Hurricane Isabel in 2003 had the biggest storm surge and Hurricane Sandy was second of all tropical systems in the past 40 years.

The research also showed that the upper Chesapeake Bay and upper Delaware Bay, as well as the lower Chesapeake Bay and lower Delaware Bay, are impacted similarly by the same storm systems. The correlation between the upper Chesapeake Bay and the upper Delaware Bay, for instance, is greater than the correlation between the upper and lower sections of the Delaware Bay.

Callahan said that it could benefit planning, public awareness and emergency management preparation to have the regions in the Northern Chesapeake Bay and the Northern Delaware Bay — as well as their southern counterparts — to do joint announcements and have their emergency management agencies collaborate on storm response.

“Right now, we don’t do that. It’s very state oriented,” said Callahan. “Whenever we have a system come in, we look at the whole Delaware Bay, and Maryland looks at the whole Chesapeake Bay. But it might be beneficial to focus on addressing the question: ‘Is this an upper bay event? Or is this a lower bay event?’ And maybe we can work between the states.”

Having been involved with research on sea level rise for the State of Delaware for many years, Callahan said it is important to have this information on coastal flooding available because when paired with the rising sea levels, there could be disastrous consequences for the First State.

“Sea level rise is just going to make the coastal flooding worse, including from both minor, high-tide flooding and major storm surge events,” said Callahan. “This is a type of hazard that we get along the coast that is not only the most destructive, but I’d say it’s the most frequent. And it will get worse in the future.”

The other two papers the team put together on coastal flooding in the Mid-Atlantic include Skew Surge and Storm Tides of Tropical Cyclones in the Delaware and Chesapeake Bays for 1980–2019 and Estimation of Return Levels for Extreme Skew Surge Coastal Flooding Events in the Delaware and Chesapeake Bays for 1980–2019.